Countering field rotation

[frugal]

I have been following the various discussions about how to use Registax to process images of the moon, and I have been having a go. Last night I took 2 batches of 200 frames of the moon with my DSLR and a 300mm zoom lens. The one batch using Backyard EOS to hold the mirror up for 2 seconds before taking each shot to reduce vibration, and one set when I just turned on the camera motor drive and held the shutter button down until it had taken 200 frames.

Stacking the rapid shots using the motor drive was fine, however I can not stack the Backyard EOS ones because it took an hour to shoot them and the moon rotated during that time.

Before stacking I pre-processed the frames in PIPP to centre and crop the files. However even if I use the "rotation" check box in Registax it fails to align them. Is there an accepted way to remove field rotation of planetary objects before trying to stack them? Can it be done in Registax/PIPP, or do I need another piece of software. I really don't fancy manually aligning and rotating 200 frames in Photoshop by hand

[IanL]

At the risk of stating the obvious, until you find a way to sort out the rotation, you should be able to salvage something by stacking a smaller set that are closer together in time.

[brantuk]

The preferred method of planetary imaging is with a webcam. If you can get 20 frames per second, your 200 shots would only take ten seconds and planet rotation wouldn't be a problem.

Field rotation is when you get elongated stars on a long exposure of a deep sky object - best solved with accurate polar alignment and guiding. Sorry I can't help with the software - though I do know you can get programs that remove field rotation. Someone else might suggest a good one to use.

[cgarry]

PIPP does not have any function to remove field rotation.  I thought about adding something to do this in the past but never did!

With regards the 'Estimate Rotation' in RegiStax I have not played with that myself.  But I believe you also need to go into the stacking options and select 'De-rotate images'.  See here http://www.astronomie.be/registax/previewv6-2.html

I am assuming that the rotation that RegiStax is attempting to correct is field rotation and not planetary rotation.

Another random thought - Do the shadows on the moon change much over an hour?  

Cheers,

Chris

[frugal]

Planetary movement: good point, I had not considered that over the hour the terminator would have moved across the face of the moon. I will have to check the first and last frames tonight to see if I can spot a difference.

It was field rotation I was trying to correct, not lunar planetary rotation. Over the hour the moon would have effectively rotated 15 degrees due to the rotation of the earth. If I had the camera on a tracking EQ mount, then it would not be a problem, but unfortunately I do not, it is on a simple camera tripod.

[glowingturnip]

Another random thought - Do the shadows on the moon change much over an hour?  

should be unnoticable surely ?  moon is captured rotation so only rotates on its own axis every 28 days.  Over the course of an hour it'd be in the same relative position to us and the sun, and only have rotated by a tiny amount - only thing that moved noticeably is us spinning on our own axis.

[cgarry]

should be unnoticable surely ?  moon is captured rotation so only rotates on its own axis every 28 days.  Over the course of an hour it'd be in the same relative position to us and the sun, and only have rotated by a tiny amount - only thing that moved noticeably is us spinning on our own axis.

I am not sure how unnoticeable it would be, especially along the terminator.  For example, the lunar X feature is on visible for a few hours each 28 days.

Cheers,

Chris

[frugal]

Here is the first and last frame from the long stack (about an hour apart).

The focus is not really good enough to make much of an estimation as to how much the terminator has moved.

[stelios_t]

At the risk of asking something silly, why did you not use BYE in planetary mode? In that mode it takes an AVI which you can then stack.

[frugal]

At the risk of asking something silly, why did you not use BYE in planetary mode? In that mode it takes an AVI which you can then stack.

Purely because BYE uses the live view mode to do the planetary capture, which is about 1050x700 for my camera. However with a 300mm lens the moon only takes up 1/6 of the frame width. So in planetary mode the moon comes out at less than 200 pixels wide ;(

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[glowingturnip]

should be unnoticable surely ?  moon is captured rotation so only rotates on its own axis every 28 days.  Over the course of an hour it'd be in the same relative position to us and the sun, and only have rotated by a tiny amount - only thing that moved noticeably is us spinning on our own axis.

actually, I've worked it out - with a 27.3 day rotation and a circumference of 10,921 km, the terminator on the moon moves at just over 10 mph at the equator, less towards the poles).  For comparison, Aristarchus is 25 miles across.  Mind you, those elongated shadows of mountains near the terminator will appear to move faster than that.

 On earth, on the equator at equinox, our terminator moves at 1,035 mph !