PNS MANUAL: Taking Science Exposures

Important note on Object Targetting

Because of the field distortion it is very desirable to put the object on the same place on the chip each time (except for arsec shifts for dithering). The co-ordinates do not guarantee this (pointing models can differ by 1 arc minute!). For each object, enter into the database the PROBE COORDINATES and the PIXEL COORDINATES on which you are guiding, e.g.

M94
PROBE   34000   102000
294.8   126.4

and use this to acquire the object the next time.

About field centre:

When the filter is tilted at a non-zero angle, the passband depends on position over the field, so the target will typically need to be placed off-centre (in the y=spatial direction) in order to have the optimum bandpass for its velocity. This is accomplished using a programme called filtercutoff.c. The aim of this programme is to make sure that, over some range in y, 99% of the PNe are within the bandpass (i.e., the central velocity +/- 2.68 * the central velocity dispersion, assuming a radially constant velocity dispersion, which is very conservative for some galaxies). Reported is the number of Reff that are observable with this criterion.

Important note on Position Angle:

Combining data taken at different position angles is VERY complicated, even if the position angles differ by exactly 180 degrees (left arm and right arm swap role under a rotation of 180 deg, but the different cameras will have slightly different distortions). So if data are to be combined, ALWAYS use the same PA! The list of targets previously observed also mentions the PA that was used.

Important note on the Autoguider:

The telescope operators are not used to an instrument with large field of view at Cass and may select an object for guiding on which causes the guide probe to vignet part of the field. So ask the operator to choose an object with autoradial larger than 30000 if possible.

How to take on-sky science exposures

The PNS shutter is rather slow, and is controlled from the PNS PC which does not interact with the CCD controllers. This makes for a slightly messy procedure, which involves synchronizing by hand the start of the integrations and the opening of the shutter. The procedure described below uses a few scripts to make this as painless as possible.

Perform the following commands, in sequence:

  1. Once target acquisition is complete, make sure A&G mirror is OUT (check A&G mimic display). If it is not, issue AGMIRROR OUT on A&G console.
  2. Prepare two command lines without hitting return yet (where time is the desired exposure time in seconds):
    1. on DAS prepare expose time "title" - Note (i) that the ""s are necessary if the title contains blank spaces
      (this will start exposures of time+30 and time+70 secs on PNSL and PNSR respectively, to give the shutter time to open and close and to ensure consecutive readouts of the CCDs to avoid crosstalk);
    2. on PNS PC prepare expose time
      (this will open the shutter, count down time secs, close the shutter, and make a horrible noise.
  3. Now hit returns: first on DAS, and once both CCDs have started their integrations also on PNS console.
  4. At the end of the readout, the shutter closes automatically, the PC howls, and the CCDs read out, first PNSL and then PNSR. If all is well then the readout starts only after the shutter has closed, and the readouts are sequential.
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