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Need advice for coma correcting Barlow purchase

Accessories Astrophotography DSO
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#1 antariksha

antariksha

    Sputnik

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Posted 12 May 2021 - 02:00 AM

I am upgrading my primitive astrophotography setup. I began with GSO 8 in, F4 OTA, Canon EOS M3. My new camera QHY 268C is just arrived.

My current interests are DSOs ( primarily galaxies and bright but tiny nebulae). An an example, Messier objects and NGCs which are of similar size of Messiers. ( at the moment, I am excluding too faint and tiny NGC).

 

Now my main problem is as follows. Considering the GSO F4 OTA, there is a need for Coma Corrector. Further, considering 268C CMOS size 23.5mm X 15.7mm, the approximate FoV would be 1.66deg X 1.11deg which is huge for the proposed DSOs. So I need a Barlow say 2.5X or more, to be able to map intended DSO as a sizable portion in the captured frame.

One of the solutions came up was "coma correcting Barlow" by APM. They offer 2.7X Coma correcting Barlow. But this is 1.25" with clear diameter 22mm as per the APM specifications. 

Will this APM barlow work well for my setup, without vignetting? 



#2 Tapio

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Posted 12 May 2021 - 04:27 AM

I would say that you are good regarding vignetting (and at least when you use flats).

But I question your thinking - do you really want barlow and make your scope slower ?

Remember, double the F-Ratio, 4x the exposure time.

And depending your use of images (web pages or prints) you could crop your images to show your target bigger (you have plenty of pixels).


Edited by Tapio, 12 May 2021 - 04:37 PM.

  • michael8554 likes this

#3 antariksha

antariksha

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Posted 12 May 2021 - 03:43 PM

Thanks so much. You have addressed the main concern. I definitely take a note of your suggestion that Barlow will slow the scope down. So, I would take pics without Barlow and try to crop and enlarge them. Possibly I will try with Barlow too, to get more feel and experience of DSO photography



#4 TelescopeGreg

TelescopeGreg

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Posted 13 May 2021 - 04:53 PM

Thanks so much. You have addressed the main concern. I definitely take a note of your suggestion that Barlow will slow the scope down. So, I would take pics without Barlow and try to crop and enlarge them. Possibly I will try with Barlow too, to get more feel and experience of DSO photography

Correct.  You have a nice fast scope, don't "ruin" it with a Barlow.  Unless your skies are incredibly still (excellent seeing), your primary limiter of fine detail is going to be the sky, not the camera, and total integration time.  The Barlow will do more harm than good, as it can't un-do the sky's effect, and it will make it harder to gather more data.

 

If you have the budget, your choice of Coma corrector may give you a little bit of magnification without sacrificing too much speed.  I had good results with the Paracorr-2 corrector on my 8" f/5 Newtonian.  15% Barlow, and excellent correction.  It also fixed the need for a special focuser, in order to get my DSLR to prime focus, but the field of view was a little tight for many objects.

 

You may also be surprised at the number of deep sky targets that are still going to be too large for your field of view.  For me, your focal length is just about ideal.  Capturing the context around the target (image composition) can be as important as the target itself.  Enjoy it!




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