Instrument Science Report ISR 95-003
Descope of the STIS calibration system
M. Clampin and S. Baum
July 22, 1996
ABSTRACT
We review potential impacts of deleting the HITM calibration sub-system from STIS. The
primary impact is increased wear on the CIM mechanism which it is estimated will experience an additional 18000 cycles in 5 years. Failure of this mechanism would leave STIS
unable to observe at all if the CIM failed to move out of the beam, or, with no means of
wavelength calibration and MAMA flat fielding, which would severely limit the science
that could be done with STIS.
1. Introduction
In the course of recent discussions of options for descoping the Space Telescope Imaging
Spectrograph (STIS) it has been proposed that one of the two calibration sub-systems on
STIS be deleted. This instrument science report reviews the design of the two calibration
systems and discusses operational impacts which will result if, as is currently proposed the
HITM system is deleted. We will address only the impact on STIS science operations and
do not consider in detail the technical and schedule implications of the proposed change.
2. The STIS Calibration system
The internal calibration systems on STIS are designed to perform three functions:
•
Wavelength calibration of spectra
•
Flat fielding of MAMA and CCD spectroscopic and CCD imaging modes
•
Illumination of the spectroscopic slit during target acquisition
The design of STIS incorporates two calibration systems, the Calibration insert mirror
(CIM) system and the “Hole in the mirror” system (HITM). The CIM system comprises
lamps for flat fielding, namely Deuterium, Krypton and Tungsten, and a PtNeCr lamp for
wavelength calibration (Wavecal). A new Wavecal is required whenever the mode select
mechanism (MSM) is moved, since its positioning is non-repeatable.The CIM lamps illuminate the STIS focal plane via the calibration insert mirror which is inserted into the
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beam to direct light from the flat field or Wavecal lamp to the STIS focal plane. In doing
this the CIM blocks light from the telescope reaching the detector, so it also used as a shutter to protect the Band 1 MAMA. The HITM system comprises 2 PtNeCr Wavecal lamps
which illuminate the rear of the second corrector mirror (CM2). A hole in CM2 permits
the light to pass through to the STIS focal plane to illuminate the slit. There is no mechanism to block the light when using the HITM system, so light from the target is imaged
simultaneously with the lamp.
The primary advantage of the HITM system is that it permits the slit to be illuminated by a
PtNeCr lamp during target acquisition, thus avoiding the need to move the calibration
insert mirror into the beam in order to illuminate the slit.
The proposed descope is to delete the HITM sub-system and change the lamp complement
of the CIM system which currently has only one Wavecal PtNeCr lamp. The CIM’s tungsten lamp assembly would be replaced with a second PtNeCr lamp assembly to provide
some redundancy. The technical feasibility of making this change is under investigation by
the STIS team.
3. Descope Impacts
Deletion of the HITM system will incur significant impacts to STIS operations, which are
now discussed in turn.
Target Acquisition
Since the current target acquisition scheme uses the HITM system to illuminate the slit
during target acquisition, a CIM insertion cycle will be required for every target acquisition, so that the slit can be illuminated via the CIM system instead. It is estimated that this
will add ~2x16 seconds to the basic target acquisition time. The last wavecal in an alignment could be done during occultation, which would save 16 seconds per orbit.
Wavelength calibration
In the current scheme of STIS operation Wavecals for the 1st order spectroscopic modes
are routinely done using the HITM system, while the CIM system is used for the echelle
modes to take advantage of the CIMs high lamp flux levels. The descope would require
that all Wavecals would be done using the CIM system bringing into question the lifetime
of the CIM mirror mechanism and the CIM PtNeCr lamp lifetime, if not augmented by a
second PtNeCr lamp. The HITM system is currently used mainly for 1st Order Spectroscopic modes, so it is important to ensure that 1st Order modes can be calibrated using the
CIM system without saturating the MAMA detectors.
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Mechanism lifetime
One of the major issues is the additional wear on the CIM mechanism implied by this
descope. Every time a Wavecal is done the CIM mirror will have to be inserted into the
STIS beam. In order to estimate the number of additional cycles we have assumed the typical orbit defined in the STIS Design Reference Mission (ISR 95-002), which estimates
that, averaged over five years, there would be one target acquisition and 2 MSM positions
per orbit. This implies two Wavecal measurements per orbit, as every new MSM position
requires a Wavecal, and consequently two CIM mirror cycles. Thus, if all of the wavecals
using the HITM have to use the CIM instead, there will be an additional 3 cycles (1 cycle
= 2 moves) per orbit. Assuming 5 prime STIS orbits per day for 5 years, this equals an
additional 27,000 cycles on the CIM due to the descope. Current estimates suggest that
about half of the STIS modes (i.e. echelle) will use the CIM anyway since it delivers more
flux to the detectors, so if an equal distribution of the modes over five years of observing is
assumed, then there would be only an additional 2 cycles per orbit or 18,000 cycles in 5
years. Ball’s estimate of the CIM lifetime is 17,525 (PCM 94.550).
Clearly, the CIM lifetime is, therefore, the primary concern. The motor has built in redundancy which should preclude it from failing while blocking light from the telescope. This
scenario assumes that the motor itself fails rather than a power supply or control system. A
more likely scenario is that anomalous telemetry from the CIM would require that it not be
used to prevent a situation arising in which it fails while blocking the beam. This would
leave STIS with no means of wavelength calibrating any mode, thus introducing large
wavelength uncertainties in the calibration of all STIS spectroscopic observations. We do
not believe that the MSM is sufficiently repeatable to warrant the assumption that wavelength calibrations will only be required infrequently. An uncertainty of even +/-2 detector
pixels will impact a significant fraction of STIS science programs.
In addition, it would no longer be possible to flat field the MAMA detectors to the required
signal to noise, even on external targets. In summary, the loss of the CIM mirror system
would severely compromise the quality of science that STIS could perform. The HITM
system cannot be descoped unless concerns regarding the CIM mechanism lifetime are
addressed. Note also that we have assumed the CIM design would be changed to include a
second PtNeCr lamp. If this is not done the lifetime of the single PtNeCr lamp in the CIM
system is also called into question.
CCD flat fielding
In order to address the issue of the CIM’s PtNeCr lamp lifetime, a component of the proposal to descope the HITM is the removal of the tungsten flat field lamp assembly so that
it can be replaced by another PtNeCr lamp. This assembly is used to flat field the spectroscopic and imaging modes of the CCD detector (STIS Band 3 and Band 4 modes). A work
around for the imaging modes would be to obtain earth streak flats for the [OIII] and [OII]
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filters. An extrapolation from the [OII] and [OIII] flat fields to the longpass filter and coronographic flat fields would be required since the longpass filter and unfiltered CCD
bandpasses are both too broad for earth streak flats.
In the case of the Band 3 and Band 4 1st order spectroscopic modes the loss of the tungsten lamps is more serious, since earth streak flats would be probably be difficult to obtain
and interpret. The deuterium lamp might provide the CCD some illumination at shorter
wavelengths, but it would not provide coverage over the whole 310-1000 nm. To fully
evaluate this impact the effectiveness of the Deuterium lamp for doing CCD flats needs to
be investigated. In summary, the loss if tungsten lamps will have a serious impact on Band
3 and Band 4 spectroscopic modes, with the possibility that the Band 4 modes, in particular, could not be flat fielded without the use external sources which would require
significant amounts of external pointed time to scan standard stars along the slit. A program which would be severely impacted by the loss of tungsten lamps is the STIS IDTs
Galactic Dynamics program.
Commanding
The HITM lamp is currently used to illuminate the slit to find its projection on the detector. This is done autonomously by the STIS FSW, initiated by a single macro command.
The only change to the stored commanding sequence would be to select the LINE lamp
instead of the HITM as a parameter on this macro. Presumably the exposure time being
sent in this macro would also have to change in switching lamps.
The HITM lamps would have to be removed from the PDB and instructions. Five macros
would be removed from the PDB and few instructions would need to be reworked which is
not likely to take more than 1 - 2 days.
A timely decision regarding whether or not to descope is required since an extended discussion lasting more than another week is likely to impact the development of target
acquisition stored commanding and the implementation of Wavecals which follows Target
Acquisition on the commanding development schedule.
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