HST Cycle 28 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. 

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, and the STIS/NUV has a prism covering the range between 1150 Å and 3000 Å with R ~2500. 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. For 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 Cycle 28 Observation Types and Special RequirementsDetailed 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.

Next: HST Cycle 28 Primer: Scientific Instrument Comparisons

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