Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/ops/swat/safing/00305/options.txt Дата изменения: Wed Nov 15 14:29:31 2000 Дата индексирования: Sun Mar 2 01:20:06 2014 Кодировка:

Attached below is the combined impact assessment of the various WFPC2
shutter operations options as they affect STScI ground
systems, NSSC-1 FSW, and HST science and calibration.


Combined Impact Statement for WFPC2
Shutter Operations Options

Due to the recent WFPC2 shutter anomalies, several options for
modifying the use of the shutters have been identified and
investigated. The initial discussions occurred during a telecon on
October 2, 2000 between STScI Commanding, Instrument Scientist, and
Engineering personnel and GSFC NSSC-1 FSW personnel. A preliminary
impact assessment for STScI ground system changes was distributed via
email on October 16, 2000; we are now providing a combined assessment
of the impact of each option on the STScI ground systems, NSSC-1
flight software (FSW), STScI proposal processing, and WFPC2 science
and calibration.

The impact analyses have been performed by the following persons:

STScI Ground Systems: Wayne Baggett and Vicki Balzano (STScI)
STScI Proposal Processing: Denise Taylor (STScI)
NSSC-1 FSW: Michelle Troeltzsch (GSFC)
Science and calibration: Stefano Casertano (STScI)

For each subsystem affected we are assessing impact by categorizing it
into one of three ranges:

Small impact -- 1 to 2 weeks of effort
Medium impact -- 3 to 4 weeks of effort
Large impact -- greater than 1 month of effort


Option 1: AP-17 in "manual" mode
---------------------------------

This option requires that every exposure taken with WFPC2 would use
the "manual" mode (serial clocks on) of AP-17. AP-17's action in this
mode is to perform the initial shutter open command, to respond to the
TDF with shutter close commands, and to close the shutter at the end
of the exposure. The WFPC2 microprocessor is not used for any shutter
commanding in this mode, and thus the shutter error will not be
generated since the microprocessor check is never made.

A. The STScI ground system impacts of this change are --
(Change of default from serial clocks off to serial clocks on)

Changes to Proposal Instructions text for the
proposer-specified parameter for setting the state of the
serial clocks. This is a small impact.

Changes to the proposal processing software (Preprocessor) to
implement the new default. These changes mostly involve
error-checking. This is a small impact.

Changes to the proposal transformation software (TRANS) to
implement the new default. These changes involve parameters
passed to commanding for configuring WFPC2, and also the
alignment times. This is a small impact.

Update the commanding instructions and/or PLCP groups to force
the use of AP-17 manual mode in all cases. The following
cases currently do not use serial clocks on:

BIAS
Internal Flat (INTFLAT lamp)
Dark
Earth Calibration
External < 180 sec
External > 180 sec, specifying CLOCKS off
VISFLAT (VISFLAT lamp)
UVFLAT (UVFLAT lamp)
KSPOT

This is a large impact (about 6 weeks).

B. Proposal processing impacts of this change --

Identify all proposals whose science would be significantly
affected by exposing with serial clocks on. Some instrument
characterization may be required before this identification
can be made. This is expected to be a large impact.

Find all exposures which explicitly turn serial clocks off and
assess them and rewrite, if possible. This is expected to
be a medium impact.

All WFPC2 proposals will require reprocessing, unless a gradual
conversion to the AP-17 shutter control is acceptable. This
is a large impact.

Proposals which use exposure times less than 1 second will need
to be identified and rewritten. This is expected to be a
medium impact.

C. NSSC-1 work to implement the change --

If minimum exposure time of 1 second is ok, no change required.

If minimum exposure times of less than 1 second are required:
When the unique RTCS is built at the beginning of the exposure to
open the blades, calculate the end exposure time, and put the
close blade command in the same sequence. The granularity on the
exposure would be in units of 25 milliseconds. Since the sequence
would take longer to execute, it would nest the UIDLE sequence for
a longer period of time. We would want to set up an agreed to
exposure time that would cause us to take this path in the code.
This is expected to be a large impact.

D. Science impacts of this change --

Scheduling efficiency is decreased, which could lead to the
expansion of the cycle duration and less WFPC2 science being
performed.

May make programs requiring short exposures less efficient
and/or impossible. Possibly large impact on planetary
science and on bright galactic stars and planetary nebulae.
Affects 3% to 5% of proposals, and may be mitigated by the
NSSC-1 change which will allow short exposures by combining
the shutter close is the RTCS for the exposure start.

