- - SET/IRSPEC flat=....
The InSb array used in IRSPEC is not linear and the relative response
of its pixels (i.e. the flat-field) varies slightly with the level of
the signal (i.e. the count-level) of the image. The flat used should
therefore have a count-level close to the astronomical frames, better
if within a factor <2.
There is also a small
dependence on wavelength which makes it advisable to have at least one
flat per photometric band (J,H,K,L). Up to K (
m)
the flats can be derived from short exposures of the halogen lamp;
at longer wavelengths one can directly use the sky frames.
- - SET/IRSPEC dark=....
The dark frame is used in
SKYSUB/IRS
and
CALIBRATE/IRS ... mode=d
and must be taken with the same on-chip integration time (DIT) as
the astronomical frame(s). However, in SKYSUB/IRS you can avoid
caring about the dark if -- and only if -- you use this command
to make a straight subtraction (see last example of TUTORIAL/SKYSUB),
in which case the dark frame is effectively not used.
In case you do not have darks at a given DIT you may try to derive them
by scaling darks taken with different (longer) DITs.
- - SKYSUB/IRSPEC
When you have more than one pair of object and sky frames it is not
trivial to decide whether it is better to apply SKYSUB to each pair and
then average them or first average the objects and the
skies together and then apply SKYSUB (N.B. you must average, not sum
the frames otherwise you
will get into trouble with the flux calibration). The best approach
is probably to first use the second method (SKYSUB on the averaged frames)
and try the second if you have obvious problems with the cancellation
of the sky lines.
- - CALIBRATE/IRSPEC
The wavelength calibration of IRSPEC data is much simplified by the
fact that the pixel-wavelength dispersion is linear to very high
accuracy, and by the very precise mechanical positioning
of the grating. The on-line (mechanical) wavelength calibration that
you retrieve using
CALIB/IRS image
is already quite
accurate (usually within one pixel) and may be enough
for many purposes. It can still be improved using by manually determining
with
CALIB/IRS ima_ref mode=d
the shift (error) on an image containing one or more reference lines
(e.g. the OH airglow lines in a sky frame); with this you can easily
achieve accuracies of 1/3 of a pixel. When using this mode remember
to apply -- using the above command --
the same calibration to the object and to the standard otherwise
you will not be able to run FLUX/IRSPEC. Small shifts between the
object and standard frames can be corrected within FLUX/IRSPEC
using the shift=... option.
- -RECTIFY/IRSPEC
The analytical formula used to derive the tilt angle at a given central
wavelength is quite accurate and should produce rectified
spectra within a fraction of a pixel. The errors introduced by assuming
that the slit images are straight and parallel to each other are also
small.
- - STANDARD/IRSPEC
I decided to force the users to create `flux' files themselves
containing their best guesses for the flux of the `standard' stars
because the use of the photometric points is not necessarily accurate,
even though there is little more one could do.
I hope that a list of spectrophotometric -- or even just
spectroscopic -- standard stars for the infrared will be soon available.
In the meantime,
I apologize to the users for the inconvenience which, however, should
make them aware of the problems one encounters to find a
star with a precisely known flux distribution and with a featureless
spectrum.
- - TUTORIAL/SKYSUB, TUTORIAL/CALIBRATE
These procedures are meant to give you a complete list of examples for
the use of these relatively complicated commands. To work they need
a number of small files (called irstut....) in the `DEMO' area
(.../midas/demo/data), contact your local wizard to have them installed.