3.4 Designing STIS Observations

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In this section, we describe the sequence of steps you will need to take when designing your STIS observing proposal. The process is an iterative one, as you trade off between maximum spatial and spectral resolution, signal-to-noise, and the limitations of the instrument itself. The basic sequence of steps in defining a STIS observation (see Figure 3.3) is:

  • Identify science requirements and select basic STIS configuration to ­support those requirements.
  • Estimate exposure time to achieve required signal-to-noise ratio and check ­feasibility, including saturation and bright object limits.
  • Identify any additional non-science (target acquisition, peakup and ­calibration) exposures needed.
  • Calculate total number of orbits required, taking into account the overhead. Use APT to obtain accurate estimates of orbit length.

3.4.1 Identify Science Requirements and Define STIS Configuration

First and foremost, you must identify the science you wish to achieve with STIS. Some basic decisions you will need to make are:

  • Spectroscopy or imaging?
  • Wavelength region(s) of interest?
  • Spectral resolution and spectral coverage required?
  • Nature of target—extended source (long slit or full aperture) or point source?

In addition you will need to establish whether you require:

  • High signal-to-noise ratio
  • Time resolution
  • Coronagraphy
  • High photometric accuracy

Figure 3.3: Defining a STIS Observation.


As you choose your science requirements and work to match them to the instrument’s capabilities, keep in mind that those capabilities differ greatly depending on whether you are observing in the NUV/optical/NIR with the CCD or in the FUV/NUV with the MAMA detectors. Tradeoffs are described in Table 3.1.


Table 3.1: Science Decision Guide.

Decision

Affects

Tradeoffs

Wavelength regime

Detector and gratings

1640–10,300 Å — CCD
1600–3100 Å — NUV-MAMA
1150–1700 Å — FUV-MAMA

Spectral resolution

Detector, gratings, aperture

R < 20,000 (first order) with CCD, NUV or FUV-MAMA,
or R ≥ 30,000 (echelle) with NUV or FUV-MAMA only.
Wider apertures generally yield slightly poorer resolution, with more prominent low-level broad wings in the PSF

Spectral range

Gratings

Spectral range covered in a single exposure differs radically for different gratings.

Extended or
point source

Gratings

First-order gratings designed for spatially resolved and point source observations.
Echelle gratings designed for point source observations (long-slit echelle spectroscopy will suffer order overlap for extended sources, but can be done for sources with weak continua).

Time resolution

Detector

If time resolution <20 seconds required, must use NUV or FUV-MAMA.

Coronagraphy

Detector and aperture

Bright-object1 coronagraphy available with CCD only.
Coronagraphic imaging available with CCD only.
Barred coronagraphic spectroscopy available with all detectors.

1 The bright object limits for MAMA observations apply to coronagraphic observations as well, i.e., coronagraphic observations of targets which are too bright for the MAMA detectors are not allowed.

Spectroscopy

For spectroscopic observations, the configuration is: detector (CONFIGURATION), operating mode (MODE=ACCUM or TIME-TAG), slit (APERTURE), grating (SPECTRAL ELEMENT), and central wavelength (CENWAVE). In Chapter 4 we provide detailed information about each of the spectroscopic grating modes of STIS.

Imaging

For imaging observations, the configuration is detector (CONFIGURATION), operating mode (MODE=ACCUM or TIME-TAG), and filter (APERTURE); the mirror will be used as the spectral element for imaging observations. Chapter 5 presents detailed information about each of STIS' imaging modes.

Special Uses

We refer you to Chapter 12 if you are interested in any of the following special uses of STIS: slitless spectroscopy or extended-source echelle observations, time-resolved work, bright object or high signal-to-noise observations, planetary studies, parallel observations, coronagraphy, spatially scanned spectra.

3.4.2 Determine Exposure Time and Check Feasibility

Once you have selected your basic STIS configuration, the next steps are:

  • Estimate the exposure time needed to achieve your required signal-to-noise ratio, given your source brightness. (You can use the STIS Exposure Time Calculator (ETC) for this: see Chapter 6 and the plots in Chapter 13 and Chapter 14.)
  • For observations using the MAMA detectors, assure that your observations do not exceed brightness (count rate) limits (see Section 7.7). (You can use the STIS ETC for this.)
  • For observations using the MAMA detectors, assure that for pixels of interest your observations do not exceed the limit of 65,536 accumulated counts/pix per exposure imposed by the STIS 16 bit buffer (see Section 7.5.1).
  • For observations using the CCD detector, assure that for pixels of interest, you do not exceed the per pixel saturation count limit of the CCD. (You can use the STIS ETC for this.)
  • For MAMA TIME-TAG exposures check that your observations are feasible and do not violate any TIME-TAG specific count rate or data volume constraints (see Chapter 11).

