SpeX Observation Calculator

Web-based input form for SpeX Observation Calculator - 06 March 2015

This form can be used to estimate (1) the limiting fluxes of objects that can be observed with SpeX for a given signal-to-noise and exposure time; (2) the integration time needed to reach a requested signal-to-noise for an input source flux and temperature; or (3) the signal-to-noise resulting from an input source flux and temperature and integration time. The output data file generated by this routine (spex.plt...dat) can be saved to your machine from your browser window once the program has been run.

Note: The calculations now incorporate paramaters appropriate for the new detector installed in SpeX. These include a lower readnoise, a lower dark current, and a higher throughput.

When estimating (1) the limiting flux, the user should specify the observing mode, the slit size, the seeing in the K band (in arcsec), the desired signal-to-noise ratio (per resolution element), the integration time for an individual image (in sec), and the total integration time for the observation (in sec). (The source flux and source BB temperature fields may be left blank.) At each wavelength point over the observed range, the limiting flux is calculated for the input parameters. Magnitude limits are given for the nominal centers of each broadband filter covered (e.g., at 2.2 microns for the K band). Saturation will occur if the single frame integration time is too long. This can be seen in the plot as a red background for those wavelengths that are saturated. (Not yet implemented.) Wide slits will also increase the background.

When estimating (2) the integration time needed to reach a specified signal-to-noise ratio (per resolution element), the user should specify the observing mode, the slit size, the seeing in the K band, the desired signal-to-noise ratio, the source flux at 2.2 microns (K band), and the effective blackbody temperature of the source. (Note that Vega has a mag of 0.03 and a flux of approximately 4.14e-10 W/m2/micron, or 655 Jy, in the K band.)

When estimating (3) the signal-to-noise ratio per resolution element, the user should specify the observing mode, the slit size, the seeing in the K band, the source flux at 2.2 microns (K band), the effective blackbody temperature of the source, the integration time for an individual image (in sec), and the total integration time for the observation (in sec). Saturation will occur if the single frame integration time is too long. This can be seen in the plot as a red background for those wavelengths that are saturated. (Note that Vega has a mag of 0.03 and a flux of approximately 4.14e-10 W/m2/micron, or 655 Jy, in the K band.)

For background-limited exposures shortward of about 2.5 microns on-chip exposure times should be less than 300 seconds for good OH line cancellation. A 120 second on-chip exposure saturates the detector at wavelengths longer than about 4 microns for a 0.5 arcsec slit. The flux limits assume perfect flat-fielding and telluric division, and a nominal instrument performance. Throughput is particularly sensitive to seeing. Currently, the median seeing at IRTF is about 0.8 arcsec at K. Note that the plate scale for SpeX is 0.15 arcsec/pixel.

The program uses ATRAN (Lord 1992) to generate a model of the atmospheric transmission as a function of wavelength for an altitude of 14000 ft, an airmass of 1.2, and a zenith water vapor content of 1.2mm. The model transmission hase been smoothed to the requested resolution (which depends on the slit size). The calculations assume perfect flat-fielding and telluric division, and nominal instrument behavior. The agreement between the observations and the predictions is reasonable.

IMPORTANT NOTE. The calculated integration time is always less than the clock time required to do the observation due to the overheads required to for non-destructive reads, coadding, data display and data storage. See the SpeX Manual (Section 2.5) for estimates of these overheads. They need to be included in the overall request for observing time in addition to overheads for object acquistion and calibrations etc.

Questions about SpeX and its performance should be directed to the John Rayner (rayner@irtf.ifa.hawaii.edu). If you have problems with this form, please contact webmaster (webmaster@irtf.ifa.hawaii.edu).



Input Observing Parameters

Select the quantity to be estimated:

Select an Observing Mode:

Choose a slit size (arcsec):

Seeing (FWHM) in arcsec at 2.2 microns:

Required Signal-to-Noise ratio:

Single frame integration time (sec):

Total integration time (sec):

Source Flux at 2.2 microns:

Source blackbody temperature (K):



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