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 13.1: Supported Instrument Parameters for NICMOS
Table 13.2: Predefined Sample Sequences for MULTIACCUM Mode (seconds)
Table 13.3: Quantized Legal NICMOS TPG Expose Times
Table 13.4: NICMOS Apertures
Table 13.5: Spectral Elements for the NICMOS
Figure 13.1 NICMOS Coordinate System.
Figure 13.2 Definition of Orientation for NICMOS.
Figure 13.3 Guide-Star Availability with NICMOS Patterns.
13.1 Introduction to NICMOS
There are three cameras available for use on the NICMOS. The choice of camera will be dictated by the desired filter and field of view.
- Camera 1 (NIC1) provides the highest available spatial resolution with an 11x11 arcsec field of view and pixels that subtend 0.043 arcsec on a side. NIC1 will fully sample a diffraction-limited image at wavelengths 1.0 microns or greater.
- Camera 2 (NIC2) provides intermediate spatial resolution with a 19.2x 19.2 arcsec field of view and 0.075 arcsec pixels. NIC2 will fully sample a diffraction-limited image for wavelengths 1.75 microns or greater. NIC2 contains a coronographic hole and must be selected for all observations that require use of this feature.
- Camera 3 (NIC3) provides a large field of view, 51.2x51.2 arcsec, with 0.200 arcsec pixels. NIC3 will undersample the Point-Spread Function at all wavelengths, though some recovery of information may be possible by employing dithering techniques. NIC3 must be specified for all observations that use the grism Spectral Elements (see Table 13.5: Spectral Elements for the NICMOS).
Note: Target flux for NICMOS observations must be given in units of erg/(cm^2 sec Ĺ) (see 3.9 "Flux Data [Flux]" or 4.8 "Flux Data [Flux]"). The NICMOS units conversion tool on the STScI WWW pages can help you convert your source flux from J magnitude or flux in Janskys into this flux unit.
The following table lists the permitted Instrument Configurations, Operating Modes, Apertures, Spectral Elements, and Optional Parameters for the NICMOS.
Table 13.1: Supported Instrument Parameters for NICMOS
NIC1, NIC2, NIC3 ACCUM See Table 13.4 See Table 13.5 CAMERA-FOCUS, NREAD, OFFSET MULTIACCUM See Table 13.4 See Table 13.5 CAMERA-FOCUS, SAMP-SEQ, NSAMP, OFFSET NIC2 ACQ NIC2-ACQ See Table 13.5
The following sections provide further details of the entries to be made on the Visit and Exposure Specifications when a particular NIC1, NIC2, or NIC3 Mode/Configuration is chosen. The two modes that will be used by the majority of NICMOS observers are described first: ACCUM Mode in Mode = ACCUM Configuration = NIC1 or NIC2 or NIC3, and MULTIACCUM Mode in Mode = MULTIACCUM Configuration = NIC1 or NIC2 or NIC3.
- ACCUM is the simplest operational mode for NICMOS observing. ACCUM exposures begin with one or more initial readouts of the array, followed by a specified integration period, and end with one or more readouts of the array (the number of initial and final readouts is the same).
- MULTIACCUM is a more flexible operational mode which allows for a wide variety of initial, intermediate, and final readouts. More information on ACCUM and MULTIACCUM Modes can be found in Chapter 8 of the NICMOS Instrument Handbook.
- ACQ is a specialized mode used for coronographic observations, and is described in Section 13.4. More information on this Mode can be found in Chapter 5 of the NICMOS Instrument Handbook.
13.2 Mode = ACCUM
Configuration = NIC1 or NIC2 or NIC3ACCUM is the simplest readout mode available on NICMOS. One or more non-destructive readouts occur at the beginning and at the end of the exposure. ACCUM Mode will be appropriate for many targets, particularly for short integrations of relatively bright targets.
13.2.1 Aperture or FOV
See Table 13.4: NICMOS Apertures.
13.2.2 Spectral Elements
See Table 13.5: Spectral Elements for the NICMOS.
13.2.3 Wavelength
The Wavelength parameter is not required for NICMOS observations and should be left blank.
13.2.4 Optional Parameters
CAMERA-FOCUS
=DEF (default); 1-2Specifies the NICMOS focus position to be used. If DEF is specified or this parameter is omitted, NICMOS will be focussed in the best position for the selected camera. A value of 1-2 selects a compromise focus position between the optimum foci for cameras 1 and 2. This will mainly be useful with coordinated parallels, but CAMERA-FOCUS may be specified only on exposures using the primary NICMOS detector in the sequence as defined in 6.3 "Coordinated Parallels".
