TPoint Modeling with Paramount ME
The Problem :
I used to deploy a TPoint model with only about 80points and ProTrack tracking had been excellent. Any 5 minutes unguided exposure was always
trail-free. This old Tpoint model was created with data points collected by a ToUcam. After changing to the ST-10XME CCD I did a short mapping run
and this old model still worked well.
After adjusting my camera assembly by placing the focal plate out some 25mm or so (i.e., adding some backfocus using an extension adapter tube), I
also modified the RA and DEC axes balanced positons concomitantly. After such adjustments the old TPoint Model of course became inapplicable. So I
spent a whole night collecting some 250 data points with the ST-10XME ccd for a new "Grand" model, hoping that this one would be an improvement
over the old one. With my last experience with the old model, I expected that this new Grand Model might enable exposure for 10 minutes or so
The RMS and PSD figures are similar in the new and old models. Both figures are around 30 in both models. I also add similar terms in both models.
However, I noted that in the new Grand Model there is a definite wider spreading of data points in the N-S direction (longitudinally in the scatter
diagram) that can't be eliminated by adding suitable terms.
To my dismay this new "Grand" TPoint Model with 250 data points performed much worse than my old model with only 80 data points. The pointing is OK
with targets almost centered always. However, the tracking accuracy has become unstabe : an exposure could return no trailing while the immediately
following exposure of the same target would return obvious trailing. Good tracking in one part of the sky could deteriorate to obvious trailing after
slewing to another part of the sky. All in all there is always trailing with just 2 to 3 minutes exposures !!!
The Solution :
I posed this issue to the Software Bisque Support Corner and have received first class support. With the help of Thomas M. Bisque (also feedback from
Patrick Wallace and Kevin) I 've learnt the proper way of adding "suggested terms" and incorporating the most significant terms in the TPoint Model.
Moreover, I've also been made aware of the vital role of Periodic Error Correction in tracking and learnt to train the periodic error correction of the
Paramount ME using the CCDsoft autoguide/tracking logfile (which I've found to be the most accurate way to record and program PEC).
The Right TPoint Model for my Setup:
1. An 250 data point "Grand" model with a short mapping run of 30 additional data points (the latter to correct for a change in optical configuration
after the collection of the 250 data points for the "Grand" model).
2. 18 terms used in the final "Hybrid" model, viz.,
6 Equatorial Terms
5 "Lagacy Terms" which have proved functional in my old model
1 term suggested by Thomas M. Bisque (DAF-flexure of cantilevered dec axis)
5 "Suggested Terms" using the right procedure learnt from Thomas M. Bisque
1 term suggested by Patrick Wallace (HDCD5)
The RMS of this "Hybrid" model is 19.69 while the PSD is 31.41 : The "Hybrid" Model.
3. An CCDsoft tracking logfile was recorded and used to program the Periodic Error Correction of the Paramount ME. The periodic error curve is
very smooth and indicates an periodic error of +1.0/-1.0, which amounts to 2.0", at the low side of the PE for the Paramount ME.
4. The cooling fan of the ST-10XME was turned off to prevent vibration which was suspected to be the culprit of eclectic trailing (i.e. unstable tracking
performance : an exposure could return no trailing while the immediately following exposure of the same target would return obvious trailing).
5. "Outliers" in the data points was deleted by trial and error with empirical tracking performance confirmation. The finalized TPoint model gives an
RMS of 16.97 and a PSD of 29.4 : Finalized Hybrid Model.
The Results :
Casual 6 minutes' unguided exposure (with cooling fan OFF) with ProTract and the finalized "Hybrid" TPoint Model always returns images of round
stars with no visual hints of trailing. 8 minutes' and even 10 minutes' unguided exposures are visually acceptable, saved for deterioration in image
quality due to seeing, i.e., atmospheric turbulence. Mirror flop might also distort the final images in a random manner.
M39 : 6min unguided exposure with the "Hybrid" TPoint Model and ProTrack
Paramount ME PEC on and ST-10XME cooling fan off
(click image for the FITS file format)