Fine Guidance Sensor Instrument Handbook for Cycle 14

Chapter 6: 
Writing a Phase II Proposal

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6.1 Phase II Proposals: Introduction
    6.1.1 Required Information
    6.1.2 STScI Resources for Phase II Proposal Preparation
6.2 Instrument Configuration
    6.2.1 Optional Parameters for FGS Exposures
6.3 Special Requirements
    6.3.1 Visit-Level Special Requirements
    6.3.2 Exposure-Level Special Requirements
6.4 Overheads
    6.4.1 Pos Mode Overheads
    6.4.2 Trans Mode Overhead
6.5 Proposal Logsheet Examples
    6.5.1 Parallax program using FGS1r in Pos mode
    6.5.2 Determining a binary's orbital elements using FGS1r in Trans mode
    6.5.3 Mass determination: Trans and Pos mode
    6.5.4 Faint binary with Trans mode: special background measurements

This chapter is a practical guide to the preparation of Phase II proposals, and as such is relevant to those researchers who have been allocated HST observing time. For details or more general information, we refer the reader to the HST Call for Proposals and Phase II Proposal Instructions.

6.1 Phase II Proposals: Introduction

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6.1.1 Required Information

The information required for Phase II proposals are:

• Precise target information: positions accurate to ± 3, and V magnitude accurate to ± 0.5 magnitudes. If possible, the (B - V) color should be specified when know. The color is important for archival considerations as well to help the FGS Instrument Scientist determine if the standard calibrations support the needs of the proposal.
• Configuration and scheduling instructions for each exposure and visit, expressed in the specified proposal syntax.
• The relationship between the visits: scheduling, relative orientations, etc.
• Justification for special scheduling or configuration requirements.

Important: A clear explanation for: 1) the use of special requirements; 2) the special design of a program; or 3) the calculation of an exposure time, orient angle, or target offset, etc., will help STScI during the review and verification process. Text can be included in the "Additional Comments" section of the proposal or in the justification for real-time and special requirements. If an action is critical for the successful implementation of an exposure line, a short one-line comment (in the comments syntax) should be placed on individual exposure lines.

6.1.2 STScI Resources for Phase II Proposal Preparation

• APT: The Astronomer's Proposal Tool is the STScI interactive graphical software interface for both Phase I and Phase II proposal preparation. It provides proposal preparation tools and several verification and error-checking phases: proposal syntax, operational feasibility, and schedulability. APT also will offer an option to prepare your Phase II program in a text format whose syntax is similar the previous STScI proposal preparation system, RPS2. Examples of proposals in this syntax are provided in Section 6.5, "Proposal Logsheet Examples," on page 99. You are strongly encouraged to make full use of the APT software. It is available on the World Wide Web at:

 http://apt.stsci.edu/

• Program Status: For proposal status, scheduling (visit level) information, weekly calendars, recent proposal listings, please consult the Hubble Space Telescope web pages at:

 http://www.stsci.edu/hst/

• Contact Scientist: After HST observing time has been allocated by the TAC, a Contact Scientist may be assigned to the proposal. The objective of the Contact Scientist is to ensure the scientific return of the observing program is maximized.

6.2 Instrument Configuration

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This section is designed to complement the section on FGS instrument parameters contained in the Phase II Proposal Instructions. Table 6.1 summarizes the FGS instrument parameters.

Table 6.1: FGS Instrument Parameters

Config	Aperture	Sp_Element	Opmode	Optional_Parameters
FGS	1, 2, 3 (FGS1=FGS1r)	F5ND F550W F583W F605W(FGS1r,FGS3) F650W (FGS 2) PUPIL	POS	ACQ-DIST, ACQ-MODE, COUNT, FESTIME, NULL, LOCK
			TRANS	ACQ-DIST, ACQ-MODE, SCANS, STEP-SIZE

Aperture

The aperture must be defined as either "1", "2", or "3". Only FGS1r (aperture=1) is calibrated as a science instrument. FGS2 and FGS3 should not be used for science observing, For illustrations of the FGS1r field of view in the HST focal plane, refer to Chapter 1.

