Help - Multiple images problem

[JimS]

Hi all,

Starting to try my hand at deep sky imaging over the last couple of months and I seem to be running into a consistent problem.

I appear to be getting multiple images within a single exposure. I doesnt look like a poor polar alignment as the error is random, it's almost like the mount has jumped part way through the exposure.

I have a CGEM with a 8" SCT 2000mm focal length and Canon 450D at prime focus with a F6.3 focal reducer. As of yet I am not tracking, just polar aligning using the CGEM polar align routine.

Here are some two minute exposures of the Dumbbell I took last night. None are processed, just the original images converted to JPEG to attach here. There are three images and also close ups of the artifacts i am seeing.

There was absolutely no wind last night and I used the mirror lock up feature on the camera to reduce to a minimum any vibration.

Coould this be an issue with the CGEM itself ie does it need a strip down to remove any manufacturing debris from within the gears?

Cheers for any help

Jim

[Martin-Devon]

Hi Jim, I think you are actually tracking but not guiding - is that correct? You don't say the length of the exposures either? The images do look rather odd as you point out, and I suspect that something is either not bolted down properly or is actually too tight. The CGEM is an excellent mount, but there are a couple of the attachements that you have to tweak a little sometimes to optimize it's performance and these can vary from mount to mount. The first one is the attachment bolt that actually screws the mount into the tripod - this is the the screw-thread one that goes up vertically into the bottom of the mount, sometimes this can be too tight, and it may help to just back it off 1/2. The other area where you may want to try is the counter-weight shaft, sometimes if this is also screwed in very tight it can cause tightness and jumping - some folks also back this off 1/2-1 turn or put a small washer in there so that as the thread tightens up at the end of the shaft it is not impacting any movement inside. Another possibility is backlash causing the jumping, so to minimize this always come to your target using the right & up buttons on the handset, over-shoot if necessary so that you always arrive getting the target in the crosshairs of your CCD/chip or eyepiece by only using up/right as the final movements. Good luck!

[JimS]

Hi Martin,

Cheers for the reply. As mentioned in the post the exposures were two minutes each, and yes it was just tracking not guiding. I'll give your recommendations a go. I was also considering getting a Hypertune kit for the CGEM anyway, worthwhile?

I'm going to make the switch to guiding soon. I presume that should help to alleviate the problems as well?

Cheers

Jim

[ian_bird]

Jim

Might be worth balancing your mount East Heavy. If the balance is too close to spot on then the mount "bounces" back and forth on the RA worm gear. Balancing East Heavy means the worm gear is always "lifting" the RA gear - so no "bounce".

I had similar images from my CGEM DX at Olly Penrice's Les Grange a month ago. Olly said it looked like a classic "bounce" problem and sure enough - when balanced east heavy it went away.

I am about to HyperTune my CGEM DX as it has a horrible slop in RA. Will let you know how I get on.

Cheers

Ian