Is this a tight dec worm?

[1ponders] (Paul) · #1 27-10-2010, 09:06 PM

I'm after confirmation (or otherwise) that this sawtooth shape to my dec graph is caused by a tight spot on the worm/gear mesh. I only seem to get it at around -69 dec in dec (NGC2070 & NGC1910) that I'm aware of. The balance in dec is good and even if it was out the orientation of the mount atm means there is little weight on the dec axis as the RA is almost vertical. The first graph is from tonight at the described coordinate the second is from a week ago shooting M20
.

Oh and its not polar alignment. According to the Gemini I'm only 8' out in elev and 4' out in az. Drift alignment was also confirmed a few nights ago using K3 and was spot on. drift of less than 0.7 arcsec/min

h0ughy (David) · #2 27-10-2010, 09:12 PM

well something is different

bojan · #3 28-10-2010, 05:47 AM

I think it is imbalance
On a fist image, telescope is "falling", while on the second one, drive is working against the load.

The balance of the mount should always be ideally right or adjusted such that motors are working against the load just a little. They should never be "helped" by imbalance.

Or there is a dent in worm wheel teeth in that particular place

...
Another option may be something is loose.. and it demonstrates itself when telescope is in that particulat position (imbalance issue again).

Dismantle and have a look...

multiweb (Marc) · #4 28-10-2010, 07:42 AM

What's your guide scope image scale and was that a full revolution of the worm (i.e 4min)?

[1ponders] (Paul) · #5 28-10-2010, 07:45 AM

Thanks bojan, but I'm very confident its not balance. I'm much more convinced its something either to do with worm or possibly settings in Phd. I turned of the dec adjustments for a while and the graph flat lined, no drift or movement evident. Not a bump apart from normal seeing fluctuation. When I turned it back on it started again. I check my phd settings and found that at some point I'd adjusted my min dec movement to 201.

I noticed the problem earlier in the week, when the graph started doing smaller jumps. It would move one way, settle back and then move the other way. (See graph.) The min Dec move was set to 150. Increasing it made it worse it seems. But it still doesn't explain the initial movement. I suppose the only way to check is to slew to -69 dec and then loosen the worm slightly and see if the problem disappears. Certainly having the higher min dec movement made the problem much worse.

bojan · #6 28-10-2010, 09:03 AM

Yes, PHD settings may be another explanation. Feedback loops can behave like this sometimes ..
Try tracking without guiding - the scope will drift away but you will be able to determine if mechanicals are OK or not.

[1ponders] (Paul) · #7 28-10-2010, 09:07 AM

yes I did that.

Quote:

I turned of the dec adjustments for a while and the graph flat lined, no drift or movement evident.

and it was fine. I'll loosen the worm off a thou today at that spot and see how it pans out tonight. Its just strange that everywhere else I've guided the dec is fine, its just that spot.