Saturday, June 1, 2013

Setting PEC

To improve guiding more, I want to finally train and set my PEC (Periodic Error Correction). Hope that it will reduce the RA error more. I am evaluating PEMPro for this. I could connect it fairly easy to my mount, but the camera was a problem. It only works with video cameras, Maxim, CCDSoft or Astrosoft. I have none of these. So, I installed a test version of Astrosoft - that works.

So, getting ready. Starting the Calibration Wizard. Slewing to the correct place in the sky works. First, taking a picture without tracking and measuring star trails works. But the next step (taking a picture while slewing the scope) does not work. I always get the error message "Telescope not connected - Line: 1". Tried everything - nothing :-(

Posted a question in the CCDWare forum - waiting for an answer...

After I turned the camera, I tried again. This time with the "ASCOM RA" option and it worked. Ran through the calibration wizard and could finally start measuring PEC. The good news is that the lines in PEMPro looked very straight. The bad news is that they looked too straight and that the guide star slipped out of the image too quickly. In fact, it looked as if the scope did not track at all.

The next run worked better - I don't think I did anything different though :-S

Here are the recorded graphs:

The resulting PE curves are:

This doesn't look useful at all ...

... I asked about this on the PEMPro forum and Ray Gralak pointed out that I have the infamous CGEM 8/3 frequency here that can't be fixed by PEC. But that PEMPro should be able to improve the other (integer-based) frequencies.

This can be seen when looking at the frequency spectrum:

There are several spikes at full frequencies. There is the 8/3 frequency (2.66) which is inherent in the CGEM mount. But there is also a really larger spike at ~0.2. I have no idea what this is...

I loaded the PEC curve into the mount and did another PEMPro run to see if and how tracking improved. The result wasn't too convincing. The overall RMS error stayed at 4.5 and I couldn't see any improvement when I guided with PHD.

I asked again at the PEMPro forum and Ray pointed out the individual amplitudes roughly doubled (e.g. for the 1 frequency, the amplitude went from 2.053 to 3.739). Which indicates that my curve has to be inverted (which can be done in the upload functionality of PEMPro) in order to cancel out the periodic errors.

OK, doing that...

... and now errors from all frequencies are below 1 !!!

So, now to increase accuracy, I will run this for a few hours, program my mount with the new curve and then check again.

And this is what I got:

Nice! All the integer frequencies are way down. But the big distortion at ~0.2 is still there :-(