Posted: Fri Dec 10, 2010 4:51 pm Post subject: SOA + DAOPHOT
So I'm now trying to make a light curve. So I have a bunch of fits files and I want to know the magnitude of the one particular star After a lot of searching I found out that there is a package called DAOPHOT, thought most of the good manuals on how to do this stuff exist for the of DAOPHOT: II The next generation.
From what I can tell I must run phot, psf, doafind, and pselect. and then run allstar to get the magnitude. The manual I have says that the first for "can be run interactively" - using SOA (ds9). But does not say how?! does anyone know how to run these through SOA (ds9)
I guess you're doing all this with IRAF/DAOPHOT, since the DAOPHOT algorithm is also available for IDL for instance.
First of all you have to decide which kind of Photometry you want to do. Depending on your data this would either be PSF photometry (in crowded fields) or aperture photometry. I hope that your frame is not very crowded, and that your object of interest is not contaminated by another star, since time series PSF photometry is very hard to accomplish!!
A rough guide on how to use aperture photometry in IRAF/DAOPHOT:
1) Set all parameter sets (datapars, findpars, fitskypars, etc.) depending on your data
2) Run daofind to detect the sources in the frame
3) Run phot with the generated coordinate list
4) use txdump to dump all information you need out of the phot-output
Here is a guide to that:
If your field is not crowded, I would suggest to stick to simple aperture photometry.
P.S.: Never heard about anything you can do in batch mode with ds9....
Then use xtdump to get the magnitudes for each and m error. All being well the magnitude of my star should vary from image to image but the sample stars should not (assuming I have not chosen other variable stars). But my problem is a) it seems to simple and good to be truly that simple b) are these the true magnitudes or do I need a star in my sample that has a known magnitude of x and I have value y. Then I use these to scale my magnitudes by taking away the magnitude difference (x-y) from all the magnitudes of my stars?? Hence getting the true magnitude Or would I simply plot the relative magnitude of the variable star against date it was taken to produce a light curve???
Also at no point did I tell phot the gain or read out noise of the detector - this should be included at some point, no??
The GAIN and RDNOISE are important for the correct error model and building a good analytic PSF, so its better to fill them in, but not crucial for you.
I think when you've done the aperture photometry what you get is a bunch of files, each containing the measured magnitudes for a specific star...?! IF this is correct, the you are half way through the process. What you get out of phot is only instrumental magnitudes, and these of course will variy with time over the night. For example you can plot instrumental magnitude against airmass to see this effect very well!
The further steps depend on what you want to do with your data. In case you want the true magnitude of the star, you would have to look for constant stars in the field, and determine the magnitude offset between measured magnitudes and true magnitudes, but I guess you only want a light curve, so you do differential photometry.
What you need are comparison stars:
Stars in the field which are constant, and then calculate the differential magnitude:
x_obj - x_com = differential magnitude
(in case of a single comparison star)
where x_obj is the instrumental magnitude of your variable and x_com is the instr. magn. of your comparison star at a particular time.
First order effects should cancel out in this operation. Most of the time its better to use several comparison stars. Just calculate the mean of them for a very simple differential photometry. In fact it would be much better to weight them according to their error.
It can be very time consuming to find good comparison stars, but since you are not in a crowded field, just take what you've got.
AnTaR3s thanks for your help so far - you have been amazing!! I do have one final question - I've looked at the output and the standard stars are all over the place, looked into this and found that the air mass varys in the same manor - there is definitely correlation. Looking into phot it appears while you can input the air mass it does not actually use it in the magnitude calculation?!
So is there another package that does take the air mass into account?? Or can i correct for it knowing the value??
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