Wednesday, May 29, 2013

More guiding work

Doug Munday on the CelestronCGEM Yahoo! group helped me to analyze my PHD logs more to understand what's going on.

First of all, he pointed me to PHDLab - a log analyzer for PHD.

The good news is that with the work of the last few weeks, my guiding accuracy improved from 10 pixels peak error down to 4.7 pixels peak error (imaging pixels - not guiding pixels). That's good, but I should be able to get it down more.

Doug thinks that it is mostly external events that impact my guiding (because most of the time, RA and DEC at the same time need to correct large values and because there were some intervals were guiding was much better). A couple of thoughts:
Furthermore, he recommends that I set RA max duration to 2000 and that I should set RA backlash to 0. The latest hardware for the mount motors can't deal with backlash in DEC and in RA.

With that, I got my errors down to 3.83 (combined peak) or 1.11 (combined RMS). I can now take 10 min exposures and keep the stars pretty tight. That's awesome!

Three things still make me concerned:
  1. One thing that I didn't notice before is that when PHD calibrates DEC, the star doesn't move for the first 1-2 exposures, then it moves. When PHD switches direction, the star continues to move for 1-2 exposures into the first direction. Then it stops, but never moves back. I suspect that this started after I set DEC backlash correction to 0.
  2. Also, I can still see that a couple of times, the DEC axis correction tends upwards until at some point it drops. It seems as if I should allow PHD to make larger corrections faster. Is that something that I could improve by switching the DEC algorithm to "Lowpass filter"? Though I read somewhere that this setting doesn't work yet.
  3. When slewing in RA, the mount is making some "sounds". Not the clicking sounds that I heard after hypertuning the mount, but some sounds that  don't sound "smooth". That can't be good!!
On the CelestronCGEM Yahoo! group I received some useful feedback:
  1. It's normal that you don't see any movement in the second step of the DEC calibration. It's a little weird, that my mount seems to move longer into the first direction though.
  2. I tried the "Lowpass filter" option, but that lead to several runaway guides, i.e. where PHD had to apply more and more correction in the DEC direction but never caught up. I stopped that experiment.
Interestingly, the DEC error is not consistently small then the RA error:
DEC: 1.8 peak, 0.4 RMS (arcsec)
RA: 2.2 peak, 0.6 RMS (arcsec)

Next I will try if periodic error correction can help me to reduce my RA errors more.

Friday, May 24, 2013

Imaging night cut short

Last night, I setup everything to take images throughout the night of M51.

... only to find in the morning that Windows installed an update and rebooted by itself at 3am :-(

Later when I checked the "Windows Update" settings, I noticed that the default is: a) Install updates automatically, and b) Install updates at 3am... Changed that to only download updates but ask for installation.

Tuesday, May 21, 2013

Calibrating my screen

I'm doing almost all my work on my Samsung laptop. So, I wanted to make sure that it is correctly calibrated. I purchased Spyder 4 Express. Installed it, let it run and it set everything automatically. The resulting image looks a little dull, but that is probably right as the default of the laptop was very bright.

The whole process took just a few minutes - cool!

Saturday, May 11, 2013

Lodestar gone noisy

Tonight, I could not get autoguiding to work. I connected PHD and started to take images and saw many stars. But every time I tried to calibrate, the mount would not move. First, I was afraid that the new autoguide cable would be broken as well. But then I moved the scope manually and realized that almost all of the stars did not move. And that the only stars that moved were VERY faint. I took a series of dark images. Combined they lead to this:

I tried several times - always with the same result. Then I thought that my scope might be "leaking somewhere" and took darks in a dark room:


I have no idea what to do here.