NOTCam User's Guide
Content of this page:
Check also these links:
NOTCam is a multi-mode instrument for use in the short-wave infrared (SWIR)
region of the electromagnetic spectrum (0.8 - 2.5µm). It is based
on the Rockwell Science Center "HAWAII" array with 1024×1024×18.5µm pixels in HgCdTe. NOTCam is capable of:
Wide field imaging (4'×4' @ 0.234"/pixel)
High resolution imaging (80"×80" @ 0.078"/pixel)
Low-resolution (R = 2500) WF-cam spectroscopy (Y,J,H,K)
Medium-resolution (R = 5500) HR-cam spectroscopy (J,H,K)
In the future we expect to upgrade NOTCam with Wollaston prisms for
imaging polarimetry and spectro-polarimetry.
On entering the front window of the dewar, light passes through the
following components before impinging on the detector
engineering grade array (SWIR1) ,
first science grade array (SWIR2) ,
new science grade array (SWIR3)
- the aperture wheel
- At the telescope focus, carries
and masks. There are 4×50-mm and 8×18-mm slots.
- the collimator
- the shutter
- A rigid spinning disk.
- two filter wheels
- Each with 16 slots of 25mm diameter allowing
for the use of up to 30
- the pupil wheel
- At a pupil plane, carries Lyot stops and other
masks. There are 16 slots.
- the grism wheel
elements. There are 16 slots. Check
- the lens wheel
- Carries two sets of imaging optics, one for
each plate scale (the WF and HR cameras), and the pupil re-imager.
Since NOTCam is kept cold for long periods at a time, we do not change
filters, slits, grisms etc. more often than absolutely necessary (see
the current setup).
The original optical design is described
here (postscript document).
This entire assembly is mounted on an optical bench which is in thermal
contact with an LN2 tank. Additional cooling is provided by
a pulse-tube refrigerator (PTR) from Iwatani. The optical bench is
covered with a radiation shield and this and the LN2 tank are
surrounded by a superinsulating blanket. The final enclosure is the dewar
skin which maintains a high-quality vacuum surrounding the instrument.
Mechanics and Cryogenics.
The SWIR detector
The NOTCam detector is a
HgCdTe Rockwell/Teledyne "Hawaii" array with 1024 x 1024 x 18.5 microns
Together with the upgrade from SWIR1 to SWIR2 in December 2007, there was also
upgrade of the NOTCam Array Electronics.
The first science array, SWIR2, failed after 6 months and was replaced by a new
science array, SWIR3, which has been used since December 2007. Each detector
has (had) its particular characterics, described in the above links to the
detector verification pages. An
upgrade of the clock boards in January 2010 improved the performance of
the currently installed SWIR3 detector in removing the shift-register glow and
lowering the read noise.
The behaviour of the detector is continuously monitored in our
Detector Quality Control.
Schematic drawing of the detector with the four quadrants marked
in the numbering system used by Copenhagen University Observatory (CUO)
for NOTCam. (Note that Rockwell/Teledyne numbers from 1 to 4, but in the
same order.) The arrows show the direction of the fast readout and the
corners in which it starts. All four quadrants are read out simultaneously.
Left: Valid from 2001-2005. The lower left corner of quadrant
#1 is the location of pixel (x=1,y=1).
Right: Valid since Jan 2006. The lower left corner of quadrant
#0 is the location of pixel (x=1,y=1).
The x-flip was introduced together with the Multi Extension Fits (MEF)
data format in Jan-2006 in order to adopt to standard for MEF. In the
MEF quadrant numbering system, the right hand image is numbered as
follows: LL=1, LR=2, UL=3, UR=4. See fits headers.
The liquid nitrogen vent tube of NOTCam will spill LN2 when moving the
telescope in certain directions. Observations made at an airmass larger
than 2 and at given rotator positions (especially between -20 and -80)
may cause substantial spilling. A default telescope rotator angle for
NOTCam (field-r -90) was chosen to minimize this spilling.
Since January 2003 NOTCam is running on the PTR also while mounted on the
telescope, and the problem of spilling LN2 is no longer that critical in
terms of temperature stability of the detector. For typical nights the
detector temperature is as stable as +- 0.1 degree. Tracking a target at
very low airmass for a considerable time causes some spilling and the
detector temperature deviation will be larger, but in general this is not
a problem. NOTCam is re-filled with LN2 in the mornings in order to
make sure that any deviations due to spilling are corrected before the
NOTCam filling (internal page with limited access).
In 2009 the vent tube got an extension fitted such that LN2 will spill
from below the instrument to avoid dripping on sensitive parts.
Shutter monitoring - "the buzzer"
The first half year of NOTCam commissioning we had problems with a
malfunctioning shutter. Since Dec-2001 we have seen problems with it
only a couple of times. Since there is no closed-loop feedback on shutter
position a device was designed to warn the observer if
the shutter is not correctly positioned. This warning will be given as a
loud "buzzing". If this happens, you first have to check the behaviour of
the LED lights on the device to check if something really is wrong. The
device is found on the wall in front of you, and it has a reset switch
for turning off the "buzzing". The correct behaviour is that the right
LED is lit during every exposure and the left LED is lit during every
If the left light is merely blinking (not stable during the time of an
exposure), this is the first indication that the shutter is out-of-sync.
If this happens, try first to initialize the shutter by clicking on the
Initialize menu in the UIF, and then on the Init Shutter
button. If this does not help, then the array controller may need
resetting. Contact staff for this. It can also happen that the shutter hangs
open or closed. If the lights behave correctly, however, and the "buzzer"
still went off with a laudible alarm, just turn off the alarm by pressing
the reset button on the "buzzer", and continue to observe, as it probably
just picked up noise somewhere. Please, note that you can not turn off
the alarm during an integration, only after. Sorry!
The default field rotator orientation is field-r -90. Of course,
you can use any field orientation you want, which you will for instance if
you do spectroscopy on the parallactic angle. Originally, field-r -90
was selected as the default since it causes less spilling of LN2.
NOTCam orientation on the sky for different field-rotation values.
If you wish to rotate the image on the DS9 display to have North Up and East
Left, you can click on Zoom and then tick the Align button, or
alternatively, using the command line, you can toggle alignment on/off with
the sticky command notcam.wsc-align yes/no.
field-r -90 gives South left and East up in the images displayed
both on the BIAS DS9 (left) and the post-processing DS9 (right). Since
Jan-2006 when changing data format to Multi-Extension-Fits
(MEF) the stored images are flipped in X with respect to the images
in the real time buffer (left DS9). Note that the image shown on the BIAS
DS9 (left) is flipped in the display only in order to avoid confusion! (Its
X values are not flipped and X increases from right to left! NB! You
should not use the BIAS DS9 to measure positions!)
