Position of focus - Sharpstar 15028HNT or TS Hypergraph 6

[GalaxyGael]

Query for owners of this scope.

What position on the draw tube indications do you find focus when you have the fixed required backfocus of 55 mm? Let's assume that we are all at 55, not likely to be more than 2 mm different or thereabouts.

With the ASI2600 that has an internal backfocus of 17.5 mm, I add extenders to sum to 55mm from the M48 thread on the adapter ring that screws into the focuser covering the corrector lens.

Assume that the corrector lens is fully screwed in.

@CCD-Freak was kind enough to show a photo where focus is at ~15 mm, using the ASI1600 MM with a 6.5 mm sensor backfocus. Assuming his optical train between the scope M48 thread and the sensor is longer than mine by 11mm, he is also working at a notional 55 mm from the M48 thread to sensor, so same physical distance. Again, corrector assumed to be the same distance down the drawtube.

I assume also the scopes are all identically made in dimensions everywhere.

I reach focus at ~5 mm on the focuser, with 55mm from the M48 thread to my sensor also. 

If anyone else uses this scope, might you be able to chime in with you rough focus position indication please? 

My images, even when in good collimation, have a flaring that is reminiscent of a drawtube in the light path, although it doesn't look to be since these tubes are wider than the mirror cell, which sits nestled in the centre of the back of the tube., like this:

 

By getting nearly everything optimized, this niggly issue is remaining and while I don't mind the flaring, the asymmetry that follows either a circular or elliptical path makes the stars look a bit annoying. They are slipped at 12 o clock for stars at 12 o clock in the frame, clipped at 9 for stars on the left, and so on. Here is an example. Often it can be seen even when stars are in fairly good order inward and owtward of the clipped flares.

cheers for any help.

[Clarkey]

I do not have the scope, but looking at the star shapes it appears to me like an issue with the inbuilt corrector. The flattened section of the stars move round the centre of the image in a similar way to a slightly wrong spaced coma corrector or field flattener. Being a very fast scope getting perfect field correction is not going to be easy. (I am no expert on this, but that is how it appears to me). Is it possible to move the corrector distance in or out slightly which might help? Looking at the spot diagrams for the scope the full field correction is not perfect and with the larger 6200 sensor this will make the problem more noticeable.

[GalaxyGael]

Thanks @Clarkey, that was the next step after getting some values for the focus position. the clipping follows a circle or ellipse (probably circle) and when I first saw it I thought it tracked the vignetting of the focuser. Upon testing everything else, from collimation to tilt in optical train and camera (which was improved recently by mild adjustment to sensor tilt), I have with the corrector position (effective working distance from the corrector lens factoring the 55 mm M48 to sensor distance), an issue with pinching in the corrector (probably unlikely), or the position of the primary up or down the tube. The last option could be the case if tighten everything inwards only to allow outward changes for primary alignment, but maybe unlikely that a few mm would shift the focus in by so much (if indeed mine is in compared to others). The focus position differences in the two data points I have are very large, still curious to see what other owners have, at least to see if my drawtube in further in that most. I would guess not, but....

Anyway, moving the correct is a good idea. I did this at one stage and its not so easy as the hard stop for that corrector in the drawtube threading doesn't seem to be movable. I can unscrew a half turn or a single turn knowing that the M63 has 0.75 mm thread. I did this before, unscrewing to move corrector closer by about 1.5 mm or so, stars went very wonky, but the clipping didn't go away unfortunately. And, the corrector could have a bit of wobble floating in the threads maybe. I don't know without looking into it more, whether increasing or decreasing the effective working distance is the best way to start the tests without yo-yo-ing the corrector. The spot diagrams are interesting, certainly not ideal for full frame. Mine is APS-C, which is pushing it a little. The scale on those spot diagrams is also large, 100 microns, which I didn't appreciate on first look. Thanks again. Fiddling continues...