TSL Control of Calibrations
HET calibration data potentially comprise wavelength
comparison arcs, flatfields, biases, darks, skyflats,
and flux, radial velocity, and telluric standard stars.
Obtaining the right combination for the requirements of
your science data requires a minimum understanding the
interaction of the TSL target submission language with
the service observing practices. For a refresher on
the most relevant kinds of calibrations, consult the
Phase I pages.
The first distinction to be aware of is that a calibration
can be requested in one of two possible modes:
as an "standalone" target in its own right, or by the
addition of suitable keyword attributes to a related
science target. Using the first mode, that of cals as
targets (cf. TSL keyword TYPE), the PI has the greatest
control over all details of the execution of the calibration,
but at the possible cost of TAC time and of some RA
inconvenience from the queue being encumbered with a lot
of non-science "targets."
Using the second mode of requesting cals via keyword
attributes given to science targets, the PI has a choice
of three options within this method. The first and
most customary option, is to request nightly cals
via the STDCALS keyword. These "standard cals"
consist of an RA-chosen appropriate number count and
exposure level of comparison arcs, flats, and biases.
The HRS standard complement includes ThArs, flats, additional
gascell-in flats if appropriate, and biases. The MRS
standard complement includes ThArs and flats illuminated
through the FIF from an interposable lamp, viz. fifthars
and fifflats, in contrast to the HRS standard cals which
are generated internally to the spectrograph, as well as
biases. The LRS standard complement includes appropriate
selections of Ar, Cd, Hg, Ne, and Xe comparison arcs, as
well as flats and biases.
The exposure durations are RA-chosen to be tailored to
the detailed instrumental configuration, and are only
rarely adjusted (e.g. for changes of lamp output).
The currently exact count number and exposure values are
available upon request, or may be inferred from recent
observing reports. What if the PI accepts the RA-chosen
standard exposure lengths but wants MORE than the number
of instances provided by the standard cals? A second
optional approach is to select from a set of keywords
that provide for adding more (including more than zero)
instances of most types of cals. Specifically, the keywords
THAR, FF, GASCELL, AR, CD, HG, NE, XE, BIAS, DARK, FLUX,
RV, TELL, and SKYCALS (i.o.w. skyflats), when added as
attributes of a science target, tell the service astronomer
to carry out the indicated cals on that night. If the standard
cals already supply a standard number count of such cals
(e.g. of flatfields), then the keyword and accompanying
count figure is interpreted to be requesting ADDITIONAL
instances in excess of the standard number count.
Clearly it is a mistake to use this option unless you
truly want MORE THAN the standard number provided.
Note as well that this option is NOT the way to request
cals specially carried out as time-ADJACENT to a given
science target (see below).
(The g1, g2, and e2 grisms receive standard comparison cals
of AR NE XE and a simultaneously exposed CD+HG+AR combination
designated CMA (cadmium mercury argon) in the night report. If wanted,
distinct cals of CD or HG can be requested via those keywords.
The e2 grism receives standard comparison cals of AR and NE.
If wanted, additional cals of XE CD or HG can be requested via
those keywords. Warm-up and integration lengths are aimed at capturing
the largest range of lines at a good level, with particular attention
to wavelength coverage. When line strength disparity justifies it,
two contrasting exposure durations are recorded, with
negligible added overhead.)
The keyword EXTRACALS provides a third keyword-mode approach
for specifying cal requests, and it offers the additional
capability of allowing PI-chosen exposure lengths as well.
(Cal exposure lengths supplied by PIs in any other contexts
are merely placeholders for the standard cals exposure lengths
that will be executed.) Use this option if added very long or
very short exposure cals offer advantages beyond those of
standard exposure length.
Calibrations are typically finished off at the end of the
night, but may be executed at ANY convenient time opening
in the course of the night. In order to secure particular
calibrations carried out time-ADJACENT to particular science,
include a phrase early in the COMMENTS keyword text space
(e.g. "ThAr before" ... "ThAr after" ... etc). Such incidental
cals are not counted in the cals tallying described above.
The alternative mode to cause cals to be carried out time-ADJACENT
to particular science is the above-mentioned one of creating
actual cals "targets" and then grouping them with science targets
using the GNAME and GTYPE keywords.
Besides registering the need for time-ADJACENT cals, the
COMMENTS are the best place to request HRS fifflats, fifthars,
or pfipflats, as these are not provided with their own dedicated
Some Miscellaneous Cautions --
- If the required standard star setup is in any detail different
from the associated science target setup, a special step may be needed.
Most conveniently simply note the requirement at the start of the
COMMENTS. Alternatively create the individual "standalone" targets for any
standard star setup that departs in any way from the associated science
target setup. However, in the case of telluric standards, extra
keyword values have been coded (GC0, GC1, GCboth) which afford a full
choice of using the iodine gascell in the standard star spectrum,
independently of the gascell role in the science spectrum.
- If requesting a specific standard star time-adjacent to a science target,
be aware that this can engender some scheduling difficulty in the case
of intense queue competition for that sidereal time range. Possibly a choice
several acceptable adjacent standard stars (in a POOL-type group) might be
- A "comparison sky" is not usually counted among the scientific data
calibrations, because it is always treated as an individual
"standalone" target grouped (SEQ-type) with its science object. Realize
that the observer may have to secure the science target earlier or later
in the track than the anticipated ideal time. Thus to prevent a time
collision or gap, the observer may choose to execute the "sky target"
at a right ascension shifted from the nominal, but an empty sky field
will nonetheless be carefully selected.