If the Sidereal rate is 15 arcsec/second then a 60 second image will have a movement of 15 x 60 = 900 arcseconds. If the error is 0.001 then the drift will be 900 x 0.001 = 0.9 arc seconds which is about 2 pixels on my plate scale. A 5 minute image will then be 0.9 x 5 = 4.5 arc seconds .. or about 10 pixels .. this is what I am seeing. But what if the drift is lower than this .. say 0.0001 then the tape measurement must be accurate to 0.05mm. I think when a mount of this calibre has encoders (at over $6000 cost) and can easily determine pulses per period of time .. why don't we have this? This is not confidential information, just a simple metric to make sure everything is calibrated properly. I think using a piece of tape is not the method a mount of this calibre deserves.
