How come so many smaller ones instead of one bigger one?
Because it's very hard to accurately track for 30 minutes, whereas a
reasonable well polar aligned mount will track adequately for 30 secs or so.
Also the s/n ratio is improved by the square root of the number of
exposures. Additionally there is much less chance of a plane or satellite
(gust of wind, brief cloud cover etc) ruining the shot during the 30th
minute of exposure! (just throw out the short exposures that are ruined).

Regards
Ian
http://www.astro-sharp.com

"Robert Geake" wrote in message
...
"Chris Taylor" wrote in message
...
Apologies for posting the second link twice, this was an afterthought.

The unprocessed image prior to digital enhancement.
http://homepage.ntlworld.com/taylor_.../M31unproc.jpg

The processed image
http://homepage.ntlworld.com/taylor_...31_resize2.jpg

Hope this demonstrates the rationale behind the application of digital
filters. I still hope to improve on these techniques someday. Its a
lengthy
learning curve.

Regards


Chris



Chris,

I noticed you had about 30 odd minutes of expsoure (74 x 30s) on your
image!
How come so many smaller ones instead of one bigger one? Again, i realise
that filters have their place and obviously in this case have improved
detail no end. I have (somewhere at home) a 15 minute ISO 400 film
exposure(200mm FL) of M31 that i took on a crystal clear night(similar to
last night after rain that shows similar detail to your modified image.
I
will dig it out and scan it and publish a link to it over the weekend.

We have seemed to miss the main point of my post! The question i pose is
basicly how much proccesing is too much. The differences between your M31
pre and post proccessing are clear, at that point i would say thats far
enough, the image looks far better than the original and indeed, far
better
than any of us will see with our 1/60 eyes!

R