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Field Derotator

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Posted by Glenn Mitchell on October 8, 2002 17:34:27 UTC

Only some of those comments are accurate.

(1) The LX200GPS supports PEC training in both RA and Dec. This is one advantage of LX200GPS over the NexStar. Right now, PEC has some problems that are being addressed in coming firmware upgrades.

(2) Tracking is a problem with a field derotator because it is dependent upon the accuracy of the alignment you do. The Field derotator communicates with the scope to determine the alt-az coordinates where the scope is currently pointing. Some complex calculations are then done to rotate the camera opposite the amount of field rotation for those coordinates. Since field rotation is not constant across the sky, if you do a mediocre job of alignment, the tracking will suffer.

(3) You will still need to guide for long time exposures. The derotator will rotate the camera to comensate for field rotation. It will not correct for tracking errors.

(4) The field derotator is a big unit. It has a large motor that's used to rotate your camera. As a consequence, you loose 20 or more degrees each side of zenith because the camera will contact the fork base. The 10" will have less problems, since it uses the same fork as the 12" scope. The 12" and 8" are especially affected.

(5) Back focus is a consider, especially if you add an off-axis guider, filters, etc.

All this said, that's why you do not see serious astrophotographers using a field derotator.

Real astrophotographers do it on a polar mount. 80)

Long exposures are hard enough with a polar mount, and you only have one axis to worry about (if you get you scope accurately polar aligned).

Mitch

Glenn E. Mitchell II, Ph.D. (aka Mitch)
10" f/10 Meade LX-200 GPS (Big Blue) s/n 01088
Supercharged at ASO by Dr. Clay Sherrod, # 2314-2010
N 30 30' 49.2" / W 084 20' 13.8"

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