11.5.3 COS Apertures

There are a few apertures available for HST's COS detectors, and this articles describes those apertures.

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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.

Valid COS apertures are listed in Table 11.7: COS Apertures. Science observations should use the PSA or BOA apertures. The BOA aperture is the same size as the PSA but contains a neutral-density filter that attenuates the source by a factor of ~200. The WCA is intended for wavelength calibrations using the WAVE target. 

Table 11.7: COS Apertures






Primary Science Aperture

2.5 arc-seconds

Default science aperture


Bright Object Aperture

2.5 arc-seconds

Contains neutral density filter (ND2) for bright objects

Not permitted for grating/wavelength combinations that default to lifetime position LP5 or LP6 ; those need to request use of LP3 or LP4 respectively.


Wavelength Calibration Aperture

20 microns (dispersion) × 100 microns

Wavelength Calibrations

Restricted FCA

The FCA is to be used with the DEUTERIUM target for flat-field exposures.






Flat-Field Calibration Aperture

0.75 mm × 1.75 mm

Flat field Calibrations with the deuterium lamp

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Change Log

Version Cycle 30 April 2022

       PROPINST-91380 Document COS FUV Lifetime Position 6