The command notcam.teloffset x y moves the target(!) a given number
of arc seconds in the detector X,Y directions independently of the
field-r orientation. For example, teloffset 10 10 moves
the target 10'' to the right and 10'' up on both the BIAS DS9 and the
For example, for field-r -90, the telescope(!) moves 10 arc seconds:
- South with teloffset 10 0
- East with teloffset 0 -10
- North with teloffset -10 0
- West with teloffset 0 10
If you are using another field orientation and want to know exactly in
which cardinal direction you are moving the telescope, you can do the
following test. Set previous image for sky subtraction on the DS9 with
setskysub 1 and take an exposure. Move the telescope 10'' in any
direction using teloffset, and take another exposure. On the DS9
you will see the current position and the previous position (negative)
and by clicking the Zoom and the Align buttons on the DS9
it should be evident in which direction the star has moved, meaning the
telescope has moved in the opposite direction.
If you are preparing your observations and you want another field orientation
it is advised to use our
Pointing Script Generator where you can indicate your sky position angle
in degrees and the corresponding telescope field-rotation is calculated.
Flatfields should be obtained in a differential mode in order to subtract
out the temporally and spatially variable components from thermal emission
and possible stray light. It may be possible to use the sky frames
(obtained from the target frames) for the flat fielding, as well, but then
the dark must be subtracted first. For broad-band imaging the airglow seems
to be spatially uniform over the FOV on time scales of at least a few
minutes. In principle, local sky frames can be used for flat fielding after
subtraction of the dark current. If this flat fielding strategy is chosen,
the target frames should be dithered in a random fashion
to avoid grid patterns in the flat field. The dark current measurements with
NOTCam are not properly understood, and we do not recommend correcting for
dark current by darks taken at different times. To be on the safe side,
it is always recommended to make differential twilight or dome flats.
In addition, such high signal-to-noise images can be used to evaluate bad
We recommend differential twilight sky flats rather than dome
(lamp ON-OFF) flats.
Dome flats are not perfectly flat at the NOT. Thus, if weather allows in
the evening, point to a selected
in the east about 10-15 min before sunset and then open the dome (i.e.
select a field with RA a couple of hours more than current ST to make sure
you avoid pointing towards the sun in the west). For the
morning twilight point to the west (RA of blank field being 2-3 hours less
than the current ST) about 30-40 min before sunrise and remember to close
the sideports first. Also, close the dome completely before parking
the building in the morning if you continue until after sunrise!
Use the NOTCam Sequencer observing script:
which takes 8 exposures of t seconds with 10'' telescope dithering
in between. A good value of t for JHKs and the WF-camera is 3-20
seconds. Too short exptimes are not recommended, and too long exptimes
will take too long. Run the script twice per filter in the order
KsHJYZ - KsHJYZ for evening twilight and oppositely for morning
twilight. In this way you have 8 bright and 8 faint images per filter.
There is a script (mkflat.cl) available in the
IRAF package notcam.cl to make master flats from
differential sky flats.
NB! It is essential to use the same integration time on the bright and
faint images to ensure a proper subtraction of the thermal background and
possible straylight contributions.
The New Science Array (SWIR3) flatfield (WF camera, Ks-filter) is
flat to 1% in 20x20 pixel areas and flat to 3% across the FOV, and looks
about the same in all filters. Flat field corrected science frames are
typically flat to 0.1%. See
SWIR3 flat field.
The observing script notcam.skyflat t should take about 3 minutes
to perform. This short duration ensures a sufficient sky gradient
for each filter within 10-15 minutes elapse between the first and second
measurement, which is acceptable for subtracting the thermal contribution,
while it allows time for 2 cycles of 3-5 broad band filters.
The script notcam.skyflat automatically saves the raw NOTCam flats
to the /data/service/calib/ directory.
If you could not get skyflats for all your filters, the second best option
is to use differential domeflats. Because of the structure on the inside of
the hatch, we recommend setting the telescope to alt 45 or so, where
the inside of the dome is a bit "flatter". Differential dome flats can be
taken with the NOTCam QC lamp (adjustable from the control room) ON and OFF
in each filter. NB!
The integration time must be the same for the ON and OFF frames. For the
HR camera, it is useful to use mexp 20 10 for all filters, lamps
turned on full for K-band and a bit down for J and H. For the WF camera
mexp 10 10 is useful for the same adjustment of the lamps.
There is also a NOTCam skyflat archive with master
flats for the most commonly used filters taken at different dates.
Since 2005 a focus pyramid is installed in the grism wheel of NOTCam.
We recommend to focus every evening using the WF-camera and the Ks band
and apply the focus offsets for the other filters as found in the table
below. This is done automatically when using the setup
scripts notcam.setup-ima and notcam.setup-spec,
where the corresponding focus-delta offset is applied, taking as a
reference the focus found for WF camera imaging in the Ks band, which
is defined as the default NOTCam focus.
The focus offset is wavelength dependent so for the narrow-band filters
we use the focus offset for the closest broad band filter.
- If you have a suitable target for focusing you can use it or go to one
of the NOTCam standard stars that are high up. Autoguiding starts
automatically if you use:
where my-object is the name of the object in the TCS catalogue.
- The following scripts sets up
the instrument for focusing the WF and HR camera, respectively. It puts
in the focus pyramid and the Ks band filter and sets the internal focus
and the aperture mask according to the camera chosen.
where camera is WF or HR.
- Take an exposure
- Move the telescope about 15'' in any direction to be able to do sky subtraction.
teloffset 0 -15
- Set the sky-subtraction mode to use this image for subtraction and take a
- Determine the offset from the best focus, manually or automatically, or both.
The value that comes out is the offset from the actual focus position. You must
add this number to the current focus value to get the new focus value.
- Change the telescope focus tcs.focus-position <new-focus-value>
and iterate at least once. Don't forget to take the focus pyramid out.
Imaging mode focus offsets found in good seeing (fwhm:0.3"-0.5").
| WF Camera
|| HR Camera
|| Internal camera focus (fixed)
|| 22560 *)
|| 22560 *)
|| Telescope value for K,K',Ks
|| K, K', Ks
*) This is the default NOTCam telescope foc-pos value, but it may
vary by up to around 100 units, and this is the focus value you should
determine every night by focusing the telescope in the Ks filter.
Usually, the accuracy of the focus correction is around 10-15 steps, but
note that with a seeing of FWHM=0.3'', telescope focus steps as small as 5
units make a difference.
Remember to take out the focus pyramid when finished.
