Sunday, June 29, 2014

Motorizing a Vixen Polaris Mount, Part II

I finally got around to re-making the motor bracket, and made a tripod attachment to the Vixen ring mounting points (there is no saddle).  The Polaris carries the Stellarvue 80ED and DSLR quite well.  I also used an intervalometer from e-bay to allow repeated bulb exposures.  Power is (literally) provided through the USB port on the Arduino; because the stepper motor is only 4V, providing 12V through the barrel jack on the Arduino causes a lot of noise and motor heating.

I took some 10-minute long exposures with the DSLR (an exposure this long is normally not practical, as thermal noise in the DSLR becomes unmanageable).  The reason for these long exposures was to measure the periodic error.

Unfortunately, the total PE is a massive 52.5" peak-to-peak over the 10-minute worm period.  This means that the maximum error is 0.175" / second or about 0.012X the sidereal rate.  With my (old) Canon 40D and the SV80ED, the plate scale is 2.1" / pixel. Assuming star FWHM of 4" and a maximum eccentricity of 0.60, this means stars would be 4" x 7" at worst - a drift of 3".  This corresponds to a maximum exposure time of only 17 seconds, which is pretty bad.

This means that PEC is not optional, but mandatory.

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