42 filters
WFC3/UVIS · 42 Filters
The ultraviolet and visible channel of Hubble's Wide Field Camera 3, whose Selectable Optical Filter Assembly holds exactly 42 full-frame filters (plus 5 quad filters and 1 UV grism).
Hubble's Wide Field Camera 3 looks at the sky through a stack of filter wheels. In the ultraviolet and visible channel, those wheels hold 48 pieces of glass: 42 full-frame filters that cover the whole detector, 5 quad filters that each split the field into four smaller bandpasses, and one UV grism for slitless spectra. The Space Telescope Science Institute prints that inventory in the camera's own handbook: "42 full-frame filters, 5 quad filters, and 1 UV grism." The filters were chosen by a scientific oversight committee and have been flying on Hubble since 2009.
What it supports
An exact intentional design count printed in the flight instrument handbook itself (WFC3 Instrument Handbook §2.3.2) and again in the Data Handbook (§1.2). Table 6.2 lists every UVIS spectral element; counting the non-quad filters gives 6 long-pass or extremely wide + 12 wide + 8 medium + 16 narrow = 42. Same grammar as Kepler's 42 CCDs and the Allen Telescope Array's 42 dishes: an instrument built with forty-two of something.
What it does not
Not 48 (that is the total SOFA element count: 42 filters + 5 quads + 1 grism). Not 63 usable bandpasses (the quads add twenty more bandpasses at the cost of a smaller field). Not the infrared channel's fifteen filters and two grisms. Not a part number or a software version. Filter counts on other cameras take many values, so this is one design choice, not a rare constant of nature. Not Adams.
ExactnessCount
Form42
Unitfull-frame filters (UVIS SOFA)
Strengthhard
When2009
Hung12 October 2026