Spectral Element

"Spectral Element" refers to filters, etc. chosen for the observation. For FGS1r, the available filters are: F583W, F5ND, PUPIL, F605W, and F550W. Only one filter can be in place for an exposure, though multiple filters can be used during a visit (check that calibrations are available for the configuration). Recommendations for specific mode and filter combinations are discussed in Chapters 5 and 7. Table 6.2 presents a summary of the calibrated modes and filter configurations.

Table 6.2: Summary of Calibrated Mode and Filter Combination

Mode	Filter	Status	Comments
Pos mode:	F583W	Calibrated across entire FOV	OFAD in F583W only
Pos mode	F5ND	Calibrated at the center and a small number of selected locations in the FOV	Cross-filter calibration with F583W
Trans mode:	F583W, F5ND; Others as needed	All calibrations at FOV center Select reference S-Curves from library of standard stars If needed, propose for calibration time to observe a reference standard star at the same epoch as the science visit (see Chapter 7).	Availability of standard stars with appropriate spectral energy distributions, i.e., colors of reference star and science target should differ by no more than (B-V) ~ 0.3 for a given filter (F583W or F5ND).

Mode

Two operation modes are available: Position mode (POS) for measuring relative astrometric positions of targets in the FGS FOV and Transfer mode (TRANS) for obtaining high angular resolution interferometry.

6.2.1 Optional Parameters for FGS Exposures

A program is customized by specifying optional parameters for each exposure. Table 6.3 and Table 6.4 list the optional parameters, default values, and recommended settings for Pos and Trans modes, respectively.

Many of the optional parameters have default values. In such cases, the entry for that optional parameters may be omitted. If an observer wishes to override the default value, the parameter and its value must be specified.

Table 6.3: Pos Mode Optional Parameters  

Optional Parameter	Allowed Values	Comments and Recommendations
ACQ-DIST	DEF (default=10), 0.0 to 90.0 Units: arcsec	Determines the size (in arcseconds) of the search region. For ACQ-MODE=SEARCH, ACQ-DIST is the maximum radius of the search spiral. Recommendation: Although Search radii up to 90 arcsec are allowed, larger values must be used with caution, i.e., the larger the search radius, the more potential to acquire a spurious nearby star. If the target coordinates are expressed with respect to the Guide Star System, the default value of 10 arcsec (radius) should be sufficient to find the target. Typically the target is found with 0.3 to 1.0 arcsec. Note that large search radii incur a large overhead (time) cost. Reference: Appendix 1: Target Acquisition
ACQ-MODE	SEARCH (default) Units: None	Determines the strategy for locating the target. The Search Phase is the initial outward spiral to find and identify the target. The sequence of Search, CoarseTrack, WalkDown, and FineLock is the standard astrometry target acquisition scenario. No other options are available for GO programs. Requirement: Default "SEARCH" Reference: Appendix 1: Target Acquisition
COUNT	DEF (default), 1-2621400 Units: FGS PMT counts	Expected count rate [target + background]. When the default is set, the count rate is calculated (by the proposal processing software) from the target V magnitude, filter, and aperture. The default background rate (from in-orbit measurements) is 712 counts/sec for all four PMTs. Recommendation: Allow the system to select the default based upon the target's specified visual magnitude. The default values are calculated using data from on-orbit photometric calibrations. The value of COUNTS is also used to determine the default FESTIME. A few special cases may require non-default values and are discussed in the text. Reference: Chapter 7: See sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.
FESTIME	DEF (default), 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 Units: seconds	Sets the averaging time for the Fine Error Signal (in seconds). The FESTIME also determines the rate at which the star selectors are adjusted to null the fine error signal. The default value for Pos mode is calculated from the COUNT parameter if explicitly entered, or if COUNT defaults to the V magnitude and the filter. All Trans mode observations use FESTIME = 0.025 seconds. Recommendation: The algorithm that selects the default FESTIMEs from the specified brightness of the target is based on photon statistics and a conversion from photon noise to equivalent positional error. For fainter targets, longer integration times are supplied by the default algorithm to assure that the FGS can track the fringe and remain in FineLock. For non-point sources, the S-Curve amplitude is reduced because of the increased contribution of the background or companion. In these cases, it may be desirable to adjust the FESTIME. Please consult with STScI. Reference: Chapter 7: See Sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.
LOCK	FINE (default), COARSE, 0.0-1.0 Units: None	The method used by the astrometer to track the science or reference target. (Note, this parameter does not specify the HST guiding mode). Because of the excessive wear to bearings inside the instrument, CoarseLock, which is inferior to FineLock, is no longer allowed. Requirement: Fine. Reference: Chapter 5 and Chapter 7.
NULL	YES, NO (default) Units: None	Determines whether the next FESTIME begins immediately after the previous one or not until the star selectors have been repositioned. This parameter may be applicable for very fast moving target. Observers should consult with STScI before specifying non-default values. Recommendation: This use of this parameter is limited to a few specific observation scenarios (tracking solar system objects). Most standard astrometry programs benefit from NULL=NO. Reference: STScI will alert the proposer if a non-default value is required.

