NICMOS Detector Operations Overview

	Detector Operations Modes

		ACCUM

		MULTIACCUM

		RAMP

		BRIGHT OBJECT

		TARGET ACQUISITION

		Loss-of-Lock Processing

	Optional Parameters for Background Observations

	Phase-2 Proposal/Visit Exposure Logsheet - Simplified Example

	HST/System Resource Limitations
	Defining Observations: Visits/Orbits/Exposures

Operating Mode: ACCUM (aka MIF - Multiple Initial and Final)

Instrument Configuration: NIC1/NIC2/NIC3

Simplest Observing Mode: Single Sampling Period, Single Image Array

Specific Use: Faint Targets, Long Integration Times - NREAD large,
	      High S/N Targets -NREAD small; minor CPU overhead

Specific Function: Reduces R/O Noise by Performing & Averaging Multiple
                   Initial and Final Read Outs. 
                   (when read noise > dark current + photon noise)


Time_per_Exposure                       Optional Parameters
EXPTIME: >  RS * NREAD	                NREAD = 1 - 25

      RS ~ 0.27s for READ-RATE = FAST	READ-RATE = FAST,SLOW

         ~ 2.70s for READ-RATE = SLOW	BCK-OFFSET = FOM, SAM

  May be any value, but preferably	BCK-SCALE = # arc-sec

  from indexed table for calibration	FOMXPOS = # arc-sec

  and memory efficiency.	        FOMYPOS = +/- # arc-sec

Operating Mode: MULTIACCUM

Instrument Configuration: NIC1/NIC2/NIC3

Exploits Non-Destructive FPA Read Out Capability

Specific Use: Large Dynamic Range, Ground Processing for CRs and Saturation

Specific Function: Multiple Images at Specific Times as Integration
                   Progresses


Time_per_Exposure                       Optional Parameters
EXPTIME: >  RS * NREAD	                NSAMP = 1 - 25

      RS ~ 0.27s for READ-RATE = FAST	SAMP-TIMES(i) = # seconds

         ~ 2.70s for READ-RATE = SLOW	READ-RATE = FAST,SLOW

	                                BCK-OFFSET = FOM, SAM

	                                BCK-SCALE = # arc-sec

	                                FOMXPOS = # arc-sec

	                                FOMYPOS = +/- # arc-sec

Operating Mode: RAMP

Instrument Configuration: NIC1/NIC2/NIC3

Exploits Non-Destructive FPA Read Out Capability

Specific Use: Large Dynamic Range, Uncertain Target Flux, 

              On-Board CR Elimination and Saturation Detection; CPU intensive



Specific Function: Slope Computation Reduces Data Volume, Provides Variance
                   and Sample Count Arrays







Time_per_Exposure                       Optional Parameters
EXPTIME: >  (RS + OV) * NREAD	        NSAMP = 1 - 255

      RS ~ 0.27s for READ-RATE = FAST	CR-ELIMINATION (see table)

      RS ~ 2.70s for READ-RATE = SLOW	READ-RATE = FAST,SLOW

      OV ~ 5s * number of detectors     BCK-OFFSET = FOM, SAM

                                        BCK-SCALE = # arc-sec
CR-ELIMINATION	                        FOMXPOS = # arc-sec

NONE Disable CR-Elimination	        FOMYPOS = +/- # arc-sec

MARK Flag pixel as bad	

RETAIN Use previous mean([[Delta]] signal), discontinue for that pixel

CONTINUE Reject individual sample if > threshold, continue processing

Operating Mode: BRIGHTOBJ

Instrument Configuration: NIC1/NIC2/NIC3


Specific Use: For Bright Targets Which Would  Saturate FPAs in Other
              Observing Modes with Shortest Integration Times

Specific Function: Reset/Read/Wait/Reset Each Pixel Sequentially in Each Quad



0.001024s < EXPTIME < 0.26112s

(Maximum Time Required = 4278.19s)

Operating Mode: ACQ

Instrument Configuration: NIC2


Specific Use: Autonomous Acquisition of Brightest Target in 128x128
              pixel region (9.6") in Camera 2 for Coronographic Imaging


Specific Function: Results in Vehicle Offset Maneuver or FOM Motion to Place
                   Target Behind 0.3" Radius Occulting Spot


Internal Use of ACCUM Mode with NREAD = 1 to Obtain Acquisition Image


Brightest Object Located Using Target Centroiding with 3x3 Check Box



Target Location and Acquisition Precision to 1/4 Pixel Precision (18.75 mas)


Downlinks Initial Target Location ACCUM Images (Initial and Final)


Note:	Performing "back-to-back" ACQ mode exposures would result in finding
        the second brightest target in the field

Loss-of-Lock (TDF Down) Processing

ACCUM:        After last Initial Read, Before first Final Read, Take
              SINGLE Final Read Immediately.  EXPTIME reduced,
              R/O noise not reduced as planned.
 
MULTIACCUM:   Read Out Current Sample Immediately.  Reduced Sample
              Duration. Remaining Planned Samples Not Read Out.

 
RAMP:         Ignore Read when TDF down. Treat as CR Hit for ALL Pixels.

BRIGHTOBJ:    No TDF Processing. Spatial Positioning of Post-TDF Down
              Pixels Degraded.

