Monthly Archives: May 2015

Changed scopes for summer

It was clear and warm last night, but a full moon.  So no serious imaging but a good night to adjust things.  With winter skies basically done, I decided to take off my widefield refractor and go with a longer focal-length OTA for the summer season of galaxies and nebulae.  So, the SV80S is put away, and I mounted the AT8RC 8″ Ritchey-Chretién, and rebalanced the mount accordingly.

M67-3x3-10secs--15C-LuminanceThat creates new re-adjustments that are needed, of course.  I re-built a V-Curve model for FocusMax, and got nice round stars on a quick 10-second test image of M67.

I was going to do a new TPoint pointing model, and also use that to re-check the polar alignment, but I became distracted.  I tried a quick test of autoguiding, intending to confirm that my previous autoguiding problems were field curvature on the SV80S.  They were – the AT8 produces a nice flat field, and the guide starts weren’t too badly distorted to use.

The problem was that I tried doing a guider calibration, and it didn’t work.  That ended up distracting me for the rest of the night, as I experimented with the parameters that control the amount of movement the calibration routine injects before measuring direction and distance.  No matter what I specified, even absurdly long displacements, I got the same error:  “Error, insufficient motion in X-direction during calibration”.

Guider-calibration-motionWhich is clearly not the case.  As these two guider frames show (click for animated gif), there is good motion happening in both axes between calibration images.  I puzzled over this for an hour or so, then it clouded over so I shut down and went to bed.

This morning an idea hit me which I think is the solution.  The guide stars aren’t in focus. That, alone, shouldn’t be a problem, as the guiding software should calculate the centroid of each star.  However, there are bright hot pixels in the image.  I bet the guider software is selecting and guiding on a bright pixel, which doesn’t move, not on a star.

I’ll test this next clear night:

  • Adjust focus of guide camera to get nice bright pinpoint guide stars; and
  • Use a dark frame with the guide camera to calibrate out the hot pixels.

We’ll see.