OAG or guidescope

[benklerk]

Just going over finer details now

GSO 14 RC with a Moravian G4-16000 camera.

I'm using a Skywatcher 72ED on my portable setup as a guide scope with a ZWO 174mm mini guide camera.

I live in Bortle 3/4 skies and my seeing can get as low 1 arc sec avg is around 2".

The mount i'm getting will be the new JTW friction drive mount.

I have attached the image scales using different setups.

If I don't get the OAG adapter I can get the 290mm mini guide camera instead as they cost the same.

So my question is, is going OAG worth it? The mount im getting will be very solid and I can do longish unguided. So when I do guide I can set guided pulses to 15 or 20 seconds or longer?
Im just coming from the idea that the mount will compensate the use of an OAG as the mount will be good enough to do long guide pulses corrections.

[Paul Haese]

Using an OAG in short is worth the little bit of hassle to set it up; once you have it sorted you will never look back. I have been using OAG orientated guiding for many years now and would never consider using a guide scope again. The hard part is getting focus sorted for the OAG guide camera. It takes some patience.

[benklerk]

Hi Paul

I bought a ZWO OAG to use on my RC 6. I stopped using my ZWO OAG on my 6" RC because it didn't find a guide star. I can guide 10min subs with no star trails using my current guide scope.

I just found it a big pain, I got focus on both camera and guide camera. But PHD2 would not choose a star to lock onto.

[jwoody]

Having an OAG with a larger pick off prism may help. I swapped out my QHY for one of these https://www.omegon.eu/off-axis-guider/omegon-off-axis-guider-off-axis-guider-with-microfocus/p,49752
Has a 12.5x12.5mm prism.
So far so good

[Slawomir]

My vote goes for OAG, since it eliminates a possible problem of flexing between the two focusers (guidescope and the OTA) plus possibly influence on guiding of some other mechanical factors such as slight mirror shift etc.

Can pixels be binned in your guide camera to improve sensitivity? A popular lodestar has pixels with nearly three times the surface area comparing to 174mm plus a similar QA, meaning lodestar could be two-three times more sensitive?

[RobF]

For the FL of the C14 you'll need a modern (sensitive chip) guide camera for those times there are now bright stars visible in the subset of your main scope FOV the OAG prism can see.

As people have said, if your FOV is large enough that a larger prism doesn't intrude into lightpath then that improves your chances.

Rotation of the prisim is an option on some OAGs (but its manual).

If you're lucky enough to have a rotator than you have another chance at getting a FOV with a guidestar.

I've never been able to match guiding performance with guidescope versus OAG, but they can take quite a while to get right in the first instance. You shouldn't need to fiddle after that one off set up however.

[benklerk]

Thanks for the replies, looks like OAG is the way.

Few things, the G4 16000 has its own OAG and I think it’s the only OAG that will fit it.
Without spending even more money I will be using my 174mm as it has a larger FOV. I’ll be aim to do 10 second guide subs, so that could be enough time to pick up faint stars. I can even bin it.

https://www.gxccd.com/art?id=436&lang=409

[Slawomir]

Those Moravian cameras do look interesting. I would definitely bin the guidecamera, possibly 3x3 since your main camera has significantly larger pixels and both cameras will be imaging at the same FL, and guiding camera does not need to resolve as finely as the main camera due to guiding software detecting sub-pixel deviations.

[g__day]

If you have long focal length and / or the chance of mirror flop - would go for OAG over guide scope every time.

[SuperG]

Many say the hardest part is the focusing. I don't think that's true. That's the easy part. Trying to find a guide star is the hardest part. I also find that stars are very very dim in the oag camera and definately not round. The one image I got using an OAG is very good but I was faffing about for ages trying to trying to find a guide star. And that was in acrux????? With the weather the way it is, I haven't gone back to the OAG.

Oh and I was using a C6 SCT .

[Sunfish]

I am definitely no expert on guiding but I find the OAG works fine on both the 100mm refractor and the C8 with a reducer. Guiding for five minutes with poor PE and poor PA seems to work surprisingly well with the most basic of setup.

The problems arise without the reducer on the SCT which seems to reduce both brightness and field of view, so the number of available stars . The other problem is the flaky nature of the zwo USB connections .

I would definitely get a guide camera better than a ZWO 120 and start with the reducer, but the celestron OAG with good focussing , all required adaptors and rotating connectors works well on an SCT.

[Slawomir]

Indeed, small corrected imaging circle does make guiding with an OAG more tricky as stars will have significantly distorted shapes away from the optical axis at the distance where OAG operates. Binning, possibly 3x3, would have helped to increase SNR in the guidecam. Guidecamera with big pixels and a scope with a large corrected imaging circle both make life easy when using an OAG.