K3's AstroPhotography
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Tests and Analyses of Homemade GEM1 Guiding System

Finally I had chance to test my homemade GEM1 guiding system. Although seeing conditions were not very good (soft wind), I could not resist an opportunity. I put my Philips Vesta 675SC2 webcam at prime focus of my 8" F6 Newtonian. The resolution of such system is 0.96arcsec/pixel. I selected Altair as guiding star:

RA 19h 50m 47s
DEC +8 52' 06"
Alt ~ 40
Az ~ 229

Here are results of my tests. Click the pictures to see full sized pictures.
The X coordinate shows time in seconds, Y shows error in arcseconds.


At first, guiding was switched off. You can see 2 peeks of periodic error at start (30s; 630s). Also permanent RA drift (caused by motor speed error and not exact polar alignment) is visible.
At 720s I switched guiding on. At about 1620s I manually changed scope's RA, DEC to test ability of system to recover from error.

Detailed graph of guiding error during 300s. The amplitude of error is about +/-5 arcsec.
Standard deviation is only 1.6 arcsec. The regulation can be further tweaked by adjusting guiding parameters in K3CCDTools.

Scope's RA, DEC was changed to test ability of system to recover from error. At time 0 the guiding was switched on. The response in RA was less than 10s. The response in DEC was 40s for rough correction and after 150s the DEC error was in required tolerance. The DEC drive slow response is caused by slack in drive system (motor, worm wheel gears) when direction of motor rotation is changed. In RA axis only RA- and STOP commands are used for guiding, so slack doesn't affect performance.
Fortunatelly, such high transition doesn't occur in real guiding. It was only measured, if the system is stable and is able to return to normal operation.

The second thing, which I decided to measure, was effect of guiding on result photograph. I captured 2 AVI files of Altair with 40 and 120 short exposures (with period of 1s) and summed the frames to simulate long exposure. It represents long exposures of 40s and 120s. The PSF (Point Spread Function) was measured for both raw and summed frames. Here you can see the results:

The following table shows differences between raw frames, summed frames (simulation of long exposure) and summed aligned frames (simulation of long exposure with perfect guiding).

40s AVI
Raw frame Summed 40 frames Summed 40 aligned frames

400% Zoom

400% Zoom

400% Zoom
120s AVI
Raw frame Summed 120 frames Summed 120 aligned frames

400% Zoom

400% Zoom

400% Zoom

Conclusion: Although the guiding system is not perfect, it offers long exposure star image with FWHM only by 1.3 pixel worse than aligned summed image.

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Last Update: 22.09.2002