If you are doing
NOTCam spectroscopy the internal focus setting differs between camera,
filter, and slit used, and the easiest way to set both the internal camera
focus, as well as the telescope focus offset, is to use the setup script
notcam.setup-spec. Also for spectroscopy, the telescope foc-pos
value should be set to the default or the one found for the night by doing
focusing for the WF camera and the Ks band.
The sky background
The most critical step of infrared observing is to make sure that the high
sky background can be properly subtracted. The sky background (or more
correctly: foreground) is a combination of the atmospheric emissivity
(airglow, mainly vibrational transitions in the OH radical between 0.9 to
2.5 micron), thermal emission from telescope and
atmosphere (dominates longward of 2.3 microns), and any possible straylight.
This combined background varies significantly, both temporally and spatially.
It is therefore essential to allow for subtraction of a local (in time and
space) sky. A local sky frame can be obtained by median filtering several
dithered frames. Depending on the type of target, proper background calibration
puts certain constraints on both the exposure times and the observing strategy.
Some kind of dithering technique is always applied,
either small step dithering around the same field, repeated raster scans or
mosaics, or "beam-switch" mode.
Average sky background level (electrons/s/pixel) with the New
Science Array (SWIR3, gain=2.5e-/adu). Note that the sky background is
|| 110 ± 28
|| 618 ± 100
|| 706 ± 66
This corresponds to roughly J = 15.9, H = 13.9, and
Ks = 13.2 magnitudes per square arc second.
Individual exposure times - Number of
The minimum recommended individual exposure time is the one which
gives a background limited rather than read-noise limited signal. We
define background limited performance (BLIP) when the signal is
higher than 3 times "the dark plus the square of the read noise". The
read noise is around 10 electrons, i.e. close to the detector
specifications given by the manufacturer. The effective readout noise is
expected to be even less for the ramp-sampling readout mode (see below
data-acquisition.) For a monitoring of the readout noise check the
following links (one for each readout mode):
- Monitoring of
- Monitoring of
The absolute minimum exposure time handled reliably by the cold shutter is
0.6 sec. However, the shutter has a tiny delay. Without correcting for this one
the accuracy of a 2 sec integration is only 3%. For exptimes larger than
3 sec the accuracy is 1%.
The sky background is high in the near-IR, and a sufficient amount of counts
to reach BLIP is usually obtained within a few seconds for broad band imaging.
For narrow-band imaging, on the other hand, the time to reach BLIP should be
Zeropoint magnitudes and maximum exposure time to have the
sky backgrounds within linear range. The last column gives the exptime
which saturates on the
background for a cold winter night. Data from 13-Dec-2007 (SWIR3).
| Linear time
| Saturation time
For more information on sensitivity and zeropoints with recent updates
check page: NOTCam sensitivity .
The maximum individual integration time is the one where the signal
is well below saturation (which starts at 56000 ADU for the new science
grade array), and preferrably in the linear range of the detector, which is
somewhere below ~22000 ADU (for the new science grade array). For stars the
maximum integration time you can use without exceeding these limits is
highly seeing dependent. In practice, the limiting factor for faint sources
is usually the need to move the telescope (dither). It is
stay at most 1-2 minutes per sky position before making a dithering offset
to allow for good sky subtraction.
The background is highly variable, and strongly temperature dependent for
the K band. The airglow lines are most prominent in the H band and their
brightness may change by up to 50% during a night, while the typical amplitude and
period are 10% and 5-15 minutes, respectively. For a typical cold winter
night with an ambient temperature of 3 deg Celsius, the background will
saturate in individual exposure times of 1000s, 235s, and 160s for J, H,
and K, respectively. These saturation times will be shorter for warm summer
nights. In addition, you will typically want to have the background within
the linear range of the detector, limiting the exposure times to 360s,
80s, and 70s in J, H, and Ks for cold winter nights. In warmer nights,
when the water vapor content is higher, or in the case of thin clouds,
you may have to limit the exposure times much more, for instance 30s in
H and Ks is sometimes found to be the limit.
The detector is non-linear in response, and staying below ~20000-25000
adu only means staying inside the range where the non-linearity is < 1%.
It is possible to correct for the non-linearity, pixel by pixel, and the
non-linearity correction coefficients can be downloaded form the
NOTCam Calibrations page.
The memory (or
charge persistency) effect is less than 1% and cleared on the first
read for non-saturation levels. For saturated pixels, however, there is
a positive memory which persists for many reads, although at a relatively
moderate level of less than 0.03% in the 6th readout. It is recommended
not to saturate the array, if possible. If impossible to avoid, and if
the saturated pixels are close to your target, then it is recommended to
clean the array with the command clear which resets and reads the
array once without showing or storing the image and which
takes 3.5 seconds to perform. Depending on the expected severity of the
memory, the clear command may be applied once or several times. There is
also a script called notcam.clean3 which makes 3 clear commands and
then a dark 0 to show you the result.
It is generally stated that as long as one is in the BLIP regime, it is
better to take many co-averages to increase signal-to-noise, rather than
increasing the individual integration time. This is not entirely practical
with NOTCam. The reason is the large overhead owing to a relatively long
readout time (3.6 sec per read, 7.2 sec in total per image) limited by the
current detector controller. We expect a readout time of < 1 sec with the
Note that the ramp-sampling mode gives a large
dynamical range, since you save a number of integration times in the fits
file (see below). If you saturate (or enter non-linear mode) for very bright
targets while you want to go deep for faint targets in the same field,
you can select to use one of the first reads in the cube for the bright
Note, however, that saturation will produce memory effects in several
consequtive images, and you may want to add in the clean array command
clear once or more between each exposure to minimize this effect.
Because of the variable sky background, it is recommended to stay as short
time as possible on each sky position before dithering. In practice, one must
make a compromise for observing efficiency. Also, the quality of the night can
be quite variable, and this is normally assessed only when reducing the data.
Thus, to be on the safe side, if you stay about 1-2 minutes per sky position
for broad-band imaging, you'll have 10-20 different sky position frames to
average within a 10-20 minutes timespan, which is reasonable in terms of the
spatial and temporal variability. On time-scales of a few minutes the airglow
seems to be spatially uniform over the field of view of the detector, at
least for broad band imaging.
Make sure that you repoint the telescope to the start position at the end of
every observing script, or to any position you want, such that you keep track
of where you are upon possible repetitions of the macro. Between every
repetition of a dither pattern you may do a manual offset to some roughly random
direction by just typing notcam.teloffset x y in the Sequencer window.
In this way the same pattern is not repeated on the same pixels. You can use
our available template scripts such as for instance:
all described under
sequencer scripts. For all scripts you chose the readout mode and the
exposure time, the dithering step size, and in the case of beamswitch, the
beamswitch step and direction. It is also possible to tilt/skew the grid to a
desired extent with the skew parameter, which is useful in order to avoid
that the same source falls repeatedly on the dead column.
