Pointing
We have a licensed and copyrighted copy of Patrick Wallace's
TPoint software
to determine the pointing coefficients for the telescope.
It lives in the directory ~$TELESCOPE/etc/tpoint
This directory also contains his star catalogs and the
manual.
Here is a graph with some
TPoint terms.
The TPoint data file has a header with site information,
followed by data for each star.
Each star has three groups of information.
- geocentric apparent position
- telescope observed raw position
- the sidereal time
Where raw position means without any telescope corrections,
just what the ha and dec encoders read
with the correction for local sidereal time.
Procedure
Determining the telescope pointing corrections involve six steps:
- select the stars (or fields)
- put them in a sensible order to observe
- point telescope at star
- center the star or just take a picture (determine position with astrometry)
- record the observed telescope position
- analyze the results using TPoint
- incorporate them into the telescope control code
The Programs
Several scripts have been written to do most of the above:
- align - centers the star
- makepointing - generates and orders the star list
- dopointing - takes the data
- fitstotpoint - analyzes annotated fits images and generates TPoint file
- fitstorms - analyzes annotated fits images and show rms pointing errors
- strips - generates strip like star lists
- watchem shows progress of pointing measurements
Traditionally the steps had been:
makepointing - generate pointing coordinate file
dopointing - point telescope and align each star and generate TPoint file.
More recently this has been replaced by a simpler procedure using the align procedure to take
a bunch of images, these are then processed to generate the TPoint file.
fitsfix - converts images to WCS with Gaia stars
fitstotpoint - generate TPoint file from fixed images
Converting FITs files to TPoint Run file
The dopointing program has an option of generating FITs images of sky fields
that are properly annotated and converting them to a TPoint data file.
This has to be done in several steps.
Either images that are deep enough to have enough stars to be recognized must be
taken or if the FK5 star lists are used having an identifiable brightest star available.
If the former can be done they can be analyzed by the
wcsfix.html>wcsfix
program which will update the header with accurate coordinates.
If the latter can be done, they can be analyzed by the
onefix