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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:--
- ENSURE THAT THE IPCS SHUTTER IS CLOSED WITH IPC.
- Insert the acquisition and comparison mirror in the A&G box
with AGCOMP
- Switch on the tungsten lamp with COMPLAMPS W
- Set the grating angle to about 6000Å. The illumination is
more uniform in the red.
- Set the Dekker to a long slit position, such as 6, but not to
position 0.
- Set the slit width to around 500 microns.
- Insert neutral density (at least ND3.0 is required initially)
with COMPFILTA, COMPFILTB, or MAINFILTND.
- Ensure that the dome lights are out and that it is dark in the
dome.
- Cautiously open the shutter with IPO. Be prepared to close
immediately with the ``Panic Button'' if the level of illumination
is too high.
- 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.
- 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.
Previous: Switching on the IPCS EHT voltage
Up: Observing with IPCS only or with CCD and IPCS
Next: Acquiring data on the sky
Previous Page: Switching on the IPCS EHT voltage
Next Page: Acquiring data on the sky