Table 6.4: Trans Mode Optional Parameters  

Optional Parameter	Allowed Values	Comments and Recommendations
ACQ-DIST	DEF (default = 10), 0.0 - 90.0 Units: arcsec	Determines the size (in arcseconds) of the search region. For ACQ-MODE=SEARCH, ACQ-DIST is the maximum radius of the search spiral. Recommendation: Although a search radius of up to 90 arcsec is allowed, larger values must be used with caution, i.e., the larger the search radius, the more potential to acquire a spurious nearby star (spoiler). If the target coordinates are expressed with respect to the Guide Star System, the default value of 10 arcsec (radius) should be sufficient to find the target. HST's pointing accuracy usually places the target within 1 arcsec of the expected location in the FGS. Note that large search radii incur a large overhead (time) cost. Reference: Appendix 1: Target Acquisition
ACQ-MODE	SEARCH (default) Units: arcsec	Determines the strategy for locating the target. The search phase is the initial outward spiral to find and identify the target. The sequence of Search, CoarseTrack, WalkDown, and FineLock is the standard astrometry target acquisition scenario. No other options are available. (Trans mode fringe scanning commences after the FineLock acquisition.) Requirement: Default "SEARCH" Reference: Appendix 1: Target Acquisition
SCANS	1 (default)-200 Units: None	Determines the number of separate scan lines. Successive scans are taken in opposite directions along a 45 degree diagonal path in the FGS detector space coordinates. Recommendation: Determine the number of scans to achieve the required signal-to-noise. Reference: Chapter 7
STEP-SIZE	1.0 (default) 0.3-10.0 Units: mas	The average angular distance (in mas) stepped by the IFOV along an interferometric axis in 0.025 seconds. To achieve the best results, step sizes of 0.3 or 1 mas should generally be selected. Recommendation: Value depends on the science target and goals of the observation, required resolution and signal-to-noise. Reference: Chapter 7

6.3 Special Requirements

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In this section, we describe several special requirements which are often needed in an FGS Phase II observing proposal.

6.3.1 Visit-Level Special Requirements

Many visit-level special requirements are outlined in the Phase II Proposal Instructions. Those most applicable to FGS programs are:

Spacecraft Orientation ORIENT:

• Definition: The angle from North to the +U3 axis of the HST measured in the direction of +U2 (counter clockwise when North is up and East is to the left). The ORIENT special requirement is useful for orienting HST so that astrometric reference stars fall within the FGS FOV or for aligning a binary star's position angle relative to an FGS axis. Along with the ORIENT angle, a tolerance must be specified.
• Calculation: Two angles must be known in order to calculate a special ORIENT angle: the angle from North to the eigenaxis of the target or target field (measured in the direction of East), and the angle between the +U3 axis to the FGS +Y_POSTARG axis measured in the direction of +U2. (Note that the values are different for FGS1r and FGS3.)
• Accuracy: The HST roll angle precision depends on the relative guide star position errors and the FGS alignment calibration errors (when two guide star FineLock is used). The pre-designated roll angle for a two-guide star FineLock tracking will be accurate to  0.04 degrees.
• Recommendations when ORIENT is used in the proposal:
  • Explain, in the "Additional_Comments" text section of the proposal, the method used to calculate ORIENT so that STScI schedulers and instrument scientists understand (and can defend the requirement.
  • ORIENT is considered a Special Scheduling Request and as such, must be justified in the proposal and will affect the schedulability of the visit. Setting the ORIENT tolerance to as large a range as possible (and still be within the bounds of the scientific requirements) will help to lessen the scheduling impact.

