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Old 01-07-2009, 03:34 PM
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Jeffkop (Jeff)
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Flexure

I see many people talking about flexure ... whether it is because they suspect they have it .. or they are discussing ways to minimize it or avoid it. I have a general perception. I wonder if any of the community would care to reply in this thread reasonably verbosely ... educate me and anyone interested.

Thank you
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Old 02-07-2009, 06:23 PM
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I think you are saying you wish to know what it is ?.
If so,.. then ...

Its basically the bending of either the support holding the guider scope mounted on the main telescope, or the focuser bending slightly due to the fact the camera is weighing it down (Droop). But this has more effect when the focuser is at low angles (Like near the horizon), as this is when you get the most flexing of the focuser. The flexing of the support can be at many angles, due to the way it is supported.

Theo
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Old 02-07-2009, 06:38 PM
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And...... a typical imaging SCT is heavy at the back end and a typical refractor guide scope is heavy at the front end, so they droop differently, hence differential flexure (plus what Gama said).

This means that the guide star is kept in place in the guide scope with autoguiding, but the imaging OTA is outside the control loop so any movement between the guide scope and imaging OTA (flexure) shows in the image from the imaging OTA. Mounting stiffness between the guide and imaging OTA then becomes critical, this can be done in many ways, a never ending drama.
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Old 03-07-2009, 10:36 AM
Hagar (Doug)
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Flexure is a problem I really have trouble coming to terms with. Flexure between guide scope and main imaging scope should by all rights be almost nil due to the fact that flexure requires inertia. With a guided telescope moving a grand total of 15 degrees an hour there would be almost no inertia in the direction of travel. Things like cabling and obstructions could add to flexure problems. Flexure in a focuser could well leave a camera sitting with a slight tilt or sag but this would be inherant in all images and would only really become variable as the angle of the focuser/camera change.
To my way of thinking flexure would or could be evident during a slew but when the scope is guiding at sireal it should be almost nil.

Am I wrong in these assumptions. There has been a lot written about flexure but may it be actually other outside stimulus such as missaligned worms, gear imperfections, cable drag or just minor vibrations not related to tube or guidescope flexure?
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Old 04-07-2009, 01:23 AM
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One thing to take into consideration Doug. Metals expand and contract at different rates, and the amount of force applied at the time has a direct effect on the amount of travel/flexure. Also the thicker the alloy, the different rates again.

Theo
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Old 04-07-2009, 02:52 AM
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This Guy found poor support of his imaging mirror was causing his flexture problems.

Exactly how does it appear in an image? eggy stars?
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Old 04-07-2009, 05:54 AM
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With large truss dobs, we talk about flexure in terms of the truss poles warping slightly under the effect of gravity acting on the UTA. Like other examples given here, it is likely to be more pronounced when the scope is pointing lower.

If used, the TPAS function on the Argo Navis compensates for flexure.

To minimise flexure and create greater stability, the truss poles in my SDM scope were changed to slightly thicker ones several years ago. I haven't looked back.
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Old 04-07-2009, 09:47 AM
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Quote:
Originally Posted by Tandum View Post
This Guy found poor support of his imaging mirror was causing his flexture problems.

Exactly how does it appear in an image? eggy stars?
Good link, and mirror flop/flexure is outside the guide loop too.

Yes, usually the RA direction shifts the most, so along with PE, you get eggy stars. If DEC also flexes, then you get fat eggy stars.

This is all mainly shows at or over 2.5 m FL and say 5 or 10 min exposures and longer (thats when you use internal guide/OAGers etc, so the imaging train is in the control loop, all flexure is compensated for).

Shorter FLs (wide field) and/or shorter exposures are proportionally less critical.
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