2
Your Orion Field Flattener for Short Refractors corrects for Petzval field curva-
ture, which if uncorrected leaves the periphery of the image frame slightly out of
focus. This Field Flattener’s doublet optical lens configuration provides the correc-
tion necessary to yield sharp stars across the entire image frame – without affect-
ing the focal length of the refractor.
Results will be especially noticeable
when using cameras with APS-C and
full-size sensors.
The Field Flattener fits 2" focuser
drawtubes, and its barrel is internally
threaded to accept 2" Orion filters. It
has a 55mm backfocus distance, which
means that the camera’s sensor must
be set 55mm behind the Flattener’s
T-thread flange to achieve optimal
focus. The Field Flattener now comes
with interchangeable M42 (standard
T-thread) and M48 (“wide” T-thread)
male thread adapters (Figure 1). The
M48 adapter is recommended for use
with full frame camera sensors.
Attaching the Field Flattener to
a DSLR Camera
An appropriately sized T-ring (M42 or M48) for the make and model of your camera
is the only required accessory to attach the Field Flattener to your DSLR camera.
The distance from the camera’s sensor to the front plane of the T-ring will be 55mm
– a perfect match for the backfocus distance of the Field Flattener (Figure 2.1).
Attaching the Field Flattener to an Astronomical CCD/CMOS
Camera
Most astronomical CCD or CMOS cameras require less backfocus than a DSLR
camera. That means you will need to add spacers as needed to fill the 55mm of
required distance to the Field Flattener (Figure 2.2). T-thread spacers are avail-
able from several astronomical equipment dealers.
For example, if a camera has 20.1mm of backfocus – obtain this spec from your
camera’s manufacturer – you would need to add approximately 35mm of spacer
length between the camera and Field Flattener to achieve the proper distance (20.1
+ 35 = 55.1mm). Note that if you do not reach 55mm perfectly, the Field Flattener will
still work, but will gradually lose effectiveness the farther from 55mm (in either direc-
tion) your camera is positioned. Your CCD/CMOS camera must have compatible
female T-threads (usually M42) on the camera body to attach to the Field Flattener.
Remove the camera’s nosepiece (if equipped) and thread the camera body onto the
back of the Field Flattener or T-thread spacer.
M42 "T-Ring
Adapter"
M48 Adapter
Figure 1.
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