EDIT: If the main body of this post isn't useful to you, I'd appreciate if you'd check the bottom 3 paragraphs for a question I have to see if you have any input. Thanks.
This is just a quick post to help anyone else looking to put the following setup (which I imagine must be reasonably common, or at least something with the same or similar backfocus) parfocal so that the guide camera and imaging camera are both in focus.
I just set up the following (roughly, in daylight, on a distant terrestrial object):
QHY 268M (17.5mm backfocus)
QHY CFW3M-US (7x 36mm unmounted type (17mm backfocus)
QHY OAG-M (13mm backfocus when including the 3mm thick front M54 adapter)
So the above were bolted together for a total of 47.5mm of consumed backfocus.
Initially, I expected, that since I have a short backfocus (approx 11.5mm) QHY guide camera (5iii-174), it would come
to focus in the OAG-M's helical focuser with no further adjustment. Wrong. In fact, the guide camera needed to be moved "in" (further forward in the light path), such that it would bottom out in the OAG when the main camera was focused without getting anywhere near focus.
I do, before anyone asks, have the OAG stalk fully retracted to be as short as possible and it is right at the edge of the field (which is ideal anyway, since my calculations showed this would give enough clearance for light cones as fast as around f/3 without the prism casting a shadow on the main sensor, and I wanted the imaging train to be capable of that in case I buy a hyperbolic newt like an epsilon at any point).
I worked my way up gradually, and eventually have needed 8mm of spacing between the filter wheel and the OAG (you could alternatively put this behind the filter wheel between it and the camera, but then you'd be increasing the chance your filters would vignette) to get both cameras to focus at the same point. Remember that is with the OAG stalk fully retracted, as far out to the edge of the field as it will go, if you wanted it further in, you'd need more spacing.
Hopefully that number might help save someone some of the trial and error I went through.
Now I have:
M54 adapter 3mm
This bolts together well with the 23.5mm/24mm M3 bolts that come with the 268M. You must take care to ensure your bolt ends do not penetrate too far into the CFW, otherwise you will hit the filters, or jam the wheel with the tips of the screws. I measured the front-plate of the CFW as 3mm thick, you can afford your bolts to go 1mm or so beyond (into the wheel) but no more before they would interfere with something.
Given that a frequent focal reducer backfocus is 55mm, and I'm aiming for 56mm allowing for filter thickness, that is useable, but I had hoped when I initially bought this setup that I would not need the spacing, and would be able to squeeze more into the imaging train. For example, I have some manual rotators (about 12.5mm thick) that I might just have got away with (with a flattener that said 55mm but really needed more like 58mm) if I didn't need the spacing.
In my opinion, QHY ought to provide better technical drawings so this can be determined in advance. I'm not trying to have a bash at them specifically, since it's normal with any companies system for this to require some tinkering, but especially since a selling point of the system is that it bolts together with no wasted backfocus, and that I have paired it with the shortest possible backfocus QHY guide camera, I had been expecting it to reach focus without additional spacing.
With the 8mm of spacing, although I have not yet made a final nighttime adjustment of focus, on a distant daytime target I need about 1mm of extension of the helical focuser on the OAG to focus the guide camera. That's ideal and will give me a little room to adjust either way tonight for final focus.
I have one question, if anyone got this far - the 5III-174 has a threaded-on IR cut filter. Removing this would have got the guide camera 6mm deeper into the OAG, hence reducing the spacing required to only 2mm. I have not done this, because I'm concerned that letting IR get to the sensor will mean my guide stars will bloat, leading to worse guiding performance.
I am actually wondering if I'm thinking about that wrong, since bloated guide stars aren't always such a bad thing - centroid calculation can deal with very defocused guide stars, the 174 is extremely sensitive and has big pixels (so I would still expect enough SNR to guide on) etc.
Has anyone actually tried removing that window to get their guide camera further in and reduce the required spacing? Does anyone have much to offer regarding my thoughts above about removing the IR cut?
Edited by RaulTheRat, 14 January 2022 - 01:14 PM.