Note that the observing efficiency needs to be included separately.
At thermal wavelengths the
is typically ~60% due to
the short on-chip integrations (~0.2sec) and so the exposure time
calculated with the calculators needs to be increased by this factor.
For on-chip integration times longer than one second or so the observing
efficiency is close to 100%.
Up to 1700 DN the array is linear with an RMS scatter of better than 1%.
Therefore the data need not be corrected for linearity up to 1700 DN.
(It is very difficult to get flat-fielding better than 1% across the array so 1%
is a realistic limit for photometry when using stars across the field.)
At 2000 DN the counts are about 2% below the 200-1700 DN straight-line
So for photometry not to be limited by linearity to the level of 1% keep the
counts below 1700DN, for 2% keep the counts below 2000 DN.
There might be some oddities at well depths <100 DN but for faint objects
the sky should fill the well to above this level.
Dark sky flats work best at JHK. Typical integration times of 60s, 60s, and
120s respectively. Flat fielding does not work in the thermal (L'). In this
case dither the object around the field to improve the RMS.
The gain for the SpeX Guider is estimated to be 14 e/DN. The plate
scale has been measured to be 0.1185 arcsec/pixel.
Below are the results of measurements
of background count rates for the Guider/Imager through the broad-band
filters. Also given are the estimated Zero Point (ZP) values for the filters.
For accurate photometry take a standard star measurement.
Background = background count rate in electrons per square arcsecond per second (assuming a gain of 14 electrons/DN). Zero Point = value to be added to the instrumental magnitude to obtain t mag = -2.5 log (DN/s) + ZP These values correspond to an airmass of 1.0. Values for an airmass of 0.0 can be obtained by adding the extinction coefficient given below to the given ZP value. Filter Background [magnitude/sq arcsec] Zero Point Mag Extinction Coeff (DN/sec/pixel) ------ ---------------------------------- --------------- ----------------- J_MK 3.8 [ 15.5 ] 21.15 0.102 H_MK 1.6 x 10(1) [ 14.2 ] 21.24 0.059 K_MK 7.7 [ 13.9 ] 20.57 0.088 Long6 2.2 x 10(3) [ 5.8 ] 18.6 - L'MK 7.5 x 10(3) [ 4.9 ] 19.12 0.093
See here for a list of available filters. Long6 (3.13-3.53um) is an L-like filter in the Order Sorter Filter wheel which can be used for imaging.
Questions about these values should be directed to
Maintained by John Rayner.