Santa Barbara Instrument Group (SBIG) - Internal Filter Wheel for the ST-402ME with custom BVI+C photometric filters

Although priced as an entry level CCD camera, the ST-402ME is an exceptionally sensitive and
capable instrument.  The KAF-0402ME CCD is a scientific grade full frame sensor with the same
quality, quantum efficiency, and performance of sensors used in cameras costing many thousands of
dollars more.  Except for its size (field of view), the CCD characteristics are essentially the
same as the main sensors used in the ST-8XME, ST-10XME and STL-6303E.  It has very high quantum
efficiency, high linear dynamic range, full well capacity of 100,000e-, dark current of 1e- or
less at 0C, and typical read noise less than 15 e-.  The ST-402ME now has an enhanced cooling
package and photometric filter wheel option, giving performance similar to our ST-X series but at
a significant savings.  The ST-402ME also uses a fast USB 2.0 interface with full frame downloads
in less than 1 second.  The ST-402ME is an excellent choice for making high precision photometric
measurements where its extraordinary sensitivity can be exploited with professional results.

The ST-402ME has an internal filter wheel that was originally designed with custom RGBC filters
for color imaging.  When installed, the filter carousel replaces the shutter wheel and acts as
both filter wheel and shutter.  Due to the filter/shutter wheel design, the filters must be very
thin, about 1mm, and only 85mm in diameter.  They are also permanently mounted to the carousel. 
This means it is not possible to add ”off-the-shelf” photometric filters to the camera without
buying an external filter wheel because most photometric filters are about 5 mm thick and made
for 1.25” filter cells.  There is no provision for adding standard 1.25” filters inside the
camera, although if one is using only one filter, such as a V filter, it can be screwed into the
nosepiece for single color measurements.

Previously we have recommended the CFW10 filter wheel with 1.25” filters for multi-band
photometry with the ST-402ME, or the ST-7XMEI camera (which uses the same imaging CCD) and CFW8A
filter wheel.  In either case, the cost of the system, including filters began at about $2800. 
In response to requests for an easier and lower cost option, we have designed a photometric
filter set permanently mounted on a replacement internal carousel for the ST-402ME.  By saving on
the cost of an external filter wheel and larger filters we can offer the ST-402ME Photometric
camera for over $1000 less than the same or similar system with external filter wheel.

In the 1950’s Harold Johnson (Yerkes and Macdonald Observatories) established three photometric
bands, the U, B, and V based on the sensitivity of the photomultiplier tube that he used at the
time. Later on he established red and infrared (R and I) bands using a PMT with enhanced red
sensitivity. In the mid-1970’s, A.W.J. Cousins and John Menzies (South African Astronomical
Observatory) used different filters, that when used with a newer, better detector, would
reproduce the Johnson bands. Then, in the 1980’s CCD detectors were beginning to replace the
photomultipliers so a new set of filters was required that, when used with the CCD’s would give
the same results as the older filters when used with the PMT. Bessell (Mt. Stromlo and Siding
Spring Observatories in Australia) did this in 1990 (PASP, 102, 1990, 1181).   It is Bessell’s
filter definitions that are the industry standard today when using CCD’s.

Based upon the recommendations of Arne Henden, Director of AAVSO, the photometry set designed for
the ST-402ME consists of B, V, and I band filters, and a clear filter.  The filters are high
quality, parfocal, AR coated photometric filters made by Custom Scientific to SBIG’s
specifications.  Because the typical photometric filter is made from thick glass that absorb the
out of band light, a custom curve is necessary for the I filter in order to make this filter set
only 1 mm thick for use in the ST-402ME filter wheel and still match the performance of an
equivalent CCD and Bessell filter.  The B and V filters are identical to the Bessell filter
curves.  If one compares the shape of the curve of the custom I filter to the standard Bessell I
filter, the custom filter has a sharper cutoff in the near IR whereas the Bessell filter
continues to pass light well beyond 1000nm.  However, when the QE curve of the camera is taken
into account, there is virtually no difference in spectral response between the custom I filter
and the Bessell filter.  This assures that data collected with the ST-402ME+BVI filters will be
consistent with other systems using the Bessell standard filter definitions.

The clear filter is AR coated and unblocked.  The BVI carousel can be easily installed by the
user and exchanged with a plain shutter or an RGBC filter carousel at any time.  Update June 1,
2006: These custom filters are now available and include a free Special Edition of MPO Canopus
photometry software.

Note:  The filter carousel is designed to work only with the ST-402 version of this camera, for
the ST-1603 and ST-3200 an external filter wheel is required due to the size of the sensors.