CPC slew, mount drift or backlash

[rodrigol 519]

I'm experiencing an issue with my CPC800 which is I can only describe as an issue with slew, mount drift or backlash: After a sucessfull PA (ASPA or 3 Star Align) I get the scope to GOTO quite accurately. I use the Celestron CrossAim 12.5mm eyepiece (CCelestron CrossAim 12.5mm Illuminated Reticle Eyepiece) to align on each star as accurately as possible and this has helped enourmously with GOTO accuracy. If the GOTO target is to east of the meridian the GOTO and slew are bang-on - tracking starts when the target is at the centre of the FOV. However, if the target is on the west the slew continues past the target. To stop this I have to press the handset button opposite of the direction of slew and then the tracking starts. I'm afraid I can't describe this any better - I wish I could post a video.

Please, can any explain what is going on and perhaps suggest a solution?

[Kropster 444]

Is the mount and scope balanced? If it is way out, that may explain it, but it seems a bit odd.

[rodrigol 519]

HI Kropset,

Thanks for replying. It's balanced (a bit back-heavy). Yes, it's odd I think.

[UtahGooner 2]

Have you found a solution to this problem? I seem to be having the same problem with my CPC-800 and the HD Pro Wedge.

I don't recall having the problem at all without the wedge.

I'll play tonight and see if I can isolate it to a weight/balance problem.

[UtahGooner 2]

I found a post on another site indicating balance may be a problem as well, and did indeed notice that when mounted on the wedge my CPC800 does tend to rotate to having the motor side down (opposite side of the carrying handle). I added some weight (about 2 lbs) with a velcro strap to the handle and it did help, but not quite enough. I still get some drift after a polar alignment when pointed toward the western sky, but 2-3 minute exposures to the east seem to have no drift now.

The scope is also a bit back-heavy and top heavy - I've only got the default finder-scope and a small camera mount, and still think a bit of the drift might be associated with that imbalance as well.

[Kropster 444]

Putting it on the wedge must show up the imbalance that would not matter in alt/az use.

Sounds like needs to balanced to counteract the motor/gearbox.

[UtahGooner 2]

This looks like it could be a good solution - I haven't researched to see if anyone else has a balancing system, but this (or a home made version) could be a great tool.

http://starizona.com/acb/basics/using_balancing_fork.aspx

[UtahGooner 2]

And the other link I meant to post: http://starizona.com/acb/Counterweight-Set---Celestron-CPC-800---Complete-Kit-P2720C725.aspx

[rodrigol 519]

The prices for balancing counterweights are too steep! 

I will stick to wrist and anckle weights for now....I have the sliding rail ATM solution for the CPC800 but not a top weight.

[Northern Soul man 350]

Hi Rodrigol,

Another thing you could try is when searching/viewing objects, try staying in the same quadrant of the sky - I know that this probably isn't the best way of doing things, but I've found - even in Alt/az mode, that staying within the same area of sky keeps the accuracy better than moving through large "Portions" of sky - not sure if you was experiencing this in Alt/az mode or not - if so, then the wedge is set up ok, but if its only started since moving to the wedge, then as mentioned, probably balance.

Next clear night out with your scope, try to just pick objects out in each quadrant, (North, south, East and West) and do a few GOTO's within close proximity of each other - try these and see how accurate the mount is then.  

I know its a little time consuming, but then switch the mount off and do a 2 star align in the part of the sky you want to try next - say change from objects in the east - to objects in the West - on alignment pick 2 stars in only the West to align on, then try GOTO for the objects only in that part of the sky - this will give you better accuracy, I've done it a few times when moving to different parts of the sky (only in Alt/Az mode) and used the "Sync" function a few times to not so much re - position the scope in that area of the sky, but to try to lock on to closer objects to maybe refine the accuracy of the mount.

I know its a little time consuming, but I've found that using this method makes the mount much more accurate, just then trying really bright objects for GOTO accuracy as these are easier to see for me under light polluted skies - there's nothing worse than not knowing if the objects are invisible due to its faint nature or just out of the FOV (trust me, I think everyone has experienced this from time to time.

Paul.

[rodrigol 519]

Paul: many thanks for this very sound advice! Just got back from holidays and will setup soon. As soon as the weather permits. Thanks again!

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