Photometry Page

On that page:>If you have a DSLR camera and would like to learn about how to get started from scratch, >post a message in our photometry forum! OK, here I am :-). I've got an Olympus E 420 (unmodified, that one has a so called "Four-Thirds" sensor, which means it will be a bit more noisy than APS-C chips of comparable resolution, 10 Mpix in this case).I've also got two zoom lenses (17.5 - 45 mm 1:3.5-5.6, 55 - 200 mm 1:4-5.6) , and a 6" f/5 Newtonian reflector that I can use with the E420.So.....how can I use this to make measurements?CSBikeman

There are several things you will need to do. Experiment! Forget the telescope. Put the camera on a tripod. Make sure you have some image processing software like AIP4WIN and have a good knowledge how to use it. Experiment with taking images of the star system and find an exposure where the peak ADU are under 40,000. Use max aperture and sensitivity. Make sure you do not have Capella in the image as that would be much brighter and give you a false max for epsilon. Once you have a good exposure take several dozen images and stack them. With AIP4WIN spearate the RGB image and save the G plane.This will be you V filter data image. Use AIP4WIN to get the star-sky ADUcounts for the program (epsilon) and comparisons star(s) eta (and maybe zeta) Aurigae. Do a raw magnitude calculation for each star. m= -2.5 log (ADU counts). Get the difference between the program and comparison stars raw magnitudes. Normalize the results to the comparison star's published magnitude. Average and and SD the measurements and report. Questions?

Thanks for the kind reply,>Questions?Many :-) What about focus? Should I deliberately de-focus the image to spread out the star's image over more detectors than on a focused image? Might make stacking more difficult, tho.CSBikeman

This is where you need to experiment. You need to be concerned about under sampling. That is a loss of light when too few pixels are illuminated and light falls through the spaces between the pixels. With a fast system, the star image will be very small and under sampling will be more of a concern. That is one reason why using a longer focal length lens may help. Again, experiment. If you put this on a tripod the Earth's motion may "smear" the image enough to reduce under sampling. If not, then try defocusing. To experiment just consider stars of known magnitude. Your results should produce those magnitudes. If the resulting magnitude is fainter than the published magnitudes, the problem may be under sampling. Use star close to the zenith to minimize extinction. In fact until you can reproduce those publish magnitudes, any work on epsilon Aurigae will probably produce poor data. I used a DSIPro CCD camera with 50mm lens mounted on my LX200 and tracked the star. I had to defocus considerably, but it worked. Experiment! Jeff

Thanks again,Got my copy of The Handbook + AIP4Win today and will start experimenting once I get clear skies again over here. I guess I'll first try to get reproducible results on non-variables of around mag 3, then move on to short period variables and then finally to eps Aur.Thanks for setting up this interesting project

Where can I get the AIP4WIN software from? At the Willman-Bell site, it was listed at $100. I'm not really on a big budget though, is there any other place I could get it from?

I recommend you get it from Willman-Bell. If you get a second hand copy, the book will be fine, but the software will probably not work. You have to log on to get the software activated for a specific computer. If you are serious about CCD/DSLR photometry, it will be a good investment.

Folks, For many models of Canon Point&Shoot camera, there is a free "firmware-addon" called CHDKwhich dramatically increases the functionality including RAW image output, scripting, and so on. See the webpage: http://chdk.wikia.com/wiki/CHDK RAW data output is the most desirable for the most useful brightness measurements. Cheers, Doug Welch