5 Optimized PHT observing strategies

Staring observations are currently the best understood and calibrated observing modes. It is recommended to take the following considerations into account:

Use telescope nodding in clean fields to cancel out drift effects

• Note that the logic does not take into account the repeated observation of the source position: the logic calculates the S/N for a individual raster pointing, disregarding the repetition on-source.

Use raster mode, if ISSA maps indicate cirrus contamination or confusion. In case of raster mode apply:

• for the C-arrays a maximum raster step size of 1 pixel size or less to get a proper flat-fielding from the redundancy in coverage of central position
  

                C100:    46",  23",  15"
                C200:    92",  46",  31"

• the following recommended minimum raster dimensions

  PHT-P:

  C100: , 46" raster step size yields a pixel map with central position covered by all 9 pixels or
  , 23" raster step size yields a 1/2 pixel map with small pixel centred on central position

  C200: minimum , 92" raster step size yields a pixel map where the central position is covered by all 4 pixels. For a better background subtraction use raster with 92" raster step size which yields a pixel map with the central row of 3 pixels covered by all 4 pixels or use
  raster with 46" raster step size yields a 1/2 pixel map with small pixel centred on central position

• the following recommended integration times per pointing:

  in PHT-P raster sequence (no overlap):
  the required detection time

  for staring or nodding sequence:
  a minimum of 64 sec, preferentially 128 sec, per pointing

  for C100 raster mode:
  a minimum of 40 sec, preferentially 64 sec due to non-white glitch noise

  for C200 raster mode:
  32 s to 64 sec which yield 128 to 256 sec on-source integration

Use of the chopper should only be considered if the logic determines a low chopper frequency (plateau time of 64 sec or more)

• The chopper frequency in chopper mode is not user selectable. It is derived by the AOT logic taking into consideration several criteria with respect to the number of chopper plateaux and number of ramps per chopper plateau. These criteria are the reset-interval length and the on-source integration time. The chopper plateau times can be checked using the ISOPHOT time estimator program ``PHTAOT'' in the output file ``INSTVAR.DAT''.

We acknowledge input or comments by E. Becklin, M. Beckwith, M. Linden- Voernle, M. Meyer, M. Rowan-Robinson and coworkers, M. Silverstone, and all visitors of the ISOPHOT data center (MPIA Heidelberg) who complemented several aspects of our study or brought special items into focus.