CN FORUMS ARCHIVE Telescope Reviews: Need tips

Stacking photos increases the signal to noise ratio, which can accomplish two things:
- Stretch the image with software image processing to reveal deeper details
- Reveal less detail but with far smoother results (less 'grainy' or 'noisy')

http://www.samirkharusi.net/sub-exposures.html
This webpage has examples of what stacking does to reduce noise. Also explains how long your exposures should be based on the amount of light pollution / skyfog at your imaging location.

Mathematically, the simplest form of stacking is to use addition. You first line the images up (aka registering or aligning), then at each pixel location, the software adds the brightness values (with values for the R, G, B components, i.e. 255,255,255 is white, 255,0,0 is red, and 0,0,0 is black etc). Addition is the best stacking method with regards to signal to noise, however in practice rarely is the best method.

Cosmic rays, satellites, airplanes, meteorites show up in exposures (aka subexposures or subs), cosmic rays being the most common. When you add your stack, all of these defects in every sub shows up. To beat this there are a variety of statistical sampling methods which reduces to varying degrees the end result signal to noise ratio:
- average (this will produce the smoothest result but fainter details will be lost)
- median
- min-max rejection (removes the highest and lowest values at a given pixel location, effective at removing cosmic rays)
- sigma clipping (generally best at removing bright pixels without sacrificing too much signal to noise)

Software programs which do stacking:
- Iris (free! Google Iris and it's in the top 10)
- ImagesPlus
- DeepSkyStacker
- MaxImDL
- Astrostack
- Keith's Image Stacker
- PixInsight
- Photoshop (using a cumbersome manual process)
etc etc

Some of these programs do more than image stacking. For example, ImagesPlus will also automate your DSLR exposures (provided you have the right cables and a laptop at the telescope).

ALWAYS SHOOT ASTROPHOTOS IN RAW!!! NEVER JPEG!!
JPEG destroys information, and makes dark frame and flat frame subtraction impossible. JPEG changes the pixel brightness values at a given location when it performs the compression, and this change is irreversible. Save as JPEG as the absolute last, last step for presentation on the web or to friends in email.

http://www.saratogaskies.com/articles/cookbook/
This site will tell you how to use Iris, which will read your camera raw files, do all the necessary processing, and save it as a TIFF or PSD file to import into an image editing program (such as photoshop).

You can always re-process an image; you can never take that same photograph again. Shooting in RAW means you'll have the best data possible. When you learn more image processing you can always re-use the raw file. You can re-use a raw image next year when you re-photograph an object; you can't re-use or stack a JPEG. Collect good data now, figure out the processing part later.

And finally, take TONS of photos.
There's no way around it. I tried, I tried, I tried.
50-60-80-100 pictures over one or several nights of the same object. This can only be beat by astro-cooled CCDs on 40" research scopes. It sucks, it takes forever, but its the nature of the hobby. Autoguiding and DSLR Focus or DSLR control or a Canon Remote timer makes this way less labor intensive and way more spouse-friendly

It can be done manually but you'll be at the scope all night.
You won't get good results until you do this. Heck, you may not get good results if you DO do this

Even a stack of 10 is better than a single image (1-hour exposure using a $12,000 Paramount ME mount and a pristine dark sky site being the exception)

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LX200 8" classic, f/10, Meade eq. wedge, .63x FF/FR
Canon 40D (LifePixel clear glass mod) and 5DMkII, unmodified
Canon EF 200/2L IS, 400/5.6L, 100/2.8 Macro, 50/1.4, 85/1.8, 35L, 24L, 17-40L and Peleng 8mm fisheye
Orion Apex 102mm (4") Mak-Cass
Pimped out with accessories and bling
My DSLR Astrophotography Webpage and photo bucket with full equipment list