CFHT, Instruments, Imaging, FOCAM, Instrument Set-Up.

Canada-France-Hawaii Telescope

A User's Manual for the CFHT Visible Imager: FOCAM

Instrumental Set-Up

InstrumentOptical and Mechanical Layout

FOCAM is essentially a big filter wheel in front of the CCD (illustration). The filter wheel can accommodate up to 12 50.8x50.8x6 mm filters simultaneously, and in addition one open and one blocked positions are provided. Note that the filter wheel is the only remotely controllable FOCAM device.

FOCAM can be installed either at the prime or Cassegrain focus. At prime focus, FOCAM+CCD provides a large field of view (currently up to 7 arcmin with a 2Kx2K, 15 micron pixel CCD) with a spatial sampling of 3-4 pixels/FWHM, depending on the seing and the CCD being used, adequate for most imaging programs. At the Cassegrain focus, FOCAM+CCD provides a smaller field of view with a spatial sampling of 6-8 pixels/FWHM, which might be needed by programs requiring a very well defined point spread function.

Prime focus

The optical/mechanical configuration of FOCAM in a direct imaging mode at prime focus is displayed in this figure. The wide field corrector shown in this figure is always installed with FOCAM in this configuration and the following figure shows the light throughput of the combined telescope+wide field correcor.

FOCAM is installed on top of the prime focus bonnette which provides the following functions:

The bonnette is currently controlled by the telescope operator. The FOCAM cabling at prime focus is shown here (note that this setup is for RCA2 and the old Photometrics controller. RCA2 is still available at CFH but the GENIII CCD controllers are used most of the time).

Cassegrain Focus

The optical/mechanical configuration of FOCAM in a direct imaging mode at the Cassegrain focus is displayed in this figure. No field corrector is necessary at this focus within the FOCAM field of view. The throughput of the telescope at the Cassegrain focus was displayed in this previously shown figure.

FOCAM is installed on the Cassegrain bonnette which provides the following functions:

The bonnette is currently controlled by the telescope operator.

CCDs and FOCAM: Summary

FOCAM can hold any of the CCDs currently available at CFHT. The link below leads you to a summary of the CCD characteristics.

CFHT has renewed his dewars. G. Luppino of the University of Hawaii has been contracted to fabricate and provide the CFHT with 6 dewars of his design. We have received 4 at CFH containing: Loral3, Loral4, Orbit1 and TEK3. STIS2 is being integrated in a dewar at IfA. The sixth dewar will contain UBC1.

Click here@ for a table of the CFHT CCD's characteristics.


Installation: Up to 12 filters can be installed at a given time. Your filter list should be provided to your support astronomer well in advance so that we can plan on filter installation. Most of the time your filters will be installed at the beginning of a FOCAM run (including several science runs). Filter changes during the course of your run should also be planned in advance with your support astronomer.

Filter Specifications: Focam has tight tolerances for filters to fit in the filter wheel. If you plan on bringing your own filters or have them fabricated for your observations, they should have the folllowing specifications (CFHT standard):

Standard U, B, V, R, I filter set: The standard U, B, V, R, I filter set is based on the following photometric systems:

There are 2 UBVRI filter sets for FOCAM at CFHT. The first one has been at CFHT since 1981 and the second since 1990. Both have closely matched characteristics. Appendix A presents the transmission curves of the latest U, B, V, R, I filter set.

CFHT filter list:This hyperlink@ gives the current list of CFHT filters.

Observed image quality with FOCAM

The image quality obtained with FOCAM is automatically measured for each night of observations, thanks to J. Bouvier who initiated a procedure which has been regularly upgraded by the CFHT software group. The scripts select every frame with more than 100 seconds of integration time, find stars, measure the FWHM of each star, and compute the mean FWHM of each image. One line of data per ccd frame is kept in a log file that can be referred to when need be. We then have available at the end of each night the histogram of measured image quality, with as many measurements as there were exposures.

The mean image quality with FOCAM at prime focus is constantly around 0.75 arcsec as illustrated by this histogram, representing unbiased image quality measurements with FOCAM from 1991 to 1993 (all measurements for exposure times > 100 sec., all filters, nocorrection for airmass).

Please send comments to Dennis Crabtree at