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Old 11-09-2012, 01:19 AM
mook (Paul)
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Location: Geraldton, Western Australia
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Oval Stars Near Zenith

Hi All,

I have an NEQ6 mount on which I use an Skywatcher ED120 piggybacked by an Orion ST80 guided by PHD Guiding and have had pleasing success for the last year.

I now have a C11 on the mount piggybacked by the ST80 and I'm getting oval stars around either side of zenith, but if I continue tracking toward the horizon the stars resume their sharp round shape.

Since it has guided perfectly from all parts of the sky with the ED120, are the oval stars a side affect of the extra weight of the C11 when the RA axis is parallel to the ground or could it be some other factor?

Hope someone can work their magic for me?

Thanks in advance.

Mook
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  #2  
Old 11-09-2012, 07:56 AM
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rat156
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Hi Mook,

Are they oval in RA or in DEC?

You have a huge difference between you guider focal length and you imaging FL, a small, probably unnoticed variation before is now a few pixels at the longer FL of the C11.

It does sound like you scope is having balance issues at Zenith, it may be unbalanced ever so slightly the wrong way, so that the gears loose mesh whilst tracking, this then takes a couple of seconds to catch up etc...

Sometimes, with cameras etc hanging off the scope, getting the scope balanced is challenging.

It could also be a flexure problem with your guidescope, though I've never really used a guidescope, so I don't know what the symptoms would be. Someone else will be able to help here.

Cheers
Stuart
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Old 11-09-2012, 10:27 AM
mook (Paul)
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Location: Geraldton, Western Australia
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Quote:
Originally Posted by rat156 View Post
Hi Mook,

Are they oval in RA or in DEC?

You have a huge difference between you guider focal length and you imaging FL, a small, probably unnoticed variation before is now a few pixels at the longer FL of the C11.

It does sound like you scope is having balance issues at Zenith, it may be unbalanced ever so slightly the wrong way, so that the gears loose mesh whilst tracking, this then takes a couple of seconds to catch up etc...

Sometimes, with cameras etc hanging off the scope, getting the scope balanced is challenging.

It could also be a flexure problem with your guidescope, though I've never really used a guidescope, so I don't know what the symptoms would be. Someone else will be able to help here.

Cheers
Stuart
Thanks for the reply mate.

The stars are oval in RA, and the guide scope is pretty snug in its rings and because of the return in good guiding as it tracks closer to the horizon, I thought that would take the guide scope out of the equation, but maybe not?

My PHD graph looks pretty good, and the numbers for Oscillation Index and RMS are good too. I have a Losmandy style saddle on the mount and Losmandy dovetail for the C11, as I knew tube flexure would be a problem.

I've tried balancing the RA axis as even as possible with all of my imaging gear set up on it, and I've also tried having the East side heavier to make sure the worm gear is always in contact. I don't know how much heavier it has to be however. I only had it slightly heavier so maybe I need to increase the East side weight?

I also thought that vibration suppression pads might be a solution.

Cheers

Mook
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