Monthly Archives: July 2019

First round, new Periodic Error analysis and correction

Wednesday, July 3

Continuing from last night – now that polar alignment is close, I’ll do a first pass of Periodic Error analysis.

First, with all corrections off (no PEC, TPoint, Protrack, or guider relays), I collected 4 worm rotations’ worth of tracking data. Plotting this with TheSky’s PEC analysis, I get the following:Uncorrected PE graph

Peak-to-peak error, completely uncorrected, is 6.8 arc-seconds.  This is within spec (7 arc-seconds) for this mount, for the first time – so this is very good news, although I think the mount can do still better.

Using this data to program a Periodic Error Correction curve into the mount, I then did another 4-cycles worth of data collection, this time with PEC running.

Corrected Periodic Error is 2.9 arcseconds peak-to-peak.  That’s good, and I feel good about the improvements that the maintenance has generated.  But I’m left with a few questions:

  1. Most important, why the corrected PE isn’t even better than that.  A good, pure, periodic error should be almost completely eliminated by PEC.  This wasn’t.
  2. This makes me think that the error in the uncorrected data isn’t entirely composed of repeating periodic error, but rather that there are other factors still being injected.

On my next clear night, I’ll re-check the tightness of the pier mounting bolts, re-check equipment balance, re-check spring plunger settings,  then do a further refinement of TPoint/Protrack and polar alignment.  We’ll see what that does and, if the data are still puzzling I’ll look for other causes such as loose connections in optical chain.

Meanwhile, a few unguided test exposures with PEC and ProTrack running, just for fun.  These are all single 60-second exposures, binned 2×2, PEC running but no guiding, and corrected by dark and bias frames (no flats).

Since I was pointed right next to it for testing purposes, I started with M10:

Pretty nice, unguided.  That’s only a 60-second exposure, and the very slight star elongation I can see would no doubt get worse with a longer exposure.  It’s tempting to just throw guiding at it – but I want unguided performance to be as good as I can get it first.

Next, not far away in the sky, M12:

And, just to try something on the other side of the meridian, M27:

In all of these test photos, you can see the slight star elongation goes left-to-right, almost precisely along the horizontal axis.  Since the camera was rotated with polar North up, within a small fraction of a degree, this means the horizontal axis in the image corresponds almost exactly to Right Ascension.  So this elongation still corresponds to RA:  RA error, which is likely, but not necessarily, Periodic error.

There was also substantial Dec drift during my data gathering.  This doesn’t affect PE, but leaves me wondering why.  I’m guessing not-yet-perfect polar alignment is a factor.

A few days of clouds and thunderstorms are now forecast, so more testing later.

Polar Alignment and TPoint model

Clear tonight, but not dark until quite late.  (Duh. That’s what I get for re-starting the observatory only a few days after Solstice.)

I did new polar alignment tonight:

  • The “rough polar alignment” featur in TheSky, which slews to a computed star position which you then correct by adjusting the mount alt and az controls;
  • Then a TPoint model, about 40 observation points around the sky (semi-automated with camera and TheSky “Image Link” feature);
  • Then, with the TPoint model calculated, it makes an  “accurate polar alignment” feature available, which uses another star and continuous imaging to fine-tune the alt/az positions.

After that I did a few test slews.  Slewing to M27 from far away, it landed dead-on, precisely centred.

Just for fun I did a quick 60-second exposure while I was there.  It looks pretty good, so I’m hopeful I’ll see improvements.  This image is uncorrected (no darks or flats) and, more important, has no Periodic Error Correction running.  PEC training will be my next evening’s activity.  I won’t really know how the mount is performing until I see that Periodic Error graph.