Off Axis or Piggy Back guide scope?

[Swoop1]

With an off axis guide set up, is it just a case of placing the OAG into the image train between the scope and camera or am I likely to need other gubbings to make the kit work such as extension tubes or adapters?

[michael8554]

A lot depends on which scope you attach it to.

It may fail on the 150P due to the usual Newtonian in-focus problem, I'm not acquainted with the 150P.

If the OAG is connected directly to the C6 and Sony, the C6 should be able to focus.

Now the 290MC - a colour camera is often not sensitive enough to reliably pick up a guide star on every target, mono is better.

The key to OAG is getting the distance from the prism to the Sony sensor, and from the prism to the guidecam sensor, exactly the same, so that they're both in focus.

Depending on the OAG that first distance would be roughly half-OAG thickness + T-Adapter + Sony E Flange Distance,  say 5 + 11 + 18 = 34mm.

If you can't get the guidecam that close to the prism, then you'll need a spacer on the imaging camera path, so OAG + Spacer + T-Adapter + Sony = more than 34mm.

Michael

 

 

 

[Tomatobro]

I have tried both but for shear simplicity have gone back to a guide scope. I change cameras quite a bit and with OAG have to set it up each time but with a guide scope its just plug and play. The numbers from PHD say the OAG is slightly better but the images look the same from both setups.

 

[Swoop1]

Thanks both for your information.

Being a bit of a thicko where maths (or even simple arithmetic) can be concerned, I might be better off sticking with the guide scope and camera route for now until I get a handle on some of the other witchcraft that is astrophotography.

[BrendanC]

That's exactly the route I took for exactly that reason, and my guiding is fine with a guide scope. 

[Tomatobro]

wise decision

[nfotis]

For long focal distance scopes (above 1500mm or so) it's suggested to use an OAG.

There's a rule of thumb that for guiding you need the guide scope to be not much shorter than 1:4 in focal length (so, for a 2000mm imaging scope, you would want a 400-500mm guide scope or so). There's also the matter of the combined weight and center of gravity if you add a large guide scope.

The OAG requires that you first focus your imaging camera, *then* your guiding camera should be positioned at the same distance from the OAG prism as the imaging camera (it's not much fun, I am informed). And the prism should not intrude into your imaging camera field of view. The good thing is, you can move the whole train to another scope as a unit, and it will always work at a common image scale.

There's also the ONAG method, if you want something different:

https://skyandtelescope.org/wp-content/uploads/ONAG.pdf

 

N.F.