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Old 30-08-2012, 07:35 PM
Carl
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elongated stars again after tests

I posted this problem about 3 weeks ago and have tried a few tests since without luck.
Problem: elongated stars across entire field. noticeable more when shoot M8 pointing west. Aimed at Tuc 47 and centre is ok but elongations towards edges. The elongation is all in one direction across the entire filed. This problem has only reared it's ugly head over the past 2 months since i rotated my Moonlite focuser shooting M8 and M20.

Tests:
1. Recalibrated guide star when pointing West
2. Re balanced scope
3. Added weight to east when pointing west to counter backlash
4. Removed and re fitted Hotech field flattener with rubber expansion ring. Camera attached tight.
5.Checked for any looseness or drawtube slop on anything on my Moonlite focuser

I dont believe flexure should be a problem as it has not shown up before.

I'm using an eq6 mount.
As i said this has only happened over the past few weeks. My next test will be polar alignment but my scope is locked down on a permanant pier so i can't see how things could have changed.

Any help will be appreciated. Cant guarantee a fast reply as lots on my plate.

Damned frustrating hobby this, even worse than golf!

regards
Carl
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  #2  
Old 30-08-2012, 08:46 PM
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billdan (Bill)
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Hi Carl,

I would check your drift alignment and if you are using PHD have a look at your dx/dy graphs and confirm that DEC is stable ( with DEC guiding turned off).

About 2 weeks ago I had to redo mine as our soil had dried out so much ( due to no rain for 6 weks) it is shrinking. At least every 3 days I've had to readjust my elevation knobs ( just micro turns) to bring it back to alignment.

Regards
Bill
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  #3  
Old 01-09-2012, 03:48 PM
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tempestwizz (Brian)
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I'd check the direction of elongation also. ie is it in the Dec plane or the RA plane or flexure if in line with guide-scope gravity. That would give a clue what to look for most.
Is the extent of elongation related to length of exposure? If not, it could be hysteresis in guiding.
What do you get with no guiding? RA drift, Dec Drift, or both? Better to look near Zenith when checking this.
But, since you notice effective rotation when pointing South towards Tuc47, it indicates your polar alignment is off.

In defining exactly what the problem is, you will probably identify a solution.

Cheers,
Brian
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  #4  
Old 02-09-2012, 07:53 AM
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gregbradley
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An image would help analyse it.

Permanent setups can shift so don't assume it won't because it may have. Wet weather, dry weather can cause soils to change and cause movement. Bumping into your mount can also.

I assume from your post it was once good and then went bad or was it never good?

Elongated stars are caused by:

1. Bad polar alignment.
2. Unbalanced scope (if you have a piggy back system then centre of gravity is high and your setup can be unbalanced at the angle you are imaging at yet balanced when horizontal so check balance at the angles you are imaging at).
3. Cable drag.
4. High periodic error of mount, ie. your mount is inaccurate.
5. Flexure.
6. Poorly setup guide camera, not callibrated, too long exposure times, double star used for guide star, hot pixels picked by software instead of gudie star, guide star too dim.
7. No PEC or PEC done wrong - east/west setting back to front so guider is correcting the opposite of what it should.
8. Mirror shift for SCT scopes.

Are you using a guide scope? Does it flex? What sort of main scope is it?

Elongated stars in corners may be coma not guide errors.

Greg.
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  #5  
Old 02-09-2012, 04:42 PM
Carl
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elongated stars

Hi Brian
Thanks for your input.
Answers to your questions
1. elongation is same length regardless of exposure time
2. elongation is one direction only. When i turn off PHD guiding the elongations can be seen more clearly in RA only
3. I don't see any field rotation or distortion across the field of view for example stars elongating out at an angle at the edges, it just seems to be constant across the image.

If the skies are clear Monday night i will do another polar alignment. I always get confused with my direction when doing a polar alignment but get there in the end.

I'll report back when skies are favorable

Regards
Carl



Quote:
Originally Posted by tempestwizz View Post
I'd check the direction of elongation also. ie is it in the Dec plane or the RA plane or flexure if in line with guide-scope gravity. That would give a clue what to look for most.
Is the extent of elongation related to length of exposure? If not, it could be hysteresis in guiding.
What do you get with no guiding? RA drift, Dec Drift, or both? Better to look near Zenith when checking this.
But, since you notice effective rotation when pointing South towards Tuc47, it indicates your polar alignment is off.

In defining exactly what the problem is, you will probably identify a solution.

Cheers,
Brian
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  #6  
Old 02-09-2012, 04:47 PM
Carl
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elongated stars

Hi Greg
Thanks for the feedback.
If the night is clear on Monday I'll give a polar alignment a go again.
My scope is an ed120and i'm using a modified finderscope as a guide scope, never had flexure issues before and would be surprised.

I'll report back after my tests


Quote:
Originally Posted by gregbradley View Post
An image would help analyse it.

Permanent setups can shift so don't assume it won't because it may have. Wet weather, dry weather can cause soils to change and cause movement. Bumping into your mount can also.

I assume from your post it was once good and then went bad or was it never good?

Elongated stars are caused by:

1. Bad polar alignment.
2. Unbalanced scope (if you have a piggy back system then centre of gravity is high and your setup can be unbalanced at the angle you are imaging at yet balanced when horizontal so check balance at the angles you are imaging at).
3. Cable drag.
4. High periodic error of mount, ie. your mount is inaccurate.
5. Flexure.
6. Poorly setup guide camera, not callibrated, too long exposure times, double star used for guide star, hot pixels picked by software instead of gudie star, guide star too dim.
7. No PEC or PEC done wrong - east/west setting back to front so guider is correcting the opposite of what it should.
8. Mirror shift for SCT scopes.

Are you using a guide scope? Does it flex? What sort of main scope is it?

Elongated stars in corners may be coma not guide errors.

Greg.
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  #7  
Old 02-09-2012, 04:53 PM
Carl
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elongated stars image

Hi all
Attached is a jpg as big as i can get uploaded. You will notice the elongation is the same across the field.

Hope this helps

Carl
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