When using the telescope simulator to rotate the frame for astrophotography composition, how is the indicated angle applied to the telescope? My scope has a rotation scale, but I don’t understand how it is set up or aligned to begin with. Is zero degrees on the telescope scale supposed to be aligned with the North celestial pole?
You’re correct, zero degrees means the North Celestial Pole is “up” when seeing your frame in landscape mode (we’ll assume 180 degrees is the same as 0 degrees, because the rectangular shape is just the same).
From there, the Position Angle “turns positive into the direction of the right ascension”, as defined by the IAU. This means the PA grows East of North - with the caveat that East is left of North in the sky (because we’re looking at the celestial sphere from the inside) as opposed to the Earth surface, where East is right of North because we are seeing the sphere from the outside.
To make this more confusing, some astronomy software got this backwards and now it’s very difficult for them to fix, because of the number of integrations they already have in place.
Here’s a topic where I explain the Position Angle in great detail: Position Angle / Rotation Angle - Is it backwards?
I hope this helps!
Actually yes..,that did all make sense. Thanks for the info. Now I think I can grok my way having control over the rotation, and thereby composition of my frames.
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I’ll add (because it’s not very intuitive) that in the usual home position (telescope pointing north and counterweight bar downwards), the telescope points towards the celestial north pole, and therefore there isn’t really a “up.”
In this situation, an angle of 0° corresponds to a sensor placed vertically (relative to the ground). As in this example:
