
	User's Guide Contents
	Return to ETC Welcome SYNPHOT SNR and Exposure Time SNR Time Optimal SNR Spectroscopy Target Acquisition Basic Use Page Heirarchy Reference Extraction Regions Spectral Distribution User Spectra Other Spectra Extinction Normalization Background CCD Parameters ID Number Calculations SYNPHOT Calls Call Descriptions Call Parameters NICMOS Spectroscopic Extraction Regions Tables Spectral Elements NICMOS Filters NICMOS Dispersers ACS Filters and Dispersers COS Dispersers WFC3 Filters and Dispersers STIS Filters and Dispersers NICMOS Tables COS Tables STIS Tables and Topical Help

ETC and SYNPHOT

The ETC relies heavily on two synthetic photometry software packages, pysynphot (COS ETC) and SYNPHOT (all other ETCs). Both packages simulate photometric data and spectra as they are observed by the HST instruments and rely on throughput data from the Calibraton Data Base System (CDBS) at STScI. The ETC requests photon count rates from the synthetic photometry package for each term in the SNR/Exposure Time equation. SYNPHOT is part of the Space Telescope Science Data Analysis System (STSDAS) and is developed and maintained by the Science Software Branch (SSB) at STScI. STSDAS is an external package layered on the Image Reduction and Analysis Facility (IRAF) software package, which is developed and maintained by the National Optical Astronomy Observatory (NOAO). Pysynphot is intended as a modern-language successor to the IRAF/STSDAS SYNPHOT package, and is also developed and maintained by the Science Software Branch at STScI.

Except for the details of the SYNPHOT call descriptions and parameters, the ETC documentation describing SYNPHOT applies equally well to pysynphot.

The diagram below depicts the relationship between the ETC and SYNPHOT. The user first directs his/her web client to the ETC web server. The user then inputs instrument and exposure level parameters to the ETC input form that are then transmitted to the ETC application web server. The parameters are then packaged up as SYNPHOT request(s) and transmitted to the SYNPHOT server. The SYNPHOT response, which is a count rate in photons per second, is then retrieved by the ETC application and used in the final calculation and in the SNR/Exposure Time Report.