Tag Archives: tpoint

Testing unguided performance

Sunday night I used the removable dome cover (via the PZT) to enable two automated TPoint data-collection runs.  (The PZT makes this practical because I can start the run and then go indoors while it operates.  Before the PZT I had to stay in the observatory and adjust the dome position for every “sweep” of data.  This was both inconvenient, and also probably affecting the results through the vibration of moving the dome.)

The first run was 56 data points, just used to refine polar alignment, which was reported as “Excellent” in azimuth and needing small adjustment in altitude.  After doing the polar adjustment, I did another, large, TPoint run of 360 data points. This confirmed polar alignment was now excellent in both axes, and I then used this to build a high-quality TPoint “supermodel”.

What’s all that mean?  It means that

  • The scope control system now has a detailed model of how theoretical and actual pointing of the telescope differed at 360 different locations around the sky.  The recorded differences indicate where alignment is off, where something is loose or sagging, where non-linearities in the optical system affect pointing, etc.
  • Using this model, the scope control system can compensate, resulting in extremely accurate go-to pointing anywhere in the sky.
  • A feature called ProTrack also uses this model to adjust the drive tracking of the scope while it is in operation, keeping tracking accurate without the use of Autoguiding.  (This doesn’t prevent autoguiding, or PEC;  all 3 can be used together, and each contributes something to the accuracy of tracking.)

With a good TPoint model and ProTrack, the mount is supposed to be capable of unguided imaging of fairly long exposures.  I decided to test that, by taking unguided images of M15 of various exposure lengths.  In the following images, PEC, TPoint, and Protrack are turned on, but not autoguiding.  These are unprocessed.

Before PEC, TPoint ,and Protrack, 20 seconds would be the longest I could go without noticing distortion in star shapes.

To my eye, the above are showing no star distortion up to and including the 300-second (5-minute) exposure, but the 600-second (10-minute) one is starting to show distortion.  So it would seem I can do unguided imaging for exposures up to, and slightly over, 300 seconds.  Wow.

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.