Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

11.2 Exposure Sequences and Contemporaneous
Calibrations

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There are several instances when a series of associated STIS exposures (rather than a single exposure) will be taken. The data from these exposure sequences are generally processed as a single unit through the STScI calibration pipeline, with the scientific data from the multiple associated exposures appearing in a single file (for a high-level overview of the STIS calibration pipeline and the data-product format see Chapter 15). While you do not have to specify that you plan a series of associated exposures in your Phase I proposal, it is helpful to know about these sequences when planning your proposal. In Phase II, once your proposal has been accepted and you are working on scheduling your observations, you will be able to see and use these sequences. All are generated from a single exposure logsheet line in your Phase II proposal.

We discuss several types of associated exposures below:

• Automatic wavecal exposures taken with scientific data to allow calibration of the spectroscopic and spatial zeropoints.
• CCD CR-SPLIT exposures taken to allow removal of cosmic rays in the scientific data during post-observation data processing.
• Multiple identical repeat exposures, which can be taken to provide time resolutions of tens of seconds (CCD) or minutes (MAMA).
• Pattern sequences, in which the target is stepped, for example along the slit to mitigate the impact of hot pixels or perpendicular to the slit (to map a two-dimensional region) for spectroscopic observations, or in a dither pattern for imaging observations.

In addition there are two types of contemporaneous calibration observations that observers may take with their scientific observations in special circumstances:

• GO wavecals, taken if exceptional wavelength accuracy is required, or for slitless spectroscopy.
• CCD fringe flats (CCDFLAT) which need to be taken for near-IR (>7500 Å) observations in the grating modes if high signal-to-noise is required.

11.2.1 Auto-Wavecals

The STIS optical path from source to detector passes through the aperture (slit) wheel (where the filters for imaging also are housed) and then reflects from one of the elements mounted on the Mode Selection Mechanism (MSM) which houses the first-order gratings, the prism, the cross-dispersers for use with the echelles, and the mirrors for imaging work (see Figure 3.1). Lack of repeatability in the MSM causes the center of the spectrogram (as defined by the aperture and wavelength centers) to fall on a slightly different detector location each time there is a movement of the MSM (the MSM-induced offsets in dispersion and the spatial direction have been measured to be ±3 pixels or less). In addition, for MAMA first order spectrographic observations, the aperture location on the detector is deliberately shifted each month to ensure equalization of extracted charge across the detector.

To allow calibration of the zero point of the aperture location and the zero point of the wavelength scale for spectroscopic observations, a line-lamp observation (so called wavecal) is taken automatically each time the MSM is moved. In addition, if a series of exposures or a single long exposure is taken at a single MSM setting, then an additional wavecal will automatically be taken when there is a pause in data taking if 40 minutes of exposure time has passed since the previous wavecal. Here, 40 minutes is the time constant for thermal changes which might affect the wavelength accuracy. Testing in orbit has shown that in extreme conditions (when there is a swing from hot to cold), worst-case thermal shifts of roughly 0.3 pixels/hour can be seen; however, monitoring shows that under typical observing conditions thermal drifts are of the order of 0.1 pixels/hour (see also the Monitoring page on the STIS web site).

To summarize, each set of spectroscopic scientific exposures taken at a given grating tilt (i.e., MSM position) will be accompanied by at least one automatically taken wavecal exposure, and if the exposures extend over a period of 40 minutes or more, multiple wavecals will be taken. These wavecal exposures will be processed along with the scientific data, and they will be used by the pipeline to automatically correct the zero-point offsets in the wavelength and spatial scales (see Chapter 15).

The automatic wavecals are designed to be of sufficient duration to produce spectrograms which contain at least 3 emission lines with 3 counts per pixel and 50 counts summed over the line. In those regions of the spectrum where 3 lines are not obtainable, there will be at least 1 emission line with 18 counts per pixel and 300 counts summed over the line. For the CCD where integration times are short, the automatic wavecals will typically be taken to ensure roughly 8 times this signal.

The combination of thermal changes between the wavecal and scientific exposures, coupled with the ability to measure the zero points in the wavecal exposures, limits the accuracy of the absolute zero points to 0.2 pixel (see Section 16.1). In addition to the automatic wavecals, observers can also take their own wavecal exposures, using the WAVE target option (see GO Wavecals, below) if they desire more accurate wavelengths than will automatically be provided, or they are particularly concerned about the time variation of the zero point.

GO Wavecals

Only if you require particularly accurate wavelengths do you need to consider using the TARGET=WAVE option to insert additional wavecal exposures into your observing sequence.

The wavecals taken with TARGET=WAVE are identical to those taken automatically (i.e., the auto-wavecals) with two important exceptions. First, you can explicitly specify which aperture (slit) you wish to use for the TARGET=WAVE exposure (whereas for automatic wavecals the program slit or a pre-defined alternative for each grating is used). Second, you can take longer exposures, increasing the signal-to-noise of the lamp exposures or possibly saturating some lines to bring out weaker lines near astronomical lines of interest.

