How can you adjust tracking rate on the EQ5 upgrade kit?

[almcl]

Has anyone ever felt the need to adjust the tracking rate on the Skywatcher dual axis motor drive upgrade kit?  The instructions say 'only by an experienced technician' (which suggests that it is possible) and mention a potentiometer on the circuit board.

I've got a little bit more work to do before I can prove that my controller's rate is off, but the evidence is beginning to point that way. 

Presently I do a PA with the polar scope using the info from Polarfinder.  The other night I followed this up with drift alignment using the 'out-and-back' ccd/dslr method outlined here http://scopetrader.com/?l=/messageboard/view.asp&ID=110">http://scopetrader.com/?l=/messageboard/view.asp&ID=110.  Now have not completely got my head round this, but a 4 minute example (2 out, 2 back) below doesn't seem to have much drift error.  Unfortunately I don't have an east horizon (house in the way) and on both occasions there was cloud in the west (where there's not much of a horizon view, either) so haven't been able to carry out the second part of the DA process to definitely prove the PA is OK. 

Usually I can get to 30 second subs without to much problem but on several nights I have tried taking longer subs than normal, and all of them show star tailing in the same direction:  stars gradually move east on the image.  Now if it was my PA that was off (and it may well be - I am no expert), I'd expect the error to be a bit more random but the fact that it seems to be always in one direction suggests to me that something is consistently off and that this might be the mount not tracking at the appropriate speed.

If anyone would care to point out the flaw in that above argument or to tell me how they adjusted their tracking rate, or explain how to do a DA with no east or west horizons, that would be much appreciated.

The tracking motor(s) is/are running off a 6 volt mains-fed supply rated at 2.5 amps and the error doesn't change when switching to a battery supply.  The final polarscope part of the process is done with the EQ5 mount fully loaded with OTA and camera (combined weight 7.5kg) and the scope balanced.

[ollypenrice]

Hi Monsieur  ,

Remember that a 4 minute 'out and back' is only the same as a 4 minute classical drift test so longer might refine PA. However, if it's your PA which is out then the error won't be all in one direction, it will be rotational. The easiest way to check is to centre a known  star in a starfield and take, say, a one minute sub. Wait for a decent while - maybe an hour - and then recenter the known star and take another. If PA is out the stars in the corners will have moved but the centre one won't. Or you could ask a stacking software to stack the two based on star pattern and you'd see that the edges of the frame were now rotated relative to each other.

As for the logic of your analysis, I'd say that it doesn't sound like PA for the reasons you mention. It is all in one direction. However, it may not be to do with motor speed but with simple periodic error. Here's the logic; the full worm rotation takes about 8 minutes and you can expect a deviation on and EQ5/6 of maybe +/-30 arcseconds during that time. If you take a (say) three minute sub the drift may be going all in one direction during those minutes. It may then reverse. Or, if your sub happened to be partly outbound in deviation and partly homeward bound, how would you know? You'd still get a line trail.

The thing about the EQ5/6 is that it has huge periodic error (the bad news) but that this error autoguides out very effectively (the good news.) I have a Tak EM200 with a claimed PE of1/10th that of the EQ6 but, under autoguiding, they are not that different. 

Olly

[almcl]

Thanks, Olly, I think I understood that!

Here's a 10 minute sub taken a little after the DA check (cloud came and went a bit during the exposure which might explain the gaps) but does the brightening and darkening suggest the periodic error you mention?

[ollypenrice]

That looks very like backlash to me. The bright star is the clue. It appears twice at a slight separation. This would be consistent with the mount spending some time on one side of mesh and then falling across the backlash to the other side for the rest. It doesn't seem to spend long in between, which is how backlash manifests itself.

You should ideally try to identify whether it's in the RA or the Dec axis. The defence against RA backlash is to run the mount out of balance and east side heavy so that it falls onto one side of mesh and tends to stay there. Of course you can always try to tune it out. There's Astrobaby's guide to the EQ mounts out there in netland.

Olly

[Stainlesssteve]

If you turn the tripod a bit so the PA is out, a star in the south should move vertically. Any periodic error will produce a zigzag in a very long exposure.

[almcl]

Thanks to all for replies, I took Olly's earlier advice and have started guiding my EQ5. 

Nothing very sophisticated, Shoestring's interface and handset mod, a cheap web cam and PHD, but  I am amazed by the results, 6+ minute subs now with nice round stars. 

I think Steppenwolf mentions this in his book and elsewhere, too, but the damascene moment when PHD reported @Guiding' and the error graph settled down to an OSC of 0.24 and RMS of 0.19 was quite good.