Generate Calibration Information

The next task is to generate a simple ASCII text table containing two columns. For consistency with later steps, the file should be called swarp.zeropoints. The first column contains the names of the single-extentions FITS files you wish to coadd. The second column contains the zeropoints measured for those frames. Note: in this scheme, the zero-point is measured for each frame relative to all the others in the set of frames you wish to coadd. Absolute calibration will come in a later step, once the individual exposures have all been coadded.

The way in which the swarp.zeropoints file is generated has recently changed. The method is as follows:

Suppose the files you're reducing are named SSA22.????.zpcor.fits . From the linux prompt, type:

zptmatch SSA22.????.zpcor.fits

...which takes a minute or two to finish. An outcome will be an ASCII file called Zpt.out with the relative zero points listed in order. They are referenced to the first file in the input list. In other words, the first entry should be zero.

This is a significant improvement over the old procedure, because the relative zero points are computed without using SDSS as an absolute reference. In other words, the Megacam frames need not be coextensive with SDSS, and there is no worry about filter conversions corrupting the relative zeropoints (the SDSS filters are close to but not quite the same as the Megacam filters).

You should take a few moments to verify that the relative zeropoints are reasonable. Do they make sense in light of the trends in airmass, exposure times, and weather? Sometimes the default settings for zptmatch cause it to use an inappropriate brightness threshold for the sources it automatically photometers and compares from one frame to another; in particular you must be careful to avoid saturated sources. You have some control over what sources are used for this comparison through the -m (instrumental magnitude) flag. I have found that the default value (12) works well in many situations, but just to take one recent example, I had to run with -m 10 to get good results for a set of 300 and 500 second z' exposures. The default value made zptmatch think that 300 s and 500 s frames were equally sensitive. Ergo:

zptmatch -m 10 SSA22.????.zpcor.fits

Further details on the usage of zptmatch can be had simply by typing zptmatch with out any arguments.

When the Zpt.out file is created, you can make the swarp.zeropoints file simply; this is one way:

ls zptmatch SSA22.????.zpcor.fits > x
paste x Zpt.out > swarp.zeropoints
rm x

...so that ultimately, swarp.zeropoints should look something like this:

SSA22.2595.swarp.fits 0
SSA22.2597.swarp.fits 0.008665
SSA22.2624.swarp.fits 0.0238651
SSA22.2625.swarp.fits 0.0266661
SSA22.2626.swarp.fits 0.0237131
SSA22.2630.swarp.fits -0.0233248
SSA22.2631.swarp.fits -0.0685924
SSA22.2632.swarp.fits 0.0273393
SSA22.2633.swarp.fits 0.0307617
SSA22.2634.swarp.fits 0.0319809
SSA22.2635.swarp.fits -0.0762956
SSA22.2636.swarp.fits -0.170042

, etc.

When you have a zeropoint measured for every file to be coadded, move on to the next step. Constructing/Verifying the Total Coadd.

Last updated December 5, 2007.