OAG-S with QHY294C

OAG-S with QHY294C
« on: September 16, 2020, 10:56:14 PM »
I have what should be a fairly common and straight forward setup for a QHYCCD camera owner:
-a QHYCCD camera (the 294C)
-a Sky-Watcher refractor with the 0.85x focal reducer/flattener

The presence of the flattener in the image train dictates that the QHY sensor must be 55 mm from the flange of the flattener.

I bought my 294C used but the person from whom I bought it also sold it with an OAG-S and a set of spacer rings (0.5, 1, 2, 3, 7 mm) plus the 020056 M42m flange adapter.

I have to say I am quite disappointed with OAG-S and its frankly absurd system of mounting itself in the image train.

Now, according to the page https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=193&id=113 "Adapters Specifications", the QHY294 "used a bigger housing body only and the adapter remain the same of small sized camera." Ok, so that would mean it should work with the OAG-S, right?

Well, no, not really, but we'll get to that.

Next, on the page "Adapters for small sized cams" (https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=193&id=114), we see that my setup falls under 'Scenario IV: SMALL SIZED CAMERA + OAG". Here is how that is illustrated:

So note what's going in here: the OAG-S (10 mm) and the two included M42 thru-bolt rings (3 mm each) are mated with the 020056 M42m flange adapter, which replaces the M42f 020085 flange adapter that came with the 294C. This here is just bizarre: why does part 020083 have an M42f rather than an M42m thread? What purpose is being served by this? To alternate between using or not using the OAG-S now requires the fiddly changing of 020056 and 020085 as they're both screwed in to the 294C.

But worse, this illustrated setup is actually impossible:

Re: OAG-S with QHY294C
« Reply #1 on: September 16, 2020, 11:03:50 PM »
The OAG-S drawtube strikes the camera body, so it won't fit. The illustration for 'Scenario IV' is thus false and misleading.

But there's a different instruction on the QHYOAG User Guide (https://www.qhyccd.com/index.php?m=content&c=index&a=show&catid=30&id=251)

It tells us to use all the rings from the 020086 ring set, with this being the finished condition:

And here's how that looks on my camera:

Re: OAG-S with QHY294C
« Reply #2 on: September 16, 2020, 11:24:01 PM »
It's off-square and the pickup prism blocks part of the sensor.

This is happening because the OAG-S is threaded into the camera without there being a way to adjust the radial position, so unless the thread start and end positions are perfectly set, there's no way to avoid it being in a sub-optimal position.

Looking into this further, it appears the OAG-S is really just designed to work with the filter wheel. The OAG-S bolts onto the filter wheel without 020083 while leaving 020085 on the camera itself. The filter wheel attaches to the camera not through the threads of 020085 but rather its external cam, so the rotational adjustment is built into the filter wheel.

This is what the OAG-S lacks on its own - an ability to adjust its radial position.

Somehow, and I don't really where, I found reference to part 02001, "M42 (F) Centre Adjust Ring" (see attached). If I bought an M42m to M42m adapter and this, this part might be able to latch onto 020085 on the camera? It's similar to 020002, which is for medium sized cameras and included with them, but, unlike the medium variant it doesn't appear to have the bolt holes.

Here's how it works for medium sized cameras:

Why didn't you just do the same with small sized cameras? It's just so frustratingly stupid and frankly I could never recommend that anyone buy an OAG-S if they're not getting the filter wheel.

Re: OAG-S with QHY294C
« Reply #3 on: September 22, 2020, 11:58:07 AM »
I am going through the same issue but with QHY268C.  See my thread in the mechanical section. 

I can tell you that I have tried the method on the bottom picture you cite.  The guidecam (I have tried with a QHY5 and a ASI1174MM) is much too far from the focal plane with respect to the imaging camera to achieve focus.

The sensor on the OAG is much further from the pick-off prism than the sensor of the main camera.  The only way is to "pad" out the main camera from the camera side of the OAG but the adapters supplied are nowhere near enough to push the camera far enough back so that it matches the OAG.  It needs to be pushed back by almost 60mm from my estimation.

Even if a EFW was in the path, even that would not add 60mm.  This system is either nuts, or I am missing something although struggle to see what though since I have other OAG setups and I am familiar with it.
« Last Edit: September 22, 2020, 12:24:08 PM by kaiser-sose »