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


I think there is likely some tolerance issues at play here amongst all added components and the scope. I can't imagine holding exact tolerances to the 3rd decimal or even +- 1mm across all platforms. Even the coefficient of expansion on any given day may account for deviation off stated nominal specifications especially in aluminum. -Best, Robert

On 06/18/2021 7:27 AM Dale Ghent <> wrote:

The 35FF is a 1.1x flattener. My 130GTX+35FF works out to a focal length of 871.5mm, which gets me a focal ratio of 6.703. Yours is not so different from this, at f/6.730.

From what I understand, backfocus distance is influenced by the focal ratio, so the manufacturing and figuring variances across different combinations of OTA and flattener. This means that the stated backfocus distance (80.8mm) is more of a guideline than something you have to strictly measure to.

The 111.5mm you state isn't too far off from the 102.9mm distance of the 35FF/QTCC sans extension. Are you confusing this with the 80.8mm backfocus distance when using the spacer?

On Jun 18, 2021, at 09:01, Chris White <> wrote:

Sorry for the repost and deletion of my previous topic. I cleaned it up a bit after learning a little bit about what to expect.

The following is my experience dialing in the Field Flattener.

I've had a paucity of clear nights since I purchased my 130GTX so it has taken me a while to get spacing and tilt dialed in, but I finally have succeed in getting it pretty darn good. I initially started out at 103.9mm of spacing (per AP spec) which accounts for a 3mm thick filter in the optical path. Stars looked pretty bad. I was able to get a slight improvement by adding 2mm of spacing, which a friend told me that fixed his spacing issues. Then I saw Roland post on here that one would actually want less spacing than specified for a very large chip. (I'm using a 36mm x 24mm IMX455 chip camera which has 3.76um pixels which is capable of revealing the warts of any system.)

So, I started looking at spacing a couple of mm below spec and stars looked much worse. So I decided to throw out all of my assumptions with respect to spacing specification and just started adding spacing one night using the fact that stars were pointing inward from the corners as my guide. To my surprise, every time I added another mm of spacing, stars looked incrementally better. All the way out to 111.5mm of spacing! This shocked me, as its 7.6mm longer in spacing than specified!

Last night I was able to dial in the tilt that I had in the system which required a 100micron adjustment on two of the three screws on my Gerd CTU. Stars now look very good to my eyes, and I'm a pixel peeper. I've included the aberration inspector below so you can see. Stars look very round to the corners. They look tight and in focus. There is a tiny amount of flaring, maybe... but its hard to tell. I'll need to make an integration to really see if this is an issue. But overall, stars look pretty good, and this is Bin1.

So a question for AP is, is it normal to see a 130GTX so far out of spec? Is this caused by the scope or the flattener being a little out of spec? I have not tested the QTCC yet with this spacing, but do you think this could be a problem when it comes to reducing the scope? Stars look pretty good to me with the field flattener, but could they be even better? Moshen suggested that I platesolve to confirm actual focal length, and it was 875mm, which is pretty much spot on to spec. So why the large backspacing discrepancy? (I've triple checked my measurements from the sensor to the shoulder of the flattener (where the threads start emerging from the flattener. 111.5mm for sure. )

The AA below is from a 600 second Luminance filter shot on a 900GTO. There might be a little bit of flaring on some of the brighter stars, but hard to tell with a single sub. Also included is the CCDI maps. Looks pretty good, although there is still a little bit of remaining tilt that I might be able to improve on... although not sure if my eyes can even see this as an issue. I'll need to do an integration to see if it is meaningful.

Here is the original fit file from camera in case anyone is interested in a closer look at the file:

Thanks in advance for any input.


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