GPUSB and EQ4 (orion skyview pro)

[pfile]

hi -

i've used the GPKIT to modify my dual-axis hand controller for use with GPUSB, apparently successfully.

are there any hacks for this hand controller to reduce the drive rate? according to people on the stark labs group (for PhD) the minimum speed of 2x for this controller may be too high.

i am trying to debug PhD driving the mount via GPUSB and using a meade DSI + 9x50 finder to provide the guiding image. somehow even though everything is communicating properly and PhD can calibrate, my subexposures exhibit the same drift whether guiding is turned on or not. my imaging focal length is only 200mm so i would have expected everything to look better, given your stated +/- 4 arcsecond guiding accuracy when using this kind of mount. initially it would appear to be entirely RA drift, but i have to do more analysis of the images.

[dpanderson]

I'm not aware of a hack to adjust the guide rate, but if I could find a schematic for the handcontroller, it might be possible to do something. If you find a schematic, please let me know.

[pfile]

in your instructions for the kit, you say +/- 4 arcsecond accuracy was achieved... over what length of exposure are you talking about?

[dpanderson]

Its been a long time since I made that measurement, but as I recall, I was using an error logging function to monitor the accuracy. An example of an error logging function would be the "Enable Graph" feature found in the Tools menu of PHD Guiding. I usually run such graphs for at least 8 minutes.

[pfile]

okay. i have been using PhD's graph and i can see that in fact it seems to be guiding well. i'm not sure what the scale is, though.

i'm trying to debug a problem where even when guiding is turned on i'm seeing pretty significant eastward RA drift. to me this would indicate that perhaps the RA motor is turning too slowly, though i don't understand why autoguiding would not compensate for this. i'm left wondering if i'm simply running up against the accuracy of this particular mount. i did check my battery pack and under load it's a bit low (4.5V vs. nominal 6V). i don't know what the minimum voltage is for the electronics in the hand controller to function properly...

i also wonder if i somehow damaged the hand controller while making the modification.

[dpanderson]

If PHD is constantly trying to apply an RA correction, but the correction is not happening, this would be a clue that either the modification for that direction is not correct, or there is damage to the handcontroller. If you can still manually push the scope around with the handcontroller buttons, then I'd say the handcontroller is OK.

[pfile]

well - that's why i'm so confused. PhD can definitely move the mount. it can calibrate successfully in both axes. when it's guiding, the graph goes right down the middle, give or take. if i tell it to stop sending guide commands, i can see the RA oscillating pretty wildly with a period of about 1-1.5 seconds, which i assume is periodic error. i'm not sure what the scale of PhD's graph is though...

by all accounts, PhD is doing what it's supposed to be doing, and the mount is responding.

what confused me initially is that an unguided sub looked pretty much the same as an unguided sub. given that PhD is clearly doing something, i would have expected there to be some difference between guided and unguided, especially at such short focal lengths (now 280mm). it may be the case that my unguided drift and guided drift just happen to have similar absolute error, but they are in slightly different directions.

as it turns out, last night i got more subs and i can see both RA and DEC drift in these. i should plot all the subs' image centers on a graph to make sure it's really linear, but between the first and the last sub (1h7m difference between the two) there are 17 arcseconds of RA drift and 1m24s of declination drift. on average that works out to ~7 arcsec of dec drift per 6 minute exposure. image scale is 3.6 arcseconds per pixel. so i suppose stars that are "fat" by 2 pixels are within your measured tolerance.

there's at least one other person that was using the same mount, GPUSB and modded hand controller that was getting satisfactory results at 1000mm. given that it seems that i can do better, but at this point i'm at a loss to understand what. i guess if i keep repeating this and find that the drift direction is random, then it could be due to flexure. that's a very real possibility because my mount setup is very much a hack. i just thought flex would not manifest itself at such short focal lengths. probably i'm wrong.

[pfile]

well, i can report that it all seems to be working okay now. i improved my mounting, tied down all the cables, and put fresh batteries in the drive motor pack. i think i must have just had flexure. the guided subs actually were different; the direction of drift was subtly different. it was just hard to see eyeballing it. once i started platesolving i was able to figure that out.

[dpanderson]

Glad to hear you are up and running.