The Periodic error in the Eq1 mount (>100 arc sec peak to peak in this example) and the difficulty in trimming the speed of the basic RA drive supplied, prevents exposures longer than about 10sec being achieved. I therefore decided to investigate if the Vixen Skysensor 2000 system from the main scope could be fitted to the mount. Since the mount would be polar aligned only the RA drive would be required.

 I was surprised how simple it was to fit the ss2k motor to the mount. A simple bracket was fabricated out of flat steel which bolted onto the mount using the existing bolts which hold the RA worm gear assembly in place. The motor bolts to this bracket using the existing threaded mounting hole on the motor. The hole in the bracket is slotted to allow adjustment

The gear from the GPDX mount was exactly the right diameter to fit on the Eq1 RA shaft and the motor was adjusted to set the clearance between the intermediate gears. A new set up was configured on the ss2k controller with the worm to gear ratio changed from the 144 for the GPDX to 100 for the Eq1.

The mount now has a precise RA drive with Backlash Compensation, Periodic Error Correction and RA axis GOTO capability, with the Dec axis being set manually based on the Dec reading given on the ss2k handset.

In practise, the mount is polar aligned as normal and the ss2k run in polar aligned mode.

The ss2k is instructed to GOTO a chosen alignment star. The star is centered in the eyepiece (using the ss2k controls for RA and the manual DEC controls on the mount and an alignment is done.

The RA setting circle can be adjusted to match the reading on the ss2k. The DEC setting circle should match the result on the ss2k

It is now possible to use the GOTO capability of the ss2k. The mount will drive to the correct RA but the DEC will need to be set manually based on the readout on the ss2k.

(NOTE:- Because the the ss2k does not receive any DEC feedback, it will not beep when the object is reached and I have found that for some reason, the manual RA buttons will no longer operate at full slew speed after a GOTO. To get round this, press the align button to get a beep after each object is reached. The ss2k will then return to normal operation. NB you will only ever get a one point alignment because of the lack of DEC feedback)

The periodic error correction can be trained in the standard way. (Only movements in RA are corrected for, any DEC drift is ignored. ie the guide star is kept on a vertical line (in RA) rather than a single spot) The worm takes approx14.5 min for one rotation.

Unfortunately, the ss2k controller only stores one set of PE corrections so it has to be retrained each time the controller is swapped between mounts. (Please let me know if you come up with a way round this)

The graph below shows the Periodic Error as measured using a webcam and K3CCDtools software.

Although the speed of the ss2k motor was much more accurate than the basic Eq1 drive, the periodic error was no better at approximately 100arc sec peak to peak. (This suggests the main errors are in the worm/ring gear on the mount rather than in the motor/gearbox.) With PEC training however (in this case done manually, observing the image on the laptop screen), the PE was reduced to 15 arc sec pp, and < 10arc sec pp for most of the time. With good polar alignment, it should be possible to take >1min unguided exposures with the wide field refractor; remarkable for such a simple mount!

26th May 2003

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