- notcam.9point (a 3x3 grid)
- notcam.5point (a 5-point dice)
- notcam.beamswitch (a beamswitch script for extended sources)
- notcam.abba (ABBA dithering along the slit for spectroscopy)
In general, autoguiding is always recommended. Note that since 2007 there is
no additional overhead in dithering sequence owing to autoguiding.
Script Generator is a useful tool which also gives you an idea about the
For point sources the most efficient method is to obtain an image from a set
of slightly dithered images. One or more images are obtained per sky position
before moving the telescope. The total time per sky position should be at
most a couple of minutes. It is recommended to use a step size of at least 10
times the FWHM of your objects. This permits using the target images themselves
to evaluate a local sky frame to be subtracted (median filtering). Therefore
this method is more time-efficient than taking off-field sky images, which is
needed for extended objects. A frequently used mode is the 3x3 dither with
10'' step sizes. For standard stars and bright sources the 5-point (dice) mode
is usually sufficient.
If the target is not contained fully within one field, make a mosaic. The
mosaic can be made as a raster scan which can be repeated again and again
until you reach the desired sensitivity. Note that you should dither the
telescope a small amount between each repetition of a raster scan to be
able to remove the bad pixels and bad features from your sources.
For each scan, depending on its size, all or groups of the target frames
can be used to create a local sky frame (provided they don't contain
extended emission). If there is a small amount of extended emission in a
small fraction of the N mosaic frames, these can simply be excluded when
making the sky frame.
Note that depending on the dither steps you plan to make, the corresponding
guide star area must be selected when doing target acquisition. This is
explained in the Autoguiding section in the
Step-by-step observing guide.
Extended sources or crowded regions
If the target is extended or very crowded (e.g. a strongly centrally condensed
globular cluster), then small step dithering is not sufficient. In this
case, so-called "beam-switch" observations are required. This means
observing one or several sky fields off the target itself. The off-field(s)
must be sufficiently away from the target that no extended emission is
included. Observations must be alternating between target and sky staying
at most a couple of minutes in each position, and each sky and target
position must dithered (step size at least 10 times FWHM) to filter away
stars (and bad pixels). Note that the Script Generator is not yet capable
of producing beam-switch scripts, but there is an available template script
called notcam.beamswitch that can be used.
NOTCam data acquisition
With NOTCam there are three different commands to start an exposure. Users
familiar with BIAS
(Brorfelde Image Acquisition System) will recognize two of these from
the BIAS versions in use with CCDs (exp, mexp). For IR arrays
the data acquisition is different, however. In contrast to CCDs, where
the same Si array is used to collect the photons and to read out the photo
electrons, IR arrays are hybrid devices with one array for the detection
of photons (HgCdTe layered on a a sapphire substrate) and another for
reading the photoelectrons (the multiplexer constructed using conventional
Si based CMOS technology). The two are manufactured separately, then aligned
and connected pixel by pixel with small bumps of indium.
Each individual photodiode (the IR sensitive HgCdTe) and its corresponding
electronics on the multiplexer (together comprising one pixel) possess a
given electrical capacitance C. The voltage V across this
capacitor is set to a given value when the pixel is reset. The reset
switch on each single multiplexer circuit is closed at the beginning of an
integration, thus charging the capacitance. During an exposure, the reset
switch is opened and incoming photons release electrons which discharge the
capacitor ( dV = dQ/C). At any time during the integration
(before the next reset) the multiplexer can read the detector, i.e. measure
the voltage across each capacitance, without changing its value. This is
called non-destructive readout.
The multiplexer is divided into four independent quadrants, each with its own
clock and bias lines and its own output amplifier. Each quadrant is read
out simultaneously. There are many different readout modes for multiplexed
arrays. With NOTCam we offer two modes:
- A basic reset-read-read mode (also called
What it does is: reset the capacitor, read out immediately after (the
so-called reset frame), then integrate for as long as wanted, then read-out
again after the end of integration. This method efficiently eliminates any
possible long-term drifts as well as the kTC noise, which arises from
a fundamental limit to the accuracy of the reset process and causes an
uncertainty in the voltage level. One exposure thus consists of two readout
frames, one reset frame which must be subtracted from the end of
exposure frame in order to obtain the desired signal. This subtraction
is done on-line, but the reset frame is kept and stored as a second extension
in the [1024,1024,2] MEF file. This readout mode is used with the two
commands exp, mexp.
When using the
mexp t N command, be aware that there is usually an anomaly with
the first of the N images. This one should then be skipped in
the final averaging.
- More sophisticated is the ramp-sampling
mode, where the signal is sampled many times during the exposure in order to
reduce the noise. A linear regression analysis applied to the data points
provides a mean flux rate and decreases the noise by a factor sqrt(N) where N
is the number of such multiple non-destructive readouts. In NOTCam this mode
is used with the exposure command frame t N, which produces a final
image of exposure time t * N seconds. What the command does is: reset
the capacitor, read out immediately after, then integrate for as long as
wanted, and during this integration read out at every time step t.
The minimum value of t is 3.6 (since the readout time is 3.6s) and
the maximum number of N is 14. The frame command produces one output
file, a MEF fits file with N+2 extensions. The first image extension
is the final result of the linear regression analysis, then follows the
N non-destructive read outs (reset-subtracted), and finally the reset
We strongly recommend to save all individual images.
The different exposure modes with NOTCam.
| Read mode
|| Output files
|| Order in fits cube or
Image extension order
| exp t
|| 1) reset subtracted integration
2) reset frame
| mexp t N
|| N * [1024,1024,2]
| N files as above + 1 average image
| frame t N
|| t * N
|| 1) final image (result of lin. regr.)
2) first non-destructive read
N+1) last non-destructive read
N+2) reset frame
To each of the three exposure commands there is an associated dark exposure
command, which works in the same mode but leaves the shutter closed. These
dark t, mdark t N, and dframe t N .
Please, note that the dark current measured with NOTCam is non-linear,
variable and higher than the specifications (which state less than 0.1
NOTCam Darks - internal summary for more details about the darks, and
NOTCam Calibrations for examples of dark images.
For imaging in the background limited case, we thus do not recommended to
correct for the dark level by taking separate dark images that are aquired
at a different time. By using differential flats and sky subtraction, as
usual in infrared image processing, the dark level will automatically be
For spectroscopy mode, however, we recommend to take a set of darks with
the same readout mode and integration time as your observations, although
spectroscopy is also done in differential mode (subtracting a dithered
image). But since the number of dithers is less, for practical reasons,
darks are useful to estimate a hot pixel mask. The number of hot pixels
increases with the integration time. For this purpose, the calibration
script notcam.notcam-calibs at the end of the night is useful.
