Re: Changing tracking rate in app

Terry Fishlock

Thanks Alex, Will certainly try your suggestions. Much appreciated.


On Tue, Apr 7, 2020 at 3:15 PM Alex F <phobos226@...> wrote:

[Edited Message Follows]
[Reason: Added image clarification of 'east heavy']

I have been having the most success with my EQ5, 130mm Newtonian and 50mm guidescope combination using all three of refraction compensation, OnStep PEC playback and Predictive Pec (PPEC) in PHD2. I have been recording and averaging my PEC curve over about 5 or 6 sessions and it seems to have smoothed out the transient errors to reflect the natural periodic error of my RA drive by now. Most nights I am averaging around 0.4-0.8 arc seconds in RA which isn't bad for a 130mm on an EQ5 in my opinion and well within my imaging scale of 1.81 arc sec / pixel.

Every system is different though, so do some PEC recordings over a few nights to average out the polar alignment errors, wind etc. and incrementally update the PEC accuracy as you go. I can definitely recommend PPEC over the default Hysteresis algorithm for an EQ5 though regardless, definitely give it a try.

I'd suggest an analytical approach on a multiple night basis, keep a little notepad or text file and ignore the graph, focusing on the RMS values and the images you capture. Sometimes my graph looks horrific but the RMS values are tight and the resulting images have good star shapes.

When you're dithering then even the RMS values aren't anything to go by as it ruins my stats, but provided you leave a long enough 'settle' time after each dither you won't affect the quality of your images once guiding stabilises itself before the next exposure begins.

My approach was this:
  • Polar drift align the mount, get it to within 5 minutes of error to minimise the effects of polar alignment on your readings
  • Set guide speed to 0.5x
  • Choose a target with minimal atmosphere in the way to reduce that variable on your guiding as well, you can image at the same time but don't expect much usable data, this is more about the mount than imaging at this point
  • Balance the RA axis East heavy so the worm drive stays engaged at all times during guiding, so that when it's commanding a West guide pulse the teeth aren't disengaging and falling due to gravity
  • Enable the Hysteresis algorithm on RA
  • Run the Guiding Assistant for just over one full worm rotation period, on my EQ5 with 144 tooth RA axis this is just under 600 seconds or 10 minutes, I'm not sure what that is on the CGE
  • Fill the min-move with what the assistant tells you to, adjust your guide camera exposures as directed, I run 2.5sec exposures 90% of the time, 3sec the other 10%
  • Keep OnStep PEC playback turned off, start guiding and start recording PEC data
  • Guide through 4 or 5 worm periods to get a nice averaged RMS result, don't do any dithering!
  • Note down your stats
  • Turn PEC on and repeat, compare the stats for improvements
  • Write PEC data to EEPROM
  • Turn off PEC playback in OnStep Switch the algorithm to Predictive PEC, repeat the observation and note taking
  • Turn PEC back on in combination with PPEC, take notes again
  • Compare all your results and see what is working and what isn't
It's a right pain to sit through but it works out in the end, and it's how I settled on my current combination of Predictive PEC, OnStep's PEC playback averaged over multiple sessions and Refraction compensation turned on. The next few nights are perfect for it as it's a full moon and you're not going to be imaging much anyway, except maybe the moon itself.

Hope this helps!

EDIT - Clarification of 'East Heavy' if you're unfamiliar. Just in case -

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