ACQ:          No TDF Processing. Probable Acquisition Failure on
              TDF-Down.

 
Guiding Performance Since Re-Enabling of SAGA-2 (from 6 months of data):


   * 5 true LOL events in ~2500 Fine-Lock Orbits (~1 event/500 orbits) 
   * Maximum LOL duration = 200 seconds, most < 1 minute 
   * Otherwise Level 1 guiding specification met: < 7 mas RMS jitter in FL
Note: Observations which are declared failures due Guide-Star Loss-of-Lock may be repeated (by petitioning the TTRB) without GTO/GO time charged to the failed observation.

Optional Parameters for Background Observations

Assumption: One or more scalable "standard patterns" of field-offsets, to provide, a median-filtered sky background is sufficient for most observations requiring background subtraction.

Geometry of Standard Background Patterns TDB, but are expected to
contain 16 or 25 pointings.

"Preferred" Background Offset Method is via SAM (S/C Small Angle Maneuver)
   * FOM is left at MIQ Position (no change to PSF or degraded image quality) 
   * Preserves FOM lifetime (limited life item) for required use 
   * Small Offsets (< ~ 2') may be done on single GS pair w/o re-acquisition 
   * New Guiding scenario requested of GSAWG for large SAMs (FL / Gyro hand-off) 
   * Observer-Specified "Pattern" via POSITION TARGET Special Requirements
Sample (4x4) Background Offsets with Small SAMs 

 Background Offsetting via FOM (maximum displacement 26"): 
   * Maintains Fine Lock on Guide Stars - No Additional PCS Overhead 
   * REQUIRED for Parallel Programs (which do not control the S/C pointing)
   * Observer-Specified "Pattern" via FOMXPOS, FOMYPOS Optional Parameters

Phase-2 Proposal Visit/Exposure Logsheet - Simplified Example

HST/SYSTEM Resource Limitations

Observational sequence definitions may be constrained by time or data capacity limitations.

Maximum Number of Images in NICMOS Internal Data Store (IDS): 83 
   * This is total, NOT per camera.
   * Images must be off-loaded to Science Tape Recorder (STR) Before IDS fills.
 
Science Tape Recorder Has Maximum Capacity of ~ 600 NICMOS Image Frames, but:   
   * STR is a resource shared with other SIs (1 4-CCD WFPC image ~ 40 NICMOS Images.
   * Time-averaged data allocation for PRIME observations divided equally among all SIs.
   * Parallel (i.e., survey or snap-shot) programs will have lesser priority than Primes.
   * Nominal operations employ two-STR mode, one STR must be dumped before switching. 
   * Dump of FULL STR requires ~ 20 minutes. 
   * Downlink opportunities must be managed with uplink requirements. 
 
NSSC-1 (HST SI Flight Computer) Has Only ~ 6K Words of Stored Command Memory 
   * Stored command memory is a shared resource with other SIs. 
   * Memory partition between Absolute and Relative Time processors is equally divided. 
   * NICMOS, STIS, and WFPC-2 primarily use ATP; FOC primarily uses RTCSs. 
   * Many short exposures will "use up" stored command memory quickly. 
   * Operational throughput of command uplink management must be increased. 
 
NICMOS CPU is NO Speed Demon (16 MHz, 80386) 
   * iRMK operating system and task handling imposes some significant CPU bottlenecks.
   * Some multi-camera detector operations can reduce efficiencies (notably RAMP mode)

What's The Bottom Line?
   * Greatest impact for Parallel (e.g., survey) programs.
   * Define EFSW Macros for executing such programs.
   * Consider benefits of reduced data volume offered by RAMP mode when appropriate
   * Understand timing ramifications of multi-camera operations (notably RAMP mode)

Defining Observations: Visits/Orbits/Exposures

A New Method of Defining Observations Was Adopted by STScI for HST Cycle 5. 

   * Driven by allocation of GO time in "Orbits" rather than S/C time.
   * Replacement of RPSS with RPS-2 + Orbit Visualization Tool.
Vehicle Slews to Targets are NOT Considered Part of the Observation. 
ORBIT = Min (Time between Start of Guide Star Acquisition and Earth Occultation,
             Time Between S/C Ascending Node Crossings {~96 minutes}
Note: All Orbits are NOT Created Equal 

VISIT = Time Interval While HST Attitude is Maintained by PCS for One Target
   * Visit may be one or more orbits (GS Reacqs contained within one Visit). 
   * Vehicle Small Angle Maneuvers permitted within a Visit.

EXPOSURE = Sequence of SI events driven from a single Exposure Logsheet Line
   * e.g., FOM Move, Filter Selection, Detector Operation. 
   * Multiple image frames may result from one Exposure (e.g., MULTIACCUM)
All Three NICMOS Cameras May Be Operated Simultaneously 
   * Simultaneous operations for a gven program must be defined for the Visit.
   * Specification of simultaneous operations currently via RPS-2 scheduling requirements.
   * STScI is developing a GUI to simplify defining simultaneous operations.
   * Parallel operations with an unused camera may be non-intrusively "Attached" to Primes.
     (Due to CPU loading some multi-modal attachments may be restricted)
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