Plate solving and field rotation

[MLM]

I'm trying to build a sequence for an Alt/Az mount (TTS-160) which has both an optical tube rotator and a Falcon camera rotator. My goal is to capture multiple groups of say 30 x 120 sec exposures. The optical tube rotator, which compensates for field rotation during exposures, has a range of motion limit of 26° before it stops. After that it must be rewound.

In SharpCap a plate solve and sync command after each block will recenter the target if there was any drift, AND will automatically rewind the optical tube rotator back its starting position.

However, my next block of images will be now captured with a different rotation angle of the camera than the first block. This can be corrected by rotating the camera in the opposite direction than the rewind of the optical tube rotation. SharpCap does have a move rotator command in the sequencer to do this. The problem is to determine how much to rotate the camera, because the rate of field rotation varies with target location, and the extent of rotation depends on the cumulative duration of exposures during the prior block.

When plate solving, would there be a way to determine the rotation angle of the first image in the starting block, and automatically rotate the camera after a plate solve so that the 1st image in the next block overlays the images in the prior block?

[admin]

Hi,

in theory the data is there - if you do the plate solve then look at the results (either in the notification bar in SharpCap or in the log), you should see a rotation measure in each plate solving result - 'Up is XXX degrees E of N'. Now, if you plate solve both before and after the block of exposures, I think that the rotation you need will be the difference between the XXX as measured before and the XXX as measured after. However, I'm not sure if the direction to move the rotator would be well defined - ie if the difference is 12 degrees then should the rotator be moved +12 or -12?

Anyway, if you have time to test the theory above about the plate solving direction change matching the rotation required, please let me know the results. It may be possible to build some sort of sequencer based solution to this that can plate solve every N frames and derotate as well as reposition.

cheers,

Robin

[MLM]

I always wondered what the "Up is XXX degrees E of N" indicated. I'll experiment and get back to you.
I've been told that NINA handles this automatically, but since I only use SharpCap Pro, I don't know the details.
I'll try to find that out as well.