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Instrument Calibration Data

Decisions which have to be made before beginning an observing run with the IPCS include:--

• The grating to be used, and the central wavelength that it will be used at.
• The dichroic, flat mirror, or folding prism to be used.
• The format of the IPCS data window, in terms of area and resolution. If the window is larger the frame time will be longer, and thus the linearity at high count rates worse. Each CCD pixel is subdivided into a number of smaller pixels which is a power of 2 up to a maximum of 8 in each direction, these factors are known as X- and Y-resolution factors. The X-resolution factor is normally set to 8, which offers the maximum sampling in the spectral direction, and gives 2560 pixels along the spectrum. In normal use the Y-resolution factor will be either 2 or 4, giving a spatial pixel size of 0.7 or 0.35 arcsec respectively.

At the beginning of the setup, and when moving to a new target whose brightness is unknown, it is advisable to run the IPCS in full frame readout mode. To do this type OVERSCAN, but note that unlike IPCS-I on the INT, there is no way of observing the whole of the output of the image intensifier tube; the ``overscan'' mode on IPCS-II simply offers a readout of the whole of the CCD. To resume observing after running in full frame readout mode it will be necessary to repeat the setup of the detector format with IPCSFORMAT.

Procedures for configuring the spectrograph for the beginning of observing are described in Part iv. The centre of the slit on the detector can be defined by observing the tungsten lamp through a narrow Dekker and appropriate neutral density filters. The IPCS data window format can be set with the interactive ICL procedure IPCSFORMAT, and adjusted until the narrow dekker spectrum falls in the centre of the data window. Note that there are some restrictions upon the parameters supplied to IPCSFORMAT, these are detailed in the description of the procedure given in Part v of this manual, and explained in detail in Chapter 4 of the IPCS-II User's Manual.

The IPCS-II dark count is negligible in use on ISIS, and any attempt to measure it will result instead in a measure of the light leaking into the detector. Three types of calibration exposure are useful when observing with the IPCS:-

• Flat field exposures. Because of the low granularity of the current IPCS image intensifiers, the introduction of ``dither'', and the low count rate limit imposed by the hardware; it is very hard to get sufficient counts in a flatfield actually to improve the data. An integration time of at least three hours would be required to give 1000 photons/pixel in normal observing format (in which each CCD pixel is divided into 16 or 32 data frame pixels). To obtain a flatfield:--
  
  1. ENSURE THAT THE IPCS SHUTTER IS CLOSED WITH IPC.
  2. Insert the acquisition and comparison mirror in the A&G box with AGCOMP
  3. Switch on the tungsten lamp with COMPLAMPS W
  4. Set the grating angle to about 6000Å. The illumination is more uniform in the red.
  5. Set the Dekker to a long slit position, such as 6, but not to position 0.
  6. Set the slit width to around 500 microns.
  7. Insert neutral density (at least ND3.0 is required initially) with COMPFILTA, COMPFILTB, or MAINFILTND.
  8. Ensure that the dome lights are out and that it is dark in the dome.
  9. Cautiously open the shutter with IPO. Be prepared to close immediately with the ``Panic Button'' if the level of illumination is too high.
  10. If the level or uniformity of illumination are too high then cautiously adjust the grating angle and the amount of neutral density. The count rate should not exceed about 3 Hz/CCD pixel, which is equivalent to 0.1 Hz/data pixel. THE SHUTTER MUST BE CLOSED WHILE MOVING EITHER THE GRATING OR THE FILTER WHEELS OR SLIDES.
  11. Begin a long exposure with EI. It is your duty to ensure that the level of illumination in the dome remains low during the flatfield exposure. The dome lights are interlocked so that they should not come on with the IPCS EHT on, you must check that this interlock is not overridden. You must also ensure that nobody will open the dome before dark if a flatfield is in progress.
• Twilight sky exposures. These are used to determine the uniformity of illumination along the slit. It is safer to obtain these at the end of the night, when the sky brightness gradually increases, rather than at the beginning when it is difficult to estimate the level of illumination. Twilight sky exposures should be taken with the comparison mirror out ( AGSLIT), and with the slit width set to the value that has been used during the night. They should be taken with the telescope tracking and pointing at a blank area of the sky; there is a list of these in the WHT Users' Manual.
• Calibration lamp exposures. These should be interspersed with observations of astronomical targets throughout the night. The normal comparison lamps are Copper-Argon and Copper-Neon; to maintain a safe level of illumination it may be necessary to use these in conjunction with neutral density filters, especially when working at low dispersion. To obtain sufficient counts for a good wavelength fit a comparison lamp exposure time of at least 100 seconds is required.