CAMERA-FOCUS is allowed only with NIC1, NIC1-FIX, NIC2, or NIC2-FIX as the chosen aperture.
NREAD
=1 (default), 9Each ACCUM exposure is preceded and followed by a set of detector readouts, which are used to determine the initial and final pixel values. NREAD specifies the number of readouts. Multiple readouts may be used to reduce read noise at the cost of extra overhead.OFFSET
=SAM (default)Specifies the method of FOV offset to be used in a predefined pattern of offsets.If the default value of SAM is used, offsets from the target that cannot be supported by small angle maneuvers (SAMs) while locked onto the target guide stars will be executed under gyro control. After this happens, any subsequent pointings in the pattern within range of the target guide stars (such as returns to the initial target position in a chop pattern) will reacquire the guide stars.
13.2.5 Time Per Exposure
Exposure times in ACCUM Mode are quantized. The exposure consists of a set of NREAD initial readouts, followed by a period of data accumulation, followed by a set of NREAD final readouts. The Time_Per_Exposure (in seconds) refers to the total integration time, which begins at the start of the first initial readout and ends at the start of the first final readout. The NICMOS Timing Pattern Generator (TPG) uses a list of discrete values for the time between the last initial readout and the first final readout, known as TPG expose time (TPG_TIME), which is given in Table 13.3. The specified exposure time will be one of a set of possible values allowed by the following table:
NREAD Exposure Time (seconds) 1 TPG_TIME + 0.598 9 TPG_TIME + 5.158
For example, suppose the desired exposure time is 10 seconds. With NREAD=1, the ideal TPG expose time is 9.402. The closest TPG expose time to that, from Table 13.3, is 9.117. So the corresponding exposure time is 9.117 + 0.598 = 9.715 seconds.
With NREAD=9, the ideal TPG expose time is 4.842. The closest TPG expose time to that is 4.781. So the corresponding exposure time is 4.781+ 5.158 = 9.939 seconds.
If the exposure time entered by the user is not one of the values allowed by the above formula, it will be reduced to the next lowest legal value. It is illegal to specify an exposure time which corresponds to a TPG_TIME below the minimum value of 0.0. In other words, do not specify an exposure time shorter than 0.598 sec, which is the fastest ACCUM read time for NREAD=1. If the brightness of the source requires a shorter exposure time, MULTIACCUM Mode or BRIGHTOBJ Mode is suggested instead.
13.2.6 Special Requirements
The three NICMOS detectors may be operated in parallel. Coordinated parallels are not possible with certain special requirements; see Chapter 6: "Parallel Science Exposures" for details.
13.3 Mode = MULTIACCUM
Configuration = NIC1 or NIC2 or NIC3MULTIACCUM is a flexible mode that allows multiple non-destructive readouts of the array during integration spaced at user-specified intervals throughout the integration, with the results from each readout being recorded onboard and returned to the ground for analysis. See the NICMOS Instrument Handbook for more information. This mode differs from the use of multiple readouts with NREAD in ACCUM Mode, because it actually produces multiple images whereas the multiple initial and final readouts in ACCUM Mode are used to reduce read noise in the generation of a single image. MULTIACCUM Mode may also be used with a single sample time to achieve integration times as short as 0.203 seconds (the shortest allowed in ACCUM Mode is 0.598 seconds).
13.3.1 Aperture or FOV
See Table 13.4: NICMOS Apertures.
13.3.2 Spectral Elements
See Table 13.5: Spectral Elements for the NICMOS.
13.3.3 Wavelength
The Wavelength parameter is not required for NICMOS observations and should be left blank.
13.3.4 Optional Parameters
CAMERA-FOCUS
=DEF (default); 1-2Specifies the NICMOS focus position to be used. If DEF is specified or this parameter is omitted, NICMOS will be focussed in the best position for the selected camera. A value of 1-2 selects a compromise focus position between the optimum foci for cameras 1 and 2. This will mainly be useful with coordinated parallels, but CAMERA-FOCUS may be specified only on exposures using the primary NICMOS detector in the sequence as defined in 6.3 "Coordinated Parallels".
CAMERA-FOCUS is allowed only with NIC1, NIC1-FIX, NIC2, or NIC2-FIX as the chosen aperture.
SAMP-SEQ
=SCAMRR, MCAMRR, STEP1, STEP2, STEP8, STEP16, STEP32, STEP64, STEP128, STEP256, MIF512, MIF1024, MIF2048, MIF3072, SPARS64, SPARS256Specifies the name of a predefined sequence of times from the start of the exposure at which the nondestructive readouts (samples) are performed. The number of samples (up to 25) taken for each exposure is controlled by the NSAMP parameter (see below). Table 13.2: Predefined Sample Sequences for MULTIACCUM Mode (seconds) gives the sample times (from the start of the exposure) for each sequence. SAMP-SEQ is required.
