RASA8 Curious Optical Aberration

[tomato]

I captured a batch of subs with my RASA8/QHY268c/NBZ combination in an unattended session last night, they all have a curious aberration.

The stars through the central region appear to be duplicated or maybe triplicated(?) whereas those on the vertical edges are unaffected, although they show some elongation in the corners. Guiding was within tolerable limits throughout the session (~0.5 arcsec total RMS) and every sub was affected. This is a single stretched sub, so it is not a registration or stacking effect. I think I have found the cause but wondered if anybody would like to take a stab at what it was? (Hint: it was something I introduced to fix a previous problem).

[ollypenrice]

Something moving/flopping? Mirror. Camera on the corrector plate. Something to do with the cable routing in front  of the corrector? It's certainly a new one, which takes some doing. Congratulations!!! 

😁lly

[tomato]

You are pretty close Olly. I’ve since decided it’s not very helpful to not immediately publish the cause of an issue such as this so without further ado:

A while back to help fix tilt issues with the QHY268c/RASA camera adapter I introduced some quarter circle shims between the camera and adapter. For speed and convenience these were cut from some 0.3 mm thick clear acetate.

Last night it looks like one of these shims came adrift as the scope slewed and ended up across the centre of the secondary lens and in front of the sensor. The transparent nature of the shim has caused a refraction/reflection resulting in duplicate stars in the centre of the image and the sides remaining unaffected. The offending shim fell to the floor as I removed the camera, but a test tonight should confirm this.

The problem is certainly exacerbated by QHY’s rather dubious design of mating the OSC camera to the adapter by means of a tapered flange and three clamping screws. It is very hit and miss and makes it difficult to provide a compression between the two mating surfaces, hence the shim being free to float around. Why they don’t supply the camera with a simple m48 thread connection like their mono version, beats me. 

[Tomatobro]

Can we engineer the problem out?

[tomato]

4 minutes ago, Tomatobro said:

Can we engineer the problem out?

It will be difficult because the tapered wedge is incorporated into the body of the camera, I’m not sure how close to the sensor mounting we would get if drilled and tapped the body to permanently mount a threaded adapter. You could of course leave the wedge connection alone and carry out tilt adjustment elsewhere but it already consumes quite a bit of back focus. It is worth a look though.