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Different trails lenghts for same time sub?


Astronutjob
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Hello i was out imaging the other night, M45 unguided. My polar alignment was fairly good, I set the camera remote timer to take 1m 30s subs, but looking at the images when i got in on the computer screen there seems to be trailing in some and none in others ( i hadnt moved anything)? I would have expected them all to be trailed or none?

Any suggestions on what might be causing it? Would slight mis ballance of the mount cause it? or loose gears? Its an HEQ5 for info? Just wandered if anyone else has had this problem and remedies:icon_scratch:?

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i would be concerned on the following.

polar alignment.drift align for even better results

poor balance. balance your equipment better.

flexure somewhere at your system, maybe at the focuser or mounting rings of the scope

bad tracking of the mount. this is the last of your concerns and can be solved by servicing your mount.

also these tracking errors are common if you don't guide with a guide camera so if you don't have a guidescope i would suggest to buy one along with a guide camera to increase the sub frames exposure time.

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Thanks kookoo, I find it strange that i took a 2 min sub on another object, there was no trailing then took another 2 min sub of the same object and it showed trailing? Im using a heq5 with a megrez 72 and 450D so there isnt much weight on the mount. It doesnt seem to be consistent? Im hoping maybe my ballance wasnt set up correctly?

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My CG5 is much less able than your HEQ5, but on my mount It would be:

Most likely - wind gust - doesn't take much ...

Next - vibration - moving anywhere near the mount, especially on earth/grass

Next - After a GoTo the mount tracks nicely, but

After a GoTo if I touch the hand control to centre the target, the mount will trail for the next 4 or 5 frames - around 2 minutes before moving accurately again.

I had an HEQ5 and mine tracked very well up to 2 minutes at 1,000mm focal length.

You should set up your PEC (Periodic Error Correction) if you haven't already.

I don't have PEC on my CG5 so have to rely on short exposures ...

Hope that helps

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All the above suggestions will contribute but the phenomenon you have discovered is entirely routine and affects all mounts to some degree. The worm takes what, eight minutes, to go round once. During that time it speeds up and slows down in both systematic and random ways, mostly systematic due to the varying accuracy of the worm wheel and its mesh. Some of your short exposures will be taken during the best part of the cycle and some during the worst. Simple as that. It is very well known. When you run an autoguider you want a mount with a smooth, jump-free set of errors that the guider can correct easily. The amplitude of the error matters less than its smoothness. Large smooth errors guide out easliy. Small sudden ones don't.

If you have Periodic Error Correction you can train the mount, so to speak, by recording a series of hand made corrections during one cycle of the worm. These corrections will then be played back on each successive cycle, reducing the systematic errors of the system. Use as long a focal length as you can for the training and use a star close to the celestial equator. (Polaris describes a very small circle over a day, Beltelguese a very large one.)

Backlash occurs when the mount oscillates between being 'pushed' and 'pulled' by the drive pinion. Play in the gears allows the mount to fall onto mesh then away from it, etc. (Like a car and trailer, sometimes the car pulls the trailer, sometimes the trailer pushes the car). The usual solution is to run the mount a little heavy on the east side so the pinion is always pushing the RA worm. I find it can also help to run camera end slightly heavy as well, for the same reason.

But I have to say, an autoguider is great...

Olly

Edited by ollypenrice
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I concur with all that Olly has said. I have an HEQ5 which is motorised but not guided and I always get exactly the problem that you have described. I often run either 1min or 2min subs and with either I reckon on getting roughly a 40% failure rate (ie 40% have slight trailing). My solution, at the moment, is to take at least twice as many subs as I think I need and ruthlessly delete the poor ones - In the mean time I'm saving my pennies for a guiding system!

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There is a fix for star trails in Ps. Not nice but these things never are...

One use for your trailed subs, though, would be to stack them and layer mask in only the faint data on generally low detail parts of the image where noise is a problem. Waste not want not!

Olly

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I've had this inconsistent trailing problem on my eq3 mount. I eventualyy tracked it down to being a combination of factors:

1. Mount stiction - meaning the polar axis rotates in small jerks periodically instead of smoothly.

2. Wind gusts blowing the scope, although this usually just gives bloated stars not defined trails.

3. Balance of scope causing more stiction.

4. General tracking inaccuracies of mount.

I was able to overcome these issues by starting to do guiding. I also stripped and greased my mount but i don't know how much that helped compared to guiding. I suspect guiding was the main factor.

If you are using a longer focal length scope, say over 1000mm you'll also find it difficult to avoid trails since the slightest inaccuracy causes scope motion relative to the stars and due to the greater magnification the effect is more obvious.

As a low cost way to start guiding i used a 50mm finder with a webcam coupled with PHD on my laptop. Works really well.

The other thing to consider is whether the trails are due to poor polar alignment. Since they are inconsistent and your subs short this is probably not the cause, but if the trails are in the DEC axis only, you've got a polar alignment issue too. This is easier to spot if you are actuslly guiding with something like PHD and can see where the EA and DEC axes are oriented.

David

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Basically you just make it "out of balance"!!

By that we mean if you have the dec axis horizontal (so the scope is on one side of the mount and the counterbalance weight on the other) and the RA axis clutch undone, the scope (or counterbalance weight) that is on the east side of the mount shoud be set to be "heavy" and should JUST slowly "drop" under gravity. Hope that makes sense :(

Edited by Bizibilder
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Heavy in the east so light in the west!

Polar mis alignment alignment will show in longer subs. Not all stacking software can rotate (they only move on x and y to align) so long runs of subs can also show polar misalignment rotation.

DSS, which is free, can rotate and align.

Olly

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