Celestron oag - difficulty matching camera to prism distances

[Ade Turner]

I’ve just bought a Celestron oag to use with my Edge 8 HD, 0.7 reducer, ASI 533MC-Pro (image) and ASI 178MM (guide). I’m having difficulty getting an equal distance between the cameras and the prism.

I can easily achieve the required image camera back focus of 105 to within a gnat’s whisker (104.8). 

The distance from the main sensor to the centre of the prism is 65mm. However, my guide camera sensor is 78mm to the prism, a 13mm difference. I’m assuming that’s too great a discrepancy?

With the helical focuser already fully screwed down I see only two ways to reduce it:

a) Remove the screw in IR filter and the nosepiece to gain about 2.5mm. I’m reluctant as then I wouldn’t be able to orient the guide sensor to the prism.

b) Swap the 178MM for my 120MM mini which is only 8.5mm to its sensor. That would make the sensor to prism distance 74mm saving 4mm making the difference 9mm . But again I’m loathe to do this as I want to use the better 178MM to guide. 

I’m still setting up in the house atm so nothing’s been tested.

Any suggestions please? Thanks.

[Stefan73]

Wouldn't it be easier just to lose the helical focuser and use extension tubes / spacers to get the cameras parfocal?  You only need to get them parfocal once (and you can do it in daylight on an aerial or something).  I've never missed not being able to orient the guide camera yet for my OAG...

[Ade Turner]

9 minutes ago, Stefan73 said:

Wouldn't it be easier just to lose the helical focuser and use extension tubes / spacers to get the cameras parfocal?  You only need to get them parfocal once (and you can do it in daylight on an aerial or something).  I've never missed not being able to orient the guide camera yet for my OAG...

Thanks Stefan, that’s an idea, thanks. I unscrewed it but I don’t think the thread is standard. It’s smaller than M42, around 38mm I’d reckon. 

[MikeBY]

Keep the helical focuser.

Remove the nosepiece and I/R filter from the ASI178. Thread the filter into the internal threads directly in front of the sensor.

Remove the 1 1/4 nosepiece thumb screws on the helical focuser. 

Thread the camera all the way on to the focuser and check orientation to the OAG prism. It should line up!  If it does not, get a very thin m42 threaded stop ring and thread it all the way on to the helical focuser then thread on the camera and back the camera off only enough to get it oriented with the prism. Tighten the stop ring against the camera to secure it to the focuser. Set the helical focuser to mid point (+4 mm).

Note, the back focus of the ASI178 is split. The sensor is set 5 mm behind the front surface of the sensor window. The depth of the M42  threaded nose of the camera is 7.50 mm. 

If the helical focuser's M42 threads are 7.5mm or more and the camera threads completely on, the minimum backfocus of the helical focuser plus camera is 37 +5 + = 42 mm. Maximum focus distance is 45 +5 = 50mm.  If a stop ring is used or the focuser threads are not deep enough you will need to measure this distance: The minimum depth of the focuser from the OAG frame to the front most edge of the camera and add 12.5 mm to get the this distance and add 8mm more to get maximum (8mm helical focuser range)

The last adjustment is the prism depth.  It looks like the prism tube is fully extended. (6+14+6=26 mm). Focus distance (6+14 +0) = 20 mm.

Focus range *fully threaded camera on helical focuser* is 

62 to 70 mm  ( you can march focus with the main camera at 65 mm)

However, the prism looks a bit low and may interfere with main sensor. Adjust it

Loosen 2mm set screw(s) on the OAG frame that lock the prism in place. This may be in on the camera side or may be on the OAG frame edge.  BE CAREFUL. the prism carrier will be loose and can easily fall out. Do NOT TOUCH the prism. Surface! 

Move the prism further away from center by 3 mm (shaft measurement now 11mm).  This puts the center of the focus range right at 65 mm.

Focus main camera 1st. Then guide camera.

Check for vignetting on both the main camera and guide camera. (During day) 

Main camera vignetting, move prism away from center and refocus guide cam.

Guide camera vignetting, move prism towards center. Refocus Guide cam.

Tips:

Be sure prism is perpendicular to the focal plane when adjusting length. Prism may extend into guide camera. if adjusted to high (large main sensors)  the top of prism may hit filter or sensor. BE CAREFUL.

. If you have tilt adjustments, take test shots and adjust out any tilt. Tilt set screws can also be used as fine adjustment of Guide camera backfocus avoiding the 42mm stop ring.  Similarly some OAGs have tilt/stop screws to adjust the stop position of the  helical focuser where it threads on to OAG . 