TARGET=WAVE exposures cannot be taken with all slit-grating combinations as the line lamps can be too bright for the MAMA detectors when used with wide slits. Therefore only certain aperture-grating combinations can be used for MAMA TARGET=WAVE observations (all are available for the CCD). Tables of lines and observed count rates from the line lamp for each grating mode for several different apertures and the complete list of allowed combinations are provided in the "Phase II" area on the STIS web page. Although the slit-wheel repeatability is very high (see Slit and Grating Wheels), observers wishing particularly accurate wavelength calibrations are best off using a slit for their scientific exposures for which there is an allowed slit-grating wavecal; otherwise, the slit wheel will be moved each time they take a wavecal exposure, producing an additional uncertainty.

TARGET=WAVE exposures are processed through the STScI pipeline as individual (unassociated) exposures and are not used to calibrate the scientific data in the pipeline itself. For this reason, the x1dcorr and x2dcorr steps will not be performed by the pipeline when this setting is used.

11.2.2 CR-SPLIT

In order to allow rejection of cosmic rays in post-observation data processing, observers using the STIS CCD should always try (as much as possible given signal-to-noise ratio constraints when in the read-noise-limited regime) to obtain at least two-preferably three or more-identical CCD exposures (see Section 7.2.3). In Phase II, the CR-SPLIT optional parameter (default value 2) allows easy scheduling of such multiple associated exposures. You specify the total exposure time and set CR-SPLIT=n, where n is the number of exposures to break the total observing time into. For example, if the total exposure time is 12 minutes, and cr-split=3, then three 4 minute exposures will be taken. Those three exposures will be associated with one another, passed through the STScI calibration pipeline as a unit, and a cosmic-ray free image will be produced during pipeline processing (see the "STIS Calibration" Chapter of the HST Data Handbook). Allowed values of CR-SPLIT are integers from 1 to 8. Note that overheads are incurred for each CR-SPLIT subexposure.

11.2.3 Fringe Flat Fields

The STIS CCD exhibits fringing in the far red, limiting the signal-to-noise achievable at wavelengths longward of ~7500 Å in the G750L and G750M spectral modes. As discussed in Section 7.2, the best way of eliminating the fringes in the far red is by obtaining contemporaneous flat fields along with the scientific observations. These "fringe flats" must be taken at the same position of the Mode Selection Mechanism as the scientific data. STIS users can insert such contemporaneous fringe flat fields into the same visits as their scientific data, as described below.

Designing your Fringe-Flat-Field Observations

Observers of extended sources will typically want to take their fringe flat fields using the same slits as they use for their scientific targets, since the flat-field lamp will then illuminate the detector in the most similar way to the targets. However, observers of point sources will typically fare better if they use small slits (e.g., those which are otherwise used for echelle observations) for their fringe flat fields. The main reason for this difference is that the PSF of the STIS CCD features a substantial halo in the far red containing up to 20% of the total source flux, which causes the fringes in lamp flat fields to behave differently from those of external sources, especially in the case of point sources (see also Section 7.1.7). Fringe flat fields taken with short slits simulate the spatial structure of point sources significantly better than those taken with long slits.

The slits supported for scientific observations with the G750L and G750M gratings and the associated slits to use for fringe flat fields in the cases of both extended- and point-source observations in the far red are in Table 11.1.

Table 11.1: Slits for Extended-Source and Point-Source Fringe Flat Fields

Supported Scientific Slit	Fringe Flat Slit for Extended Source Observations	Fringe Flat Slit1 for Point Source Observations
52X2	52X2	0.3X0.09
52X2E1	52X2	52X0.1
52X2E2	N/A	52X0.1
52X0.5	52X0.5	0.3X0.09
52X0.5E1	52X0.5	52X0.1
52X0.5E2	N/A	52X0.1
52X0.2	52X0.2	0.3X0.09
52X0.2E1	52X0.2	52X0.1
52X0.2E2	N/A	52X0.1
52X0.2F1	52X0.2F1	52X0.2F1 and 0.3X0.09 (optional)
52X0.1	52X0.1	0.2X0.06
52X0.1E1	52X0.1	52X0.1
52X0.05	52X0.05	0.2X0.06
52X0.05E1	52X0.05	52X0.05

1Short slits are chosen so as to be concentric with matched long slit. E2 positions are chosen to be concentric with 52X0.1 aperture at row 900.

