Patrick, I always apply calibration frames first... then align and stack... then stretch. There are many ways to clip the data during processing. If you stretch the frames and then add them together (as 16-bit depth individual frames), and then save the result as a 16-bit image, you will likely have added enough bright areas to exceed the white value of the 16-bit depth. However, if you always save your manipulated data as floating depth (IEEE Float), then you will never clip as a result of inadequate bit depth. Just pay attention at every step to make sure you are not forcing your hard-earned data to be black or white if it isn't black or white in the frames. My guess is that you will do better with fewer frames of longer duration. That is generally true as long as you are not saturating too many stars in the image or other areas of interest. Remember that the camera has a fixed noise level that is independent of the exposure length (bias) as well as noise from the exposure length (darks). The more separation you can get between the noise and the signal, the better off you will be and the smoother your image will be. I hope that makes sense. Cheers, Tyler -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Patrick Wiggins Sent: Wednesday, March 24, 2010 2:21 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] M-82 Hi Tyler, On 24 Mar 2010, at 10:46, Tyler Allred wrote:
Patrick, I agree that the focus looks good, and you picked up some nice detail in the arms. The core only looks blown out because you keep letting your software decide how to stretch the data. If you recall, I have chastised you several times for this offense :)
Yep, and I still have the welts to prove it. :) Actually I stretched the heck out of each image and, as you suspected, the resultant images were very dark. But when I stacked the dark images the final product was still burned out in the core. I'm guessing part of the problem is that with only 2.5 minutes of data there's not much there to work with (remember I shot the pictures just to check focus). A few questions: 1) Do you stretch the individual images or only the image you end up with after stacking? 2) And, for that matter, do you stretch before applying the flat and dark? 3) Would I be better off shooting say 20 one minute exposures or 40 thirty second exposures? 4) Maybe one of these days you can do an imaging session at a SPOC star party? Cheers, patrick _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com