-- CCDTIME---Predicting Exposure Times
There is an improved and generalized version of the
ccdtime task in the
noao.astutil package for V2.10.3BETA / V2.11. This task estimates
exposure times
to achieve a desired signal-to-noise ratio (SNR) or the converse, the SNR
and magnitudes for a given exposure time. The important generalization is
that the information used to derive the exposure times are obtained from a
text database file. This allows new instruments to be added, the user to
have a personal file, and any observatory or user of an observatory to use
this task by creating and calibrating a database file. The database
contains information about the telescopes, the filters, and the detectors.
These divisions allows filters and detectors to move between telescopes;
the main change in the calculation would be the telescope aperture, scale,
and transmission. Standard files are distributed in the directory
ccdtime$ (defined when the NOAO package is loaded).
To look at one such file try:
cl> page ccdtime$kpno.dat
It is very important to understand that this task provides only an
estimate and that the SNR is the formal value for ideal aperture
photometry once the center of the star is known. The SNR may sound
good for the aperture photometry, while the per pixel SNR of even the
central pixel may be barely above the noise. Generally, objects with
SNR < 4 will not even be found, and often a SNR > 9 is required to
assure completeness in surveys. Thus, please be conservative when using
this task to estimate observations to define observing programs.
Frank Valdes
This document was translated by ms2html v1.8 on 21Jan95.