OAG star shapes

[pmrid (Peter)]

I have toyed with various OAGs over the years but never really mastered their use. Some things never seem to change. My current iteration is a ZWO.

The problem I am having is with star shapes. I have moved the pick-off mirror so it is on the long side of the imaging sensor (an ASI1600MM) and as close at it will go without obscuring it.

And yet the stars are very banana-shaped and ASIair calibration fails.

I am using a lovely SharpStar 61EDPH f5.5 triplet APO with a dedicated 0.8 reducer/flattener so it ought to be providing a flat field to the very outside of the FOV - which it does on the main sensor. But my OAG seemingly has other ideas.

The guide cam (an ASI120MM mini) is in a helical focuser and seems to be nicely secured.

I wonder what I am missing.

[Startrek (Martin)]

I don’t use an OAG ( my guides scopes on both rigs work fine ) but this video might help with a few tips ?

https://www.youtube.com/watch?v=s93x...bm5lbCBPQUc%3D

Cheers
Martin

[Joshua Bunn (Joshua)]

If the guide sensor is with in the image circle if the imaging sensor, and still ordering bad stars, I would be looking at pick off mirror orthogonality to the main imaging light path, or guide sensor tilt. What happens to the stars on the guide chip if you rotate the guide camera? Do they bad star shapes follow the guide sensor or change their place relative to it?

[AlexN]

For what its worth Pete, I had the Zwo OAG just recently, and have updgraded to the Player One OAG that bolts to my camera rather than threading on.

With the Zwo OAG, star shapes on the guide sensor were pretty terrible, and on the same scope with the Player One OAG, the stars are pinpoints, so as Joshua said, OAG orthagonality is probably the issue here.

The other factor is this - with the ZWO OAG and terrible star shapes on the guider, I was still guiding at 0.3~0.5" RMS error...

To show how similar my scenario is...

I was using the ZWO OAG, a 4/3" sensor (same size as the 1600mm) the Askar 65PHQ with 0.75x reducer (65mm f/4.8 ~312mm focal length).

This scope with (or without) the reducer, provides pinpoint stars to the edge of a 35mm camera sensor, so the 4/3" sensor is no match for it whatsoever, however, the ZWO OAG always produced very odd stars.

I have not used a guide scope since the first time I got an OAG in 2008/2009, and I can safely say that on most of my scopes over the years, guide star shapes were a bit screwy, if not completely terrible (I'm looking at the newtonians!!) and guiding was always just fine regardless..

The OAG that bolts directly to the camera body is the only one that's ever had guide stars looking as good as the stars on the imaging sensor..

[pmrid (Peter)]

Thanks guys.
As you would expect, the rain gods have decided that I can't continue to fiddle with this OAG for what is shaping up to being a long wet time.

But it's a problem that won't go away so sooner or later, I will come back to it.

[pmrid (Peter)]

Originally Posted by Startrek

I don’t use an OAG ( my guides scopes on both rigs work fine )

I have guide scopes that would work fine but actually mounting them on smaller scopes is difficult if they don't have multiple mounting shoes to accommodate both an ASIair as well as a guide scope. Each of mine only has one and I find the fiddling required to fabricate something for the second item is a PITA. So an OAG is a simpler solution if it can be made to work.

Peter

[PRejto (Peter)]

Re the Player-One OAG, I started this post at CN: https://www.cloudynights.com/topic/9...x-light-leaks/

Good luck with that OAG! It looks good, but isn't really in my opinion.

Peter

pS: For what it's worth, the OAG inbuilt to the StarlightXpress wheels have always worked well for me. They are simple, but not terribly robust.

[Peter Ward]

Ever thought of going next level and getting a SBIG star chaser?

Internal optics give you a wider field plus independent placement and focus of the pick-off prism.

Not inexpensive but there is no better system on the market IMHO. Oh...and you can add an AO

[PRejto (Peter)]

Hi Peter,

That SBIG system is quite interesting. From a cursory reading I didn't get any sense that it could be used with a non-SBIG camera. Is that correct?

Ta
P

[Peter Ward]

Originally Posted by PRejto

Hi Peter,

That SBIG system is quite interesting. From a cursory reading I didn't get any sense that it could be used with a non-SBIG camera. Is that correct?

Ta
P

You'll need custom adapters for non-SBIG camera heads, but that's all.
it has its own power supply and software and works independently of the camera. Make sure you choose the correct back-focus variant however

[AlexN]

Originally Posted by PRejto

Re the Player-One OAG, I started this post at CN: https://www.cloudynights.com/topic/9...x-light-leaks/

Good luck with that OAG! It looks good, but isn't really in my opinion.

Peter

pS: For what it's worth, the OAG inbuilt to the StarlightXpress wheels have always worked well for me. They are simple, but not terribly robust.

I've not had that issue with mine at all. Maybe a variance in quality.

[PRejto (Peter)]

It does seem to vary a lot between users...some, no issue, others, severe.

I do think one possible light leak is between the camera and FW due to the tilt adjustment plate but that wouldn't have an impact on the OAG. The stalk seems to be the issue.

P

[Leo.G (Leo)]

Originally Posted by pmrid

I have guide scopes that would work fine but actually mounting them on smaller scopes is difficult if they don't have multiple mounting shoes to accommodate both an ASIair as well as a guide scope. Each of mine only has one and I find the fiddling required to fabricate something for the second item is a PITA. So an OAG is a simpler solution if it can be made to work.

Peter

Can you not use a tube ring to mount a guide scope to without the need of an extra shoe?


I've not done any guided imaging since I played with a 35mm camera attached to a small refractor in the early 90s (manual guiding with reasonable results (I had better eyes 35 years ago)) .
I have often had the stupid idea I could offset my guide scope on the mounting to align with any easily located star in the field of view instead of going with an actual OAG.
Is this a totally STUPID idea, from people who know and do this stuff?
I'd like to use my green laser for guiding accuracy.

I guess in effect it is a poor mans OAG?