Possible Issue with new OTA

[Ryderscope]

I've just noticed an issue with my new Askar FRA300 OTA which I'm diagnosing and some input from others here would be handy. I have contacted the vendor with my report but whilst I am waiting for a response it would be good to see if there is anything that I may have missed or should look at.

The Askar FRA300 is paired with a ZWO ASI6200 MM Pro and 2" filter wheel with Astronomik 50mm filters. I have installed a ZWO EAF for auto focusing and the ZWO M68 OAG for guiding. SGP Pro is used for image acquisition and auto focus.

The issue is that I am getting what appears to be collimation errors in the blue channel only. See attached images which are a 200% zoom of the field. Stars in the centre of the field are fine. The aberrations in the blue channel start to become noticeable about 2/3 of the way across the field and become progressively worse towards the edge of the field and in the corners. No such aberrations appear in the red or green channels.

Here is a link to a Dropbox folder (https://www.dropbox.com/sh/zpvoos4ikydo48o/AACzY5hEnLYlM31EyVQXuRyja?dl=0) with a sample of each of the red, green and blue subs if you want to have a closer look.

I have not encountered these sorts of issues previously in one colour channel only. Any experience that others can bring to the fore would be appreciated.

Thanks,
Rodney

[PRejto]

Hi Rodney.

Well, obviously something is wrong with either the design of this petzval APO or this particular example. How disappointing!

A wild guess, might your Astronomik blue filter be transmitting more UV than a competitor's filter? The UV might not be within the corrected range of this APO.

As a temporary cure, try running BlurXterminator on just the blue channel. You can set it only to correct star shapes and not deconvolve.

Good luck,
Peter

[kencas]

When toggling between the images I can also see a slight shift in the green channel. The red is perfectly circular, the green is showing some centre weighted abberation, and as you have pointed out the blue is quite elongated.

[Ryderscope]

Peter, Ken, thanks for your responses. I would like to get to the bottom of this and see if I do have an issue with the OTA or not. As a first stab I will try with adjusting out the small amount of tilt that is there and see if that makes a difference.

[AdamJL]

Keen to see how you get on with this.. I have seen the same star shapes with my Askar FRA 400... I've only shot OSC with it right now, but I noticed it straight away.

[croweater]

Hi Rodney, with the triangular look of the stars I immediately thought it looked like pinched optics but I guess it can't be considering it's only in one colour channel. Good luck with the diagnosis mate
Cheers, Richard

[Ryderscope]

Interesting Adam. Its worth following through in case it is an OTA issue.

[Ryderscope]

There in lies the conundrum. Why is it only in one colour. Before I agitate more on this I will be triple checking the tilt adjustment and then see where we go.

[Joshua Bunn]

Thats very interesting, its not only in the blue filter to, the red filter is showing just as much elongation in the top right corner. Did you refocus between filters?
Does this filter wheel and filters work well with a different scope?

[glend]

I found a discussion on Cloudy Nights forum on this scope, along with a proper set of Spot Diagrams.

Have a look here:

https://www.cloudynights.com/topic/823745-is-there-an-ideal-small-refractor/page-2

[Ryderscope]

Thanks for the link Glen.

[Ryderscope]

Most definitely interesting to be sure and puzzling :question:
To respond to your points:

- Yes, my auto focus routine is set to refocus between filter changes.

- Prior to switching to the FRA300 I was imaging with a Canon 200mm prime lens and had not noticed this level of aberration in one colour channel. Just to be sure though I will go back and check the data.

- Agreed that the red filter has obvious tilt in the top right corner. And, as Ken points out, there is a slight shift in the green channel as well. The difference here though is that the red channel shows what I would call a classic tilt in that the issue is quit apparent in the top right but the rest of the field is ok. Also, the star shapes are elongated as opposed to the ‘triangular’ shaped that is evident in the blue channel. From my experience, the triangular shaped stars only come from collimation errors or pinched optics. Again, this last point brings up the conundrum of why is one channel affected more than other?

Notwithstanding all of the above, I am going to tweak the tilt adjustments to see if this evens the channels out and then re assess where I take things from there.

[Nikolas]

I have the same scope and notice no aberrations but I do use it with a 294mc Colour camera so it's not a full frame like yours

and the L-Ultimate. I haven't noticed any aberrrations and the field is extremely flat right up to the corners.
I think the trail off could be a filter/fullframe issue and maybe some tilt in the image train?
Blue is always the most notorious channel for star bloat as it is.

[Ryderscope]

Thanks Nik, yes the 294mc would be much kinder on the OTA that the ASI6200. That said, if there were any serious issues with the OTA you should still see them. Thinking about this, I could try out my Canon 60Da with the scope and see how that goes. The 60Da has a crop sensor so would still give the OTA a good test.

[glend]

The rear mounted Pretzval element can, in my experience, be susceptible to colour shift because of the nature of the optical design. The Spots show some shift. If this was an APO with good glass, and a traditional Reducer or Flattener, you may not notice anything.
I note that the FRA600 is much worse in terms of Spot offsets, and this was mentioned on Cloudy Nights as well. So is it a 'feature' of this design?

[Ryderscope]

Thanks Glen. Obviously these OTAs are built to a budget so would not expect the same performance as a $6-$8k OTA of similar focal lengths. Hopefully the issues that I'm seeing are not a feature though :D I have been advised that I need to get the tilt sorted first before I reach any conclusions so I'm proceeding down that path.

[Ryderscope]

Further to your point Glen, attached is a diagram from the Askar site on the FRA300 that shows the way that the OTA focuses the different wavelengths. If I am correct in interpreting this diagram, it is indicating that the blue light is focused differently than the red and green. If this is the case, it could explain why the blue channel is affected more so by tilt.

[glend]

Yes I have seen that diagram. Assuming it is correct then the blue light seems like it should be the reference (being the one centred in the field), and red and green are skewed. I suppose which ever ones you can manipulate would give you a result.
When I used to shoot narrowband through my old Bresser f5 (which had a Pretzval rear lenses as well), I had to align the image layers in Photoshop to clean it up.
Have fun.

[AdamJL]

Maybe Bing can help, Rodney ;)

I'll be trying this soon!

[Ryderscope]

Well I'm impressed :D

[Joshua Bunn]

I don't believe this is the case Rodney, its just indicating, in a rather crude way, that off axis light is focused to the same plane, mostly flat field.

[AdamJL]

Hey Rodney. How did you go getting this issue sorted?

[Ryderscope]

I’ve had to hit a pause on this one at the moment Adam as I’m away travelling and won’t be back until the beginning of May. My intention is to make sure that I can eliminate (as much as is practicable) the tilt from the image train before I make the final call on the performance of the OTA. Joshua Bunn has made up an adapter for me that will allow me to use my ZWO tilt adjuster in the image train and the adapter is waiting for me at home now ready for my return. I will be exercising this as soon as I get home and will report on progress forthwith.

Clear skies,
Rodney

[AdamJL]

Good luck with that Rodney.

In the meanwhile, they've come out with something that allows you to interchange the objective as well.

https://www.sharpstar-optics.com/Products_1/75.html

Full kit apparently USD 1695.