Astrophotography by Tomi Virtanen - Equipment

Mount

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Updates:

7.2.2005 - created this page

GSO GEM

Periodic Error & Correction

Modifications

GSO GEM

An equatorial mount with RA drive motor, made by GSO.

This is not manufactured anywhere near Germany. The "German" in the name comes from the axle arrangement; Righ Ascension (RA) axis is tilted to point to the 'pole of the sky' and the declination (DEC) axis is on top of that.

A very important aspect in astrophotography is adjusting the RA axis to point exactly into right direction. This mount has a polar alignment scope inside the RA axis (covered by cap in this picture) to make this task easier

This mount, with it's aluminium legged tripod, is considered to be suitable only for small and relatively light scopes, like my 150mm short Newton.

Closeup of the RA drive motor.

Periodic Error & Correction

Graph of the mount's periodic error while tracking an object at 38 degrees DEC.

Each dot represents an average of the apparent position of the tracked object over last 15 seconds. It includes also any drift that can be caused by error in polar alignment and/or inaccuracy of the RA-motor driver clock. In this case this drift is approximately 2 arcseconds / 10 minutes. This graph clearly indicates that this mount needs Periodic Error Correction (PEC). As I currently don't have PEC, I have to reject about 50% of frames when doing 15s exposures because they are blurred.

I'm in progress of adding a PEC circuit for this mount. It will include an optical worm shaft position sensor and a microcontroller that can counter the error with proper data. The goal is to achieve exposures of 1 minute at focal lenght of 750mm without too much rejected frames.

Modifications

Modified RA setting circle.

The original setting circles of the mount were rough and inaccurate. I printed new ones on paper with a laser printer and covered them with plastic film

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Folding center support for tripod.

Originally this tripod had a sheet-metal accessory tray that had to be screwed on/off with 3 screws every time it was folded/unfolded. I replaced it with this simple hinged wooden structure that folds nicely with the tripod legs.

email: tomi.virtanen@yahoo.de