Timing Requirements:

• Definition: Timing links between visits are fairly common for FGS observations. For parallax programs, timing requirements are generally invoked so that a target field is observed at times of maximum parallax factor (every six months). Trans mode observations may also make use of timing requirements, such as observing a binary system at specific orbital phases. Timing links on the Visit level include: BEFORE, AFTER, BETWEEN [dates], GROUP [the following visits] WITHIN [xxx hours, or xxx orbits], etc. See the Phase II Proposal Instructions for the complete list.
• Note: Timing requirements place restraints on the schedulability of the visit. Specify the largest tolerance on the timing constraint that the science can accommodate.

6.3.2 Exposure-Level Special Requirements

All available exposure-level special requirements are described in detail in the Phase II Proposal Instructions. Of these, the following three types are most often used in connection with FGS observations.

POS TARG:

• Definition: The X and Y offset of a target from the standard aperture reference position are the POS TARG coordinates. They are specified in arcseconds and with respect to special coordinate systems which are illustrated and described in Chapter 2 and in the Phase II Proposal Instructions. (Note, the POS TAR reference frame is not in the same coordinate system as the FGS detect reference frame.) POS TARG is used to position a target at various points in the FGS FOV. For example, a common use in Pos mode observations is to place the science target at a position in the FOV such that reference stars will also be within the FOV. In Trans mode, since calibrations are only available at the center reference point, the non default (0,0) use of POS TARG is not recommended.

SAME POS AS:

• Definition: The position and orientation of the spacecraft will be held constant over the course of all exposures within a given visit when the special requirement SAME POS AS is invoked. This requirement is virtually always used for Pos mode visits.

SEQ NON-INT:

• Definition: To ensure that all exposures within a visit are scheduled in the same orbit, the SEQ NON-INT special requirement should be used, otherwise the system may divide the exposures over several orbits, requiring guide star re-acquisitions and incurring instrumental overheads taking up to 12 minutes, as well as exceeding the number of orbits awarded by the TAC to the proposal.

Do not hesitate to explain the use of any special requirements in the proposal text. The more explanation, the easier for STScI to understand your requirements and schedule the proposal

6.4 Overheads

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The assessment of the total spacecraft time needed by a program should take into account the overheads associated with various types of observations. Overheads are incorporated into the APT proposal processing algorithm. They are explicitly pointed out here for illustration.

6.4.1 Pos Mode Overheads

Table 6.5 lists the estimated overheads associated with guide star acquisitions, the initial configuring (standby to operate) of the FGS for science observing, the time for slewing the IFOV from target to target in the observing plan, the acquisition of target stars, and finally configuring the FGS at the end of the observing sequence (operate to standby).

The acquisition time of an astrometric target depends upon the target's magnitude. Fainter stars require longer intergration times (FESTIME) of the fine error signal during the walkdown to finelock. For example, the FESTIME used to acquire a V=10 star is 25 msec, while for a V=16 star FESTIME=3.2 seconds. Columns 2 and 3 in Table 6.6 list an average overhead for each exposure and the recommended total exposure time to be entered on the phase2 proposal (see Table 4.5) as a function of target magnitude. Total time required for a given exposure is estimated by combining columns 2 and 3. The additional overheads indicated in Table 6.5 must be included when determining the total time available for collecting science data. Fortunately, APT calculates and includes all of these overheads when processing a phase2 proposal.

