The `rtp` guide

rtp is the Real Time Processing program that allows you to process fast CCD photometry images as the observations are proceeding, using aperture photometry.

`rtp` Startup

Get a new xterm window from the left mouse button menu.
Create a directory for your data processing in a location different from the data storage directory. This will be the data processing directory.
In this directory, make a symbolic link to to the directory where tcpcom stores the data and give it the name "data", e.g:
> ln -s /fastphot-data/qb07/PG1307 data
Now you can start the rtp program, e.g:
> rtp -r 12
or more sophisticated;
> rtp -MO -i 3 -r 12 -z 3 -n 50
(See the rtp options guide for an explanation on the many alternative options when starting rtp.)
Before the program starts it will give you a menu page displaying all the processing options, with default values or the values you entered on the command line. You can change these options here before the processing starts by hitting the corresponding key. Hitting 'g' starts the processing.
You will get a gnuplot window that shows the propagating light curve. Use the 'd'-key in the rtp window to switch between displaying the raw light curve, the sky subtracted data and the differential photometry. (See for details on using rtp.)
To check that the stars are well aligned in the windows and synthetic apertures, hit the 'W'-key in rtp. This will produce a file 'testview.fits' in the data processing directory (where you started rtp) that shows the current windows, the primary target at bottom, all reference stars and sky fields on top. Use saoimage (in another xterm window) to display the image. The apertures applied when processing the data are imposed on the CCD window images.
For on-line help, use the `?'-key.
when done:
> rm data

Basic startup options

The aperture radius
The most important option to use when starting rtp is the aperture radius. This is specified with the -r flag and the value is given in pixels. For ALFOSC images useful values range between 10 (good seeing) and 20 (bad seeing). It is necessary to try out different values to get an optimal noise level in the light curve. The centering of the aperture can be done manually by giving the centre coordinates for the stars with the -x and -y flags.

Centering offsets
However, it is usually much better to use the -O option to have rtp calculate the geometrical centre within each aperture. These centre coordinates are obtained from the first frame and kept throughout the run.

Moving apertures (MAP)
Even more useful is the -M option, which will apply recentering to the target star (channel #1) for each frame in the sequence, thus adjusting for minor tracking errors that often result from guiding on a star with different colours than the target. The -i [iter] option can be given if more iterations are required to get a good center position (both for the -O and -M centering options).

Extra sky fields
If more than one sky field was specified when setting up the window lists, it is necessary to provide the number to rtp with the -z [nsky] option. Only one sky field is used for the sky subtraction, even if all sky channels are presented in the ".raw" output file. To select a sky channel different than the first, use the -t [num] option.

Start frame number
If it is desirable not to process the whole data set, but only from a certain frame, the -n [num] option can be used. Processing start at this frame, but the timing is still counted from frame zero.

Thus, a good rtp start line can look like this:
> rtp -MO -i 3 -r 12 -z 3 -t 2 -n 50
This gives moving apertures with individual offsets between the targets within each window, an aperture radius of 12 pixel, specifying that three sky fields exist and that the second should be used for the sky subtraction, and that processing commences at frame 50.

Many more options are available. Refer to the options page for a full list.

Flatfielding

The rtp program can automaticly flat-field the images for you. All that is needed is to create another symbolic link in the data processing directory that points to the catalog where the flat field images are stored. For example:
> ln -s /data/data1/ka01/skyflats flats
The files must be named to match the string "flat*.fits". If other names are given, specify the search string with -f, e.g. -f "ka01*.fits". If flat-fields of several filters are kept in the same directory, the files should be renamed to match a specific string, for instance "flatB*.fits".
The flat field images located in this way are trivially averaged and normalised, the relevant windows are extracted from the combined image and all windowed CCD images are divided by the matching image section. After the flat field images have been combined, the master flat is written to the file "Flat.fits" and stored in the data processing directory. This file will be used by default when restarting rtp, unless the -f option is given.
It is also possible to process the flat field images with your favorite software, and store the image in the processing directory with the name Flat.fits, and rtp will attempt to use this for the flatfielding. Make sure that this image is not truncated on the side of the origin, as this will offset the physical pixels relative to the CCD windows.

Data files

The rtp program produces three output files. New data points are appended to these files each time a new image is stored in the data directory by tcpcom. These are the files that are continuously plotted in the gnuplot window.

"phot.raw" contains the plain aperture sums after bias level correction and (if available) flatfielding. The first column contains the time since the sequence was started, the following columns are aperture sums for target (Ch#1), reference stars (Ch#2 etc), and sky fields (Sky#1 etc). There is also a column for the bias level.
"phot.dat" contains the sky subtracted data.
"phot.dif" contains simple scaled differences between the different channels.
When running with automatic recentering, an extra file called "phot.cen" is generated, listing the computed centroids of the centering star for each frame.

Extinction corrections

The rtp program does not do automatic extinction correction. For this you need to apply the rtcorr program. See the documentation on rtcorr for info.

Fourier transforms

It is possible to obtain a Fourier transform of the collected light-curves at any time while tcpcom and rtp is running simply by hitting 'F'. This will produce a Fourier transform in the range 0 to 10000 uHz. The units on the x-axis are micro-Hertz, and the units on the y-axis are milli-modulation amplitudes. If you need more advanced options, you can invoke the rtft program with the file that you want to make a temporal spectrum of as input. See the rtft guide for info.

Image centroids and FWHM

Another useful diagnostic can be accessed by hitting the 'C' key. This will display the image centroids of the target object, x in Ch#1, y in Ch#2, as well as the FWHM of the image, the x-direction in Ch#3, the y-direction in Ch#4 and a radial fit in Ch#5.

Converting window frames

A special program has been made to convert the CCD window frames to normal FITS images that are suitable for visualisation or processing by programs like daophot. This program, rtcnv, can use exactly the same processing steps as the rtp program, but instead of calculating and storing the aperture sums, rtcnv converts the results into FITS files. See the rtcnv guide for info.

Back to the front page | Search the NOT WWW pages | Send feedback to NOT staff

Last modified: Tuesday, 31-Apr-2001 12:08:31 ACT

The rtp guide

rtp Startup