NB! Please, note that the linear regression analysis image for darks taken
with the dframe t N command is not reliable, since the dark current
is not linear (but this is not important for finding hot pixels).
What are the overheads?
The total time it takes from one exposure command to the next is
composed of a subset of overheads due to readout, file-storage,
real-time display, plus telescope offset in case the observations
are performed with dithering.
Note that the reason for having two readout overheads for the exp/mexp
commands is the use of the shutter to define the exposure time. Note that for
the frame command the exptime = t x N, while for the rest the
exptime = t.
- exp t: Ttot = Tseq + Tread + Texptime + Tread + Tsubt + Tshow + Tsave + Tteloffset
- mexp t N: Ttot = N * (Tseq + Tread + Texptime + Tread + Tsubt + Tshow + Tsave) + Tave + Tshow + Tsave + Tteloffset
- frame t N: Ttot = Tseq + Texptime + Tread + Tlinreg + Tshow + Tsave + Tteloffset
New! Since November 2011, the exp mode is
speeded up by letting the telescope move as soon as the shutter is closed, i.e.
while reading out, subtracting, showing on real-time display and storing the
data file (marked in red above). The measured total time spent is lowered as
follows: Tnow/Tbefore = 0.76 for exp 3 and 0.82
for exp 10. This does not apply for staring mode observations.
- Tseq = 0.6 - 1.5 s (sequencer startup, variable)
- Tread = 3.6 s (controller readout time)
- Tsubt = 0.6 s (subtract reset frame, get header info, etc.)
- Tave = 0.6 s (calculate average image etc.)
- Tlinreg = 0.6 s (pixel by pixel linear regression analysis, get header info etc.)
- Tshow = 0.3 s (minimum time for real time display on DS9, can be switched off)
- Tsave = 1.3 s + 0.1 s x size(Mb) (minimum time to store file)
- Tteloffset = 8s (for 10-15'' step, typical value when autoguiding, depends on how much is az and alt movement)
These are measured overheads for NOTCam (Feb 2011) :
| Single readout time (controller)
|| 3.6 s |
| Fits file storage (for 4 to 32 MB files)
|| 2 - 7 s |
| Total overhead for a exp t command
(see note 1)
|| 10-11 s |
| Total overhead for a frame t N command
(see note 2)
|| 8-13 s |
| Clean-array command clear
|| 3.5 s |
| Telescope dither 10" (ag-off)
|| 3 - 9 s |
| Telescope dither 10" (ag-on)
|| 7 - 9 s |
| Telescope dither 30"
|| 6 - 8 s |
| Telescope dither 100"
|| 8 - 10 s |
| Telescope dither 240" (ag-on)
|| 8 - 20 s |
| Changing between the WF (4'x4') and the HR (80"x80") camera
|| 20 s |
| Changing filter (see note 3)
|| 2 - 28 s |
| Changing grism (see note 3)
|| 2 - 28 s |
| Changing aperture item: pinhole/slit/mask
(see note 3)
|| 2 - 28 s |
Note 1) Time to add to the exptime when using exp/dark . This includes
the time it takes to read the array twice (2 x 3.6 s) plus the time to
store the 4 MB file (2 s) plus any possible delays in the acquisition
system. From November 2011, when these exposure commands are followed by
a telescope dither, observing is speeded up by letting the telescope move
while doing the 2nd readout, saving about 5-6 seconds of total overhead.
This does not apply to staring mode observations. (During 2008-2010 the
overhead was ~ 13 s, while during April and May 2007 it was as high as
Note 2) This includes the time needed to read the array once (3.6 s, since the
other reads are interlaced with integration), to store the large file (up
to 7s for 32 MB), and any possible delays in the acquisition system
including the linear regression analysis (up to ~1.4 s).
Note that when the time between non-destructive reads (i.e. the t
in "frame t N") is < 5 s, there are internal delays in the PC-board
(see NOTCam BIAS
for details). This may increase the overhead with 2 s per frame
The following note was valid from 2007-2010:
The total time needed per command occasionally becomes much larger if the
program experiences hick-ups (up to 150s), a problem present from Apr-2007
to Oct-2010, for the frame command. Both the average overhead values
and the frequency of hick-ups were variable with time. This additional
readout overhead was quite impossible to predict.
Note 3) All wheels can be moved simultaneously! The wheels move in one direction
only. The two filter wheels, the grism wheel, and the stop wheel all take
~ 30 s to make one full turn.
NB! The time to do telescope dithers
has increased slightly since Nov-2007. The reason is the introduction of an
extra wait such that the telescope has come to a complete rest before
the next integration starts. Tested for 240" teloffsets and short (2 s)
integrations using the HR-camera in 0.3" seeing. The earlier appearance
of elongated stars in such extreme conditions has now been remedied. Note
that the exact time needed to make a given dither depends on how much of
the movement is in azimuth. In the above table are given the value ranges
measured on a number of tests.
The frame command is normally more time efficient than the mexp
command. This is due to the long readout time, and the use of a shutter to
determine the exposure time. If you need individual exposure times shorter
than 3.6 seconds, you must use the exp or mexp command.
Nominal overheads are calculated by the
Observing Script Generator.
If you wish to make your own dither scripts it is recommended to use the
Script Generator. Knowing your proposal ID you can upload your scripts
directly to your own directory on the data acquisition computer well in
advance of your run. There is also a number of
available NOTCam dither scripts that can be run directly from the
sequencer window with your own choice of input parameters. Some useful
examples are listed below:
- notcam.9point frame 6 8 my-object 10 2 3
This does a 9point dither (3x3 grid) and at each position takes an exposure using the
exposure command frame 6 8 (which gives a 48s exptime). The dither step size is
10'' and a skew of 2'' is applied to tilt the grid. The dither is run 3 times. Recommended
for deep imaging where it is vital that the sky can be well calibrated.
- notcam.5point exp 6 1 my-object 15 3 1
This does a 5point (dice) dither, and at each position takes an exposure using the
exposure command exp 6 (note that you need to add N=1 as input). The dither step
size is 15'' and a skew of 3'' is applied to tilt the grid. The dither is not repeated.
Useful for bright targets where you don't worry so much about the sky.
- notcam.beamswitch frame 10 10 my-object S-posX 240 10 2
This is a beamswitch mode going alternatingly ON and OFF target. At each position the
exposure mode frame 10 10 (giving an exptime of 100s) is used. The OFF field is
in the direction S-posX at the distance 240'' from the ON field. Both the ON and
the OFF fields are being dithered by 10'' step sizes in a 3x3 grid fashion, using a skew
of 2'' to tilt the grids. Useful for extended targets.
