QHY268C and OAG-M >>>> guide camera cannot focus - distance too much

This my first post :)

I have a nice new QHY268C.  It is very nice.  This is my first QHY main camera :)  There is NO filter wheel of course because this is colour camera.  No need to have EFW.

I also buy the OAG-M.

I fit it all together.  Main camera is all good and I get focus.  But OAG guide camera cannot focus.  It is too far from focal plane compared to QHY268C.  I cannot get it to be shorter.

Because there is no filter wheel I need extra space between the OAG backplate on the camera side and the QHY268c to make them both same distance.  I have lots of spacers that come with camera and OAG.  I use all these spacers and in different combinations but camera is not pushed back far enough..... Why not QHY make a dummy spacer?  Prism is right want because I can look into the OAG camera hole and see just fine.

YOu can see here that from front of OAG to QHY268C sensor is about 45mm.

But from pick off prism to Guidecam sensor it is about 95mm.

Why don't you make a dummy spacer for this to make up for no EFW in situation with colour camera? 

This is when all spacers fitted.  As you can see, it looks stupid and still the guidecam sensor is much too far from the focal point compared to the QHY268C main camera.  It is almost 50mm more than the imaging sensor and this is why the guide cam cannot focus.

Best, Stephen
« Last Edit: September 22, 2020, 12:33:11 PM by kaiser-sose »

Re: QHY268C and OAG-M >>>> guide camera cannot focus - distance too much
« Reply #1 on: September 22, 2020, 02:39:10 PM »
I think you have to use a 1?" profile guide camera to use an OAG... that design puts the guide camera's sensor only a couple of cm above the prism. I sold my puck-style ASI120MM as I knew it wouldn't get to focus properly with the OAG.

If you're using a focal reducer or coma corrector in your image train, they usually have something like 55 mm or 75 mm of back focus, so you don't actually want to be adding extra distance if you don't need to.

This is in fact another bug bear I have with the default range of attachments that came with my QHY294C: they don't play nice with the focal reducers on the market. My Sky-Watcher 0.85x focal reducer/flattener has M48m threads and a 55 mm back focus. The camera itself with the flange-M42f adapter takes up 17 mm of back focus. The included 2" OD (M48f internal threads) to M42m adapter is 32.5 mm. So together they add up to 49.5 mm, which is 5.5 mm shy of the 55 mm distance that is typical, requiring one to purchase additional M48 or M42 spacer rings. The 2" adapter could have been 5.5 mm longer and the camera would then be usable out-of-the-box with many focal reducers.

Re: QHY268C and OAG-M >>>> guide camera cannot focus - distance too much
« Reply #2 on: September 23, 2020, 01:17:10 AM »
Hi Aurenth, as it happens I do have one of those bullet guide cams on another rig - an ASI 120MM guidecam. You make a valid point though about the guidecam profile and I will try it. However, I think it will only bring the sensor two or three cm nearer to the focal plane with respect to the main imaging camera.  It seems from my measurements that it needs to be much nearer than that.

I will try it all the same and thanks for your suggestion, I had not thought of that. 

I take your point about the FL reducers and not adding too much to the camera side of the OAG.  That would imply this OAG/OSC system must surely work - certainly I would have thought QHY would have tested it.   Fortunately I am not using focal reducers any more, they are far too much trouble in my opinion and this camera will always be used at the native FL of the optical systems - telescope or lens.

« Last Edit: September 23, 2020, 02:43:01 AM by kaiser-sose »

Re: QHY268C and OAG-M >>>> guide camera cannot focus - distance too much
« Reply #3 on: September 23, 2020, 07:34:21 AM »
I can confirm the OAG and the QHY268C do come into focus with the spacers in the box.  You need to backspace the camera 13mm with the spacers that come in the kit and this spacing is what a thin filter wheel would occupy.  You need longer M3 screws however, 35mm ones.  They can be bought very cheaply off of Ebay a dollar/euro/pound or two for a bag of ten - and you need six. You need a bullet guide cam though that can go deep into the OAG housing - there is no way to do it with a puck style guide camera like the QHY5 classic.

Must also remember that if you move the pick-off prism it will alter the focus of the guide camera.


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Re: QHY268C and OAG-M >>>> guide camera cannot focus - distance too much
« Reply #4 on: December 31, 2020, 10:52:15 PM »
        If the guide optic patch is longer than the imaging optic patch. You need to increase the image optic patch by insert more spacer between OAG and camera.  You may need to order the longer spacer. We should have such a space. And also the chanllege is that you need to calculate the distance and also find the suitable screw for this length .

        Another simple solution is change to QHY5III462C which has shorter back focal length.

Best regards,
Qiu Hongyun
Qiu Hongyun


Re: QHY268C and OAG-M >>>> guide camera cannot focus - distance too much
« Reply #5 on: January 23, 2021, 04:04:01 AM »
With an OAG-M , with a typical prism insertion into the beam and with a 1.25-inch barrel guider (Like QHY5III178M) the shortest back focus is about 45mm. That means that the distance from the sensor, to the mid-line of the OAG has to be at least 45 mm too.

This unavoidable truth needs to be recognized in the various assembly instructions and spacer recommendations.  I suspect that every customer patiently sits down and works this out for themselves and finds they cannot achieve guider focus unless they go back out and buy additional spacers.

In practice, this typically means that the telescope or flattener is screwing directly into 42/54 OAG top plate.
It would really be appreciated if QHY reviewed their online content and corrected the backfocus typos in specs and manuals and came up with some assembly alternatives that work for OAGs.

For instance, I have had to make these for my various setups.
Chris Woodhouse FRAS