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Old 28-02-2011, 08:44 PM
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allan gould
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Mirror flop???

Ive recently been using my 10"SCT to do astrophotography and previously with my guidescope mounted on top of the 10" Ive been able to do 10-15 min subs with no problems.
But just recently (without changing anything apparently) I find star elongation at 5 min and cant do a 10 min sub. without severe star elongation. The graph readout using PhDguiding is level and calm for the 10 min and smooth but the image shows star elongation.
Suspecting mirror flop I tried two diffeent approaches.
I used the bolt spring compression method for meade mirrors but to no avail - still got elongated stars.
Next I tried the OAG on my QSI583wsg and got perfect round stars.
Therefore the guiding is perfect and thus must be a flexture difference between guidescope and main scope.
Any thoughts on what to try?
Thanks Allan
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Old 28-02-2011, 09:09 PM
Dennis
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Hi Allan

Using a side-by-side set up I once discovered that the grub screws on my after market focuser had worked loose and the camera cable drag was sufficient to cause slight movement as the mount climbed, even though the guide scope was showing perfect tracking.

All I can suggest, is check that all fasteners, locking screws, etc. are tight, just in case something has become loose.

Cheers

Dennis
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Old 28-02-2011, 09:17 PM
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Merlin66 (Ken)
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Mirror flop on SCT's is an urban myth...
There's confusion about focus shifting and the mirror flopping around on the baffle when the scope changes orientation across the meridian.
I asked this question on another forum almost three years ago and requested copies of images showing definate mirror flop.....I'm still waiting.
I think a more sensible answer lies with the guide set-up flexing....
You can run the main mirror up and down the baffle a couple of times to minimise focuser shift - I do this when I remember.....
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Old 28-02-2011, 10:04 PM
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allan gould
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Quote:
Originally Posted by Dennis View Post
Hi Allan

Using a side-by-side set up I once discovered that the grub screws on my after market focuser had worked loose and the camera cable drag was sufficient to cause slight movement as the mount climbed, even though the guide scope was showing perfect tracking.

All I can suggest, is check that all fasteners, locking screws, etc. are tight, just in case something has become loose.

Cheers

Dennis
I tried to see where flex could be comming from but failed. I was using rings with 3 screw tighteners on each ring. Ill try solid ring mounts and see if this makes a difference, as well as the obvious stuff.

Quote:
Originally Posted by Merlin66 View Post
Mirror flop on SCT's is an urban myth...
There's confusion about focus shifting and the mirror flopping around on the baffle when the scope changes orientation across the meridian.
I asked this question on another forum almost three years ago and requested copies of images showing definate mirror flop.....I'm still waiting.
I think a more sensible answer lies with the guide set-up flexing....
You can run the main mirror up and down the baffle a couple of times to minimise focuser shift - I do this when I remember.....
Ive heard of driving the mirror up and down a few times to redistribute the grease so Ill try that. Thanks for the reminder.
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Old 01-03-2011, 03:43 PM
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Sounds like flexure to me. That is why I only use an OAG to guide with now. If you are getting elongated stars and they get longer and longer from 5 minutes to 10 minutes then it is flexure and not flop. Flop will tend to make the images shift a lot from frame to frame.
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Old 01-03-2011, 05:57 PM
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multiweb (Marc)
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I reckon also OAG is the only way to go with an SCT at long FL. Temperature shifts... there are two many things lose in an SCT design with changing temp. The corrector glass, the cork holding the corrector, the cork ring holding the primary, the grease on the baffle tube, the alu tube, the list goes on.
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Old 01-03-2011, 06:22 PM
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Bassnut (Fred)
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Quote:
Originally Posted by allan gould View Post
Next I tried the OAG on my QSI583wsg and got perfect round stars.
Therefore the guiding is perfect and thus must be a flexture difference between guidescope and main scope.
Any thoughts on what to try?
Thanks Allan
You have answered the question. Bolt the guidescope and guidecam hard down.

Mirror flop is a myth?. Um no, I got that at merdien with the mirror lock loose (Meade).

Meade has a mirror lock for this reason, Celestron doesnt, take yr pick.
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Old 01-03-2011, 06:27 PM
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Merlin66 (Ken)
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Fred,
Do you have any images which show the effects of mirror slop/ flop? I'd honestly be interested in recording it.....

If you're using an OAG and the mirror DID flop....how can/ could the guiding software make a fast enough correction to stop it showing in the final image???????
( I use a 25 -30micron slit on the spectroscope and have to obviously keep the star on the slit....so far in all these years with SCT's I've never experienced a mirror movement which caused the star to "jump" off the slit....maybe I've been lucky??)
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Old 01-03-2011, 06:45 PM
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Bassnut (Fred)
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Stuart, I was using external guide at the time, so guideing wasnt affected at all natch. I got just one very blurred sub and all subsequent subs were shifted quite a bit. Sorry, I didnt save the sub. It didnt bother me much, it just reminded me to do up the mirror lock.

Of course OAG wouldnt be fussed by this, assuming the flop wasnt big enough to lose the guidestar.
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