- notcam.loop-frame 3.6 3
Staring mode observing, using the exposure mode frame 3.6 3 (giving an exptime of
10.8s). The script is repeatedly taking exposures until interrrupted with Ctrl-C. The
same type of loop script exists for the exposure mode exp t.
Warning: When using the Script Generator to create scripts,
please, note that you must check yourself whether you have chosen an allowed
exposure mode. Also, we warn that using cut and paste from the output on the screen
in for instance a windows browser will add strange characters to the commands and
give serious problems in the execution of the scripts. Check your scripts well!
It is very useful to add one of the two following lines at the end of the
These are audible and visible alarms to the observer that the script has
Information about several calibration issues such as non-linearity correction
and distortion correction, archives and download info, is found in
For broad band photometry we recommend the JHK observations of faint standard stars in the Mauna Kea Observatories near-infrared photometric system,
Leggett et al. 2006, MNRAS, Vol 373, 781 (or for astro-ph/0609461).
These are the currently best measurements of 115 stars in the MKO JHK broad band
filters (which are used in NOTCam). Of these 79 are UKIRT standards and 42 are
from Persson et al. (1998, AJ 116, 2475).
See also: Northern JHK Standard Stars for array detectors by
Hunt et al. (1998, AJ 115, 2594) (or see local ps files: paper
and erratum ). Contains 86 stars in 40 fields, many of
which are faint enough to observe with the frame command. Another advantage is
that many of the fields have multiple standards.
We recommend using the same readout mode for the standard as for the target.
The data format
Data taken from Jan-2006:
Multi Extension Fits (MEF) format is now used. Major upgrade of the
FITS header keywords. For more information check the page
New data formats and FITS headers.
Note that the images are now flipped in the X-axis with respect to images
taken before Jan-2006.
Data taken before Jan-2006:
The data was stored in fits cubes. Check this example of an old
NOTCam FITS header which contains
explanations to some of the specific NOTCam header keywords.
Renaming of FITS header keywords Oct 2012:
Several instrument related fits header keywords changed name on October
the 1st 2012. This includes the old keywords APERTUR, FILT1, FILT1ID,
FILT1POS, FILT2, FILT2ID, FILT2POS, STOP, GRISM, LENS, CAMERA that are
now called NCAPRNM, NCFLTNM1, NCFLTID1, NCFLTPO1, NCFLTNM2, NCFLTID2,
NCFLTPO2, NCSTPNM, NCGRNM, NCCAMNM, NCFOCUS. For details see the
comparison between an image with old header
and new header.
Condensation on the entrance window
If during observations you get an image which looks like the following:
Condensation on the entrance window.
then there is water condensed on the entrance window of NOTCam. This can
happen if the humidity inside the dome is very high over a long time, or
if there is some problem with the vacuum in the cryostat. Observations
can not be continued. Call staff. NOTCam must be dismounted on its trolley
and lowered down to remove the condensation.
The appearance of a dust speck
The image below is an example of a large, out-of-focus particle. The image
is a ratio image of a WF-camera J-band masterflat from the 5/3-07 to that
of 4/3-07. The particle is just barely visible in raw images unless it
moves between the two images used to make a difference image. In the below
example the light attenuation effect is 3% in the worst part. If the
particle is moving around, its effect can not be flatfielded out, but with
a number of dithered images, the effect should diminish. If you see such a
feature in your images, please, submit a
about it. If the placement of the feature in the FOV is very disturbing,
it might be a good idea to move the telescope down and up, perhaps turn
the rotator with the telescope down, to try to get rid of it, although
this may not help.
Large dust speck. This is the ratio image
of a J-band masterflat from the 5/3-07 to that of 4/3-07. The largest
difference inside the dust speck is on a 3% level. The rest of the
image has a value of 1 within 0.1 to 1 %, and a standard deviation
around the mean of 0.004.
If the particle is on the entrance window, then NOTCam must be dismounted
and the entrance window inspected and cleaned. Before doing this, change
to the other camera and make a ratio image of domeflats with the two
cameras. This should reveal whether the particle is on the camera lens
inside NOTCam. In the example here the ratio image between a HR-camera
J-band domeflat and WF-camera J-band domeflat shows that the particle is
located on the WF-camera lens. See image below. Note that the feature has
moved from lower right to lower left quadrant. Since the particle is
inside NOTCam, we can not do much until the instrument is warmed-up and
Ratio J-band domeflat between HR-camera and WF-camera. In addition to
the large feature (now moved to the lower left quadrant) there are also
3 smaller and rounder features at about 1% level or so. The greyscale
is negative so the dark features means they are present on the WF camera
image and not on the HR-camera one.
If your images have a disturbing pattern of almost horizontal stripes this
could be the problem of pick-up
noise recurring. The problem was found to be caused by the power supply
in the electronics of the array controller.
The 9th of June 2009 event
On the 9th of June 2009 there was a sudden increase in the number of bad
pixels. This is demonstrated by the two WF-camera J-band master flats below,
where the first one is from the 8th and the second from the 9th of June.
The cause of this is not well understood, but it is believed to be related to a
shaking of the telescope caused by the failure of an altitude tacho during
powering on. The largest of the new bad-pixel areas have a very low response,
only about 40% of the surroundings. The many more lower response pixels are
probably caused by tiny dust particles deposited on the array after the
violent shaking of the telescope.
New bad pixels on SWIR3. Green circles mark already existing bad pixels,
while the red rings mark the new bad pixel areas appearing overnight. Note that
two hair-like features seem to have disappeared on the second image.
Also the flat-field structure changed at the same time. See below the ratio
of the flat in J-band from the 8th to that of the 9th (left). The response is
about 4% lower in the dark big area in the upper right quadrant. Probably this
is due to dust having fallen down on the entrance window. The entrance window
was cleaned on the the 10th daytime, and the ratio flat between the 10th and
the 9th is shown to the right. The remaining structure is due to (moving) dust
on the WF camera lens inside the cryostat. The amplitude in this case is at most
1.5%. The gradient in the lower left corner is 1%.
Ratio flats for WF-camera and J-band, between 8th and 9th (left) and between
9th and 10th (right). To the left is shown the dust on the entrance window which
in the darkest part has about 4% lower illumination than the surroundings. To
the right is the remaining structure after having cleaned the entrance window,
and this is due to dust specks on the WF camera lens that have moved slightly.
The peak to peak amplitude here is around 1%.
