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Telescope Doctor

KTC's picture
KTC
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Joined: 2010-12-08

How is your scope working?  Got any problems with it?  Maybe we can find a simple solution.

This is the June, 2012 call for 'all sick telescopes'.

This month's special topic:  Stiction in the telescope drive.

What is stiction? http://en.wikipedia.org/wiki/Stiction (difference between static and dynamic friction)

For those of us that do imaging, frequently the worst problem of stiction is in the declination drive of an equatorial mount.

Symptom:  when you reverse the declination drive direction, the scope does not immediately reverse...it actually continues in the original direction for some time, and only later does it actually reverse direction.  This causes an overshoot and delayed correction.  If you use an autoguider, you often have images with very bad trailing in the north/south direction.  (If you watch the autoguider while it struggles with this problem, it's often a slow-motion oscillation that may take 1 or 2 minutes for a cycle to complete.)

Fixes:  Software can help work around the problem, such as Maxim (guider settings to recognize and deal with stiction), but the underlying physical problem remains.  The best solution is to make mechanical changes or adjustments...but sometimes that can be an involved process...but sometimes a simple adjustment or two can make considerable improvement.

But static friction is always greater than dynamic friction, isn't it?  Yes, that's true.  But you can make your telescope less vulnerable to this fact.

Simple adjusments/fixes:

- make sure the scope is well balanced in both axes, and in all orientations.

 

Slightly more difficult fix:

- make sure your declination worm is pressing against the worm wheel with only a small and sufficient amount of force. The more the worm presses against the worm wheel...the greater the affect of stiction in terms of guiding performance.

 

Difficult fix:

- stiffen the worm wheel and its support.  (Often the problem here is a long load path from worm wheel to its support...because of gaps between parts, or loose parts.  Disassmbly, inspection, reaseembly with shims...often will make a difference.)

Each scope is unique, and I can't cover all scenarios here.  Feel free to post your questions about your sick telescope in this thread, or contact me privately.

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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484