10.5.1 ACS WFC Spectral Elements

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There are many spectral elements (filters and polarizers) allowed for HST's ACS/WFC. This article provides a list of those spectral elements for the Astronomer’s Proposal Tool (APT).

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

Table 10.3: Spectral Elements for use with ACS/WFC Configuration

WheelPositionNameDescriptionAllowed Configurations

1

2

F555W

Johnson V

ACS/WFC

1

3

F775W

SDSS i

ACS/WFC

1

4

F625W

SDSS r

ACS/WFC

1

5

F550M

narrow V

ACS/WFC

1

6

F850LP

SDSS z

ACS/WFC

1

8

POL0UV1

UV polarizer 0°

ACS/WFC

1

9

POL60UV1

UV polarizer 60°

ACS/WFC

1

10

POL120UV1

UV polarizer 120°

ACS/WFC

1

11

F892N1

methane

ACS/WFC

1

12

F606W

broad V

ACS/WFC

1

13

F502N

O III

ACS/WFC

1

14

G800L2

GRISM

ACS/WFC

1

15

F658N

Ha

ACS/WFC

1

16

F475W

SDSS g

ACS/WFC

2

2

F660N

N II

ACS/WFC

2

3

F814W

Johnson I

ACS/WFC

2

4

FR388N

O II Ramp

ACS/WFC

2

4

FR423N

O II Ramp

ACS/WFC

2

4

FR462N

O II Ramp

ACS/WFC

2

5

F435W

Johnson B

ACS/WFC

2

6

FR656N

Ha ramp

ACS/WFC

2

6

FR716N

Ha ramp

ACS/WFC

2

6

FR782N

Ha ramp

ACS/WFC

2

8

POL0V1,2

visible polarizer 0°

ACS/WFC

2

10

POL60V1,2

visible polarizer 60°

ACS/WFC

2

12

POL120V1,2

visible polarizer 120°

ACS/WFC

2

16

FR914M

Broad Ramp

ACS/WFC

2

16

FR853N

IR Ramp

ACS/WFC

2

16

FR931N

IR Ramp

ACS/WFC

217FR459MBroad RampACS/WFC
217FR647MBroad RampACS/WFC
217FR1016NIR RampACS/WFC
218FR505NO III RampACS/WFC
218FR551NO III RampACS/WFC
218FR601NO III RampACS/WFC

1, 2

1

CLEAR3

no spectral elements

ACS/WFC

1The polarizing filters and the narrow-band filter F892N induce vignetting when used with ACS/WFC: On Filter Wheel #1 (POL0UV, POL60UV, POL120UV, F892N) and on Filter Wheel #2 (POL0V, POL60V, POL120V). The vignetting results in an approximately 72 x 72 arcsec2 field of view for each of these filters. More details can be found in Sections 5.1 and 5.3.1 of the ACS Instrument Handbook, and Figure 5.10 of the Instrument Handbook shows a graphical view of the filters and their positions and relative sizes in the two filter wheels. Although the RAMP filters themselves are not vignetted, the monochromatic field of view of the RAMP filters is approximately 40 x 80 arcsec^2. More details on the RAMP filters can be found in Sections 5.3.1 and Section 7.3.1 of the ACS Instrument Handbook.

2Imaging Polarimetry is also addressed further in Section 6.1 of the ACS Instrument Handbook, and Grism (WFC only) and Prism (SBC only) Spectroscopy is further addressed in Section 6.3 of the ACS Instrument Handbook. As of Cycle 32, a new capability,  Imaging Spectropolarimetry with the POLV filter set and the G800L Grism, is also now available. See Hathi et al. (2024) for more about this new mode, and please contact the HST/ACS Help Desk for the latest information and more expert assistance if you wish to use this mode. 


3For available but unsupported parameters (CLEAR)

The observer may specify the spectral element for ACS/WFC ACCUM exposures as CLEAR if no spectral element is desired. (The CLEAR filters would then be used on both filter wheels.) But this is not normally allowed for GOs and should only be done in consultation with an ACS Instrument Scientist as STScI does not provide calibration reference files for the mode. It yields imaging with significantly degraded ACS/WFC PSFs.


Normally only one spectral element may be specified for an exposure. The exceptions are:

The polarizers must be used with another spectral element. This is required to maintain focus. The UV polarizers (POL0UV, POL60UV, POL120UV) must be used with a spectral element on wheel 2 except for: POL0V, POL60V, POL120V, and any inner or outer ramp filter segment (see Table 10.5: Wavelength Ranges for the WFC Ramp Filters. The visible polarizers (POL0V, POL60V, POL120V) must be used with a spectral element on wheel 1 except for: F850LP, F892N, POL0UV, POL60UV, and POL120UV.

For restricted parameters

When Optional Parameter CTE is specified (but not as NONE), and the Target is TUNGSTEN, other wheel 1 spectral element plus wheel 2 spectral element pairs are permitted, provided that both are legal for the WFC.

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Related Links

10.5.2 ACS WFC Permitted Filter Combinations
10.5.3 ACS WFC Ramp Filter Ranges
10.5.4 ACS WFC Allowed Combinations of Aperture, Spectral Element, and Readout
10.5.5 ACS SBC Spectral Elements
10.5.6 ACS Aperture Coordinate System



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Change Log

Version Cycle 32 April-May 2024

  1.  PROPINST-91410 - Added a new footnote to table 10.3 about the polarizing filters and filter F892N
  2. Added another new footnote about the ACS/WFC imaging spectropolarimetry capability with the G800L Grism
  3. Added a link to the HST/ACS help desk



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