of Calgary acquired one of these Baker-Nunn cameras
and decided to modernize it and put it to work searching
for asteroids and other astronomical objects. The University
staff modified the 3-axis mounting into an equatorial
fork mount using the original primary gearing.
DFM Engineering was contracted to provide new secondary
gearing and a retrofit
TCS based upon DFM's latest Windows version of their
TCS. Special provisions were made throughout the entire
project for operation in the cold weather Canadian climate.
A very interesting part of the project was replacing
the photographic film with a modern large format CCD
detector. The CCD detector, while smaller than the format
of the film, is much more sensitive and has a wider
dynamic range than film. One major problem with the
Baker-Nunn camera optics is that they produce a curved
focal plane. The film could be bent around the focal
plane curvature, but the flat CCD cannot be curved so
a field flattener operating at F/1 was needed.
Dr. Malcolm MacFarlane designed a two element field
flattener for the Baker-Nunn. Dr. MacFarlane has previously
worked with DFM Engineering on field correctors/field
flatteners and the design of the US
Naval Observatory's wide field 1.3-m DFM telescope.
The first element of the field flattener is a steep
meniscus and is located substantially in front of the
focal plane. The second element is located just in front
of the CCD focal plane and was built into the CCD camera
DFM Engineering was tasked with integrating the entire
prime focus instrument including the lens cell for the
first element of the field flattener. The cooled 4K
x 4K CCD camera, its housing, and the second field flattener
lens cell were contracted to a commercial camera manufacturer
by the University. DFM designed and fabricated the other
The prime focus instrument consists of spyder vanes,
a focus housing, the CCD camera housing, a large format
shutter, and the lens cell for the first field flattener
element. The entire assembly had to fit into the available
space and place the CCD focal plane and the field flattener
lens elements at the proper optical spacings. DFM Engineering
worked with the camera and optical manufacturers to
coordinate the various components and maintain the critical
The spyder vanes were custom designed to just fit into
the space available immediately behind the last surface
of the camera corrector lens element. Tip/tilt and centering
adjustments were built into the spyder vanes where they
attach to the original OTA structure and still maintain
the original Invar spacers used to control the focus
shift due to temperature changes.
The space available for the focus housing required
redesign of the standard DFM
20/24-inch telescope focus housing to reduce its
length. The weight of the prime focus instrument including
the CCD camera, the first field flattening lens element,
and the custom designed shutter was substantially greater
than the weight of a secondary mirror. The increased
load required some additional changes to the focus housing.
The focus housing is motor driven and includes a very
accurate focus position encoding system. The motor drive
controls and the focus position readout are performed
by the TCS.
Collimation (optical alignment) of the system is very
critical due to the F/1 focal ratio optics. Presently,
the collimation is very good, but some additional work
may improve the images. The raw data image of Andromeda
is a 1 minute integration and reaches about 17th magnitude.
The field is about 4 degrees square.