Photometry

[Karls48]

I would like to try to do photometry. For last two years I have been taking images of major stars in different constellations (as they become visible from my light polluted and obstructed back yard). By using GSC11 and lately Tycho 2 star catalogues and Maxim DL I found that unfiltered magnitude of the stars in my images can have can have up to 1 magnitude difference between my images magnitude and the star catalogue magnitude. I do understand of the reason for this, but purchase of photometric filters at around $600 set is out of question for now and for foreseeable future. After having work accident in 2002 I went back to work when the insurance company told me to go. When the condition of broken bones I suffer in the accident forced me to stop working in 2006 the insurance told me that it is natural degradation and that they are not liable. If you think that you are fully covered against industrial accident in NSW, think again. So from well-paid job I become overnight invalid pensioner. That’s the reason why I try to do astronomy on cheap. Back to the filers.

I have Apogee filter wheel and set of RBG filters and have I made IR pass filter from the piece of 35mm colour film. Can I do any meaningful photometry with this setup? Or, would I be better off with Wratten # 25, 47, 58 filters? If I calculate mean error magnitude for each of my colour filter against the star catalogue magnitude and apply the correction to my images can I get closer to the correct magnitude? Also, I reason that if I plot light curve (looking for eclipsing binary for example) it does not matter that I’m not using photometry filter, the result should be valid. Any comments appreciated.

[Terry B]

Dear Karl
You can certainly do meaningful photometry with the filters you describe.
Proper BVIR are not as expensive as you suggested though.
see http://www.astrovid.com/prod_details.php?pid=237
Even just purchasing a V filter would help.
I will respond with more info later but am short of time at present.

[Terry B]

You can use the coloured filters but they may not give you perfect results.
Take a look at this page
http://www.aavso.org/observing/programs/ccd/ccdcoef.shtml
Download and read the Lou Cohen paper for a reasonable walkthrough of how to determine the transformation coeffecients for your filters. another paper is at http://reductionism.net.seanic.net/CCD_TE/cte.html
He describes using non photometric filters and how to compensate for them.
The standard stars to image are listed here
http://www.cfht.hawaii.edu/ObsInfo/Standards/Landolt/
or at
http://www.noao.edu/wiyn/obsprog/images/tableA.html
Pick a set that are high in the sky at a reasonable hour and image them through all your filters. Let me know if you need more info.
Good luck

[Merlin66]

I've also started some "preliminary" work on photometry.
Managed to pick up a set of Schuler filters from the States ($200) and have already found that the U -series won't work for me. The response of my CCD's is no good below 400A.
One of the better ( best?) books I've found so far is "Astronomical Photometry" by Henden and Kaitchuck. It explains the issues of achieving repeatable magnitude estimates. Although a bit dated in terms of technology, it does a wonderful job of explaining all the variables which can influence the accuracy of the results.
I'm using AstroArt4 and CCDSoft with a 12" LX200 and an ATiK16ic ( the Canon 350D and the Starlight MX7c were no good - colour response etc)
Certainly not for the fainthearted, but interesting fun nevertheless.
There's a very good CCD photometry web article on the BAA variable star site.
http://www.britastro.org/vss/ccd_photometry.htm

[Karls48]

Hi Terry and Ken. Thanks for the links. I got bit of reading to do. Calculating correction for filters, telescope and CCD is bit more complicated then I have thought. But it can be done with gear I got now, so I don’t have to wait until I save for photometry filters.

[Merlin66]

The major difference between just getting some un-corrected magnitude estimates and "scientific professional level" UBV calibrated results is, as I'm finding, a long hard, stoney road!!
Having said that, the ability to use a commercial software package to "ease the pain" is very enjoyable and you can quickly see where the results could lead.