Chapter 4
FOC Instrument
Overview

The Faint Object Camera (FOC), desinged and built by the European Space Agency, is the highest-resolution imaging instrument on the Hubble Space Telescope (HST). It is a long focal-ratio, photon-counting imager operating in the 1150 to 6500 Å wavelength range with a 14 x 14 arcsecond field of view. The Corrective Optics Space Telescope Axial Replacement (COSTAR), installed during the December 1993 servicing mission, restored the two prime scientific objectives of the FOC-deep imagery and photometry of very faint celestial objects and imagery of bright objects at the highest possible resolution-which were hampered by the spherical aberration of the telescope's primary mirror. The corrected FOC offers imaging capabilities with a pixel size of 0.014" and a FWHM of 2-3 pixels, providing peak sensitivity at 3400 Å. Low detector background and insensitivity to cosmic rays allow for long exposures providing very deep photometry of point sources, reaching a S/N of 10 for a V = 26 B5V star in a 45 minute exposure.

Two cameras, named f/48 and f/96 after their original focal ratios, are available on the FOC, but difficulties with the f/48 camera have made the f/96 camera the FOC's workhorse, responsible for virtually all of the imaging. Since the installation of COSTAR, the f/48 camera has been used exclusively for long-slit spectroscopy. Observers should be aware that the names of these cameras no longer describe their actual focal ratios. COSTAR has raised the f/ratio of HST's Optical Telescope Assembly (OTA) from f/24 to f/37, increasing the f/number of the two FOC cameras from f/48 to f/75.5 and from f/96 to f/151. However, because the original names are deeply rooted in the HST ground system at all levels, from proposal entry to data archiving, we have been forced to retain these names. Table 4.1 summarizes the post-COSTAR imaging characteristics of the FOC.

Summary of FOC Performance Characteristics

Optical Modes	f/481	f/96
Focal ratio	75.5	151
Operating range (Å)	1150-6500	1150-6500
Number of bandpass filters	11	39
Bandpass FWHM (Å) max	2700	2300
Bandpass FWHM (Å) min	300	34
Maximum ND attenuation (mag.)	0	9
Field of view (arcsec) max	28 x 28	14 x 14
Field of view (arcsec) min (128x128)	3.6 x 3.6	1.8 x 1.8
Unzoomed pixel size (arcsec)	0.029	0.014
Minimum wavelength for critical sampling (Å)	6500	3250
Peak efficiency (%)	6.6	7.9
Peak wavelength (Å)	3400	3700
Limiting magnitude, point source2	27	27.5
Limiting magnitude arcsec-2, extended source3	23.5	23
Dynamic range, point source4 (mag)	20-27	19-27.5
Dynamic range, extended source5 (mag arcsec-2)	18-25	17-25
Overload magnitude	9	9
Number of polarizing prisms6	0	3

1 The f/48 mode has been available for long slit -spectroscopy in Cycles 6 and 7. 2 S/N = 5, 5 hour integration, U band. 3 Same as b. over 0.1" x 0.1" area. 4 2 counts sec-1 pixel-1 upper limit. 5 0.5 counts sec-1 pixel-1 upper limit. 6 0 degrees, 60 degrees, 120 degrees direction of polarization.