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Old 03-04-2011, 01:17 PM
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Exfso (Peter)
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Oval shaped stars

I set up last night after balancing my mount (G11 with Gemini) to what I believe is to be correct. C/W slightly heavy on east side, and Ota slightly heavy at focuser end. Clutches up tight. Yesterday I pulled out the clutch pads and removed some grease that had made its way onto them. The first two subs were great, but thereafter I had egg shaped stars. My Guiding was very good, the graph was hardly moving! Nothing is binding or catching that might cause the scope to slip through the clutches. Must admit, this one has me stumped.
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Old 03-04-2011, 05:35 PM
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wasyoungonce (Brendan)
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I really cannot say what the issue is but Occam's law is usually the culprit.

I was having trouble with eggy stars on my G11, getting slowly but gradually worse over time. Eventually found my WO guide scope rotating focuser screws were loose....I could slop around this focuser by more than 10mm at the guider camera. I had looked everywhere and tightened everything..... but the obvious.

Fixing this fixed the stars!
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Old 03-04-2011, 07:38 PM
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Exfso (Peter)
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Thanks for the suggestion, must admit never thought of this one, worth a try though...
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Old 03-04-2011, 08:00 PM
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tlgerdes (Trevor)
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How long were your subs and where was the scope pointing at the time? (Not what, but where ie straight up or north east at about 45 deg etc)
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Old 03-04-2011, 08:41 PM
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Exfso (Peter)
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Trevor, the subs were 15 mins and pointing at Centaurus A, approx 45deg elevation to SE. Cant work out why the 1st 2 subs were spot on and thereafter they were crud.
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Old 03-04-2011, 09:06 PM
Hagar (Doug)
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Pete, I really think you have a slight flexure problem with your guide scope. I know your gear and think firstly removing the guide scope adjuster and bolting the guide scope down hard onto the SBS will be the first step in the right direction. Then you can maybe lighten up on your counterbalance weights a bit. You have to be running close to the limit, weight wise.
Make sure you balance in all 3 axis as well when using the SBS. They can make it hard and deliver some strange results.
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Old 03-04-2011, 11:33 PM
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Exfso (Peter)
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Yeah Doug, I am beginning to think you are on the money on this. Looking at possibly getting rid of the SBS and doing a piggy back setup with the Tak and the ED 80. Need to find a decent set of rings for the Tak so I can bolt the guidescope on to it. Any suggestions re the rings??
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Old 05-04-2011, 11:36 AM
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pmrid (Peter)
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Peter, just a thought - it might be useful to get a sense of which axis the egg-shape is in. I would be inclined to compare a guided exposure of a few minutes with an unguided and then, with a third exposure with the mount not tracking at all - so you will get star trails. That ought to give you a sense of whether the elongation is caused by slop in one axis or the other at least. If the star trails are aligned the same way as the long axis of the eggs, then that's where you look for a cause; but if the egg-shape is a right-angles to the star trails, then you would look at your dec axis. If none of these helps, there is always the possibility that your camara is not flat.

Peter
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Old 05-04-2011, 06:09 PM
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Exfso (Peter)
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Thanks Pete, I think we can rule out the camera as the 1st two subs were spot on. I have completely re wired my gaggle going to the mount, I reckon the fact that they were hanging off the guidescope dovetail plate surely would not have helped. Will have a look tonight. Also removed the Tak Guiding mount as it is a weighty fella and would have been pushing the limits of my G11 considering I have the TOA130 and ED80 with a SBS losmandy plate, a lot of weight totalling up. Not to mention the modded 40D with its cooler et al...
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Old 06-04-2011, 06:45 AM
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Pete
I too have been suffering a bout of eggy stars, annoying to the point of sending me over the edge.
Given the equipment list above, have you thought of trying the guide camera attached to a converted finder-scope, and this finder-guider mounted on top of the TOA clamshell or rings, or under the tube somewhere (on the saddle or even under the OTA tube).?
It will reduce the weight and may not give up anything with respect to guiding resolution.
Gary
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Old 06-04-2011, 03:35 PM
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Exfso (Peter)
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Yeah Gary, thought along those lines for sure. My finderscope is a Tak 50mm finder, not too sure how I would fit a guide camera to that though.
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Old 06-04-2011, 03:58 PM
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pmrid (Peter)
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Quote:
Originally Posted by Exfso View Post
Yeah Gary, thought along those lines for sure. My finderscope is a Tak 50mm finder, not too sure how I would fit a guide camera to that though.
Peter, there's an article on this very topic in the edition of Astronomy Technology Today mag that was released today - if you have access to it. The writer puts a limit on the effectiveness of these converted finder/guidescopes - says they are OK for guiding scopes up to about 1500mm but not much beyond that. There were several threads on this topic in IIS also, a year or so back that you will surely find with a bit of searching.
Peter
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Old 06-04-2011, 08:33 PM
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mill (Martin)
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Peter i had the guiding going weird too with the orion starshoot guider untill i got rid of the guider cable and got Ascom to do the guiding.
The pulse guiding doesn't work thru the gemini.net but i am having nice round stars
One more cable i have got rid of
I am using PHD to do the guiding thru ascom now.
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Old 06-04-2011, 08:42 PM
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Martin how do you do that ???

also the GPS ap do I need to have an RJ12 to DB9 connecter

cheers

Trevor
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Old 06-04-2011, 09:13 PM
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mill (Martin)
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Quote:
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Martin how do you do that ???

also the GPS ap do I need to have an RJ12 to DB9 connecter

cheers

Trevor
Trevor in phd goto mount in the top and click ascom, then choose gemini.net and switch of pulse guiding and precision pulse guiding in the telescope setup.

In the gps app you set the comm port your usb gps has and then the com port your gemini is connected to.

Cheers
Martin.
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Old 06-04-2011, 11:35 PM
TrevorW
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I assume the cable i get rid of is the one from camera to mount

Last edited by TrevorW; 06-04-2011 at 11:51 PM.
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Old 06-04-2011, 11:46 PM
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mill (Martin)
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yep
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Old 07-04-2011, 02:12 AM
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Exfso (Peter)
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Thanks chaps. I had a go at the hamburger last night and had perfect round stars for all my subs, thought you beauty. Had another go at another object tonight without changing a thing, and yep you guessed it, oval shaped stars again. Both the hamburger NGC5128 and the other object tonight in a similar area of the sky. This one is very very strange. Just for info, my guiding is fine too, well at least according to the graph it is...
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Old 07-04-2011, 04:06 AM
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Tandum (Robin)
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Peter, what FL are you imaging at and what FL are you guiding at? It sounds like the tube rings are flexing to me. Bolt it all down hard.
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Old 07-04-2011, 05:44 AM
gbeal
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Annoying isn't it, one night great, next not.
Like Robin suggested, bolt it down. I favour "no-rings", a solid attachment if you can. What I need is a lathe, LOL.
Gary
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