Avoiding focuser sag - tips?

[MikeD]

The issue was differential flexure the egg shaped stars came from the imaging CCD moving in relation to the Guide cam.

Put both the imaging CCD and the guide cam in the same focuser and it dosent matter if the focuser flexes a bit.

Hope that makes sense.

Mike.

[lukebl]

The issue was differential flexure the egg shaped stars came from the imaging CCD moving in relation to the Guide cam.

Put both the imaging CCD and the guide cam in the same focuser and it dosent matter if the focuser flexes a bit.

Hope that makes sense.

Mike.

Right. OAG, here I come, and presumably a whole load of new problems. Well, I guess that's part of the fun.

[MikeD]

Do you get round stars on say a 20 second exposure, if so then it may well be diff flex and an OAG could be the way forward?

I would hate for you to get a OAG based on my experiences and it not solve your issues.

Mike.

[Zodiacal_Light]

Wouldn't there be not enough in-focus on a newt to use a OAG?

[Merlin66]

That's were the thin body versions of the OAG come in. Lumicon do a specific model for Newtonians and then there's the TS version.

[MikeD]

I use the TS (9mm) version but I did have to modify the focuser to get the coma corrector to come to focus.

Mike.

[lukebl]

Do you get round stars on say a 20 second exposure, if so then it may well be diff flex and an OAG could be the way forward?

I would hate for you to get a OAG based on my experiences and it not solve your issues.

Mike.

Thanks Mike, I've had a bit of a re-think. As I think said earlier on, the stars are exactly the same shape whether it's a long or a short exposure so it can't be diff flex, and so wouldn't be helped with an OAG.

I'm going to dust off my old 200P and see if I get the same results. The tube of the 250PDS does seem pretty 'flexy' around the focuser, with the camera plus filter wheel, and maybe the old 200 will be a bit more rigid. Is your 250mm the older blue model? Perhaps they have a more rigid tube than the newer ones. Not sure now why I upgraded to the 250PDS, since most of my stuff now is imaging and I've only gained an extra 20% of focal length over the 200P. I do like the smaller galaxies and galaxy clusters, though, and the slightly longer focal length of the 250 is good for those.

[MikeD]

Yes my 250px is one of the first generation blue tube ones, I got it in 2004.

Mike.

[lukebl]

Crikey. I think he's done it.

As I've already said, I've been convinced that the problem I've been having with eggy stars has been focusser sag, or possibly the focuser/secondary alignment.

I was having another play with the focuser and my Cheshire collimator yesterday, and I noticed that when I rotated the collimator, the centre of the cross-hair moved too. Basically, the cross-hairs weren't central, and were telling me that the thing was collimated when it wasn't, depending on how it was aligned in the focuser. It didn't seem to show up on a visual star test, but I guess CCD imaging is much less forgiving than visual. hence eggy stars.

So, while I wait for a new collimator to arrive, I went back to basics. I dismantled and thoroughly levelled the focusser using the three levelling screws (why on earth do you have to partially dismantle the focusser to get access to all three screws?), adjusted the secondary to get it as central to the focuser as possible, then aligned it to the primary dot as much as was possible with a dodgy collimator. Then collimated the primary.

Bingo! Nearly round stars, which should improve after collimating properly with with the new collimator, and no focus sag. So all this time it was a dodgy collimator giving false readings, and not a dodgy focuser. Well, it has confirmed that it was the focuser/secondary alignment which was the issue. Quick test here of the open cluster NGC2301, 6 x 60secs, no darks or flats, bright moonlight. Not a particularly pretty picture, but it's made an old man very happy.