Forums / The Science / Photometry / Why stack pictures?
Why stack pictures?
Hi all,
(some of you may know me as a visual observer).
Eps Aur got me interested in photometry. I want to do my first steps in photometry trying the get the right magnitude of Eps Aur. I'm going to use my Canon 400D.
I have read here a lot about stacking several pictures. I know this is a way to get a higher recording time. But let's say that I can get a good shot of Eps Aur and comparisonstars in one shot will this be sufficient enough to do the photometry? I can put the Canon 400D behind a 66 mm ED apo. After calculating this setup gives me about 3 degrees FOV. But that's maybe just to tight for a good picture.
http://www.vvs.be/wg/wvs/
Hubert,To get good V estimates of epsilon aurigae I have found, using a Canon 450D, that I need to take 50 images at the settings of 5 seconds exposure, ISO200, f5 with an 85 mm lens. This number of images is necessary to overcome scintillation. It would be very tedious to analyse the images one by one. Instead I stack sets of ten images and analyse the five resulting stacked images. These are averaged to get a final estimate. The five sets are used to calculate standard error. The whole process, with stacking, from acquiring the images to final results takes about 45 minutes. I think that this is worthwhile to get good results.Using this procedure, last night, I made epsilon at JD 5128.521 to be 3.382V. The standard error was 0.005. When I am looking at another eclipsing binary such as RZ Cas I have found that analysing sets of ten images every 15 minutes is acceptable because the main point of the study is to estimate the time of mideclipse. V magnitude in such a study is irrelevant. The higher error involved in just doing ten images is acceptable because you will be aiming to do at least 5 points on the fade and 5 on the rise after mid eclipse. 5 on either side is enough to get a good estimate of mid eclipse though I usually try for seven.Best wishes,Des Loughney
Hello Heinz and Des,I will follow the advice you gave me. I will get AIP4Win soon.regards, Huberthttp://www.vvs.be/wg/wvs/http://www.vvs.be/wg/wvs/index_en.php
Hello Hubert, I think the main motivation was to get at least (say) 30 sec of light for a measurement to eliminate atmospheric scintillation effects. Eps Aur is so bright that it's not easy to make 30 sec exposures without saturating pixels, so the idea was to stack shorter exposures and then do the photometry on the stacked image. However, software like AIP4win make it quite easy (as easy as stacking)to do photometry on a whole set of exposures, so whether you stack first and then do photometry, or do photometry on several images and combine the result by averaging, is more a practical consideration because the results should be the same more or less. Of those who reported here in the forum, all seem to be using regular DSLR lenses with f=50 .. 80 mm giving you a vast field of view that easily includes eps, eta, zera and lambda Aur all reasonable close to the center of the field. Clear skies Heinz