QHY268M-OAGM compatibility for my 3” Flattener back space

[Xsubmariner]

Problem

  After building my new QHY 268M, CFW3-M  and OAGM setup I have been unable to get a guide camera to focus through the OAG. While the camera came with layout cards showing this setup for 55mm, I also hoped this arrangement would let me use my 3” flatteners (BF 52mm) with my TS&AA 130 triplets. Is it possible?

 Having attempted this setup my conclusions are:

The minimum back focus (BF) this equipment arrangement can support is 54.5mm for a guide camera with 12.5mm BF. To achieve this BF the prism will have to be set at the very outer edge of the field of view. Note every mm the prism is extended into the optical path will necessitate the adding of a mm to the FW-OAGM spacers to keep the guide camera focus tied to the imaging camera and this will increase the overall setup BF. Whereas most OAGs keep the guide camera BF fixed while the prism is altered in the QHY OAG case it appears the guide camera is fixed in position and the BF is increased or reduced when the prism altered.

The layout card for the above equipment included with the camera should show the additional spacers arranged between the CFW3 and the OAGM, with the exception of the 5mm M48 threaded spacer which stays in front to attach the kit to the flattener/reducer.

The recent transition to short bodied cameras, QHY600M and 268M variant necessitates a redesigned OAG to deliver a short BF arrangement <55mm. 

To use this arrangement with my 3” Flatteners I will need to either cut 4-5mm off the front of my Loadstar X2 or replace the 4 round head screws for countersunk units, and use a thinner M48 threaded spacer, say 2-3mm. This would recover 3-4mm and that is what I am planning to do, or should I just butcher my Loadstar X2?

 

Back ground

  From what I have ascertained the minimum back focus distance from the centre of the OAG prism to guide camera sensor is 33mm + back focus of guide camera. This is based on assumption the adjustable prism is placed just inside the optical field of view to achieve the shortest possible guide camera BF. The 33 mm is measured from centre of the prism to guide camera housing base that the guide camera front face mates to.  A picture shows 30mm measured and there is 3mm taken up by material thickness and the 4 housing mounting screws with round heads ( countersunk heads could recover 1mm).

 The guide camera housing is 38mm deep and 39.5 with the 4 housing screws removed, so for the guide camera to reach the housing bottom the camera tube must be 38mm or greater.  This limited my choice to Loadstar X2, my GPcam 2 body was to short.

  My Loadstar X2 has a back focus of 12.5mm.  Therefore my OAGM minimum back focus is 33mm + 12.5mm = 45.5mm measured, but there is a practical need for some inward focus travel, say a minimum 1mm .  Moving the adjustable prism further into the optical tube to extend this distance risks covering or shadowing the APS-C imaging sensor. Retracting the prism to shorten the distance results in blanking a portion of the prism image. Therefore I assess the minimum OAGM back focus distance with a Loadstar is 44.5mm (C) . Noting there is 5mm from the prism centre and plus the 5mm thick M48 threaded spacer makes the minimum BF this kit can support =54.5mm (B) assuming you have the necessary 9.5mm spacers (A) available.

  To use my 3” flatteners with their 52.1mm (inc filter compensation) BF, I will need to reduce the OAGM BF by 1-2mm.  I hope to achieve this by swapping the round head screws for countersunk screws will recover 1mm and possibly shaving 1 mm off the Loadstar face. With FW-OAGM spacers (A) reduced to 8.5mm and the M48 threaded adapter swapped for a 3mm thick adapter I should be able to achieve a total back focus of 51mm, jod done. Now to find a 3mm thick M48 threaded adapter.

 

[Starflyer]

This is great information, thank you, I have the same setup on backorder and expect that it's still a few weeks away.  I don't have you challenges and only need 55mm BF but want to get the filters as close to the sensor as possible.

Like you I have a Lodestar X2, for a while I've been contemplating changing it for something with smaller pixels.  I think the ZWO ASI 174MM Mini would fit the bill and also give you another 4mm. Not a cheap solution, but a solution nevertheless.

[Fellside]

Xsub

What 130 Scopes (F number) and Reduces are you using? Have you used the scope and reducer combination already and found that 52mm works?

As Starflyer has mentioned the ASI 174MM Mini would help as the BF is listed as 8.5mm. You mentioned the guide camera housing is (with blue ring) is 38mm deep. The ASI 174MM Mini is 40.7 from the nose to the start of the Fins. So that is getting close as well. But it should be ok

I have a 268/FW and OAG on the way, I also I have as many probably will a Lodestar X2. So It looks like a change of guide camera may be coming.

Regards

Graham

[Xsubmariner]

6 hours ago, Starflyer said:

I don't have you challenges and only need 55mm BF

I would think the X2 in the current equipment setup is a close call even at 55mm BF, I only accounted for 1 mm inward focus to achieve the 54.5mm min BF with 9.5mm of spacers between the FW and the OAGM. Thanks for the heads up ref the ASI174MM, looks to be a possible solution. But at £500 delivered the cheaper option for me is to machine 4mm off the front of my X2, as long as I leave some thread I can use it with my SX setups using a 5mm extension.

 

5 hours ago, Fellside said:

What 130 Scopes (F number) and Reduces are you using?

I have the TS130 photoline F7 and Altair 130EDT also F7. I have 3x 3” Altair 1.0 flatteners (51.5mm BF) and 3x Planostar 0.79 reducers (55mm BF) for my different triplets. I understand Altair’s new flatteners were modified to give a 55mm BF. Actually it may be possible for me to machine the ends of the 1.0 flattener cone to increase the BF to near 55mm.

Edited February 13, 2021 by Xsubmariner

[Fellside]

The 174 is not cheap but it has more uses than just as a guide camera. QHY version is even more expensive.

If you can reliably hold the front section of the X2. I think machining is the neatest solution.

I have the TS-Optics REFRACTOR 0.79x 3" ED Reducer which looks like the new version of the Altair reducer you mention. On the Altair site the two examples of spacing setup are both over the 51.5mm ideal spacing. So It might be worth giving it a try. Altair quote a 5mm tolerance. I wish all Reducers were so tolerant

Initially I will fit the 268 to my TS 0.79, SW Esprit 100 combination. Then it will go on the GSO RC10 at Bin x2.

Regard

Graham