Photographing the Stars

[Don Hoey]

Hi Dave,

I am in the dark as to the absolute way stacking works. The assumption I made after reading comments from Alex was that the greater the stack the more very fine detail would be shown in the final image.

I used DeepSkyStacker ( freebee on Sky at Night magazine ) and loaded say 35 frames plus 1 darkframe. You cannot see all the frames you have loaded which is a bit of a miffer. After loading, the light frames are registered. During that process each frame is given a score and the program uses say the best 80% for the stack. During the registration process you can see the star count for each frame. I started with a count of about 530 max and ended up with a count of around 1200 by the time I was stacking previously stacked images. Darkframe subtraction only takes place during the stacking process and I included a darkframe ( same one ) for each stack operation.

I have attached the first stack I made. This was from 35 frames plus 1 darkframe using the best 80%.

Don

[Don Hoey]

Dave,

I have just put in a stack of first and last images to read the star count. Image 1 of the first stack 530 stars. Image 2 taken from my full frame posting 1815 stars. I put that down to the totally massive stack I did.

Don

[Dave Smith]

Originally Posted by Don Hoey

Dave,

I have just put in a stack of first and last images to read the star count. Image 1 of the first stack 530 stars. Image 2 taken from my full frame posting 1815 stars. I put that down to the totally massive stack I did.

Don

Hi Don

I have used Deep Sky Stacker but gave up on it as with my images I was getting in some cases double stars that were not there. That may well be due to my inexperience using it rather than the program. Some people do use it with success. I am not sure that the number of stars used is any indication of image quality.

I have just done a quick Google search on image stacking and there are many results. Here is a link to just one of them which seems to explain the process clearly and in particular the need for truly random noise for stacking to be effective.

http://www.rocketroberts.com/astro/ccdnoise.htm

I hope that helps.

Dave

[Don Hoey]

Hi Dave,

Thanks for that. I will read in a minute. I am just cobbling together a single image compaison of my fist stack with the first image. Will post in a minute once I have added explanitory text.

Don

[Don Hoey]

Hi Dave,

Side by side comparison. Quite a difference in the level of extra detail in the stacked version.

Don

[Dave Smith]

Originally Posted by Don Hoey

Hi Dave,

Side by side comparison. Quite a difference in the level of extra detail in the stacked version.

Don

With 35 images there should be quite a remarkable improvement (Look at the images further down the page of that reference). You first image seems to have double stars? The second image is so similar to the first but more sort of blurred. There is no extra detail visible that I can see. Sorry!

I am firmly convinced that there is no short cut to taking real multiple images. It is then very worth while. I'm sorry I don't personally have any examples that I have taken that I can post, my astro images so far are quite long exposures and so have not taken many to stack. (I believe it is also good practice to let the camera cool down between shots which extends the time taken.)

Here are two shots of M27 (the dumbell nebula). The first is a single processed image and the second is using a dark frame and stacking just 3 images. I will at the next opportunity take many more images to stack.

Dave

[Don Hoey]

Dave,

I processed my images as jpegs due to file size. So image does suffer from multiple process jpeg blocking as well as star movement and frame registration with my primitive kit. Next opportunity I will try a longer exposure time and stack a lot fewer tiffs.

All down to processing power. 35 light frames + 1 dark as jpeg = 120.24 mb as tiffs this would have been 2519.64mb. Each jpeg run took an hour and a half so I have no idea what a tiff run would take.

All down to Alex now if he has some samples with his new fancy kit.

Don

[Don Hoey]

For those that might be unfamiliar with jpeg blocking here is a screen capture.
This is 400 magnification of the image from the end of the first stack operation.

Don

[Don Hoey]

Better issue this as a correction. Or I'll get jumped on by Rob. Seems I am looking at individual pixel wells here.

Left hand is a tiff from the first stack. Right hand is a tiff from the raw file of the first image of that stack.

Don

[Don Hoey]

I have just found this link http://www.astropix.com/

Huge amount of detailed info on processing in Photoshop. A lot more than a 5 minute read.

Don