HATS Pointing Model

The Sky X pointing model interface

Introduction

The Pointing Model is a function that corrects the telescope pointing. The function is composed of several terms each one has two factors: a function of the telescope coordinates that can be a trigonometric or polynomial; and a parameter that multiplies the function. The final correction value is obtained by adding all the different terms.

$\Delta x_i = \sum f_i(x_1,x_2) \times A_i$

where $(x_1,x_2)$ are the telescope coordinates (they can be azimuth and elevation, or right ascension and declination), $f_i()$ are the different functions, and $A_i$ are the multiplication factors (parameters). Examples of functions $f_i(x_1,x_2)$ are

$\Delta h = -MA \cos(h) \tan(\delta) \quad , \quad \Delta \delta = +MA \sin (h)$

$\Delta \delta = + ID$

with $h$ the hour angle and $\delta$ the declination. $\Delta h$ is the correction for the hour angle; $\Delta\delta$ is the correction for the declination.

HATS currently uses 7 functions in the Pointing Model which was developed by Patrick Wallace, the expert developer and owner of the TPoint software. It is important to note that when we talk about a "model", we may refer to two different things:

1. The set of functions to use. HATS model uses 7 functions described in the TPoint manual.
2. The value of each parameter.

HATS uses TPoint to obtain the parameters values from measurements done during observations. The following table shows the parameter names and their values as of 2025-04-13 (the functions can be consulted in the TPoint manual):

  Name     Value
  ID       -815
  ME      -9106
  MA      +8852
  HHSH     -922
  HHCH    -2577
  HDSH    -7163
  HDCH    +8714

Measurements

(Needs to be written)

Model determination