Table 6.5: Pos Mode Overheads

Activity Description	Duration	Comments
Initial guide star acquisition	6 min	Two-guide star FineLock (5 min. for re-Acq)
Initial instrument configuration	4 min.	Incurred at the beginning of every orbit
IFOV slew to target	17 sec + 6 x dist (in arcmin)	for slews between targets in the FOV.
Astrometry target acquisition: Search CoarseTrack FineLock	5 sec 13, 21 sec varies as V	(Each science target and reference star) For a 10 arcsec search radius. For V < 14 and V > 14 respectively For V<12: ~3 sec; For V>16: ~400 sec
Instrument shut down	2 min.	Incurred at the end of every orbit

Table 6.6: Pos Mode Overheads and Exposure Time vs. Magnitude

V Magnitude	Estimated Overhead per exposure (minutes)	Exposure Time1 in seconds
8-12	0.6	20
13	0.7	20
14	2	30
15	3	45
16	8	45
17	8	50

1For total spacecraft time, add columns 2 and 3.

6.4.2 Trans Mode Overhead

The overheads for the activities which occur during Trans mode exposures are listed in Table 6.7. The guide star acquisitions, instrumental configurations at the beginning and end of the observing sequence, and astrometry target acquisition activities for Pos mode and Trans mode are identical through the CoarseTrack phase. The FineLock phase in Transfer mode does not depend upon the star's magnitude. (The star is not really acquired in FineLock. Setting the FineLock flag allows for the transition to fringe scanning under the control of the 486 SSM computer.)

Table 6.7: Trans Mode Observing Overheads

Activity Description	Duration	Comments
Initial guide star acquisition	6 min	Two-guide star FineLock (5 min. for re-Acq)
Initial instrument configuration	4 min.	Incurred at the beginning of every orbit
Astrometry target acquisition: Search CoarseTrack FineLock	5 sec 13, 21 sec 1 sec	(Each science target and reference star) For a 10 arcsec search radius. For V < 14 and V > 14 respectively FGS transitions to scanning from FineLock
Time for IFOV to slow and reverse scan direction	12 sec/scan	This overhead is incurred for each scan, regardless of the scan length or step_size.
Instrument shut down	2 min.	Incurred at the end orbit

6.5 Proposal Logsheet Examples

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This section provides several examples of RPS2 formatted proposal logsheets for the following types of investigations:

• Parallax program using FGS1r in Pos mode
• Determining a binary's orbital elements using FGS1r in Trans mode.
• Mass Determination: Trans and Pos mode
• Faint binary with Trans mode: special background measurements.

6.5.1 Parallax program using FGS1r in Pos mode

• Goal: Determine the parallax and proper motion of star, "OGF_1".
• Scenario:
  • The program requires three visits, at 0, 6, 12 months intervals, each of which is timed to occur at epochs of maximum parallax factors. During each visit the science target and reference stars will be observed multiple times in Pos mode.
  • A special orientation is needed to align the FGS FOV so that the target and selected reference stars fall within the FOV (and hence are observable!). The appropriate Orient range can be determined by using special tools (e.g., PICKLES or VTT)
  • Filter for all targets with V > 8: F583W.
  • Reference Star Geometry: Six reference stars near science target; POS TARG required to fit all reference stars in the pickle. See Table 6.1.
• Special Considerations:
  • The need for special orientation must be explained in the text justification section.
  • The science target must be shifted from FGS1r aperture reference point (center of FOV) in order to fit all (i.e., OGF-21-REF) stars in the FOV. Appropriate POS TARG can be determined from PICKLES or VTT. Contact STScI Help desk for assistance if necessary.
• Target Logsheet:
  • See Proposal Instructions for assistance.
  • Please include color information on all targets if available so the need for color-related calibrations can be assessed by STScI.

Figure 6.1 illustrates the field geometry and the orient angles of a typical FGS observation, while Table 6.8 summarizes the details of the exposures in the visit.

Figure 6.1: Example 1: Field of View at Special Orientation

Table 6.8: Target and Exposure Input for Example 1

Target	V	FILTER	FESTIME (default)	Exposure Time (seconds)	~NEA for Single Exposure (mas)
"OGF_1"a	9	F583W	0.025	20	< 1.0
OGF-3-REFa	11.1	F583W	0.025	20	< 1.0
OGF-7-REF1	10.3	F583W	0.025	20	< 1.0
OGF-9-REF	10.2	F583W	0.025	20	< 1.0
OGF-11-REFa	13.3	F583W	0.050	30	3.1
OGF-12-REF	13.6	F583W	0.050	30	3.5
OGF-21-REF	11.5	F583W	0.025	20	1.0

1The "REF" appendix is not required. Rather its use is suggested simply for book keeping purposes.

