Using finder/Guidescope as main imager

[Pitimax]

Hi, all...

I use a Celestron C6se (altaz GOTO mounted) for short exposure astrophotography;  (not optimal, I know)... and I find the field of view way too small. Had anyone tried reversing roles and using a 50 or 60 mm Finder as the imaging scope for ultra wide FOV (~5°)while using the main scope for guiding? ... how long till field rotation kicks in?

[vlaiv]

Hi, welcome to SGL

I guess severe field curvature, chromatic aberration and star distortion will kick in long before field rotation does.

I have F/4 60mm guide scope and it is just about suitable for quick peak on low power visual - even Moon is rather blurry at very low magnification - around x20. It does guide well, since guide software does not really care about star shapes.

Now, as always you can give it a try - nothing wrong with guiding with "main" scope (or simply put bigger one) while imaging with "guide" scope (or in this case smaller one) - but I would not expect much in terms of results.

[michaelmorris]

Piggy-backing a small scope on the back of an SCT used to be very common and can work well with the right mount.  However, as others have said, finderscopes tend be pretty rubbish when it comes to optical corrections.  Good enough as a finder or a guidescope, but really pretty hopeless as a main imaging scope.  However, one (not cheap) option to consider would be replacing your finder with a small wide field apochromatic refractor.  Examples might include -

https://www.firstlightoptics.com/william-optics/william-optics-zenithstar-61-apo.html

https://www.firstlightoptics.com/borg-astrograph-telescopes/borg-55fl-f3-6-astrograph-set.html

[ollypenrice]

2 hours ago, michaelmorris said:

Piggy-backing a small scope on the back of an SCT used to be very common and can work well with the right mount.  However, as others have said, finderscopes tend be pretty rubbish when it comes to optical corrections.  Good enough as a finder or a guidescope, but really pretty hopeless as a main imaging scope.  However, one (not cheap) option to consider would be replacing your finder with a small wide field apochromatic refractor.  Examples might include -

https://www.firstlightoptics.com/william-optics/william-optics-zenithstar-61-apo.html

https://www.firstlightoptics.com/borg-astrograph-telescopes/borg-55fl-f3-6-astrograph-set.html

Because I could never get my wedge-mounted SCT to track well enough for its huge focal length I took my first DS images using the method you suggest, guiding in the SCT and imaging through a WO ZS66. It worked well.

Olly

[Stub Mandrel]

Why not get a good 135mm prime lens, there are some brilliant ones out there and the Zeiss ones are very affordable and its an excellent focal length of wide field. With a typical 4/3 sensor DSLR youw will get about 10 by 6 degrees FOV.

Alternatively Tokina do an excellent 300mm lens,a gin cheap second hand, whioch will give you a FOV of 3 degrees by 4.5. With these lenses you will be able to use respectable exposures with an alt-az mount.

This is with a Tokina that cost me £30. The subs are only 35 seconds, I should have used a much longer exposure, I was worrying too much about burning out the core:

Edit: I should say the Tokina does get blue haloes on bright stars, but these are small and easily corrected with a photoshop action. My Zeiss Sonnar 135mm doesn't show noticeable CA, but 135mm is quite a wide field of view.