Manual mode (CLOCKS=ON) is currently rarely used, and is not
fully calibrated. While we expect the calibration impact to
be small, a partial suite of calibration observations is
required to verify and quantify this impact. Approximately
20 external orbits will be needed to verify the areas of
greatest potential impact - photometric throughput, flat
field, and CTE. Additional internal calibrations will also
be needed for bias and dark current.

This option is estimated to be a large effort on the
WFPC2 Instrument Scientist group, primarily for the calibration
observations planning and analysis, and for identifying
affected science programs.

Option 2: Single Shutter Blade Use
----------------------------------

In this option, AP-17 will require updating to command shutter blade B
only. It also requires the use of AP-17 in "manual" mode, to ensure
that AP-17 commands every shutter move. Thus, the STScI ground system
impacts for this option are the same as for Option 1, above.

The NSSC-I changes could be done in two phases a small impact where only
the data values for open/close A skeletons were changed to open/close B
commands. The second phase which would be a medium impact, would be to
correct the reporting in the SHP of which shutter blade was moved. The
ground could potentially handle the SHP being wrong.

The science impacts are the same as Option 1, with the addition that
shutter shading calibration will be required. This is expected to
require 5 to 10 external (pointed) orbits, depending on how repeatable
the shading is. May also require engineering data for shutter time of
flight for proper calibration. This work is expected to be a large effort.


Option 3: Pre-positioning of Shutter Blades
-------------------------------------------

This option avoids the current shutter anomalies by having stored
commanding preposition the shutter blades to the A closed, B open
configuration prior to every WFPC2 exposure. The ground system
currently does this for some calibration exposures, so the technique
is a known one and has been used for many WFPC2 exposures already.

Implementation of this option will result in every exposure requiring
more time (probably 60 seconds) to execute.

A. The STScI ground system impacts of this change are:

Changes to the alignment time calculations in the proposal
transformation (TRANS) software to provide time for
prepositioning the shutter blades. This is expected to be a
small impact.

Update the Commanding instructions to generate the commands for
prepositioning the shutter. This is a small impact (about 2
weeks).

B. Proposal processing impacts of this change --

All WFPC2 proposals will require reprocessing, unless a gradual
conversion to the single shutter blade operation is
acceptable. This is a large impact.

All WFPC2 exposures which currently specify the use of the
wrong blade configuration (probably a few calibration
proposals) will require rewriting. This is expected to be a
small impact.

C. NSSC-I work to implement the change --

None

D. Science impacts of this change --

Scheduling efficiency is decreased, which could lead to the
expansion of the cycle duration. This is estimated to range
from about 20% for orbits packed with short exposures to
about 2% for single exposure orbits. The average impact is
about 8%.

No additional calibration or proposal identification work is
anticipated.



Option 4: RAM Patch to Energize Sensor Earlier
----------------------------------------------

This option would implement a RAM patch to allow the shutter blade
sensor electronics to turn on approximately 10 milliseconds before the
blade position is checked. This would hopefully stop the errors by
having the LED/phototransistor package at full brightness/sensitivity
prior to making the position check, assuring that the check will not
fail. While the fix is technically made to the WFPC2 microprocessor
"code", the ground system is affected because the patch is normally
loaded during the safing recovery procedure, and the PLCP group used
to make the RAM load will require updating and verification.


A. The STScI ground system impacts of this change are:

i. PDB updates --

Update the PLCP group PURAMLD and bit validate it.

ii. Testing support for this change --

The WFPC2 safing recovery commanding will require
recertification. Test SMS's will need to be generated and
delivered to the FSW team for use in their testing and
analysis.

The combination of i. and ii. is expected to be medium impact.

B. Proposal processing impacts of this change --

No impact.

C. NSSC-I work to implement the change --

None.

D. Science impacts of this change --

The on-orbit test proposal would require preparation and data
analysis by the WFPC2 Instrument Scientists. There may
be additional calibration work if the shutter sensor LED
adds additional background to the science exposures by
being energized longer. This is estimated to be a small
impact.


Option 5: Continue to Operate as is Done Today
----------------------------------------------

This option makes no changes to the WFPC2 shutter
operations.

A. The STScI ground system impacts of this option --

No impact.

B. Proposal processing impacts of this option --

HOPR repeats for failed observations will require minimal work,
unless the failure rate increases significantly.
C. NSSC-I work to implement the change --

None.

D. Science impacts of this option --

Estimate that approximately 20 orbits of prime science and 70
orbits of parallel science would be lost for remainder of
WFPC2 usage (January 2002?). This represents about 0.7% of
the remaining WFPC2 science exposures.

The effort to define scheduling rules to minimize science loss
due to shutter anomalies would represent a medium impact.