Re: Interesting Back Spacing for 130GTX. Is this far off spec considered normal, and does it even matter?

Dale Ghent

I've been following this thread and Chris's journey. I have both the QTCC and 35FF for my 130GTX and received my QHY600 Pro a year ago.

I started out with it hanging on the QTCC, and when I reconfigured my scope for the QHY600 imaging train, I also added in a OAG. After some initial testing, some tilt was evident but what became more apparent was that the back focal distance wasn't working out in practice to what it should have been on paper. I got things to "good enough" out of the interest of moving on and actually enjoying the camera, but reviews of the mechanical distance to what it looked like the problem was (sensor too far from the QTCC) made it seem like I should have been in the right neighborhood, but the results didn't bear that out.

In the fall, I swapped out to the 35FF to see how the IMX455 did on the slower focal ratio and to get some tighter framing for the season's objects. Going from 1.3 arcsec/px to 0.89 was a nice shift in gears. It was also my first time using the 35FF. It did, however, push things to the limit with my junky US east coast skies. This also prompted me to start habitually using pointing models. I also got a Gerd Neumann CTU. Back focal distance and tilt were far more tolerable with the 35FF, and the CTU made in-situ adjustments for tilt a cinch to work out. The process really speeds up once you realize with corner of the sensor corresponds to the physical arrangement.

I switched back to the QTCC last week and, after a night of fiddling, I came to realize that my back focal distance was actually shorter than I had calculated. I had everything spaced out for 81-82mm (Chroma filters, so +1mm there) but it seems like I have /too much/ mechanical distance in there. Well, I couldn't take anything out of my train to shorten it and my QTCC -> M68 adapter from Precise Parts was already at the shortest length they will make them. I realized a solution with the QTCC's own spacer. The 130GTX's spacer is 22.1 mm, but the TEC 140's spacer is 18.3mm. I figured I could swap in the shorter TEC140 spacer and then bump things back out with 1mm spacing rings to get it right.

The hunch worked. I added 3mm of spacing and I am now *very* close. I think I need 0.3-0.5mm more and I'll be golden, and a little tweak to the CTU for tilt. It seems to be that I was indeed 1-2mm over what I should have been. I'm in the midst of a week of crappy weather and I hope that I can work out this last little bit of distance. Here's what CCD-I is telling me. The tilt issue looks large but in reality the adjustment to the CTU to eliminate it is a hair's breadth of an adjustment.

I have calipers and feeler gauges, so I'll measure the distance and the CTU adjustment gap once I get it zeroed in.

On Jul 1, 2021, at 19:13, Chris White <> wrote:

On Thu, Jul 1, 2021 at 06:03 PM, Roland Christen wrote:
I am testing the color version of this camera on various scopes, and do find one corner off, but I haven't had time to mess with it. The camera is on loan from George, so i don't want to monkey with it.
Have you or are you planning to test with the 130gtx/qtcc? I'll be interested to see how your corner stars look with that sensor. Thank you,

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