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What could have happened here?

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#1 BrendanC

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Posted 11 July 2020 - 11:31 AM

Take a look at this...

 

L_8881_ISO800_30s__15C.JPG.bb45229b982f657875f8d14be5cd8feb.jpg

 

I have trails that go horizontal to the right of the image...

 

left.jpg

 

... but then seem to curve up towards the vertical on the left!

 

up.jpg

 

The brighter stars also seem to have a 'judder' to them.  It's the same for every one of the 155 subs.

 

How can this happen?

 

This is from a 130PDS on an NEQ6 mount, camera is a modded EOS1000D with Skywatcher coma corrector. I'm using DARV for polar alignment in APT. The subs were 30 seconds, just tracking, no guiding. Control was via APT, using APT's own dithering module, via EQMOD directly connecting the laptop to the mount. There is no stretching or processing on this image, other than converting from raw to JPG to upload it. It was a really clear, calm night, no wind. The mount is on the grass, not concrete, and I made sure it was level.

 

I've been able to get 30 sec subs easily before with pin-sharp stars. I'm stumped as to how this could have happened. Tripod not level? Camera not secured? Collimation? PEC? Is it optical, or mechanical? Little goblins coming out at night and shaking the tripod?

 

Any/all suggestions welcome.

 

Thanks, Brendan



#2 the Elf

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Posted 11 July 2020 - 11:52 AM

It is clearly mechanical because you have a bright and a dim copy of every star. Wrong balance could cause this. Make sure the mount is balanced well or slightly east heavy. Check all things that might move: scope rings, dovetail, main mirror, secondary mirror, focuser, camera, counterweights. Possibly a cable that pulled the mount. Route all cables along the RA axis, not straight away from the scope to the computer. Don't walk near the rig when imaging. Start it and stay away. As you do not guide, make a test run without any cables connected to the camera, using the self timer mode that can take up to 10 images. Good luck!



#3 BrendanC

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Posted 11 July 2020 - 01:00 PM

Thanks for this, I've just been out doing exactly as you said.

 

How do you think it's possible though for the trails to be near-horizontal at one part of the image, and near-vertical at the other? That's the bit I really don't get.



#4 piaras

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Posted 11 July 2020 - 01:04 PM

Backlash in the drive. Likely too well balanced! Try with being East heavy so that the drive is loaded against the direction of motion.

Pierre



#5 BrendanC

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Posted 11 July 2020 - 02:01 PM

Good suggestion, thanks. I'd heard of this before, never actually done it though. Now I've just researched to find out how to do it, I'm going to try that tonight.

 

Quick supplementary question though: would this account for the strange different orientations of the trails? I still don't get how that could happen.



#6 piaras

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Posted 11 July 2020 - 02:57 PM

There will be backlash on both axis and it will be random how much and which direction the scope moves. 



#7 BrendanC

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Posted 11 July 2020 - 03:37 PM

OK, I see, so within that 30 second period there could be variable backlash for both the RA and Dec? Makes sense, thanks. :)



#8 BrendanC

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Posted 11 July 2020 - 06:06 PM

Hey, just thought I'd report back and let you know it worked! Thank you so much Piaras, you were bang on the money and saved me a LOT of hassle. Pin sharp stars again! :) :) :)

 

Thanks

Brendan




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