Jezphil

Yes, that's correct.

Ah - it's within striking distance of London and Bortle 4(ish) in the countryside there. I'm an MKAS member, so another Kent astronomy link there.

Thank you - yes, you nailed it. Many thanks and I tested the corrected spacing last night in a field outside Edenbridge and I now have round stars. The camera now screws rather than clamping to the filter wheel which I am sure was indeed a dodgy connection. The other clamp you highlighted goes into a Starlight Feather Touch focuser which seems very finely engineered and snug and doesn't seem to create an issue.

Okay, I ran test frames last night after switching in a 20mm spacer and screwing rather than clamping the camera to the filter wheel to avoid sensor tilt. Back focus was the correct 55mm. I have round stars in the margins at last! Thanks so much for your help on this - you totally nailed something I hadn't understood and that's been bugging me for a long time and I am hugely grateful. I am sending a virtual beer to Birmingham which, apologies, I know isn't as good as an actual one. I got back home to London, after testing in a field out of town, at 5am this morning and accidentally set off the house alarm, waking up the family. So if you also offer marriage guidance counselling... ?

Correction: I measured to the wrong part of the camera, but it is still well over... filter wheel: 22mm sensor set back 13mm cone to edge of camera housing 32mm total 67 ...so I am 12mm over. I don't have a 20mm spacer but will get hold of one. I need to figure out the correct widths and thread sizes.

Hello David Thank you for this. That total distance is: the filter wheel at 22mm + 55mm to the end of the camera housing as illustrated by your blue lines above. So that is 22+55=77mm, plus an extra 13mm to the sensor. So 90mm! And this should be 55mm? Blimey. Does this make sense? I am way, way out, right?

Thank you - yes, I need something to screw at one end to the ATIK 460EX camera and at the other end to the EFW2 filter wheel, keeping that distance 34mm so the back focus is correct. I think the rotator is less than 34mm, so I am looking into a solution. I think the weak point is likely to be the camera filter wheel connection, rather than the reducer/focus tube.

Thanks - yes I think there were indeed too many spacers. However, there are fewer spacers now than you see in the image - 2mm of spacers - as I removed a couple. It didn't make any noticeable difference to the issue. And that reduced amount was what I used for the test frame above. The cone on the reducer fits into a Starlight FeatherTouch focuser which I use as replacement for the original SkyWatcher one. It doesn't have a screw thread, just a clamp. But it's quite a high quality part with a wide opening/fitting for the wide cone, so perhaps the issue is more likely to be the camera with it's thinner cone going into the filter wheel clamp which doesn't seem to be as well engineered. It has just one screw operating the tightness for example. The star-shape issue was the same before I fitted the new focuser - i.e. when the reducer was screwed directly into the original SkyWatcher focuser. So this also makes me think maybe the camera into filter wheel junction could be the culprit. The distance between the camera with cone screwed off to the filter wheel is 35mm, so I think the rotator in the link above would need an added piece of tubing to keep the back focus correct. I will have to try to figure that out.

Ah - thank you so much guys. I really appreciate the time you have taken to respond to this issue, which has been bugging me for a long time. What an amazing community astrophotographers are. Regarding the red lines above showing the direction of elongation - yes, this makes complete sense and yes, the camera is clamped to the filter wheel. This had been a concern to me and I had tried to solve it by loading the image train vertically, and very carefully, but was always concerned that the clamping mechanism was still creating an issue. Obviously the electrical tape you can see in the image was a slightly desperate and vain attempt to keep things perpendicular. I will definitely try a threaded connection now. Regarding the SkyWatcher Evo rotator link above - thank you for this really helpful guidance and I will now go down this route. I will need to make sure the back focus is correct.

Thank you - here you go. Scope is a SkyWatcher Evostar 120ED Pro refractor with a SW 0.85 flattener/reducer. There is also an ATIK EFW 2.2 filter wheel in the image train. I have tried the delrin-type spacers in various combinations but they don't help. The camera slots into the filter wheel. Image of this is also attached.

Thank you Nigella - useful and much appreciated. Yes, there is the effect on all four sides. Oriented in different directions though - wonder whether that still means it is coma?

Am I experiencing sensor tilt here? This is a stretched single sub-frame. My guiding was good at around 0.6 px RMS. I have tried spacers in various configurations but they haven't helped. I am using an ATIK 460EX mono CCD, which doesn't have a large sensor: a SONY ICX694AQG, 12.49mm x 9.99mm. Any thoughts would be very welcome. WR134_300sec_1x1_WR134OIIILightsn2_0019 stretched.tif

13.8V supply now ordered for this. Thank you everyone for your help and advice. Much appreciated - what a great community this when you need help 👍

Mount and adapter are around two years old btw.

Yes, it's the branded one actually supplied by SkyWatcher that came with the mount and is noted on the back as 12V and 5A. I am using it purely for the mount, nothing else. Why wouldn't they do it for slightly more than 12V to avoid issues with under powering? Posing this HEQ5 with slightly more eradicates the issue, I have found. And I think that's what others have found also, from what I have seen.