Long darks (i.e. dframe 60 6) taken with the internal focus setting below 1000
units, typically used with the HR-camera, have more counts and a distinct
structure compared to equally long darks taken with the internal focus setting
is at higher values, such as 5650 which is typically used with the WF-camera.
The median pixel value in the area [260:310,650:700] is 360 adu in the left
image and 16 adu in the right one.
The problem is currently under investigation.
360 second long darks using "dframe 60 6" taken with the internal focus value
at 20 (left) and internal focus value at 5650 (right). The left corresponds to
the detector plate being at the furthest distance from the camera wheel. This is
the focus value used for HR-camera imaging.
NOTCam Quicklook reduction package
NOTCam package in IRAF
A small NOTCam package of iraf scripts (notcam.cl) is available on the
computer florence in the control room. NB! This is not a pipe-line.
The purpose of these scripts is to take a quick, but reasonably fair look at
the data obtained.
Upgraded to version 2.5 in October 2012.
Note that you need this new version to work on data taken after October 1st 2012! This is due to renaming of FITS header keywords.
Upgraded to version 2.6 in December 2014.
Available for download.
||Make a differential master flat from bright+faint flats
||Make linearity correction pixel by pixel on raw images.
||Reduce a small-step dither observation
||Reduce a beamswitch dither observation
||Sky subtraction and flat fielding of a set of images
||Quick-look photometry of individual and combined images
NB! Note that the scripts reduce and reduce_bs does the
image registration satisfactorily only on data obtained after 15-Nov-2007,
since until then the RA/DEC fits header keywords were not accurate. This
was because of a bug in the output coordinates from the TCS, although the
actual TCS coordinates were accurate.
Note that for quick-look you don't have to make badpixel and masterflat
images, but may download these directly from our archive
and bad pixel masks.
You may also wish to download the correction files for non-linearity
and optical WF-camera distortion from our archive
correction models and
Get started as follows:
- In an xterm on Florence open an xgterm by typing xgterm &
- Inside the xgterm go to directory /home/guest/
- Start DS9 by typing ds9 &
- Start IRAF by typing cl (or ecl or ncl)
- Open the notcam IRAF package by typing notcam
- Type help to get a listing of the scripts.
- List script parameters using lpar and edit them using epar
Before starting the reductions you should be aware of the optical distortion
of the WF camera (see below) and see the note about NOTCam darks.
Note on the optical distortion of the WF camera
The optical distortion of the WF camera in NOTCam is significant, especially
in the corners.
From version 2.4 of notcam.cl you can select to correct the images for this
distortion in the scripts reduce and reduce_bs. These will need
the input file "notcam.db" which contains distortion models for the three
bands J, H and Ks for the WF-camera. These models are based on high-quality
(fwhm = 0.5'') data from 2009 of a stellar-rich field (~ 300 2MASS sources).
NOTCam Calibrations - Distortion Correction for more details.
An earlier NOTCam distortion model was provided in 2005 by Magnus
Gålfalk based on his observations of B335 made with FWHM=0.5''. The
model is calculated from more than 100 stars in the field using the 2MASS
catalogue as positional reference. It is available for download at the web
page http://www.astro.su.se/~magnusg/NOTCam_dist/. Please, note that images obtained after
Jan-2006 are flipped in X and this old model must be correspondingly adjusted.
The HR camera has an excellent optical quality all over. See the
image in the H-band obtained with fwhm = 0.44'' and ellipticity typically
0.02 to 0.05 all over the FOV. The pixel scale of the HR camera is
Note on NOTCam darks
Please, note that the darks are not fully understood. We do not recommend
using darks in the data reductions of broad band imaging. The dark current
is automatically subtracted out together with the sky. The dark is also
automatically subtracted out when you use differential flats (twilight or
dome). For long integrations in spectroscopic mode, we still recommend trying
to use darks obtained with the same readout mode and integration time as the
Bad pixel masks
Darks are useful for making bad pixel masks, though. Hot pixels, which increase
with exposure time, are easy to find on dark images, while cold pixels are best
extracted from well exposed flats.
It is strongly recommended to correct the
flat field images for the zero-valued pixels before making master flats.
This is now done inside the mkflat script, and you just have to enter
the bad pixel mask to be used.
Check the NOTCam
bad pixel mask archive for the mask called "bad_zero_sci.fits". See also
NOTCam calibration images for info about bad pixels.
pixel mask archive for bad pixel masks from different dates and
Note that image lists must be having only one image name per line. The
extension ".fits" must be removed, but the images extension number, for
instance , must be given. To make such lists you may use the IRAF
files NCqd29*%.fits%% > totlist
takes all images with prefix NCqd29 and exchanges the string ".fits" with
the string "" and pipes them to the file "totlist".
Master flats - imaging
A NOTCam script called mkflat is available for making master flats.
Make sure that there are about equal numbers of bright and faint images per
filter. Also, the input individual flats of high and low intensity must all
have the same EXPTIME, otherwise the differential approach will not work.
Since version 2.3 this is checked by the script which will abort if this is
not the case. Make sure that raw flats from morning and evening twilight
are not mixed together, as the script will not check this.
mkflat @skyflats flat_j bad_zero_sci wf j suppress=yes
The script searches in the input image list "skyflats" for images with
IMAGETYP=FLAT with FILT1ID/NCFLTID1 or FILT2ID/NCFLTID2 corresponding to your
input filtid and LENS corresponding to your input camid. In
order to have useful flats, make sure that your evening flats and
morning flats are separated in two lists, otherwise they will be mixed
and the thermal contribution will not be well subtracted. The script
sorts the selected images on brightness, subtracts faint from bright,
interpolates over bad pixels using your input badpixelimage, optionally
corrects the differential images for the "dc-gradient" (if suppress is yes)
and combines all the differential images. The master flat is normalized.
The output image is diplayed on the DS9.
Since version 2.4 it is possible to autosearch=no and in this case the script
will use all images provided in your input list without searching for camera
or filter ID. This can be useful for very old data with different keywords.
If for some reason you could not get any useful twilight flat observations
during your run, check the
archive for master flats from different dates for the most commonly used
configurations. Download these to your working directory and use them as
input in the following reduction scripts.
Note that the "dc-gradient suppression" is made on all
differential flats in the archive from December 2007 (prefix ql13). See
below for a comparison of master flats obtained with suppression (left)
and without (right). This is a differential twilight flat for WF camera
and Ks band.
For more information on the dc-gradient in differential
images because of a stronger reset-anomaly since the electronic upgrade in
December 2007 see some notes on the electronics upgrade.
If you are doing quick-look in the control room on the Florence
computer you will skip this point.
If you have exceeded the linear range of the detector, and you need good
photometric accuracy, you might consider correcting your target images
for non-linearity before further data reduction. NB!
