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OAG OMG

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#1 dx_ron

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Posted 25 July 2021 - 04:55 PM

Having come into possession of a GSO RC6, I would like to move into (or at least try out) the world of OAG. I don't think there are any particular backfocus issues with the RC6/CCDT67, but if I'm doing OAG I also want to run it on my short refractor. That uses a 55mm-backfocus flattener or reducer.

 

My current train is:

M42:QHY183 (17.5+/-) - 21mm spacers* - 16.5mm M42-M48 adapter = 55ish mm

 

* the 21mm spacers are useful for swapping in and out a 1.25" L-Extreme filter. The filter is the scopestuff filter holder sandwiched between two 10mm M42 spacers. When I swap that out, the 21mm assembly gets capped on both ends (so I never have to touch the filter itself) and replaced with a 20mm spacer plus a 1mm ring.

 

I spent much of this afternoon sorting out what additional spacers would be required to make a QHY OAG-M or OAG-S work (it's complicated, but I think I have it). I also considered the Orion TOAG, but almost everyone seems to add a helical focuser to that (unless you actually have the Orion guide camera) - if I'm going to do that I might as well just go with the QHY that already has a focuser - unless there is some reason why the Orion is objectively better than the the QHY OAGs.

 

Then there's the ZWO, and here's a specific question I hope the forum can answer for me. The ZWO says it comes with an M48-M42 adapter that uses 0 backfocus.

With that adapter, does the ZWO OAG essentially screw directly into the M48 threads of the flattener? If so, the 16.5mm optical thickness of the OAG is just about exactly right.

 

That would be nice, as the money saved could be used on the likely eventual upgraded guide camera.



#2 Medic002

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Posted 25 July 2021 - 05:33 PM

yes the thread adapter screws directly onto the m48 inside of the oag so it does not change the thickness of the oag and remains at 16.5mm. I bought mine last week and works well. The only complaint with an oag is that the fov is going to be pretty much whatever the main scopes fov is going to be but the fl of the guide camera will also be the same as the scope also.. I use mine on a sct and it makes it harder to do a polar alignment for me due to my ncp being partially blocked by trees.


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#3 Alex McConahay

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Posted 26 July 2021 - 08:18 AM

I should think the helical focuser on an off axis guider would help get the focus on the guide camera. However, one must do this only once. Once it is set for any given OAG to Camera distance, it is done. Therefore, it is not nearly as big a deal as one might suggest. I would not make my decision about which OAG to buy based on this feature alone. 

 

>>>>>>>with an oag is that the fov is going to be pretty much whatever the main scopes fov is going to be 

 

I don't get this. Yes, the OAG is in general pointing to the same area of the sky, but it is by definition OFF AXIS. It therefore sees what is off to the side of the main imager FOV. Furthermore, the chip is usually much smaller, and thus, the FOV is smaller. 

 

Alex


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#4 dx_ron

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Posted 26 July 2021 - 10:50 AM

I should think the helical focuser on an off axis guider would help get the focus on the guide camera. However, one must do this only once. Once it is set for any given OAG to Camera distance, it is done. Therefore, it is not nearly as big a deal as one might suggest. I would not make my decision about which OAG to buy based on this feature alone. 

 

Thanks Alex. That should be true - even if you move the OAG + main camera to another scope. An issue with the TOAG, as far as I can tell from reading old posts, is that focusing without a helical focuser is a royal pain involving tiny set screws. On the plus side, the TOAG seems to have much more flexibility in how you position the prism.

 

But as long as the ZWO fits in my 55mm requirement for the refractor I will probably start with the ZWO.



#5 Alex McConahay

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Posted 26 July 2021 - 11:03 AM

Thanks Alex. That should be true - even if you move the OAG + main camera to another scope. An issue with the TOAG, as far as I can tell from reading old posts, is that focusing without a helical focuser is a royal pain involving tiny set screws. On the plus side, the TOAG seems to have much more flexibility in how you position the prism.

 

But as long as the ZWO fits in my 55mm requirement for the refractor I will probably start with the ZWO.

Yes, as long as you keep the same OAG to sensor to IMAGING sensor the same, things are set for life. And yes, the tiny screws are a pain in the butt. Point is, though, that you can loose fit the screws, and loosen one enough to give free travel, and then, as you find the good distance, tighten that loose one. Check the focus. ANd then if all is good, tighten any other screw(s). (It is not as if you need to worry about the whole loose screw thing at once. Just work with one loose screw. I say this not to disconvince you about your ZWO......just for lurkers who may be wondering. 

 

Alex


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#6 dx_ron

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Posted 26 July 2021 - 12:11 PM

Giving another look at the TOAG, so I downloaded the manual. I'm impressed by the seemingly important details that manufacturers leave out of the specs and instructions. For example - what is the distance (or distance range) from the prism to the 'guider base'?

 

The manual specifies that both sensors should be the same distance from the "center of the OAG" - should it be that the guide sensor distance needs to be measured from the center of the prism? In other words, if you focus the guide camera then move the prism up or down, does that change the guide camera focus?

 

As far as I can glean from the TOAG manual, the threads on the telescope side are M42. So I need to find an M48f to M42m that is 6.5mm (or smaller plus shim rings).

 

On the camera side, the connection is not really threaded, is it? There are flanges on the side of the M42m camera adapter that are held by a couple of set screws. Does that risk flexure of the main camera, or is there enough flat contact area that no notices any flex?




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