#1  
Old 31-07-2012, 10:09 PM
Carl
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Streaky stars in the west

Was imaging m20 with scope facing due east. Tracking fine with nice round stars. Flipped the scope over to face west later on and all my stars were oval.
This is a first for me, scope is accurately balanced and polar aligned.
Any members come across this before.
Regards
Carl
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  #2  
Old 01-08-2012, 05:17 PM
petershah (Peter Shah)
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that sounds like it could have too much backlash in the RA, the scope might seem balanced, but it only needs it to be a fraction out to cause a small judder after the mount performs a flip. Unfortunately baclash compensation will not fix this problem, the worm will need adjusting. This is all assuming the oval stars are in the same direction as the RA axis.

Differential flexure can also cause this problem if the scope has settled over time in the east side of the pier after a flip some times the scope need time to settle on the western side

a couple of things to think about
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Old 02-08-2012, 08:24 AM
Poita (Peter)
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Which scope and mount?
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Old 02-08-2012, 11:30 PM
Carl
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Streaky stars

Quote:
Originally Posted by Poita View Post
Which scope and mount?
Mount: eq6 pro
Scope: ed120
Guiding with 9x50 finderscope,using phd software
Camera: canon 500d
Not much weight at all. This has never happened before with a meridian flip.

Carl
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Old 03-08-2012, 03:41 AM
Poita (Peter)
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Any chance of a streaky image so we can take a look?
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Old 03-08-2012, 07:01 PM
Garbz (Chris)
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Quote:
Originally Posted by Carl View Post
scope is accurately balanced
This could be your problem. I recently read up on issues with poor guiding at zenith. The recommendation was not to actually accurately balance the scope but rather always have it heavy on the east side. This way the gears are always engaged on one side with reduced backlash.

I setup my weights accordingly to weigh it down on the east and I haven't had a problem since.
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Old 03-08-2012, 10:07 PM
Carl
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Streaky stars continued

Quote:
Originally Posted by Garbz View Post
This could be your problem. I recently read up on issues with poor guiding at zenith. The recommendation was not to actually accurately balance the scope but rather always have it heavy on the east side. This way the gears are always engaged on one side with reduced backlash.

I setup my weights accordingly to weigh it down on the east and I haven't had a problem since.
Yes I have heard of this technique but I am a little confused. If I am shooting west my scope is actually on the east side of my mount. Do I weigh it slightly to that side?
If I do that does it mean that I have to re balance my scope when I do a flip?

Is there a link to this technique
Regards
Carl
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Old 03-08-2012, 11:46 PM
Garbz (Chris)
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I have taken it to mean always east so yes rebalance when you do a meridian flip. However I have no real source for this. I think I actually found it in a post on this board while searching about my autoguiding problems, but I've also heard it mentioned in a conversation with another astronomer.

The logic is sound. If you're east heavy the RA motor is always pushing with each step. If you're perfectly balanced the scope may wobble ever so slightly as it bounces between sides of the worm gear, then you get backlash too if the worm isn't really engaged when the motor makes a step. If you're west heavy you're relying on gravity to advance your RA step.

I did see a powerpoint on the importance of perfect guiding and how to achieve it, and at the very end it said without justification that on some mounts you may want to bias your weights and that it again should be heavy on the east.
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