HST Primer: Additional Observing Modes

An overview of additional HST observing modes, including imaging polarimetry, slitless spectroscopy, coronagraphy, ramp and quad filters, and coordinated and pure parallel observing.

Imaging Polarimetry

ACS/WFC provides imaging polarimetry at 0o, 60o, and 120o relative polarization angles. WFC3, STIS, and COS do not have a polarimetric capability. Please refer to ACS Instrument Handbook for more details. 

Imaging Spectropolarimetry

Imaging spectropolarimetry is offered for ACS.  The ACS polarizers (ACS Instrument Handbook Section 6.1) can be used in conjunction with the G800L grism (ACS Instrument Handbook Section 6.3.1) to provide low spectral resolving power (R~100 @ 8000Å) imaging spectropolarimetry from ~5500Å – 8000Å. Commissioning and characterization indicate that this new mode will be capable of measuring polarization signals with a precision of ~1-2% and with similar absolute accuracy. Because this mode is slitless, it is most suited for point-sources. However, slightly extended sources up to 2-3 arcseconds have been observed as polarization calibrators during the commissioning process, and reliable results should still be obtained for such objects with polarization >~ 4-8%. Note that polarization follows a Rice as opposed to a Poisson distribution, so confident polarization measurements require an absolute minimum p/sp > 4; however, 5 is the highly recommended minimum. Spaxels may be binned to achieve this requirement, if necessary, but at the expense of spectral resolution. Details of signal-to-noise calculation per spaxel are the same as for the broadband imaging polarimetry mode as discussed in the ACS Instrument Handbook (Section 6.1, Eq. 6), and also discussed in more detail in Section 7.4 of Sparks & Axon (1999, PASP).

This observing mode is still undergoing calibration refinements by the ACS Team as of Cycle 32. Therefore, prior to proposing, potential observers should contact the Help Desk to discuss their specific goals and the current status of the mode. Note that in the future, the useful wavelength range may be extended to ~8200Å after more characterization has been completed; the polarizing efficiency of the polarizers decreases rapidly for wavelengths longer than 8000Å Figure 6.1, Instrument Handbook). Full descriptions of the spectropolarimetry capabilities of the ACS are found in the ACS Instrument Handbook, and details about the reduction of the data are described in the ACS Data Handbook.

Slitless Imaging Spectroscopy


There are several choices for slitless imaging spectroscopy in the ultraviolet (λ < 3500 Å). ACS/SBC has two prisms providing R ~ 100 spectroscopy (at 1210 Å) from 1150 Å to 1700 Å. The WFC3/UVIS has a grism for R ~ 70 spectroscopy from 2000 Å to 4000 Å.  The STIS/NUV has a prism covering the range between 1150 Å and 3000 Å with R ~2500; however, we note that this mode is Available-but-Unsupported starting in Cycle 30.  Please refer to the STIS Instrument Handbook for more information. In addition, any first-order STIS mode can be used for large aperture slitless spectroscopy over a 52" x 52" FOV (STIS CCD gratings) or a 25" x 25" FOV (STIS MAMA first-order gratings.)


ACS/WFC covers the wavelength range from 5500 Å to 10,500 Å with a grism at R ~100. The STIS CCD also has a number of gratings that can be used for slitless spectroscopy over a 52" x 52" field of view at wavelengths ranging from as short as 1700 Å to as long as 10,200 Å.


For near-infrared (8000 Å < λ < 25,000 Å) spectroscopy, WFC3/IR has a grism with R ~210 covering wavelengths between 8000 Å and 11,500 Å, and a grism with R ~130 from 11,000 Å to 17,000 Å.

Ramp and Quad Filters

Ramp Filters

ACS has a set of ramp filters covering the wavelength range from 3100 Å to 10,710 Å at 2% and 9% bandwidth. There are five ramp units; each has an inner, middle, and outer segment. The ACS/WFC can use all three segments, providing a total of 15 ramp filters. More information can be found in the ACS Instrument Handbook.

Quad Filters

WFC3/UVIS contains five quad filters. Each is a 2 x 2 mosaic of filter elements with each quadrant providing a different bandpass for the narrow-band line or continuum measurements. Please refer to the WFC3 Instrument Handbook for additional information.


STIS aperture bars allow for spectroscopic coronagraphy, and the STIS 50CORON aperture provides various wedges and bars that can be used for unfiltered imaging coronagraphy with the STIS CCD. Please refer to the STIS Instrument Handbook chapter on Coronagraphic Imaging for additional information. Since Cycle 24, the new BAR5 position on the 50CORON aperture location is supported within APT to allow inner working angles as small as 0.25” and with certain observing techniques can reach contrasts as deep as 10^-6 beyond 0.55". See the STIS BAR5 webpage and the STIS page on coronagraphy with small inner working angles for additional details.

Observations with Two Instruments - Parallel Observing

Observers are encouraged to submit programs that make use of simultaneous observations with two or more instruments. This can greatly increase the scientific value of individual programs and the public Archive. There are two ways to obtain parallel observations: coordinated and pure parallels. 

Coordinated Parallels

As the name implies, coordinated parallel observations allow an observer to use multiple instruments simultaneously in a way that optimizes the telescope pointing (e.g., dither patterns or mosaicing), with exposure and readout times that satisfy the goals of both the primary and parallel science components of a single science program. 

Pure Parallels

Pure parallel observations are proposed independently of any primary GO science program and are slightly more restrictive in the number of allowed parallel/primary instrument combinations than coordinated parallels. Implementation of pure parallel observing is done by identifying parallel scheduling opportunities that are compatible with primary COS and STIS spectrographic observations for the cycle. Accepted WFC3 and/or ACS pure parallel observations are then matched and structured to schedule simultaneously with those COS and STIS primary observations. Matching and structuring of parallel observations to prime observations, done at the start of the observing cycle, is intended to improve the execution rate for all accepted pure parallel programs.

Policies and Procedures

The policies for coordinated and pure parallel observing, including allowed instrument usage, are found in HST Observation Types. Detailed descriptions of coordinated and pure parallel observing modes, guidelines for developing a proposal using these modes, and how they are implemented and scheduled, are found in the Parallel Observations User Information Report and the individual instrument handbooks.


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