A note about your prism backfocus measurement point.  

The image seen by the guider is refracted 90° by the 45° prism surface. The measurement point for focus is the plane of the image, which is the closest point to the camera where the 45° surface starts, not the midpoint of the slope. 

Be sure everything is tight and lock the helical focuser. You should not need to adjust OAG focus again!  

Worth the effort.. 

If you switch to a DSLR the flange to sensor distance can vary slightly. Cannon EF is 44mm sensor to flange plus 11mm Flange to T2 adapter = 55 mm.

Mirrorless is only 17.5 mm. to flange. However, if your buy the Cannon mirrorless camera T adapter it adds the 17.5 mm back in (it's 37.5 mm) to maintain the industry 55 mm standard. 

 

 

[Ade Turner]

On 06/02/2023 at 20:19, MikeBY said:

Keep the helical focuser.

Remove the nosepiece and I/R filter from the ASI178. Thread the filter into the internal threads directly in front of the sensor.

Remove the 1 1/4 nosepiece thumb screws on the helical focuser. 

Thread the camera all the way on to the focuser and check orientation to the OAG prism. It should line up!  If it does not, get a very thin m42 threaded stop ring and thread it all the way on to the helical focuser then thread on the camera and back the camera off only enough to get it oriented with the prism. Tighten the stop ring against the camera to secure it to the focuser. Set the helical focuser to mid point (+4 mm).

Note, the back focus of the ASI178 is split. The sensor is set 5 mm behind the front surface of the sensor window. The depth of the M42  threaded nose of the camera is 7.50 mm. 

If the helical focuser's M42 threads are 7.5mm or more and the camera threads completely on, the minimum backfocus of the helical focuser plus camera is 37 +5 + = 42 mm. Maximum focus distance is 45 +5 = 50mm.  If a stop ring is used or the focuser threads are not deep enough you will need to measure this distance: The minimum depth of the focuser from the OAG frame to the front most edge of the camera and add 12.5 mm to get the this distance and add 8mm more to get maximum (8mm helical focuser range)

The last adjustment is the prism depth.  It looks like the prism tube is fully extended. (6+14+6=26 mm). Focus distance (6+14 +0) = 20 mm.

Focus range *fully threaded camera on helical focuser* is 

62 to 70 mm  ( you can march focus with the main camera at 65 mm)

However, the prism looks a bit low and may interfere with main sensor. Adjust it

Loosen 2mm set screw(s) on the OAG frame that lock the prism in place. This may be in on the camera side or may be on the OAG frame edge.  BE CAREFUL. the prism carrier will be loose and can easily fall out. Do NOT TOUCH the prism. Surface! 

Move the prism further away from center by 3 mm (shaft measurement now 11mm).  This puts the center of the focus range right at 65 mm.

Focus main camera 1st. Then guide camera.

Check for vignetting on both the main camera and guide camera. (During day) 

Main camera vignetting, move prism away from center and refocus guide cam.

Guide camera vignetting, move prism towards center. Refocus Guide cam.

Tips:

Be sure prism is perpendicular to the focal plane when adjusting length. Prism may extend into guide camera. if adjusted to high (large main sensors)  the top of prism may hit filter or sensor. BE CAREFUL.

. If you have tilt adjustments, take test shots and adjust out any tilt. Tilt set screws can also be used as fine adjustment of Guide camera backfocus avoiding the 42mm stop ring.  Similarly some OAGs have tilt/stop screws to adjust the stop position of the  helical focuser where it threads on to OAG . 

A note about your prism backfocus measurement point.  

The image seen by the guider is refracted 90° by the 45° prism surface. The measurement point for focus is the plane of the image, which is the closest point to the camera where the 45° surface starts, not the midpoint of the slope. 

Be sure everything is tight and lock the helical focuser. You should not need to adjust OAG focus again!  

Worth the effort.. 

If you switch to a DSLR the flange to sensor distance can vary slightly. Cannon EF is 44mm sensor to flange plus 11mm Flange to T2 adapter = 55 mm.

Mirrorless is only 17.5 mm. to flange. However, if your buy the Cannon mirrorless camera T adapter it adds the 17.5 mm back in (it's 37.5 mm) to maintain the industry 55 mm standard. 

 

 

Thanks very much Mike for taking the time out to give me such a detailed breakdown 😉

I’ll work through it and definitely give it a shot. I’m using my 120 mini in the oag at the moment - it’s OK, it works reasonably well and I’ve had decent guiding but the sensor is tiny. It’ll be good to get the 178 on board and focussed.

Cheers, Ade.