UIST – Read Speed, Exposure Times and Efficiency

UIST – Read Speed, Exposure Times and Efficiency

The information below pertains to the new ARC (formerly SDSU) controller commissioned with UIST in December 2006. For numbers specific to the old Edict system, please contact the instrument scientist.

Maximum Exposure Time 

IMPORTANT: Currently, because of memory limitations on the acquisition machine, the MAXIMUM exposure time possible with UIST is 240 seconds.

Read Speeds and Minimum Exposure Times 

The array can be addressed or “clocked” at various speeds. However, the faster readout rates lead to output coupling, where a fraction of the signal on one pixel is picked up on a pixel 8 columns away. Read speeds are therefore limited by a desire to minimise this effect.

With NDSTARE readout of the full array (used in non-thermal imaging and all spectroscopy modes), a read time of 622 millisecond has been adopted; with an additional dwell time of 378 secs, this means that the array is read out every second. With a 10 second NDSTARE exposure, the array is read 11 times (12 including the NULL read), with each read being sampled four times (the digital averaging). Faster readout is possible with sub-arrays, and of course the thermal readout modes (used only with thermal imaging), run with faster readout clocks leading to shorter read times, as shown below.

The READ, DWELL and TOTAL times in the above table represent the following:

  • The READ time is time it takes to physically read out the whole array. 
  • The DWELL is effectively a pause after the READ: 
    • The DWELL is fixed with NDSTARE – longer exposures are created by increasing the number of reads. For example, a 0.6sec NDSTARE 512 exposure would consist of a Null read, a 0.184sec read, a 0.016sec dwell, a 0.184sec read, a 0.016sec dwell, a 0.184sec read, a 0.016sec dwell, and finally a 0.184sec read.
    • The DWELL is variable with CDS – longer exposures are created by increasing the dwell time. For example, a 2sec CDS 512 would consist of a Null read, a 0.184sec read, a 1.816sec dwell, then the final 0.184sec read.
  • The TOTAL time listed in the above table is READ + DWELL, and therefore represents the minimum exposure timewith each readout mode: your exposure time can be (much) longer than this.

Readout Overheads and Efficiency 

As described earlier, digital averaging is used to beat down the read noise in each exposure. The read noise also decreases with longer exposure times, reaching an optimum above about 60 seconds. Long exposures are also by far the most efficient. Only saturation on source or non-linearity, or the desire to collect data before the sky background changes appreciably, should limit the exposure time used.

If short exposure times must be used, then it may be desirable to combine these with a number of co-adds. Although this makes each individual observe less efficient, overall this may be more efficient when considering the time taken to nod the telescope between “source” and “sky”. Increasing coadds also brings down the read noise (a little).

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 instrument configuration in an OT sequence will yield a reasonable exposure time for a specific source magnitude, you may need to fine-tune this value. Note, however, that generally the maximum possible exposure time is the optimum exposure time, as overheads are reduced to a minimum. 

Spectroscopy tests during commissioning, where the same source was observed for the same total on-source period of time, but using 10sec, then 30sec, then 120sec and then 240sec exposures clearly showed that the better detection was obtained with a few long exposures as compared to many short exposures. Indeed, it seems that 200-240 sec exposure times are optimum for non-thermal spectroscopy of faint sources, or half of this if the OH sky lines aren’t being subtracted off too well. In imaging mode, a five or nine-point jitter pattern should probably be acquired within 5-10 minutes, so that a suitable flat field frame can be created from the median average of the jittered target images.