Pinwheel Galaxy with GPCAM, Anyone know what this streak is?

[cuivenion]

4 hours ago, ollypenrice said:

People using very well polar aligned mounts still autoguide in Dec. Sure, theory says that the Dec axis doesn't do anything under perfect PA but ths is not borne out by experience! A bit of backlash is all it takes. Or wind, or vibration, yes.

You just need to work out the degree of precision needed to keep your tracking error to about half a pixel (where it needs to be) to see why we use autoguiders. This is not a 'norma'l degree of engineering precision. We want the impossible!

Olly

Ah, gotcha.

[ecuador]

Don't forget also that targets don't do perfect circles around the Celestial Pole - they will deviate due to atmospheric refraction, and the deviation will be more the lower the altitude of the target you are tracking. This deviation translates to changes in actual RA "speed", and also changes in declination. For the slowing or fastening of the RA due to refraction, some mounts (e.g. the current iOptron line-up), offer the "King" tracking rate, so that you can achieve even longer unguided exposures. But you'd ideally need some correction in dec as well which I don't think even those mounts provide, so you'd still have a little movement in dec even if everything is perfect regarding alignments/conditions/mechanics/electronics etc