 

• Reference/Check Star Pattern: Check Stars in this observation are REF-7, REF-11, and OGF_1. Observing sequence:

OGF_1--> 12 -- 11 -- 7 -- OGF_1 -- 11 -- 21 -- 3 -- 7 -- OGF_1

-----> 9 -- 7 -- 11 -- OGF_1 -- 21 -- 12 -- 11 -- 9 -- OGF_1

• The Exposure Logsheet Template: The Optional_Parameters entry is missing since, for this example, all Pos mode optional parameters are DEFAULT values

 Visit_Number: 1
 Visit_Requirements: BETWEEN 14-MAR-1998 AND 15-MAR-1998
 ORIENT -18.5D TO -19.5D FROM NOMINAL 
 Visit_Comments: Two Guide Star FineLock Required.Exposure_Number: 01 
          Target_Name: OGF_1
               Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
    Time_Per_Exposure: 20S
 Special_Requirements: SEQ 1-4 NON-INT;POS TARG -33,-60.
 
      Exposure_Number: 02 
          Target_Name: OGF-12-REF
               Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
    Time_Per_Exposure: 30S
 Special_Requirements: SAME POS AS 1; 
 
      Exposure_Number: 03 
          Target_Name: OGF-11-REF
               Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
    Time_Per_Exposure: 30S
 Special_Requirements: SAME POS AS 1;
 Comments: 
 
      Exposure_Number: 04 
          Target_Name: OGF-7-REF
               Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
    Time_Per_Exposure: 20S
 Special_Requirements: SAME POS AS 1; 
 Comments: 
 

...and continues until the orbit is filled.

6-months after Visit 1, the HST Nominal Orient Angle is 180° from the Nominal HST Orientation in Visit 1. Nominal orient angle = 247°, off-nominal orient angle = 228°, pos targ = (+60,+33).

 Visit_Number: 2
 Visit_Requirements: BETWEEN 14-SEP-1998 AND 15-SEP-1998;
                     ORIENT -18.5D TO -19.5D FROM NOMINAL 
 Visit_Comments: Two Guide Star FineLock Required. 
 Exposure_Number: 01 
          Target_Name: OGF_1
               Config: FGS
               Opmode: POS
             Aperture: PRIME
           Sp_Element: F583W
    Time_Per_Exposure: 20S
 Special_Requirements: SEQ 1-4 NON-INT;POS TARG +33,+60
 Comments: 
      Exposure_Number: 02 
          Target_Name: OGF-12-REF
               Config: FGS
 

...Continues until the orbit is filled. Visit 3 is identical to Visit 1 except that it executes 1 year later.

Running the proposal template file through APT will inform the observer whether the syntax is correct, whether the exposures fit in an orbit and how much time is left, and finally whether the observing dates are viable for the requested ORIENT angle.

6.5.2 Determining a binary's orbital elements using FGS1r in Trans mode

• Goal: Observe a known faint (V=16) close (separation ~40 mas) binary in Trans mode in order to determine the system's relative orbit from which the orbital element, and hence, the system's mass can be derived.
• Scenario:
  • Program requires 4 single orbit visits to observe the binary at different orbital phases. These shall be timed accordingly.
  • Target is faint enough (V>8) for filter F583W.
  • The expected separation of binary is ~40 mas. Its position angle is expected to be approximately 180 deg at the time of the first planned observation. This will have implications for the ORIENT of HST for the visit (see below. Note that special orients also imply scheduling constraints. Use of APT will facilitate resolving these constraints.)
• Special Considerations:
  • Include B-V color information. STScI will use such information to plan the calibration observations of the color reference standard stars.
  • An ORIENT range is used to avoid the binary's projected separation along either of the FGS axis to be less than 20 mas (to assure an accurate determination of the system's total projected separation and position angle.)
• Exposure Time Calculation
  • target magnitude = 16.0, use F583W
  • ScanLength: 0.5 arcsec.
  • StepSize: 0.6 mas.
  • target visibility period: 58 Minutes.
  • guide star acq and instrument overheads: ~12 min.
  • available observing time: ~ 46 min.
  • time (sec) per scan = 0.025 * [ScanLength/StepSize] + 12 = 33. (note, the 12 sec is the per scan overhead.)
  • the number of scans that can fit in within this observing window is Nscans = 46*60/33 = 83
  • exposure time = Nscans*0.025*[ScanLength/StepSize]=1730 sec. (note the scan's overheads are not included in the user specified exposure time)

