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Novice Challenge

Skywatcher 0.9x coma corrector ridge causes tilt

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Hi all,

The SkyWatcher 0.9x coma corrector has a ridge around it that almost exactly coincides with where the focuser tube screws hold it on my 130PDS Newt. When they grip it right, I have virtually no tilt. However, sometimes they sort of slip off it, causing the camera to slip imperceptibly, but enough to create tilt in the images. I tried a compression ring but that's even worse - you can actually see the camera 'pop' up as the ring is tightened and it slips off the ridge.

You can even see it on their promo images - that black ridge line running around the shaft.


It's a bad design! But I really, really don't want to have to buy a different coma corrector if I can avoid it.

Anyone have this coma corrector? Anyone have a similar problem (I have read about other people saying this in the past but can't find the links again)? If so, anyone have a fix? I've heard tell that a third thumbscrew can sort this out, but it would still be on the ridge, and I wouldn't have a clue how to do this.


Edited by BrendanC
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Posted (edited)

I did consider sellotape but the tolerances are very narrow - I honestly don't think I'd be able to get the coma corrector into the tube even with a very thin tape around it.

However, the parfocal ring just worked perfectly. The only problem was that the grubs also kept popping over the ridge! So I tightened them just enough to keep the ring in place, but not pop out. The ring is after all just there for the spacing, it doesn't have to be tight.

In fact, this fixed three things in one go:

  1. The coma corrector is now very well held because the screws aren't gripping at the ridge
  2. I was able to recommission an old compression ring so it has an even surer grip
  3. After chopping the focuser tube to stop it intruding on the primary mirror path a while ago, I'd left myself with very little additional play and it had popped off a couple of times during auto focus. I'd bought the parfocal rings especially to fix this, but it hadn't happened for a while so just left well alone. Now however, I've got a few mm spare which is good for peace of mind.

So, all in all, a great suggestion, thank you @michael8554

Edited by BrendanC
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