9 replies to this topic

[Opera106]

I have a Celestron EdgeHD 8; I mostly use it for visual observation and some occasional digiscoping.

 

In need of a wider FOV, after some research, I acquired an old Celestron 2” star diagonal (this one: https://telescopes.n...ror-2-inch.html) and an Omegon SWA 38 mm 2”.

 

However, the image I get is distorted at the margins. If I focus the center, the stars get progressively elongated. At the very margins they even assume the typical out-of-focus doughnut shape.

Is this combination wrong for my scope? How could I fix it?

 

Thanks in advance.

[TOMDEY]

Your telescope is optimized for imaging more than visual. The aberrtion that you describe is field curvature... your eyepiece is probably not a good match for your telescope. Your Star Diagonal also induces the telescope to operate away from its best wavefront and baffling condition. It's also quite possible that the old diagonal itself is defective (from use or abuse, may have been OK when new). Those would be some things to check.    Tom

[Jon Isaacs]

The Omegon 38 mm SWA is very likely the cause of your problem. Your scope is F/10 and quite forgiving of Eyepieces but it's a poorly corrected eyepiece. 

 

A 30 mm APM UFF would clean things up nicely. Other eyepieces like the Panoptics would also but they're more expensive 

 

Jon

[davidgmd]

Sounds like field curvature or some other peripheral distortion. I agree with Jon, it’s probably coming from the eyepiece. I have that scope and the APM 30mm UFF works great with it. 
 
The Edge HD scopes are corrected for a flat field. That’s one of the reasons (another is reduced coma) that they’re better for imaging than Celestron’s non-Edge SCTs, but it doesn’t make them worse for visual.

 

Edited to add a link to Celestron’s description of the Edge HD optics:

https://s3.amazonaws...paper_final.pdf

Edited by davidgmd, 06 November 2024 - 10:26 AM.

[SeattleScott]

The 38mm is a popular recommendation for beginners with SCT because it is affordable and performs well enough at F10, although not perfect as you can see. The Pentax 40XW is a good premium alternative. The 30UFF recommended is good also but it won’t go as wide because the magnification isn’t as low. There is a 28mm ultrawide that is a popular option too. Around 90% as wide of a view as your 38mm with greater magnification. Less eye relief though (not quite the huge eye lens for maximum viewing comfort).

[Jethro7]

Hello opera106 

I'm inclined to agree with Jon Isaacs. I have the same scope. I use the Baader clicklock visual back as well as a Baader Zeiss prism diagonal and this set up works well with a 31 Nagler as well as a 35 and 41 panoptic eyepieces. (FWIW, I no longer own the 35 and 41 Panoptics do to heavy light polluted skies that cause the views to be washed out) My set up is on the expensive side of things but I have never been disapointed buying gear that has a long track record and reputation for performance. However, folks like Jon Isaacs and Don Pensack, that have alot more experience with other brands of eyepieces can certainly be more helpful advising you on quality cheaper eyepiece and diagonal choices. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Edited by Jethro7, 06 November 2024 - 11:15 AM.

[Opera106]

Thanks for the prompt and useful answers so far.

 

To give you a better idea of the problem, here is a couple of pictures of the view through the 38mm SWA, taken with a mobile phone camera.
 

https://ibb.co/HhydDjY

 

https://ibb.co/ZWNmKNX

 

[SeattleScott]

That looks about right. Pretty sharp over most of the field, with some astigmatism coming into play near the edge. That’s a $100 2” superwide for you. If you want perfect to the edge, you have to pay more. Most people wouldn’t find that relatively minor edge issue distracting, but we all have different sensitivity.

[Chris Johnson]

That’s pretty much what I see in my C11 with a 38mm SWA eyepiece. The view is much better in my ES 34mm and my 31mm Nagler. But they cost a lot more.

[PKDfan]

That looks about right. Pretty sharp over most of the field, with some astigmatism coming into play near the edge. That’s a $100 2” superwide for you. If you want perfect to the edge, you have to pay more. Most people wouldn’t find that relatively minor edge issue distracting, but we all have different sensitivity.

Yes totally Scott! I had the 38mm SWA iteration sold by Agena for a value wide angle when i got my F/9 scope and loved its comfortable use and contrast was very good but that astigmatism bloating 'stars' past 50% was intolerable for me and induced eyestrain and headaches galore; now if i had a F/15 or F/20 Maksutov i'd probably be totally thrilled.

So i found the better corrected your scope is the easier to note an eyepieces weaknesses.


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