guidescope or OAG for longish refractor

[dan_adi]

Hello,

What do you consider is best for a refractor with a FL of 1280 mm, guidescope or OAG?

[Whirlwind]

It depends on what you are going to use.  Both can be successfully used.  OAGs mean that you don't risk flexure between two telescopes but usually have a smaller field because of the limitations as to where the pickoff prism can be.  Also an OAG adds additional back focus consideration that needs to be accounted for (though most these days are relatively thin) but can be an issue if you trying to fit something else in like a rotator.

I tend to image with an OAG as I find it easier

[dan_adi]

13 minutes ago, Whirlwind said:

It depends on what you are going to use.  Both can be successfully used.  OAGs mean that you don't risk flexure between two telescopes but usually have a smaller field because of the limitations as to where the pickoff prism can be.  Also an OAG adds additional back focus consideration that needs to be accounted for (though most these days are relatively thin) but can be an issue if you trying to fit something else in like a rotator.

I tend to image with an OAG as I find it easier

Well I have 90 mm at disposal between the flattner/corrector and the ccd sensor. I was looking into Optecs motorized Sagitta OAG but it eats up 31.7 mm. The G3 with filter wheel eats up additional 47 ish mm and I have a manual rotator with about 30 mm... So I am over the limit with Optecs Sagitta.

Would it be a bad ideea to place the OAG before the flattner? 

[Whirlwind]

31 minutes ago, dan_adi said:

Well I have 90 mm at disposal between the flattner/corrector and the ccd sensor. I was looking into Optecs motorized Sagitta OAG but it eats up 31.7 mm. The G3 with filter wheel eats up additional 47 ish mm and I have a manual rotator with about 30 mm... So I am over the limit with Optecs Sagitta.

Would it be a bad ideea to place the OAG before the flattner? 

Never tried putting the OAG in front of the flattener but you will get more distorted stars in the OAG because of the lack of flattening.  It would be better to put the rotator ahead of the flattener as long as it can carry the weight.  Also I think the G3 with filter wheel is shorter than stated unless it is a third party filter wheel?

Edited December 5, 2020 by Whirlwind

[Starflyer]

I use an OAG with both my scopes, a 1200mm fl newt and a frac at 332mm fl, it just works and I don't need to worry about differential flex.

I have a pier but have to set up the scope and camera each night I image. Using an OAG means that the camera, with the change of one adapter, is ready to attach to either scope, no moving a separate guidescope from scope to scope.

The main reason for me moving to an OAG was that I was plagued with differential flex, I tried for a couple of years to get rid of it but gave up in the end.  I'm so glad I did, I know people image very successfully with separate guide scopes, I couldn't make it work and a couple of years of throwing valuable subs away was enough for me.

I have mine set up as; coma corrector or reducer / OAG / filter wheel / camera, that way I get nice round stars in the OAG to guide on.  I guide with a Lodestar and always have at least a few stars to guide on, often many more.

[vlaiv]

27 minutes ago, dan_adi said:

Well I have 90 mm at disposal between the flattner/corrector and the ccd sensor. I was looking into Optecs motorized Sagitta OAG but it eats up 31.7 mm. The G3 with filter wheel eats up additional 47 ish mm and I have a manual rotator with about 30 mm... So I am over the limit with Optecs Sagitta.

Would it be a bad ideea to place the OAG before the flattner? 

No it would not, guiding can work with stars that are not perfect in shape, but you have to be careful not to eat too much of the light cone.

This depends on the size of pick off prism and the speed of the beam prior to flattener/reducer.

I also vote for OAG - it is actually cheaper and better solution if you can fit it in your optical train. I use this one:

https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html

There is even shorter one at 9mm optical path. Just be careful that you need adapter on camera side - I use T2 one:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

In any case, say you get above OAG - it has prism that is about 8mm wide. Say you have F/8 beam. This means that at 64mm prism to guide sensor distance - you'll have only single point being illuminated at 100%.

If your flattener requires 90mm of distance to sensor and you place OAG in front of it - it will operate at much lower "aperture" because of this distance - prism will stop it down.