Four different types of sequences are provided. The SCAMRR and MCAMRR sequences are rapid sequences with linear steps, which obtain the densest temporal sampling. SCAMRR is designed for use with a single camera and provides the densest sampling available, but may not be used with multiple cameras. MCAMRR should be used if rapid sequencing is desired with two or three cameras (NIC1, NIC2, NIC3) operating in parallel.
Sequences STEP1, STEP2, STEP8, STEP16, STEP32, STEP64, STEP128, and STEP256 begin with logarithmic spacing up to the given number of seconds (1-256), and then continue with linear spacing for the remainder of the sequence, with adjacent steps separated by 1-256 seconds. These sequences all include three readouts during the first second to compensate for any nonlinear effects which may arise at the start of the exposure.
Sequences MIF512, MIF1024, MIF2048, and MIF3072 are intended to combine the read-noise reduction of the ACCUM strategy of taking multiple initial and final reads (NREAD > 1) with the advantages in dynamic range of multiple samples during the integration. These begin with eight rapid readouts, as with MCAMRR, then take nine readouts with roughly even spacing up to the given number of seconds (512, 1024, 2048, or 3072), and conclude with eight rapid readouts at MCAMRR spacing. With these patterns, NSAMP=25 is recommended to obtain the benefits of the final rapid readouts.
Sequences SPARS64 and SPARS256 begin with two readouts during the first second, and then continue with sparse linear spacing for the remainder of the sequence, with adjacent steps separated by the given number of seconds (64 or 256). These are similar to STEP64 and STEP256, except that the linear sampling begins immediately after the first two readouts rather than being preceded by a series of readouts with logarithmic spacing.
NSAMP
=1-25Specifies the number of samples in a predefined sequence that should actually be taken. Table 13.3 defines 25 sample times for each sequence. If an NSAMP value smaller than 25 is used, samples will be taken at only the first NSAMP times from this table. NSAMP must be specified.
The number of readouts will be NSAMP plus one for the initial readout, giving a maximum of 26 readouts (the initial readout plus a maximum of 25 samples) for a single execution of a MULTIACCUM exposure. Each readout will be recorded and will appear in the final data set.
OFFSET
=SAM (default)This is the same as in ACCUM Mode; see OFFSET =SAM (default)Specifies the method of FOV offset to be used in a predefined pattern of offsets.Section 13.2.13.3.5 Time Per Exposure
Time_Per_Exposure must be DEF in this Mode. The exposure time is unnecessary, because it is specified by SAMP-SEQ and NSAMP.
13.3.6 Special Requirements
The three NICMOS detectors may be operated in parallel. Coordinated parallels are not possible with certain special requirements; see Chapter 6: "Parallel Science Exposures" for details.
The exposure time sequences denoted by SAMP-SEQ are defined in the following table. These values are approximated for simplicity; actual exposure times may be reduced by up to 7 msec.
Table 13.2: Predefined Sample Sequences for MULTIACCUM Mode (seconds)
13.4 Mode = ACQ
Configuration = NIC2This mode requests the NICMOS flight software to locate the brightest target in the acquisition aperture of camera 2 (one quarter of the full NIC2 FOV, or 9.6x 9.6 arcsec) and place it behind the coronographic spot of camera 2. It is only necessary as a preparation for coronography with the NIC2-CORON aperture. Two images will be taken and downlinked, each with a single non-destructive readout at the beginning and the end of the exposure. Two each of background and lamp-on exposures will be taken to locate the coronographic hole.
13.4.1 Aperture or FOV
The NIC2-ACQ aperture must be used in this Mode.
13.4.2 Spectral Elements
Use any in Table 13.5: Spectral Elements for the NICMOS for the NIC2 configuration.
13.4.3 Wavelength
The Wavelength parameter is not required for NICMOS observations and should be left blank.
13.4.4 Optional Parameters
There are no Optional Parameters in this Mode.
13.4.5 Number of Iterations
Always enter 1. However, two exposures of the duration given in Time_Per_Exposure will be executed, for purposes of onboard cosmic ray elimination by the NICMOS flight software.
13.4.6 Time Per Exposure
Exposure times in ACQ Mode are quantized. The exposure consists of an initial readout, followed by a period of data accumulation, followed by a final readout. The Time_Per_Exposure (in seconds) refers to the total integration time, which begins at the start of the initial readout and ends at the start of the final readout. The NICMOS Timing Pattern Generator (TPG) uses a list of discrete values for the time between readouts (TPG_TIME), which is given in Table 13.3: Quantized Legal NICMOS TPG Expose Times. The specified exposure time should be one of a set of possible values allowed by the following formula:
Time_Per_Exposure = TPG_TIME + 0.228
If the exposure time entered by the user is not one of the values allowed by the above formula, it will be reduced to the next lowest legal value. It is illegal to specify an exposure time below the minimum of 0.228 seconds.
