Monday, August 5, 2013

G11 Maintenance Part 2

Continuing the good weather, I wanted to capture a few more points that would help when going back to my notes.

I've done PEC training at .2 x RA tracking.  Drift analysis at this rate is unproductive.  The estimates keep wandering around and don't get any better.  It's interesting how the .5 x rate was so good at approaching an optimum value.

The reason why I wanted to use the slower rate is that the tracking and RMS values at .2x are significantly better than .5 x.  For example, at the higher rate, I see RMS values that are no less than .5 pixels and mostly about .7 pixels.  It's possible that the sensitivity and aggressiveness are too high, or it could be that the durations of movements are too slow to be properly handled by the computer.

When using the .2x rate, I get consistently good RMS values.  Look at this graph of 15 minutes of tracking at 2 second exposures near IC 5070 (Pelican Nebula).

Notice that the scale is set in pixels.  The RMS trend is very good.
The periodic bumps in DEC are due to polar alignment issues.  I see 4 areas of corrections that are problematic as the mount appears to not want to move.  I would like to revisit the polar alignment when the marine layer cooperates and see what is happening here.

Back to the PEC topic, the .2x rate curve was similar to the .5x curve.  I did the same method as before: sampled at .5 seconds with a delay of .5 seconds (giving samples every 1 second).  A primary curve was used and then a second run was used to modify the curve.  Note, I must INVERT the curve when uploading to the mount, otherwise it does not work properly.  The first curve is inverted, the second curve is ADDED to the first, and that result is then uploaded.

At this point, I'm happy with the results on the PEC.

There hasn't been much good weather for as long as I'd prefer to be able to do rigorous testing.  Last night, the clouds came in at 10 pm.  The night before, I was only able to get 2 images done - not enough to do real testing.  This session was the source of the above graph.

Thoughts: Alignmaster may be a good system to use now that the cables are not an issue.  Could be used with the RACI finder.  The Gemini 2 tool of PAC seems to work well, but needs at least 4 good star alignments to give results (plus a slew to the appropriate part of the sky).  Lastly, finding the ideal DEC backlash setting is a problem for both systems.  I'll revisit when I get the chance.

Update:
I checked balance on the scope assembly and I see why the .2x drift refinement was not working well.  It was happening at a point where the balance was too even.  There was minimal to no east bias at the place where the script was running.
I've changed the position of the counterbalance weight, added some marking tape, and upgraded the plumb weight to 11 lbs from 7 lbs.  Seems a bit overkill, but it works to make the system consistent across the whole range of motion.  Will try drift analysis tonight if the weather cooperates.

Also, I looked down the business end of the scope under daylight conditions and was intrigued that there were more light leaks visible.  The feathertouch focuser has a tiny line of open, unbaffled tube.  The amount of light ingress was minimal and also was not directed towards the camera sensor.  Still, it may provide some reduction of contrast.  It is most certainly the place where light can enter when taking darks during the daytime.  I solved this problem with the old sock method: I wrapped the focuser tube with an old sock to block the gaps.