I'd say that at F/8 you want your guide sensor to be about 50mm away from prism (and hence main sensor as well from OAG).

Here is example of using slightly larger sensor with this prism and about 60mm of distance:

you can see illuminated section and you can see that central 1/3 is brighter than the rest - there is some vignetting due to distance from sensor.

[dan_adi]

7 minutes ago, vlaiv said:

No it would not, guiding can work with stars that are not perfect in shape, but you have to be careful not to eat too much of the light cone.

This depends on the size of pick off prism and the speed of the beam prior to flattener/reducer.

I also vote for OAG - it is actually cheaper and better solution if you can fit it in your optical train. I use this one:

https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html

There is even shorter one at 9mm optical path. Just be careful that you need adapter on camera side - I use T2 one:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

In any case, say you get above OAG - it has prism that is about 8mm wide. Say you have F/8 beam. This means that at 64mm prism to guide sensor distance - you'll have only single point being illuminated at 100%.

If your flattener requires 90mm of distance to sensor and you place OAG in front of it - it will operate at much lower "aperture" because of this distance - prism will stop it down.

I'd say that at F/8 you want your guide sensor to be about 50mm away from prism (and hence main sensor as well from OAG).

Here is example of using slightly larger sensor with this prism and about 60mm of distance:

you can see illuminated section and you can see that central 1/3 is brighter than the rest - there is some vignetting due to distance from sensor.

Thank you for the detailed response. The manual rotator is not really a big necessity anyways, and I didn't think I can just put the rotator in front of the flattner if I really want to use it  . The scope is f 6.5 at 1280 FL, so with the above calculation, the optimal distance prism to guide-sensor would be 52 mm. And I assume the same distance applies to prism - main imaging sensor.

[vlaiv]

Just now, dan_adi said:

Thank you for the detailed response. The manual rotator is not really a big necessity anyways, and I didn't think I can just put the rotator in front of the flattner if I really want to use it  . The scope is f 6.5 at 1280 FL, so with the above calculation, the optimal distance prism to guide-sensor would be 52 mm. And I assume the same distance applies to prism - main imaging sensor.

Indeed.

If it helps, here is how I put my imaging train of my TS80 F/6 apo with all the bits . I have 2.5" R&P focuser with M63 connection. I used this rotator:

https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html

I also used this adapter:

https://www.teleskop-express.de/shop/product_info.php/info/p5144_TS-Optics-Adapter-from-M68-and-M63-to-M48---Riccardi-Connection-Adapter.html

This one has M48 on both sides - male towards the camera and female towards the scope. Handy for attaching 2" LPS filter in front of whole optical train for example. It also means that 2" flattener / reducer can be sunken into focuser if there is need for that.

Then I have reducer and after than OAG , filter drawer and camera. Reducer/flattener requires around 61mm of distance for my scope, so OAG gets much closer because with this scope after reduction I'm working at roughly F/4.8

(I use OAG on my RC8" as well - but there focuser has rotation feature so I don't need separate rotator and have much more optical path for different bits).

[dan_adi]

8 minutes ago, vlaiv said:

Indeed.

If it helps, here is how I put my imaging train of my TS80 F/6 apo with all the bits . I have 2.5" R&P focuser with M63 connection. I used this rotator:

https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html

I also used this adapter:

https://www.teleskop-express.de/shop/product_info.php/info/p5144_TS-Optics-Adapter-from-M68-and-M63-to-M48---Riccardi-Connection-Adapter.html

This one has M48 on both sides - male towards the camera and female towards the scope. Handy for attaching 2" LPS filter in front of whole optical train for example. It also means that 2" flattener / reducer can be sunken into focuser if there is need for that.

Then I have reducer and after than OAG , filter drawer and camera. Reducer/flattener requires around 61mm of distance for my scope, so OAG gets much closer because with this scope after reduction I'm working at roughly F/4.8

(I use OAG on my RC8" as well - but there focuser has rotation feature so I don't need separate rotator and have much more optical path for different bits).

I totally forgot to check if the focuser has a rotation feature :)) This is what happens when I m to tired, the neurons stop firing properly :))