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The Meade Field Derotator is my only solution to balance field rotation : I image at 1 degree latitude north where a wedge for such a heavy telescope cannot be used safely.

After fixing the problems described here, now the the unit performs well enough.

Downside #1 is that I cannot use a f / 6.3 focal reducer in the imaging branch (focuser - derotator - AO7 - filter drawer - ST8) : the branch is too long and severe vignetting occurs.

Downside #2 is that the weight balance is severely affected as the branch rotates because the CCD camera is offset to one side

 

Accessory:

The LX200GPS telescope has motors on both axes to track the stars as the earth rotates. When mounted in altitude/azimuth mode, the field derotator rotates the camera bloc to compensate for earth rotation when the telescope tracks stars during the night.

I am at the equator, so if the scope faces due east or due west, there is no field rotation, the stars just go up to the zenith, or down to the horizon.

If the scope faces due north or due south, then the scope will not move, but the field of stars will rotate around either pole.

The rate of rotation in degrees per hour is :

r = ( 360/24 ) x cos( latitude of observer ) x cos (object's azimuth ) / cos (object's altitude )

Note my imaging branch as scope - focuser - derotator - AO7 - filter drawer (with a cover to prevent stray light) - ST8

Equipment description:

The stepping motor is plugged to a gear box with a ratio of 1 / 150.

It powers, via two wheels and a belt, the worm shaft.

This shaft rotates an aluminium gear wheel. The wheel has ball bearings at contact area with the central axis, a plastic ring keeps the balls in place and the camera support plastic barrel is screwed directly on the wheel.

All parts were cleaned and grease applied where necessary.

The wheel has 180 teeth, so one rotation of the worm pushes one tooth, rotates the unit by 2 degrees.

The screw at right pushes the worm gear towards the wheel, has a small built in spring, and can be adjusted to have a smooth contact between the worm and the wheel.

When I received the unit, it was not working well, it emitted a lot of beeps, lost contact several times and had a very erratic attitude !

SOFTWARE AND HARDWARE PROBLEMS AND SOLUTIONS ARE DESCRIBED BELOW

SOFTWARE

Autostar II version 1.3t does not work for the derotator for the LX200GPS instruments:

The unit will keep loosing track of the scope pointing coordinates. The unit will emit beeps every few minutes and loose pointing information.

Autostar II version 1.6b fixed this but the unit still does not rotate correctly for an observer at the equator ( cos(latitude) = 1 )

Autostar II version 1.7i has fixed it and now the derotator works like a Champ !

For those interested, this is what the command board looks like:

Components are :

U3 : ADM232AARN, 9713, 0F69138.1
Q1 : 712NDS9959
Q2 : same as Q1
D2, D3, D4 , D5 are all the same : A6M

HARDWARE

PROBLEMS

1: The head of the screw touches the control board. The only way to assemble the unit's body is to tighten this screw all the way in, jamming the worm against the wheel. The derotator does not work smoothly then.

2: The mounting screw is too close to the small wheel. The little screws used to fix the wheel on its axis touch the mounting screw, giving unwanted resistance.

Another flaw here is that when the unit is mounted in the main frame,this wheel is in contact with the lateral frame.

3. Below this "3",you will notice the bottom end of the screws mounting the motor frame to the main frame. these holes were drilled right below the worm frame support . Not the best place.

4: The control card is jammed against the screw "1", vibrations from the mechanical parts are transmitted directly to the card.

5: The worm axis and motor axis are not parallel .

MY SOLUTION

I have shifted (4) slightly, adjusted the screw (1) so that it is not overtight and the screw does not touch the board.

I have cut the mounting support (right of 2) from the cover frame and do not use it.

I use the original cover frame but I DO NOT screw it, I have loosened it and use mounting tape to close the unit. It works.

 

LAST PROBLEM SOLVED - 2006 - :

The last problem I had was a play that exists between the two axes : telescope and camera axes :

The weight of the imaging branch strains the ball bearing element and the axes do not remain aligned. It results in a shifting of a couple of pixels between images as the unit rotates.

As I image from a light polluted area, the use of good flats is a must, and I need to shift the Flat pixels to fit the actual images, very fastiduous.

After trying several solutions, I thought of changing the bearing balls to a bigger diameter. I did it and it works ! I have to be careful when assembling the unit back to not overtighten the screws and this last problem is now fixed ...