To determine your exposure time requirements, consult Chapter 6 where an explanation of how to calculate signal-to-noise and a description of the sky backgrounds are provided. To assess whether you are close to the brightness, signal-to-noise, and dynamic range limitations of the detectors, refer to Chapter 7. For a consideration of data-taking strategies and calibration exposures, consult Chapter 11.

If you find that the exposure time needed to meet your signal-to-noise requirements is too great, or that you are constrained by the detector's brightness or dynamic range limitations, you will need to adjust your base STIS configuration. Table 3.2 summarizes the options available to you and steps you may wish to take as you iterate to select a STIS configuration which is both suited to your science and technically feasible.


Table 3.2: Feasibility Guide.

Action

Outcome

Recourse

Estimate exposure time

If too long, then re-evaluate
instrument configuration.

Reduce resolving power, or use wider slit, or change detectors and wavelength regime, or use larger binning.

Check saturation limit for CCD observations

If you wish to avoid saturation, then reduce time per exposure.

Divide total exposure time into multiple, short exposures.1

Check bright object limits for MAMA observations

If source is too bright, then re-evaluate instrument configuration.

Increase spectral resolution, or choose narrower slit, or use neutral-density filter, or change detectors and wavelength regime.

Check 65,536 counts/pix limit for MAMA observations2

If limit exceeded, then reduce time per exposure.

Divide total exposure time into multiple, short exposures.3

1 Splitting CCD exposures increases the exposure time needed to achieve a given signal-to-noise ratio because of the read noise. Splitting an exposure into multiple exposures also increases the overheads, slightly reducing on-source time.
2 See Section 7.5.1.
3 Splitting MAMA exposures has no effect on signal-to-noise ratio since there is no read noise with the MAMAs. Splitting an exposure into multiple exposures does increase the overheads, slightly reducing on-source time. See Chapter 9 for more information.

3.4.3 Identify Need for Non-Science Exposures and Constraints

Having identified your desired sequence of science exposures, you need to determine what non-science exposures you may require to achieve your scientific goals. Specifically, you need to:

  • Determine which (if any) target acquisition and acquisition peakup exposures will be needed to center your target in your aperture to the accuracy required for your scientific aims (e.g., you may wish to center the nucleus of a galaxy in the 52 × 0.1 arcsecond slit and orient the long axis of the slit along the major axis of the galaxy to some accuracy). To assess your acquisition needs, refer to Chapter 8. To determine a specific orientation for the STIS long slit, refer to Chapter 11.
  • If you require more accurate wavelength zero points than the routine calibrations provide, you can insert additional comparison lamp exposures (TARGET_NAME=WAVE) at shorter intervals or of longer duration than the routine, automatic wavecal observations. To determine your wavelength calibration exposure needs, refer to Chapter 11.
  • CCD observations longward of 7000 Å are subject to severe fringing, which can be well corrected only by flat-field exposures obtained contemporaneously with the science exposures. Hence, you should include such flat-field exposures if observing near 7000 Å or longward. Fringing is discussed in Chapter 7 and the specification of corrective flat fields (CCDFLATs) is discussed in Chapter 11.

3.4.4 Determine Total Orbit Request

In this, the final step, you place all your exposures (science and non-science, alike) into visits, including tabulated overheads, and determine the total number of orbits you require. Refer to Chapter 9 when performing this step. If you are observing a point source and find your total time request is significantly affected by data transfer overheads (which will be the case only if you are taking many separate exposures under 3 minutes), you can consider the use of CCD subarrays to lessen the data volume. Subarrays are described in Section 11.1 CCD Subarrays.

Due to the sensitivity of certain STIS electronic components to charged particles, there are some special constraints on the duration and structure of MAMA visits which preclude operating the MAMAs at all during orbits which cross the South Atlantic Anomaly (SAA). Since there are a limited number of SAA-free orbits per day, MAMA visits are limited to a maximum of five orbits. Longer programs must be broken into shorter visits. Moreover, in order to conserve orbits available for MAMA observations, programs which combine CCD and MAMA observations must be divided into separate visits for each detector type, unless the CCD portion consumes less than 30 minutes including overheads or the visit is only one orbit long (see Chapter 2).

At this point, if you are happy with the total number of orbits required, you're done! If you are unhappy with the total number of orbits required, you can re-iterate by adjusting your instrument configuration, lessening your acquisition requirements, changing your signal-to-noise or wavelength requirements, until you find a scenario which allows you to achieve (and convince the Telescope Allocation Committee [TAC] of the merits of) your scientific goals with STIS.


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