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Sleuths needed!

Helen

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Helen

Helen

Posted
Posted

I've got a problem guys, but am struggling to work out what's causing it. The problem is egg-shaped stars :). This is using the wedge-mounted Nexstar GPS8. I've got a resonable polar alignment I think, I've checked the screws, cables etc, all to no avail. The other odd thing is that occasionally I get round stars. An intermittent fault is the worst I think!

I did think overnight that maybe it was a balance problem. The mount was unbalanced by a reasonable amount towards the east when I checked this morning (it was bugging me so much I checked before going to work!). Would that be sufficient to cause a problem. And would it allow for the sometimes alright condition?

Any ideas?? (I can post some subs if that would help)

Thanks

Helen

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Milamber

Milamber

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Posted

that or PE though for PE to show up in 30 secs is scary. Maybe check alignment as Blinky suggests, or even look back at the images that have round stars and see if they are closer to the pole than these ones - if the further away from the pole star the more eggy you get then you have it, Blinky's right.

Arthur

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Helen

Helen

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I was pushing it at f10 especially to test my polar alignment, but was surprised to see a problem on short subs, and that sometimes it was OK :)

I'll have to have another go on the polar aligning. I haven't had the best of days (had a bump on the way from home from work) so tonight isn't the time to go out and try again I think. So an early night, and all set for tomorrow I hope! (its so good to have a run of clear nights to really get things sorted out!)

Helen

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steppenwolf

steppenwolf

Posted
Posted

That is quite serious trailing for a 60 second and 30 second sub. I agree with other comments that this is more like a 'jump' and this could have several causes, (some obvious for which I apologize but best to be sure):-

1. Mirror flop

2. Snagging cables

3. Loose focus tube

4. Slipping clutch

5. Debris in the worm drive (this would explain why it doesn't happen all the time)

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roundycat

roundycat

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Posted

I'm with Steppenwolf on this one. Most unlikely to be polar alignment causing field rotation as it has to be quite bad to cause rotation. Shooting near the zenith plays havoc with dec balance so try biasing your scope one way or the other either with a shock cord or a weight and pulley system so the OTA is being pulled to one side.

Dennis

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Helen

Helen

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I'm completely frustrated now! I think I'll go back to visual!!!

I've done everything I can think of, but still no joy :):(

How would I test for mirror flop, Steve? and the same for clutch slip? I did loosen the cover on the base, but was a boit wary of fully removing because of the wires attached to it!

Anyone recommend someone to give it a good overhaul?

Helen <tired and fed up>

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  • 2 weeks later...
Helen

Helen

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OK, a bit of an update guys....

I've just had a long chat with a guy at Celestron UK. I was going to take it in for them to look at, but he was rather pessimistic about being able to easily locate the source of the problem and he reckoned the investigation could cost me a lot of money. With the cost of the journey on top, and the value of the scope its probably not worth it :o. So therefore a bit more amateur sleuthing is called for.

I discussed with the celestron guy some more options around mirror flop and my loose fastar secondary. He recommended rolling the focusser to extremes a number of times to redistribute the baffle grease which may help with mirror flop. So I've got a bit of wrist-ache already and will do it a couple of times more each time I use it. On the secondary movement, I've just taken the corrector off :) and tightened the secondary holder (and given the inside of the corrector a quick clean too). It wasn't nearly as scary as I thought it would be, and I'd have no hesitation about doing it again :). It seems much more solid now, and hopefully that will keep my collimation OK.

But even if these changes don't solve the problem, I've now got my head straight on it. I get so few good imaging nights that I'm not going to waste much further time on finding and solving the problem. I think the scope will still be fine for visual - it was fun in Alt Az mode at Kelling on Friday :) and I've had a lot of value from it since I bought it. It might therefore be time to move on to a different imaging set up. Maybe I'll get an EQ mount and then I'll be able to press my other scopes into imaging service :D.

I just need some more clear nights to assess the state of play.

Helen

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Ford Prefect

Ford Prefect

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Posted

I have had a similar issues with my Mak-newt creating egg shaped stars at 5-10 min subs and was told it could be flexure in the system.

I was given two suggestions,

1) Increase the magnification of the guide scope by adding a Barlow

2) Get at off axis guider

I haven’t had time to test either out, but when I do I will let you know :o

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  • 1 year later...

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