Idling, speed and cross-talk
The array idles for about 0.2sec before the first read of an exposure, which effectively removes any latency (or persistence).
The array can be addressed at various speeds, however the faster speeds lead to “cross-talk”, where a fraction of the signal on one pixel is picked up on a pixel 8 columns away. It has been determined that a tick period slower than 1440ns leads to cross-talk of less than 1%. For all spectroscopy modes the tick period has been set to 1620ns.
Readout overheads and efficiency
UIST’s extraneous noise requires extra reads to beat down the read noise in each exposure. The read noise decreases with longer exposure times, reaching an optimum at >60seconds (see the plot in the previous section ). The noise is included in the table below for reference. For all spectroscopy modes the full array is readout every second, each read being the average of 6 digital samples.
|Exposure, sec||Noise, e-||Efficiency, %|
Minimum exposure time
Since all spectroscopy modes will use the same 6-multiread NDSTARE “waveform”, the minimum exposure time is set to 1 second. In this case the readnoise would be ~40e-.
Maximum exposure times
See the section on saturation and sky counts later in this manual.
Optimum exposure times
The optimum exposure time is dependent on many factors such as resolution, wavelength, object brightness and weather conditions. Consequently, although clicking on “Default” in the UKIRT QT will yield a reasonable exposure time for a specific source magnitude, you may need to fine-tune this at the telescope. Note, however, that generally the maximum possible exposure time is the optimum exposure time, as overheads are reduced to a minimum.
In order to be background limited in the non-thermal regime (<2.3 microns), the sky noise must be greater than the array read noise (discussed earlier). Times needed to reach background-limited performance are listed in the next section. Narrower slits will obviously require longer, though beyond about 2.2 microns, the background increases rapidly as the thermal background from the sky and telescope begin to increase, and the background-limited exposure time drops rapidly.
The drawbacks to using long exposures include variations in the sky background and OH line intensities, OH line saturation (at H and K), and the increasing possibility of spikes on individual or small groups of detector pixels. If the critical wavelengths are well clear of the OH lines, then you probably don’t have to worry about their approximate 5-10 minute variation timescales or strength for OH variations, but if you are close to one then OH variations can become problematic In addition to the dangers of sky variations, long exposure times can lead to a lot of wasted telescope time if a fault should occurs or a bad sequence be loaded into ORAC.
For spectra obtained while nodding along the slit, subtraction of the negative spectrum from the positive spectrum will remove most of the sky and OH fluctuations because both vary slowly across the rows of the array. When observing faint and compact sources it is always advisable to nod a small number of rows along the slit, so that the cancellation of sky and OH residuals is as accurate as possible. Remaining residuals can be removed by polyfitting techniques, using blank sky rows adjacent to the rows of interest (although doing this will increase the noise in the final spectrum).
The medium (few thousand) resolution grisms enable observations in between the OH lines in many regions in the 1.1 to 2.3 micron spectrum (OH line emission is not a factor beyond 2.3 microns). In most cases the lower-resolution grisms (IJ, HK, etc.) are background limited at much shorter exposure times at all wavelengths when compared to the short- and long- (moderate-resolution) grisms, due mainly to their lower resolution, which ensures that an OH line is present in almost every resolution element.