field rotation angle after plate solving

[alpheratz06]

This post just to have a confirmation of my understanding :
When PS comlpletes, there is an indication like ( e.g.) solving..... 43° E of North

My understanding is that in this case the vertical axis (which is the smaller axis on a rectangular sensor) points with a relative angle of 43° with respect to the horizon.
Am I right ?

My question is fuelled by my desire to build a straightforward, deterministic process to go from framing in e.g. Stellarium for which rotation angle is defined with respect to a reference position in which , in parking mode (CWD) , the sensor has his major axis vertically oriented.

I suppose thus that to shift to Stellarium angle to Sharcap angle one needs to do some spherical trigonometric computation.

[admin]

Hi,

the angle isn't relative to the horizon - it's relative to a line from the center of the image to the North Celestial Pole (ie a line going in the +Declination direction). It's also worth remembering that the angle is positive counterclockwise, which means (given that we view an image pointing up from below rather than down from above) East is at +90 and West at +270/-90.

If you are going to be relying on this, make sure you have an up-to-date version of SharpCap - I did quite a bit of work about 6-9 months back to ensure that the angles are worked out correctly. Interestingly, while I was doing that work, I found out that other plate solving tools don't always agree on this angle - some of them clearly calculate it wrongly in some circumstances.

cheers,

Robin

[alpheratz06]

Hi Robin !
Thanks for your information, which means that the angle in eg stellarium ( if one chose equatorial mount ) and sharpcap are read with the same reference. Sign convention isn't a big issue then because a simple test can tell the which sense to select.
Regards