HST Phase II Proposal Instructions for Cycle 12 (RPS2)
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| HST Phase II Proposal Instructions for Cycle 12 (RPS2) | |||
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Tables and Figures
Table 9.1: Supported Instrument Parameters for WFPC2
Table 9.2
Table 9.4: WFPC2 Supported Spectral Elements (Filters)
Table 9.5: Linked Spectral Element/Aperture Combinations for Wood's and Quadrant Filters
Figure 9.1 WFPC2 Aperture Coordinate System
9.1 Introduction to WFPC2
The Instrument Configurations and Operating Modes described in the following section are used to specify exposures on the RPS2 Visit and Exposure Specifications. The legal Visit and Exposure Specifications entries are summarized in the following sections. More complete descriptions of Instrument Configurations, Modes, Apertures, Spectral Elements, Detector Characteristics, etc. are available in the WFPC2 Instrument Handbook.
Note that many of the Optional Parameters have default values; in such cases, an entry for an Optional Parameter in the Visit and Exposure Specifications is necessary only if it is desired to override the default value. The physical units of Optional Parameter quantities are always implicit and should never be entered by the observer.
Optional Parameters whose descriptions begin with the phrase "USE WITH CAUTION" pose a risk to the intended science if a non-default value is used inappropriately. These parameters are italicized in Table 9.1. You may wish to discuss the use of these parameters with your Contact Scientist.
The following table lists the permitted Instrument Configurations, Operating Modes, Field of View, Spectral Elements, and Optional Parameters for the WFPC2.
Table 9.1: Supported Instrument Parameters for WFPC2
WFPC2 IMAGE WF2, WF3, WF4, WFALL, WF2-FIX, WF3-FIX, WF4-FIX, WFALL-FIX, PC1, PC1-FIX, LRF, F160BN15, FQUVN33, FQCH4N33, FQCH4N15, FQCH4P15, FQCH4W2, FQCH4W3, FQCH4W4, POLQN18, POLQN33, POLQP15P, POLQP15W Enter any Name from Table 9.4 ATD-GAIN, CR-SPLIT, CR-TOLERANCE, PRE-FLASH, SUM, CLOCKS, READ
The sections on the following pages provide further details of the entries to be made on the Visit and Exposure Specifications when a particular WFPC2 configuration is chosen.
9.2 Mode = IMAGE
Configuration = WFPC2The IMAGE Mode is the only one available to GOs with the WFPC2 configuration. The WFPC2 configuration provides the largest field of view of any instrument on HST. Three wide field CCDs (WF2-WF4) each provide a 800x800 pixel format with a pixel size of 0.10x0.10 arcsec at the center of each CCD. The planetary CCD (PC1) provides higher spatial resolution and a correspondingly smaller field of view in a 800x800 pixel format with a pixel size of 0.046x0.046 arcsec at the center of the CCD. Each CCD has an approximately 40-pixel vignetted region (i.e., a blank area not exposed to the sky) at its adjoining edge which corresponds to the facets of the pyramid mirror. The composite WFPC2 FOV is shown in Figure 9.1. See the WFPC2 Instrument Handbook for details.
9.2.1 Aperture or FOV
The specification of the FOV for the WFPC2 controls where the target is placed. The target will be placed at the default location (see below) within the specified FOV, unless the POSition TARGet exposure-level Special Requirement (see POSition TARGet <X-value>, <Y-value>) is used. The ORIENT visit-level Special Requirement (see ORIENTation <angle1> TO <angle2>) may be used to request a specific orientation of an extended scene in the WFPC2 FOV.
Two types of apertures are defined for each CCD. The first type is designed for placing targets at the "optimum center" of the CCDs, based on the current performance of the CCDs. These are identified as WF2, WF3, WF4, WFALL, and PC1. The default location within these apertures will be routinely adjusted by STScI to reflect any changes in CCD performance (e.g., new charge transfer traps, bad columns, etc.). These apertures should be used for targets which are small compared to the scale size of defects in the chips (which are on the order of 10 pixels).
The second set of apertures define the "geometric center" of the CCDs and will remain FIXED in aperture coordinates. They will NOT be adjusted for changes in CCD characteristics. These apertures are designated WF2-FIX through WF4-FIX, WFALL-FIX, and PC1-FIX. These apertures should be used to specify the location of the target relative to the CCDs. Pointings designed to position an extended scene within the WFPC2 FOV should normally be done with the "geometric center" set of apertures. At present WF3 is the only CCD for which the optimum and geometric centers differ.