13.4.7 Special Requirements
Science exposures associated with this acquisition must be identified as part of the <exposure-list> and must use the NIC2 configuration and the coronographic aperture NIC2-CORON.
The Special Requirement PARallel WITH is not allowed in this Mode.
13.5 Tabular Data
13.5.1 TPG_TIME Values
The following table gives the legal TPG expose times (TPG_TIME) in seconds, needed to calculate legal exposure times in ACCUM and ACQ Modes. These values are approximated for simplicity, so that exposure times derived from them may be reduced by up to 1 msec.
Table 13.3: Quantized Legal NICMOS TPG Expose Times
13.5.2 NICMOS Apertures
Each camera has two apertures defined near the center of the FOV. Only exposures using the corresponding configuration (NIC1, NIC2, NIC3) may use a given camera's apertures. The CAMERA-FOCUS optional parameter may be used to select a compromise focus position not optimized for either NIC1 or NIC2, but intended to work well with both.
One of the two FOV-center apertures (NIC1, NIC2, NIC3) is an optimal location near the center of the FOV considering detector efficiency and pixel quality, which will be updated by the STScI as the detectors change on orbit. One aperture in each camera (NIC1-FIX, NIC2-FIX, NIC3-FIX) corresponds to a fixed pixel at or near the center of the detector array, which will not change on orbit.
The aperture NIC2-CORON is the location of the coronographic spot in the NIC2 camera. It will be used by science exposures which have been preceded by a target acquisition exposure using the NIC2-ACQ aperture or by a real-time acquisition image obtained using the NIC2-CORON aperture. The acquisition aperture will be slightly offset from the coronographic aperture, but within the acquisition detector subarray.
Table 13.4: NICMOS Apertures
13.5.3 NICMOS Spectral Elements
The following Spectral Elements, including filters, polarizers, and grisms, are available for the three NICMOS cameras. Only exposures using the corresponding configuration (NIC1, NIC2, NIC3) may use a given camera's spectral elements. See the NICMOS Instrument Handbook for details.
Table 13.5: Spectral Elements for the NICMOS
13.6 Illustrations
Figure 13.1: NICMOS Coordinate System.
This is as projected onto the sky for the POSition TARGet Special Requirement. The POS TARG coordinate system will be aligned parallel to rows and columns in each camera as shown in the diagram above. The alignment of each camera is not exact, and the internal coordinate systems attached to each of them will differ by small rotations (probably less than 2 degrees). The FITS format data files generated for NICMOS observers will have a World Coordinate System specified appropriately for each camera. The origin of the coordinate system will be located as shown in the diagram above.
Figure 13.2: Definition of Orientation for NICMOS.
Due to the linear arrangement of the three NICMOS cameras on the sky, it is sometimes advantageous to specify a unique telescope orientation. A simple example is shown above. A binary star with a position angle (PA) of 30 degrees measured east from north is to be positioned with the southern star in Camera 3 and the northern star in Camera 2. That is, we want the line connecting the two stars to lie along the NICMOS +Y axis. The resulting HST orientation is 225 + 30 = 255 degrees. (The NICMOS offset angle for orientation specifications is 225 degrees; see Table 7.4: Instrument Orientations with respect to U3-Direction.)
Figure 13.3: Guide-Star Availability with NICMOS Patterns.
The above graph shows the probability that guide stars will not be available ("failure rate") as a function of the tolerance in roll ("roll range") that the visit allows, for targets at high galactic latitude. For visits with ORIENTation requirements, the roll range would be half the difference between <angle1> and <angle2>. Note however that other special requirements, such as SAME ORIENT, ORIENT FROM, and BETWEEN, can also restrict the orientation at which a visit may be scheduled. Unavailability versus roll range is plotted for the full range of pointings within each of the NIC1, NIC2, and NIC3 apertures. Note that in all three cases the unavailability rises dramatically as the roll range shrinks to zero. The risk of not finding guide stars is considerably higher for patterns that cover the larger NIC3 aperture. For patterns larger than the NIC3 aperture, and which may approach the maximum pointing variation of 2 arcmin, unavailability will be still higher. Therefore, observations at high galactic latitude (above 45 degrees) with large patterns and tight ORIENT restrictions carry a high risk of having to be reworked later for lack of guide stars. At lower galactic latitudes, the risk still exists but is considerably reduced.
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