Hubble’s Apertures Resources

Aperture is the generic term used for any targetable area in a Science Instrument (SI), a small slit, an occulting finger, different points on a CCD, a virtual aperture that encompasses a number of chips (e.g. WFALL), anything that can be specified in the APERTURE field of the proposal file, and a few more that can't. The scales, orientations, locations in the HST FOV, and a number of other properties of these targetable areas are calibrated and updated periodically. Though these positional calibrations largely fall under the calibration plans of the Science Instrument Groups, some special calibrations are made by the Observatory Support Group, which also maintains the aperture data and relations. This information, in the form of the Science Instrument Aperture File (SIAF), is installed into the HST Project Data Base whenever updates are calculated and needed.

HST Focal Plane

Download PDF version

Simplified SIAF Tables

Fundamental characteristics of the targetable areas of each Science Instrument is kept in the Science Instrument Aperture File (SIAF) in the project database.

The SIOPSDB page (internal-only) gives ascii dumps of the operational SIAF tables, installation dates and history comments, and explanations of the fields. It also details which data products and subsystems are fed from SIAF.

Click on the instrument in the list below to view a SIAF-related file containing zero points, scales, orientations, and the dates after which these values apply. 
Columns are ordered as follows: 
Name, Xref, Yref, V2ref, V3ref, Xscale, Yscale, BetaX, BetaY


The Instrument Apertures tables contain parameters relating the instrument coordinates, pixels or deflection offsets, to the telescope coordinate frame. This frame, designated V2,V3, represents the projected position on the sky. If the telescope roll is zero, then V3 points North, and V2 points East. (The reason for the unconventional axis order is that V2 and V3 are part of a three-dimensional coordinate system with V1 being approximately along the optical axis.) The x,y coordinate system relates to each instrument aperture and the angles Betax and Betay are measured anti-clockwise from the V3 axis to the x and y axes. 

A reference point is chosen, near the center of the aperture which has the coordinates (xr,yr) in the instrument frame, and V2r,V3r in the telescope frame. The reference point is the fiducial in the focal plane that HST will endeavor to place the target specified in the proposal. The scales sx and sy in x and y directions are not necessarily equal and are each tabulated. The transformation from any point (x,y) to V2,V3 is given to first order by:

V2 = V2r + sx sin βx (x-xr) + sy sin βy (y-yr)
V3 = V3r + sx cos βx (x-xr) + sy cos βy (y-yr)

SIAF ApertureFor some instruments, the x and y axes are flipped with respect to the V2 and V3, and may not be orthogonal, but this is expressed by the Betax and Betay angles and the above formulae remain valid.

NOTE: for later-generation HST imagers such as ACS & WFC3, which exhibit significant distortion, the ground system utilizes a higher-order transformation between the pixel and V2,V3 frames. These are characterized in the SIAF CAK records which contain the operational distortion coefficients. Note that the operational distortion model in the SIAF will usually not be the most accurate distortion solution known through science calibrations and does not allow for variations between filters but is accurate enough for targeting. For more detailed astrometric needs and for forming mosaics, filter specific solutions are contained in the Instrument Distortion Calibration (IDC) tables and are utilized in the calibration pipeline.


HST Help Desk 

Please contact the HST Help Desk with any questions.