Figure 6.2 illustrates the geometry of the binary with respect to the POS TARG and detector reference frames.

Figure 6.2: Example 2: Field of View at Special Orientation


 

• The Exposure Logsheet Template:

 Visit_Number: 1
 Visit_Requirements: ORIENT 46.6D TO 48.6D
 Visit_Comments: Two Guide Star FineLock Required. Please notify Contact Scientist if two 
guide stars cannot be found.
 Exposure_Number: 01 
          Target_Name: Close_Binary
               Config: FGS
               Opmode: TRANS
             Aperture: 1
           Sp_Element: F583W
 Optional_Parameters: SCANS=83, STEP-SIZE=0.6
    Time_Per_Exposure: 1729S
 Special_Requirements: 
 Comment: 
 Visit_Number: 2
 Visit_Requirements: AFTER 1 BY 30D TO 32D 
                     ORIENT -24D TO -25D FROM NOMINAL 
 Visit_Comments: Two Guide Star FineLock Required. 
 Exposure_Number: 01 
          Target_Name: Close_Binary
               Config: FGS
               Opmode: TRANS
             Aperture: 1
           Sp_Element: F583W
 Optional_Parameters: SCANS=83, STEP-SIZE=0.6
 Number_of_Iterations: 1
    Time_Per_Exposure: 1729S
 Special_Requirements: 
 

And similar Visits for 3 and 4.

Running the proposal template file through APT will inform the observer whether the syntax is correct, whether the exposures fit in an orbit and how much time is left if they do not; and finally, whether the observing dates are viable for the requested ORIENT angle.

6.5.3 Mass determination: Trans and Pos mode

• Goal: A bright (V=13), wide binary system (separation = 0.4") is to be observed with FGS1r. Pos mode observations of the binary and neighboring reference stars will be used to determine the binary's parallax, proper motion, and barycenter. Trans mode observations will be used to establish the system's relative orbit so that its orbital elements can be derived. Combining these data from several epochs will allow for a dynamical mass determination of the system's components.
• Scenario:
  • program requires two visits every six months for three years.
  • reference star distribution: six reference stars near science target; science target must be placed at aperture reference position for Trans mode observations.
• Special Considerations:
  • include B-V color information on all targets, if available, so STScI can determine the need for specific calibrations.
  • target magnitude = 13.0, use F583W
  • ScanLength: 1.6 arcsec.
  • StepSize: 0.6mas.
  • target visibility: 63 minutes.
  • total Trans mode exposure time: 28 minutes.
  • exposure time calculation:

T_exposure = 0.025 * [ScanLength/StepSize] * N_scans

where in this example ScanLength = 1.6 arcsec, StepSize = 0.0006 arcsec, and the number of scans N_scans = 25

Figure 6.3 illustrates the geometry of the target and reference stars, while Table 6.9 and Table 6.10 summarize the details of the exposures in the visit.

Figure 6.3: Example 3: Trans + Pos Mode Visits

Table 6.9: Target and Exposure Input for Example 3

Target	V	FESTIME (use defaults)	FILTER	Total Exposure	~NEA for one exposure (mas)
Wide_Binary1 Pos mode	13.0	0.050	F583W	20 sec	2.0
Wide_binary Trans mode	13.0	0.025	F583W	28 min	Sx = 0.008
BL-1-REF2	10.8	0.025	F583W	20 sec	<1.0
BL-2-REF	14.1	0.100	F583W	35 sec	5.0
BL-3-REF	11.5	0.025	F583W	20 sec	1.0
BL-4-REF	12.5	0.025	F583W	20 sec	1.7
BL-5-REF	13.0	0.050	F583W	20 sec	2.5

1The primary target needs only be listed once in the fixed target list. 2The "REF" appendix is not required. Rather its use is suggest simply for book keeping purposes.

