MOSSTRONG (Sep2005) Nordic_Optical_Telescope MOSSTRONG (Sep2005)
mosstrong -- find telescope tweak parameters from two images taken
with a mask and of the object field. This task allows the two
images to be binned 2x2 .
mosstrong maskimage objectimage
Name of the image taken WITH a mask of a calibration lamp or
Name of the image taken WITHOUT a mask of the target field.
User-defined high cut for colour display. Hidden parameter.
This task determines the centroid positions of the fiducial stars
and the fiducial holes on an image of the mask and an image of the
target object field. To measure the pixel coordinates on the image
the interactive task imexamine is used.
This task allows the two images to be binned 2x2 .
The differences in the measured positions of the stars and the holes
are then transformed into telescope tweak parameters: offsets and
rotation. Ask somebody who knows about it (maybe yourself) to apply
this tweak, which should bring the fiducial stars closer to the
centre of the fiducial holes of the mask.
Take an image of the target astronomical field at the required
telescope position and rotator angle, and an image of a calibration
lamp (i.e. halogen) with the mask, and do
mosstrong /data/alfosc/jc240012.fits /data/alfosc/jc240011.fits
Ask yourself to apply the recommended tweak (first the offset, then
the rotation). This tweak should bring your fiducial stars closer
to the centre of the mask holes.
Iterate the tweak by using the task mosstrong until the stars are
properly positioned in the holes.
THE INTERACTIVE PART OF THE TASK
This task will first display the mask image which contains the
fiducial-star slits/holes. The user is advised to adjust the colour
scale of the image display such that the fiducial stars AND the
fiducial holes are clearly visible.
The user has to point the cursor at the fiducial-star slits, and has
to press key -a- to activate the centroid algorithm. If key -a- is
pressed more than once, the final press will be evaluated. Each
press will be acknowledged by statistics appearing on the terminal
that runs IRAF. To advance to the next slit, press -q-. To stop
marking slits press -q- WITHOUT pressing -a- first.
Then the object field image will be displayed, with the fiducial
stars marked based on the slit positions defined as described
above. Now the user has to point the cursor at the fiducial stars
marked by a YELLOW circle, and has to press key -a- to activate the
centroid algorithm. If key -a- is pressed more than once, the
final press will be evaluated. Each press will be acknowledged by
statistics appearing on the terminal that runs IRAF. To advance to
the next star (slit), press -q-.
On the image display the centroid positions of the slits and stars
will be marked by a RED circle.
When all fiducial slits and fiducial stars have been marked by the
user, the tweak will be computed. Check the distances between slits
and stars as output in the IRAF window. Check that the suggested XY
and rotational offsets make sense, before applying the tweak with
If one of the centroid positions of the stars or holes seems wrong
(judging from the RED circles and/or the computed distances), the
task can be run again with the user discarding the outlying star.
A good tweak fit needs no less than 3 stars. Nevertheless, a tweak
can be computed on only 2 stars.
A lot less than doing it by hand.
email Jacob W. Clasen (firstname.lastname@example.org),
for bug reports and constructive remarks