The WFPC2 X,Y coordinate system defined in Figure 9.1 is to be used for the POS TARG Special Requirement.
Generally all four CCDs are read out even when only one CCD (e.g., WF2) is specified as the FOV. If data volume constraint problems are encountered during the planning and scheduling of your observations, you will be contacted by STScI to discuss subsetted CCD readouts.
Table 9.2: WFPC2 Apertures
PC1 Optimum Center of PC CCD1 WF2 Optimum Center of WFC CCD2 WF3 Optimum Center of WFC CCD3 WF4 Optimum Center of WFC CCD4 WFALL Same as WFALL-FIX (see WFPC2 Instrument Handbook) PC1-FIX Geometric center of PC CCD1 WF2-FIX Geometric center of WFC CCD2 WF3-FIX Geometric center of WFC CCD3 WF4-FIX Geometric center of WFC CCD4 WFALL-FIX Approximately 14 arcsec from apex of pyramid on WFC CCD3 LRF This aperture is required when the LRF filter is specified as the spectral element. The target position will be derived from the central wavelength specified in the Visit and Exposure Specifications. F160BN15, FQUVN33, FQCH4N33, FQCH4N15, FQCH4P15, FQCH4W2, FQCH4W3, FQCH4W4, POLQN18, POLQN33, POLQP15P, POLQP15W These apertures should be specified when a partially rotated spectral element of the same name is used. They are the optimum target positions based on the FOV/filter overlap and the response characteristics of the CCDs. See Table 9.5 for specific FOV/filter combinations which are supported. The positions of these apertures are shown in the WFPC2 Instrument Handbook. Note that only one partially-rotated filter may be specified as the spectral element for a given WFPC2 exposure.
9.2.2 Spectral Elements
The Spectral Elements for the WFPC2 are listed in Table 9.4. If a partially-rotated filter is chosen, only one such spectral element may be specified for a given WFPC2 exposure.
9.2.3 Wavelength
For the LRF filter specification, the desired central wavelength of the filter passband must be specified. STScI will translate the specified wavelength to a filter orientation and default target position. For details on the mapping from wavelength to target position, see the WFPC2 Instrument Handbook or the WWW Linear Ramp Filter Calculator at:
Note that use of the LRF filter will cause an error if you impose the SAME POSition AS <exposure> Special Requirement because the desired wavelength is achieved by positioning the source in the LRF, which may conflict with SAME POS.
For exposures using the quadrant transmission filters (FQUVN and FQCH4N), the central wavelength of the intended quadrant must be specified. It will be used as a consistency check on the aperture specified for the exposure.
9.2.4 Optional Parameters
ATD-GAIN
=7 (default); 15This parameter specifies the analog-to-digital gain of the CCD readout electronics in units of e-/DN. (See the WFPC2 Instrument Handbook for the exact gains of the readout electronics. ATD-GAIN=15 is really better approximated as 14 e-/DN, but for historical reasons the former number is used.)
CLOCKS
=NO (default); YESUSE WITH CAUTION - When taking exposures containing saturated pixels, the observer can minimize the amount of image "blooming" by leaving the serial clocks running (CLOCKS=YES). This procedure minimizes the amount of transference of charge from one column to the next through the readout channel. Exposures obtained with serial clocks (CLOCKS=YES) must be equal to or longer than 1 second. Use of CLOCKS=YES is not generally recommended. For a full discussion, see the WFPC2 Instrument Handbook.
CR-SPLIT
=DEF (default); range between 0.0 to 1.0; or NOAll WFPC2 external exposures longer than 10 minutes will automatically be split into two exposures to allow for easier identification of cosmic-ray events. The two exposures will be of approximately equal length; the maximum deviation from equal length can be controlled via the CR-TOLERANCE parameter. If the default method of splitting the exposure is not acceptable, CR-SPLIT should be used to set the fraction of the exposure time that should be used for the first subexposure. Note: If the derived exposure time does not equal one of the discrete times in Table , the next lower value in the table will be used. CR-SPLIT=NO must be entered if the exposure should not be split. The number of exposures should not count these subexposures as separate exposures, so 1 should generally be entered for the Number_of_Iterations keyword. If multiple iterations are specified, the total number of exposures will be 2 * Number_Of_Iterations.
CR-SPLIT values of 0.0 or 1.0 are not allowed.
