Image stacking

[black_z]

Someone tell me why you can't just take ONE shot, and stack it... versus taking 200 30 second exposures.

[luigis]

Noise to signal ratio

[black_z]

Care to elaborate?

[Oneslowz28]

Noise produced by the sensor is not in the same place in every exposure. By taking many exposures you are insuring that all of the places on the image with visible noise is covered up by a shot where the noise is in a different place.

To be fair I have seen awesome astro images made from a few subs but these were usually made using a dedicated ccd camera. On the other end of the spectrum I have seen simply stunning astro images made with well over 1000 subs.

Then taking darks, flats, and bias images also help to further reduce the noise in the image.

I am including a quote from a member on another forum where I asked for an explanation of the different types of images required for proper image stacking.
Here it is. BTW I stack using Deep Sky Stacker but the method is still the same.

Originally Posted by David Rosenthal over on the cloudy nights froums

Here is a brief summary of each and how I shoot them:

Dark Frames:
Used to remove thermal signal from your light frames by subtracting a master dark (average of many dark frames) from each light frame. Shoot this at the same exposure length, ISO and temperature as the light frame. Make sure the body cap is on or the objective cover is on if attached to your scope. It also helps to cover the viewfinder on the camera. Shoot as many as you can. If your camera has internal noise reduction for long exposures, turn it off.

Flat Frames:
Are used to remove uneven illumination and dust motes from your light frames by dividing a master Flat Frame into each of the light frames. I shoot these using a light box as my light source, you can also use the twilight flat method. Cover the scope with a white T-Shirt, stretch out the wrinkles, aim the scope at a clear part of the twilight sky and capture. You could nudge the scope a bit in between exposures to move any stars around. This will help the averaging remove them better, Here is how I set my camera to do this for either method. Camera mode set to Av, optical train must be EXACTLY the same as it was/will be for the lights. Same filters in place, same camera orientation and same focus. ISO set to same as for the lights, some say to use ISO 100, I use the same as the lights. Shoot as many as you can to create your master.

Bias Frames:
These are used to subtract out bias signal from darks, creating a thermal signal only dark frame. They can also be used to remove bias from flat frames. They also allow your darks to be scaled to match temperature and exposure of your lights by removing the bias component and leaving a thermal only signal component in the darks. I set the camera to manual, same ISO as the lights, and the fastest exposure possible. With my 400D, that is 1/4000 sec. As with the darks, the body cap or objective cover should be on as well as the viewfinder cover. Soot as many as you can and average them together.

I usually shoot 30 darks, 30 Flats and 30 Bias frames, in RAW format, to create my masters in Maxim DL.

I hope this is what you were looking for.

[bkburns]

Originally Posted by Oneslowz28

Noise produced by the sensor is not in the same place in every exposure. By taking many exposures you are insuring that all of the places on the image with visible noise is covered up by a shot where the noise is in a different place.

Thank you!!!

I have been looking for a clear answer as to what stacking actually did

[cmoy]

Thanks so much for the info! I have a few questions...

-Dark frames: do I take these right after my light frames or before?

-Flat and Bias frames: can I take these at another location say at my home or do I need to set up on location with the same settings as my light frames?

-"create as masters" does this mean I can reuse these for other shots at different locations and time?

-When I stack them doesn't it matter what order I put them in?

Sorry for the newbie questions. Thanks!

Originally Posted by Oneslowz28

Noise produced by the sensor is not in the same place in every exposure. By taking many exposures you are insuring that all of the places on the image with visible noise is covered up by a shot where the noise is in a different place.

To be fair I have seen awesome astro images made from a few subs but these were usually made using a dedicated ccd camera. On the other end of the spectrum I have seen simply stunning astro images made with well over 1000 subs.

Then taking darks, flats, and bias images also help to further reduce the noise in the image.

I am including a quote from a member on another forum where I asked for an explanation of the different types of images required for proper image stacking.
Here it is. BTW I stack using Deep Sky Stacker but the method is still the same.

[A.S.I.G.N. Observatory]

Go to this page to where it says, "Why combine?" and mouse over the 1,2, 4, 16 and 32 images to see the difference as you add more images to stack.