Integration Time Calculator for GSMT - Examples

INTRODUCTION

The GSMT ITCs are 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). This manual does not replace the sptime documentation but gives a few examples of how to use it. The package is flexible and therefore complex and to facilitate its use a number of scripts and 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 - Multi-Object, Multi-Fiber, Optical Spectrograph. The Prime Focus, optical, fiber-fed spectrograph. Tables are set up which describe the properties of the telescope and this instrument in some detail.

NIRDIF  - Near-IR Deployable IFU spectrometer. (ITC not ready yet.)

MIHDAS - Mid-IR High-Dispersion Astronomical Spectrometer. (ITC not ready yet)

Preparation

IMPORTANT: To enable access to the files for the GSMT ITCs, change the scripts parameter in the task spectime from the default to:

spectimedb$GSMT/scripts/scripts.cl

Type:
epar spectime
and insert the above parameter in the scripts field. Having set this parameter, invoking the spectime package will set up this pointer to the necessary scripts.

For flexibility and control, the user is urged to copy the required files from the server into her own working directory. When the spectime package is invoked (with the scripts parameter set as above), a script is made available which will accomplish this. To invoke the script type cpfiles. This creates a top level directory GSMTTEST beneath your home directory and places the default directory structure in it, with separate directories for each of the ITCs and the associated files.

These files and the parameters in the tasks together 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. Thus, after running cpfiles:

cl>ls
GSMTTEST
cd GSMTTEST
ls
GENERIC/ MOMFOS/ scripts/ <- this list will grow
cd GENERIC
ls
HgCdTe            extinct_nearir    generic_VIS       irsky1_5
bg                generic           gsmt_cass
emissivity        generic_IR_Cass   irsky

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. We will assume that the ITC in use is GENERIC because it is anticipated that this is the one where the user will want most control. But the same principles apply to all.

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: Setup then "designs" a spectrograph with reasonable collimator, camera, and grating specs. It goes on to setup two tables which describe respectively the transmission and emission of the sky in the wavelength band chosen. Where the sky spectrum is highly structured, a convolution is performed to simulate the instrument transfer function of a spectrometer with a the given slit width.

[N.B. In the other more specific ITCs, the setup will be "hardwired" in the parameters provided. Setup should not be run]

Then, to set up the spectrograph type:
setup
It will prompt you for the required parameters.

Once setup has been run one simply runs sptime. Default parameters are used for the other parameters of the spectrograph unless you change them. The task sptime does not prompt for parameters. You may want to type unlearn sptime to make sure you are starting from defaults. To set or change the observing parameters type epar sptime and edit the parameter set.

Further notes on the spectrograph "designed" by setup:
When you run setup, the blaze wavelength and the observing wavelength have been set equal for simplicity. In addition, for simplicity, the spectrograph is always working in Littrow (angle between incident and diffracted beams = 0.0). When you change the operating wavelength (in sptime) and don't re-run setup, the spectrograph parameters are fixed and the grating is automatically tilted to put the central wavelength at the center of the detector.

Examples:

>sptime time=100 - gives text output describing spectrograph and S/N data for 100 second integration. The object will be the default object (a black body spectrum with mag=10 at the wavelength specified in setup).

>sptime time=100 output=ALL - same as above but also opens a graphics window and displays a series of graphs. Quit the sequence with q; step through the graphs with any other key.

>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 sptime - 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.])
  • ADVENTUROUS NOVICE (5 minutes later) - (modifying tables and other spectrograph parameters)

    Parameters are set in two places:
    1. the parameter set of sptime
    2. the parameter set of specpars
    (Sptime "calls" specpars to get its parameters.) Generally "observing" parameters are set in sptime; telescope and spectrograph parameters are set in specpars. To see what is going on it is useful to type the parameter lists:

    >lpar sptime
    >lpar specpars | page (the list is long)

    Parameters are mostly either numbers, strings, or the names of a tables. The tables can contain parameters themselves (the same parameters which occur in sptime) but more importantly contain multi-column lists giving wavelength dependent quantities like efficiency or atmospheric emission. For an example, type irsky1_5.

    To change a parameter type epar sptime (or specpars) and edit the list.

    Examples

    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 parentheses): So, to change the spectrograph throughput to 0.3 one might type:
    epar specpars and edit the spectrograph parameter to 0.3 or more directly, type:
    specpars.spectrograph = 0.3

    [more examples]

    [discuss handling
    sky conditions
    spectral resolutions]

    ADVANCED - Using PSETS

    The virtue of this two tiered scheme is that once specpars are set up they can be saved under a new name ( e.g myspec.pars) and reused by putting that name as the specpars parameter of the sptime parameter set .

    Use of PSETs. In the GENERIC ITC, the script setup sets some parameters in the PSET specpars. You can inspect the parameters by typing
    >lpar specpars    (Type: lpar sptime | page to page through the long list).
    or
    >epar specpars
    You can also accomplish this within the sptime parameter editor by typing :e<return> in the field for 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.