This should not be done on the /data/notcam/ files from florence!
The script mklincor will take the raw image FITS file(s)
as input and append another FITS extension to the file(s), where the
linearity corrected version of the extension you select to correct is
stored. This requires you to have write permission to the raw data.
Note that you need two input correction coefficient images that can be
correction models, and more information is found in the document
NOTCam Calibrations - Non-linearity correction.
mklincor NCva0700* uj12lin-ba uj12lin-ca
The header of the addition FITS extension will store the keywords:
LINCOR-Y= 'This is the linearity corrected version of image extension: im1'
LIN-C-BA= 'Linearity correction coefficient BA used: ../nonlin/uj12lin-ba.fits'
LIN-C-CA= 'Linearity correction coefficient CA used: ../nonlin/uj12lin-ca.fits'
Reduction of a small-step dither - imaging
Note that the idea behind this script is that you reduce every typical dither
sequence (for instance a 9 point), one by one. The sky image is estimated
from all the target images in the dither and only flat intensity variation is
taken into account, not possible variations over the field-of-view. Therefore
too much time should not have elapsed since the first and the last image
included (this is strongly dependent on the sky conditions of the night).
Any small-step dither can be reduced using the script reduce.
The images are optionally trimmed to retain only the overlapping part.
If your single expose command per sky position is frame
or exp it is sufficient to give as
input the first image of the set, like in the below example, where we
find the incoming data in the /data/notcam/ directory:
reduce /data/notcam/NCpa140011 9 test flat_ks bad_zero_sci add median constant skip=no trim=no badpixfix=yes distcorr=yes destripe=no
The script is interactive and needs a DS9 to be open. It takes the 'nim'
consequtively dithered images, corrects for flatfield using the input flat,
optionally interpolates over bad pixels using the input bad pixel mask image,
and then makes a sky template using here additive scaling, subtracts the sky
(which is for each image re-scaled correctly to conserve the flux), optionally
destripes saturated stars (this will not work for extended emission),
optionally performs distortion correction on each image using the 'notcam.db'
model database, registers the images based on RA/DEC header keywords and the
interactively selected stars (place cursor on the DS9 and press 'a') or if no
stars selected/found using 'sregister', shifts and optionally trims (here no
trimming is selected) all images and finally combines all using here "median
combine". If the first image in a dither run is noisy or the array has not
had time to stabilize well, it makes sense to exclude the first image in the
reductions by setting skip=yes.
NB! If you have used several expose commands per
sky position or if you have used mexp then you
can not simply give the first image name, but have to prepare the input
image list beforehand and remove the average images per position in a list
(such as imlist in the example below):
The output result from reduce is stored in the above case as
test and displayed on the DS9, while the
individually reduced (and shifted) images are stored as test001, test002, ...., test009 . In addition, the sky
template image is stored as test.sky, and all image
files have extension .fits. (NB! Note that the sky
image also includes the dark.)
If there are no alignment stars to be found in the images, or if you do not
select any stars, but simply type "q", then in both cases the image
registration is done using the "sregister" task which uses the WCS info in
the header. This registration is less accurate, but allows the script to
work in cases with no alignment stars.
If the resulting image is plagued with horizontal stripes (i.e. from
saturated stars and crosstalk), the following workaround usually helps:
imexpr "a-median(a)" test_new test
Reduction of a beamswitch dither - imaging
The reduction of a beamswitch observation is very similar to the above case
of small step dithering. The main difference is that only the off-field sky
images are used to make the sky template. Both the target (ON) and the sky
(OFF) positions are reduced individually. In order for the script
reduce_bs.cl to work, all the sky positions must have been given the
image type keyword "SKY". This is set in the sequencer observing script
reduce_bs /data/notcam/NCql130315 18 crab flat_ks bad_zero_sci add median constant skip=no trim=no badpixfix=yes distcorr=yes destripe=no
The above command reduces the total of 18 images of which 9 are target and 9
sky. The reduction steps are as explained above for reduce depending
on the options selected (flat fielding, badpixel fix, sky subtraction, destriping,
distortion correction, image registration, shifting, trimming and combining),
except that image registration is done twice (for the target as well as the OFF
field), and this produces the output target image
crab followed by the individually reduced images
crab001, crab002, ...., crab009, as well as the
sky image crab_OFF and the individually reduced
sky images crab_OFF001, crab_OFF002, ....,
crab_OFF009. (Again, the sky images include the dark.)
Check out available
NOTCam sequencer observing scripts.
Simple sky subtraction - imaging
If you only want to do sky subtraction and flat fielding of a set of
consequtively obtained images (no shifting, trimming, image combination),
then you can use the script skysub. This
can be useful if you have made a raster scan, or if your dither steps
are so large that the
reduce.cl script above will not work. Note that if the FOV is
filled with extended emission, then you would have to use the beamswitch
observing technique and the reduce_bs.cl script. Example:
skysub @grblist grb 9 flat_h bad_zero_sci mult badpixfix=yes
takes the input image list "grblist" and the input master flat field
"flat_h", creates the sky template and subtracts the sky (which in this
case has been estimated using multiplicative scaling), and corrects all
the 9 images for the flat field. All the 9 images which will have output
names grb001, grb002, ....,
skysub /data/notcam/NCpa141011 as27 16 flat_ks add badpixfix=no
takes the input images from the directory /data/notcam/ and the first
image name is sufficient if your input images are consequtive in number
(if not, you will have to make an image list). Here the sky is obtained
from the 16 raw images by additive scaling, and the reduced output images
will be called as27001, as27002, ..., as27016 .
Check photometry of point sources
A script which checks the photometric accuracy and internal scatter of
point sources is called dophot.
NB! The user has to make sure that the alignment has been
performed correctly and that the flux is conserved! It is possible
to iterate again the image alignment of the individual files using the IRAF
NB! Combining images that are not well registered (through
taking the average or median per pixel) leads to flux losses. The WF
camera has an optical distortion and a distortion correction must be applied
in order to have a good image alignment for stars all over the FOV. If the
images are not distortion corrected, and if your target(s) are located in
the central region, then select stars only in the inner 600x600 pixels for
alignment to obtain a reasonable registration for this region.
Check the photometry of a reduction by typing:
dophot test nimage=9 aprad=11
This displays the image "test" and lets you interactively select the stars you
wish to measure (place cursor on star and type 'a'). Simple aperture photometry
(using your choice of aperture radius) of the selected star(s) is made on each
individual image as well as on the median combined image. Since version 2.3 you
can use a user supplied zero point magnitude (zmag). By default this one is set
to INDEF and then the standard JHKs zeropoints are used for approximate results.
Anlaug Amanda Djupvik