2
The Orion 2” Variable Field Flattener (VFF) for Short Refractors flattens the focal
plane for astro-imaging without affecting the focal length of the telescope. It is
designed to work with refractors of 400mm to 660mm focal length and f/5 to f/6
focal ratios. While the optics have an optimal backfocus distance of 55mm, the
housing of this unique flattener is adjustable in length, allowing the mechanical
backfocus to be precisely set to accommodate cameras with different flange-to-
sensor distances, often without the need for spacers, or extension rings to reach
the 55mm optimal backfocus. Once the mechanical backfocus distance is set,
a rigid locking mechanism ensures zero flexure without slippage for rock-solid
imaging performance. You can even adjust the VFF spacing without removing
attached cameras and other imaging accessories.
Fully multi-coated, two-element Ohara glass optics and a glare-reducing thread-
ed interior ensure maximum image fidelity and contrast. The machined aluminum
housing of the VFF features a tapered 2" barrel that fits in 2" telescope focusers.
It is threaded for use with 2" imaging filters.
interchangeable 42mm and 48mm attachment Plates
The VFF comes with two camera-side thread-on attachment plates, one
with a standard male “T”-thread (42x0.75mm), the other with a wider, M48
(48x0.75mm) male thread. The M48 attachment plate is recommended for cam-
eras with full-frame sensors, to reduce the possibility of vignetting. For DSLR
cameras, you will need a T-ring – either a standard 42mm ring or a wide 48mm
ring – for your particular camera model to couple it to the VFF.
attaching the Field Flattener to a Dslr camera
With a T-ring installed on your camera, thread on the VFF. Set the millimeter
scale on the VFF to 55mm, which is the optimal flange-to-sensor spacing for
many DSLR cameras with an attached T-ring. If your images are not perfectly
sharp, the optimal flange-to-sensor distance for your DSLR with T-ring attached
may be slightly different from 55mm. In that case you can experiment with fine-
tuning the length of the VFF while the camera is still attached, until the stars
appear perfectly sharp.
attaching the Field Flattener to a ccD camera
Most CCD cameras have a shorter flange-to-sensor distance than the 55mm of
a DSLR camera. Consult your camera’s manual to determine the flange focal
distance for your particular camera. Depending on what other accessories you
add to the imaging train, such as a filter wheel and/or an off-axis guider, you will
likely need to add spacing to fill the 55mm of required backfocus distance of the
VFF. Once you calculate how much spacing is needed, extend the VFF housing
by the appropriate amount, using the engraved millimeter scale for reference.
For example, let’s say your CCD camera has a flange-to-sensor distance, or
backfocus, of 19mm, i.e., its focal plane is 19mm behind the mounting flange.
And between the camera and the VFF you’ve got a filter wheel with a mechani-
cal length of 25mm. That’s a total of 44mm. The camera’s sensor would be
11mm too close (55mm – 44mm) to the VFF, so you must rotate the housing of
