Another newbie question.

[shropshire lad]

When I do my imaging with Canon 80D and 200mm f2.8 lens 800 iso ... on Sky Watcher AZ-GTI in Az mode .... is it best to :-

1 .. Do 400 1 sec images

2 .. Do 200 2 sec images

3 .. Do 100 4 sec images

The total image time will be the same so what will the difference in quality be, and will there be a difference in noise level.

Hope this isn't a daft question.

 

[MartinB]

These short exposures will not be sufficient to eliminate read noise regardless of total exposure time.  Given that you are using your mount in alt az you will start to get field rotation with longer exposures.  You should experiment to discover how long you can expose before this becomes intrusive and let this be your guide to your exposure times.  My guess is that 20-30 sec might be ok.

[shropshire lad]

Thank you @MartinB just need some clear skies now.

Will try EQ mode when I get a little better at setting things up, then I might be able to get even longer exposures.

 

 

[teoria_del_big_bang]

1 hour ago, shropshire lad said:

Thank you @MartinB just need some clear skies now.

Well if you get some don't keep it to yourself, send some North a bit 🙂 

Good luck with the imaging

Steve

[rickwayne]

There is an online calculator to give you the theoretical numbers for field rotation, but I like the "try it and see" approach.

There are several sources of noise in astro images, read noise happens every time you read the sensor so it scales linearly with the number of sub-exposures (aka "subs", "lights"). At a certain level (incoming photon flux times exposure time) you'll have enough signal that the read noise really is no longer a factor (often referred to as "swamping the read noise").

Of course, ya don't never get nothin' for free, longer exposures have their own problems. For one, every second your shutter is open is another opportunity for badness to happen -- tracking error, kicking the tripod, firefly lighting off inside the scope's dew shield, satellites, aircraft. Too, your sensor can only register so many photons before it's filled to capacity so that that part of the image is blown out and unrecoverable. Strange that we struggle with that problem when we're imaging dim objects, but the stars are so much  brighter than nebulae that it's a real issue.

The beginning-astro books have great discussions of this -- you don't have to follow the maths to get the gist and internalize the rules, but it really does help to be  exposed. If you prefer video, Dr. Robin Glover, author of the SharpCap imaging program, had a great talk on exposure  and related topics. Note that you're using filters, the Bayer matrix in  front of your sensor means "multiply by three". Don't worry, you'll know what I meant when you get there :-).

My favorite of the beginner books is Bracken's The Deep Sky Imaging Primer. Others will recommend Richards or Lodriguss, can't go wrong with any of 'em.