View Full Version here: : OAG question

12-08-2017, 10:32 AM
I have a question about Off axis guiding (OAG).

I currently have a scope and a guide scope so i know about the Pros and cons with this type of setup.

My new scope will have a FL of 3000mm, and people recommended I go down the path of OAG. I've been doing some reading and it seems like this could be the way to go. I find a OAG with a large prism around 12mm and get a ZWO camera with a sensor around 11mm, like the ZWO 174. This should give me a wider FOV and smaller magnification to find a guide star compared to a typical guide camera. Would this idea work in theroy? Or get a larger guide scope with a FL of 1000mm, this what be a ratio of 1/3. What are the pros and cons with a OAG setup from your personal experience?


12-08-2017, 11:25 AM
At 3m FL you definitely need to go the OAG route because of flexure between the imager and the guider. The only issues you might run into with an OAG is not enough back focus or bad field correction so it will be harder to find a proper guide star. You also need a very sensitive guider like a lodestar. Otherwise it will limit your practical frame rate. You might have to expose every 3s to start seeing any guide star. This all depends on your telescope and how wide the imaging circle is in the first place. When you choose a guider also keep in mind its dimensions. It might have to fit in tight spaces so if the body is too wide it won't work. The lodestar is the size of a small eyepiece. A QHY5/ZWO looks more like a can of tuna. I also see you can pick up a lodestar for the price of a ZWO from bintel.

12-08-2017, 12:23 PM
Using an oag at 100" fl, the only problem has been ensuring you have a bright enough guide star. Having a rotator has helped me find one each time, if I didn't have that I would need to move the scope slightly some times to ensue I could get decent guiding

12-08-2017, 12:29 PM
What the French man said.....It will be a no go with a guide scope at 3000mm focal length.

What scope are you using?what mount are you using?these will dictate how much difficulty you'll have guiding.

If you really want to have a hassle free experience guiding such a long focal length get a ONAG and forget the rest.

Hey,you'll even be able to guide with a very sensitive can of tuner:thumbsup:

12-08-2017, 12:46 PM
Scope will be GSO 16 RC

Mount will be a Mesu 200

Camera will be moravian instruments G4-16000

12-08-2017, 01:01 PM
Ah very good equipment Ben :thumbsup: now,after spending all that money don't cheap out on the guiding.....get an ONAG and be done with It!

With the 16200 chip you'll need the bigger ONAG XM ,which, unfortunately is more expensive.....but,no point of all this high end gear and egg shaped stars!