 

Table 6.10: Reference/Check Star Pattern

Mode	Overhead + Exposure Time (Minutes)
GS ACQ + Instrumental Overheads.	12
Trans mode Wide_Binary	28
Pos mode: Wide_Binary BL-3-REF BL-2-REF BL-1-REF Wide_Binary BL-3-REF BL-4-REF Wide_Binary	2 1 2.5 1 2 1 1 2

6.5.4 Faint binary with Trans mode: special background measurements

• Goal: A faint (V=15) object suspected to be a close (sep < 30 mas) binary is embedded in nebulosity which contributes about 20% of the light in the vicinity of the target. The putative binary is to be observed in Trans mode. Special additional observations are needed accurately measure the background. To do so, the RA,Dec of two points on the sky about 1 arcmin from the target and to either side of it are specified as "targets" in the proposal. These "targets" are to be observed in Pos mode after the Trans mode observation of the faint science target (the FGS high voltage, which enable the PMTs to count photons, does not get turned on until after the IFOV is placed at the expected location of the first target to be observed in the visit. The background data is to be acquired as the IFOV slews away from the science object to the reference "targets". During this time the PMTs record the background counts.)
• Scenario:
  • Program requires a single one orbit visit.
  • Target Filter: F583W.
  • Reference Star Filter: F583W.
  • Geometry: Science target is placed at center of the FGS1r FOV; the background data are obtained en route to and while observing the reference points (specified in the proposal as a pointed targets) in Pos mode for 1 minute.
• Exposure Time Calculations:
  • target magnitude is 15, use F583W
  • target visibility period: 54 minutes.
  • GS acq and instrumental setup time: 12 min.
  • available observing time: 42 min.
  • ScanLength: 0.5 arcsec.
  • StepSize: 0.6 mas.
  • time per scan = 0.025 * (0.5/0.0006) + 12 = 33 sec
  • time for two Pos mode observations (include overheads): 3 min.
  • approximate number of scans that can fit within orbit: 69

Figure 6.4 illustrates a possible geometry of the field, while Table 6.11 summarizes the total exposure times for the target and reference star, and dark sky observations.

Figure 6.4: Example 4: Trans + Pos Mode Exposures


 

Table 6.11: Target and Exposure Input for Example 4

Target	V	FILTER	Total Exposure
HDFAINT Trans mode	15	F583W	39 min.
Dark_Sky1 Pos mode	10	F583W	1 min.
Dark_Sky2 Pos mode	12	F583W	30 S

Specify the background targets to be bright (V=8) to minimize target acquisition overheads. Note: these targets will fail to be acquired in FineLock since no coherent source is present. However, the desired photometry will be recorded.

• The Exposure Logsheet Template:

 Visit_Number: 1
 Visit_Requirements: 
 Visit_Comments: 
 
 Exposure_Number: 01
          Target_Name: HDFAINT
               Config: FGS
               Opmode: TRANS
             Aperture: 1
           Sp_Element: F583W
 Optional_Parameters: SCANS=69, STEP-SIZE=0.6
    Time_Per_Exposure: 1380S
 Special_Requirements: POS TARG = 0,0 SEQ 1-3 NON-INT
 
 Exposure_Number: 02 
          Target_Name: DARK_SKY1
               Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
 Optional_Parameters: 
    Time_Per_Exposure: 60 S
 Special_Requirements: SAME POS AS 1
 
 Exposure_Number: 03
          Target_Name: DARK_SKY2
          Config: FGS
               Opmode: POS
             Aperture: 1
           Sp_Element: F583W
 Optional_Parameters: 
    Time_Per_Exposure: 60 S
 Special_Requirements: SAME POS AS 01
 

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