CR-TOLERANCE
=0.2 (default); range from 0.0 to 1.0CR-SPLIT exposures are split into two parts whose default lengths are nominally equal. Scheduling efficiency can sometimes be increased if the split is adjusted according to the scheduling situation. This parameter defines the amount of adjustment that the observer wishes to allow in the exposure ratio defined by CR-SPLIT. For example, the default value, 0.2, allows a CR-SPLIT value of 0.5 to vary between 0.3 and 0.7.
READ
=YES (default); NO
USE WITH CAUTION - The default is to read out all four CCDs for each exposure. Multiple exposures of a target on different parts of the same CCD image can be made without reading out each exposure (READ=NO). A different target position may be defined for each exposure by using the POSition TARGet exposure-level Special Requirement on subsequent exposures. The SEQuential NON-INT exposure-level Special Requirement must be included to link the exposures, minimizing cosmic-ray and dark-count backgrounds and the final exposure must use READ=YES.SUM
=1X1 (default); 2X2The default is to read out each individual pixel (1X1) resulting in a 1600x1600 format for the four CCDs (800x800 per CCD). A 2X2 pixel on-chip summation may be used, resulting in an 800x800 format for the four CCDs (400x400 per CCD). This reduces spatial resolution by a factor of two and increases signal-to-noise for cases where CCD readout noise is a large contribution to the error budget.
9.2.5 Number of Iterations
The value entered is the number of times this Visit and Exposure Specifications line should be iterated. There are many observational situations when two or more exposures should be taken of the same field (e.g., to keep a bright object from "blooming" by keeping the exposure time short). See the WFPC2 Instrument Handbook for details.
9.2.6 Time Per Exposure
The value entered is the duration of the exposure. Although the possible exposure times for the WFPC2 are quantized (see the WFPC2 Instrument Handbook), an observer may enter any value for the exposure time. However, if that value does not equal one of the discrete times in Table , the next lower value in the table will be used.
Since readout noise is present in the WFPC2, exposures are not split for scheduling reasons except as needed for cosmic-ray removal as described under CR-SPLIT. Remember that if CR-SPLIT is used, the total exposure time will actually be apportioned among two shorter exposures.
9.3 Tabular Reference Data
1
Table 9.4: WFPC2 Supported Spectral Elements (Filters)
Table 9.3: Valid WFPC2 Exposure Times (seconds) 0.111, 2 0.121, 2 0.141, 2 0.161, 2 0.181, 2 0.201, 2 0.232 0.262 0.302 0.352 0.402 0.502 0.602 0.702 0.802 1.00 1.202 1.402 1.602 1.802 2.00 2.302 2.602 3.00 3.502 4.00 5.00 6.00 7.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 23.00 26.00 30.00 35.00 40.00 50.00 60.00 70.00 80.00 100.00 120.00 140.00 160.00 180.00 200.00 230.00 260.00 300.00 350.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00 1100.00 1200.00 1300.00 1400.00 1500.00 1600.00 1700.00 1800.00 1900.00 2000.00 2100.00 2200.00 2300.00 2400.00 2500.00 2600.00 2700.00 2800.00 2900.00 3000.00 3100.00 3200.00 3300.00 3400.00 3500.00 3600.00 3700.00 3800.00 3900.00 4000.00 4100.00 4200.00 4300.00 4400.00 4500.00 4600.00 4700.00 4800.00 4900.00 5000.00 5100.00 5200.00 5300.00 5400.00 5500.00 5600.00 5800.00 6000.00 6200.00 6400.00 6600.00 6800.00 7000.00 7500.00 8000.00 8500.00 9000.00 10000.00
1These very short exposure times are affected by shutter movement; see the WFPC2 Instrument Handbook.
2These exposure times should not be used with CLOCKS=YES; see the WFPC2 Instrument Handbook.
Table 9.5: Linked Spectral Element/Aperture Combinations for Wood's and Quadrant Filters
9.4 WFPC2 Aperture Coordinate System
Figure 9.1: WFPC2 Aperture Coordinate System
Coordinate system is onto the sky for the POSition TARGet Special Requirement. The short lines indicate the blooming directions of the CCDs. Units are seconds of arc. See POSition TARGet <X-value>, <Y-value> for details.
Note that the POS TARG X- and Y-axes are not the same as the U2- and U3-axes, and that POS TARG axes are associated with each aperture selected (i.e., there is a POS TARG coordinate frame centered on each aperture).
1Exposure times below the heavy line will automatically be split into two exposures unless CR-SPLIT=NO.
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