A few notes are of importance on the use of short slits for obtaining fringe flat fields:

• Fringe removal for sources that are offset from the center of the (long) slit will not be possible with a short-slit fringe flat field; one has to use long-slit fringe flat fields for those cases. A special case in this respect is that of point source spectra with the 52X0.2F1 slit, as the 0.3X0.09 slit (which is in principle the appropriate one to use for fringe flats in that case, cf. Table 11.1) is only a few CCD pixels larger than the occulting bar of the 52X0.2F1 slit. However, a short-slit fringe flat does give a somewhat better fringe correction for the area covered by both the short slit and the 52X0.2F1 slit, so if that area is of particular scientific interest, we recommend taking a short-slit fringe flat as well.
• The limited length of the short slits used for obtaining contemporaneous flat fields of point sources (0.2-0.3 arcsec) does not allow one to sample the full PSF, so that absolute spectrophotometry cannot be performed with the short-slit fringe flat fields alone. However, a comparison with the pipeline-reduced point-source spectrograms will enable a proper flux calibration.
• At wavelengths longward of ~7500, fringing is the dominant calibration concern at high S/N, whereas imperfect charge transfer efficiency (CTE) is the dominant concern at low S/N ratios. We therefore recommend using the"E1" or"E2"pseudo-apertures for faint sources and the normal aperture positions in the long slits for high S/N observations.
• The "E2" aperture positions are, like the "E1" aperture positions, located near row 900 of the detector, and are intended to be used to mitigate CTE effects. However, in order to better align with the 52X0.1 aperture, which is used for fringe flats near row 900, the targeted position is offset about 1 pixel in the dispersion direction from the physical center of each aperture. Fringe flat alignment will be slightly better than when using the "E1" positions, although for the 52X0.2E2 aperture, the throughput will be slightly reduced. The "E2" positions should only be used for point source observations where fringe flats are needed and CTE is a concern. If a peakup is desired before using the "E2" aperture positions, it should be done using the 52X0.1E1 aperture.
• The limited length of the short slits used for obtaining contemporaneous flat fields of point sources (0.2-0.3 arcsec) imposes a minimum requirement on the accuracy of the acquisition of target point sources in the slit. The final accuracy should be of the order of 1 pixel (i.e., ~0.05 arcsec). In case the observer has to use offset acquisition targets, it is therefore recommended that an ACQ/PEAK exposure in a short slit be performed to ensure centering in both directions (see Chapter 8).

Inserting Fringe Flat Field Exposures in Phase II

You specify a fringe flat field exposure in your Phase II proposal input as follows.

• Specify Target_Name = CCDFLAT to indicate the exposure as a fringe flat field. The flat-field exposure will automatically be taken at CCDGAIN=4.
• Specify Number_Of_Iterations = 2 (to allow cosmic-ray rejection and to obtain adequate signal-to-noise)
• Specify Config, Opmode, Aperture, Sp_Element, and Wavelength.
  • Config must be STIS/CCD
  • Opmode must be ACCUM
  • Aperture must be one of 52X2, 52X0.5, 52X0.2, 52X0.2F1, 52X0.1, 52X0.05, 0.3X0.09, or 0.2X0.06.
  • Sp_Element and Wavelength must be one of the following combinations:
  • Sp_Element: G750L and Wavelength: 7751
  • Sp_Element: G750M and Wavelength: one of 6768, 7283, 7795, 8311, 8561, 8825, 9286, 9336, 9806, or 9851.
• Specify Time_Per_Exposure as DEF (Default). The default exposure time is determined from in-flight calibration data and ensures a signal-to-noise of 100 to 1 per pixel for all settings mentioned above and Number_Of_Iterations = 2.
• If the scientific data are taken in binned mode, specify Optional Parameters BINAXIS1 and BINAXIS2 in the same way as for scientific observations. Supported binning factors are 1, 2, and 4.

Two very important issues for fringe flat fields:

• Fringe-flat-field exposures are moved into the occulted period by whenever they occur as the first or last exposure in an orbit. Thus you can fill the unocculted portion of your orbit with scientific observations and take the fringe flat during the occultation by placing it at the beginning or end of the orbit.
• Fringe flat fields are effective only if taken without a move of the Mode Selection Mechanism between the scientific exposure and the fringe flat field. Observers must ensure that if the spectral element or wavelength setting is changed during an orbit in which they wish to obtain a fringe flat, then they place the fringe-flat-field exposure immediately before or after the scientific exposure(s) they wish to de-fringe. In some cases (e.g., for a long series of exposures) the observer may choose to bracket the scientific exposures with fringe-flat-field exposures to be able to account for any thermal drifts.

Please refer to the Instrument Science Reports 1997-15 for more details about near-IR fringe flats; 1997-16 which deals with fringing in spectrograms of extended sources; 1998-19 (Revision A) which deals with fringing in spectrograms of point sources as well as more general fringing analysis and details related to the 52X0.2F1 aperture; and 1998-29 which is a tutorial on the use of IRAF tasks in the stsdas.hst_calib.stis package to remove fringes.

11.2.4 Repeat Exposures

A series of multiple repeated identical exposures can be taken most easily using the Number_Of_Iterations optional parameter in Phase II. In this way, time-resolved observations at minimum time intervals of roughly 20 seconds for the CCD (if subarrays are used) and 30 seconds for the MAMA can be taken in ACCUM operating mode. The output of this mode is a series of identical exposures. If your exposure time is 60 seconds, and you set Number_Of_Iterations=20, you will obtain twenty 60 second exposures. These twenty exposures will be associated with one another and processed through the pipeline as a unit-the individual exposures will be fully calibrated and a summed image will also be produced for MAMA data and a cosmic-ray-rejected image for CCD data (see also Chapter 15).

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