Integration Time Calculator for GSMT - Examples

INTRODUCTION

The GSMT ITC is based on the sptime IRAF task in the spectime package. The documentation for sptime applies in full (the program and documentation have been modified recently). The package is flexible and therefore complex and to facilitate its use a number of tables have been created to do most of the setup work. In this description, an "ITC" refers to generic sptime plus an associated group of tables aimed at modelling the performance of a specfic range of instruments or specfic instrument.

ITCs

Four ITCs are planned:

GENERIC - A GSMT spectrograph. The user is required to specific wavelength region and resolution and very little else and an unspecific instrument is "created" by means of an IRAF script.

MOMFOS - The PF, optical, fiber-fed spectrograph. Tables are set up which describe the properties of the telescope and this instrument in some detail.

IFU Spec (??) -

HiR Spec (??) -

Preparation

For flexibility and control, the user is urged to copy the required tables from the server into her own working directory. This set of tables and parameters serve to document the state of the telescope&instrument when the calculations are made. Since the user will very likely want to modify the tables it is important that she has full control over them. The files are found in the spectime database spectimedb, sub-directory GSMT. Each ITC is in a sub directory below GSMT. Here we illustrate the case of the ITC called GENERIC:

cl>
>cd spectimedb
>cd GSMT
>ls
> GENERIC/ MOMFOS/
>cd GENERIC
>ls
HgCdTe            extinct_nearir    generic_VIS       irsky1_5
bg                generic           gsmt_cass
emissivity        generic_IR_Cass   irsky
<make working directory and go to it>
>cd <working directory>
>copy spectimedb$GSMT/GENERIC/* .

There are several levels at which the user may use an ITC. We will illustrate with a few example at different levels of complexity starting with the simplest:.

NOVICE (minimal modification of spectrograph characteristics)

For GENERIC there exists a script called setup which prompts for the minimum characteristic of the spectrograph that must be set:

* Maximum spectral resolution
* Maximum slit width (arcsec)
* Spectral resolution
* center wavelength
* slit width (arcsec)
* sampling of monochromatic slit image (pixels)

Setup then "designs" a spectrograph with reasonable collimator, camera, and grating specs.

The throughput of the spectrograph is the product of the throughput of the telescope, spectrograph and detector. At the moment, the files create a combination as follows (the defining file is in parenthese):

telescope (GSMT_spectrograph) : 1.0
spectrograph (specpars PSET): 1.0
HgCdTe detector (HgCdTe): 0.52 - 0.65 as a linear function of wavelength between 0.8 and 2.6 microns (fairly realistic).

Once setup has been run one simply runs sptime. Default parameters are used for the other parameters of the spectrograph. sptime does not prompt for parameters. Type: lpar sptime to see the parameters you may set.

Examples:

>sptime time=100 - gives text output describing spectrograph and S/N data for 100 second integration.

>sptime sn=20 - gives text output with exposure time required to reach specified S/N ratio. Note there is a parameter which sets the maximum integration time (default 3600 sec). If the requested S/N ratio is not achieve in less than the max, the number of multiple integrations required will also becomputed.

>sptime sn=50 refflux=23 - computes time to reach S/N of 50 for an object of magnitude 23 (whose flux at the reference wavelength corresponds to a body of magnitude 23).

>epar spectime - one can adjust the parameters freely. For example, one will frequently want to change the following parameters:

  • output - select different output graphs to be displayed (ALL gets everything).
  • extinction - insert the name of different atmospheric extinction tables
  • spectrum - insert the name of an (ascii) table containing the target object spectrum (the default is a blackbody spectrum and the user specifies the magnitude at the central wavelength. Read the example: spectrum_example for format. [This file is not very interesting at the moment. Would Joan or Arjun be interested in providing a useful example? The format is described in the sptime documentation.]

    • EXPERIENCED USER - (modifying tables)

    [more example]

    [discuss handling
    sky conditions
    spectral resolutions]

    PSETS

    [This section needs more. I just wanted to include enough for first users.]
    Use of PSETs. In the GENERIC ITC, the script setup sets some parameters in the PSET specpars. You can inspect the parameters by typeing

    >lpar specpars    (Type: lpar sptime | page to page through the long list).
    or
    >epar specpars

    The specpars contains most of the spectrograph-defining parameters. Note, however, that the observing wavelength, diffraction order and slit width are set in sptime, not in specpars. Also, the blaze wavelength and the observing wavelength have been set equal because I am assuming that we are not interested in grating blaze funtion effects in this ITC. I have set up GENERIC so that the spectrograph is always working in Littrow (angle between incident and diffracted beams = 0.0). When you change the operating wavelength the grating is automatically tilted to put the central wavelength at the center of the detector.