CCD telescope pairing query

[woldsman]

I’ve been reading up on pairing CCDs and scopes and am looking for further guidance as advice encountered so far is confusing! Planning to pair an Atik 314L CCD with a William Optics zenithstar 71 refractor. Also plan to use a filter wheel for lrgb.

My queries are:

1) do I need to include either a field flattener (I have an 0.85 reducer) or a Barlow in the train? Advice encountered so far is conflicting, ie use a flattener, use a Barlow, use neither!

2) will I need any spacers or adapters? 

3) is it advisable to take darks and flats?
 

According to astronomy.tools, pairing an Atik 314L with a ZS 71 results in under-sampling, and the calculator suggests using a Barlow. Interesting the calculator is actually less happy with the Atik CCD than with the Canon EOS 100d I have been using due to the fact the resolution rises from 2.12 to 3.14 “ per pixel (it gives a 0.67 to 2” range. They say the computer never lies but I find it hard to believe that the Canon would be a better option and slotting in a Barlow just feels wrong. 

Any suggestions would be appreciated! 

[The Lazy Astronomer]

13 hours ago, woldsman said:

1) do I need to include either a field flattener (I have an 0.85 reducer) or a Barlow in the train? Advice encountered so far is conflicting, ie use a flattener, use a Barlow, use neither!

What reducer do you have? I think the reducers sold for the ZS71's were reducer/flatteners. You mention imaging with a DSLR (I assume this is with the ZS71) - how are the star shapes near the corners? If they're good, then no need for a separate flattener.

As for the barlow: it has no place in deep sky astrophotography. 

13 hours ago, woldsman said:

2) will I need any spacers or adapters?

This depends on the required backfocus from your reducer/flattener. Typical backfocus requirement is 55mm to the camera sensor. For a dedicated astro cam you very likely will need spacers.

13 hours ago, woldsman said:

3) is it advisable to take darks and flats?

Absolutely yes, proper image calibration is not really optional. An advantage of a camera with set point cooling is you can create a library of darks and reuse them.

14 hours ago, woldsman said:

According to astronomy.tools, pairing an Atik 314L with a ZS 71 results in under-sampling, and the calculator suggests using a Barlow. Interesting the calculator is actually less happy with the Atik CCD than with the Canon EOS 100d I have been using due to the fact the resolution rises from 2.12 to 3.14 “ per pixel (it gives a 0.67 to 2” range. They say the computer never lies but I find it hard to believe that the Canon would be a better option

That calculator is concerned only with pixel sizes and sampling rates. It makes no judgement at all as to whether one camera is better than another, and indeed there are a multitude of other factors to consider.

In typical conditions, it is unlikely you'd be able to actually achieve the sort of resolutions that calculator might have you believe.

An example from personal experience: I image at 1.7"/px. However, based on the measured FHWM of my images, I find I can usually bin x2 in processing with no loss of actual detail (this effectively makes my sampling rate/resolution 3.4"/px).

There is also a way to attempt to recover resolution in undersampled images through the use of drizzle integration (provided you have adequately dithered data). I've never personally tried it, although it's on my list of things to do.

[woldsman]

Thanks for the reply! It is a reducer/flattener and actually 0.8 and yes the star shapes are ok in the corners.

Will find out soon enough if spacers are needed when doing a focus test.

Thanks for making the point about a library of flats and darks - not having used a ccd before this had not occurred to me as up to now I’ve been making sessional calibration frames with a DSLR. 

Thanks also for the advice on under sampling. I think I’ll just relegate this down the worry list as it seems really specialist - I’ll take a pragmatic ‘what works’ approach and see what I can get out of the kit.

 

[The Lazy Astronomer]

41 minutes ago, woldsman said:

library of flats and darks

Library of darks: yes, super easy, just take them and reuse however many times you want. 

Library of flats: yes, but only if you leave the imaging train intact between sessions. Even then, eventually you'll get some new dust bunnies (or they'll shift slightly) and new flats will be required. You should be able to get a good few months out of them unless you have an unusually dusty environment.