T-mount (also known as T-thread adapter) is defined by thread
dimensions M42 × 0.75 as
well as 55 mm Back focal Distance.
T-thread adapter for C1 cameras does not comply to the second
parameter, its BFD is only 18.5 mm. The 55 mm BFD is
not required in all applications and keeping such relatively large BFD
would make the adapter very long.
C1 camera with T-thread (M42 × 0.75) adapter and with
CS-mount adapter (left), camera with 55 mm BFD extension tube with M48 × 0.75 and with M42 × 0.75 (right)
Still, an extension tube with male M42 × 0.75 thread on one side and the
same, but female thread on the opposite side is available. This
extension tube converts the C1 camera BFD to 55 mm, required by numerous focal-reducers,
field-flatteners, coma-correctors and other optical elements.
As the M42 × 0.75
(T-thread) standard is more and more replaced with larger
M48 × 0.75 standard,
another variant of the extension tube for C1 cameras with male
M42 × 0.75 thread on one side and female
M48 × 0.75 thread on the
opposite side with 55 mm BFD is also
The extension tube outer diameter is exactly 2 inches
(50.8 mm), so it can
allow using of the C1 camera with any 2" focuser instead of 2"
New M42 adapter for C1 cameras also contains CS-mout compatible
thread inside, so it remains compatible with all CCTV lenses and other
equipment. The only exception is integrating of C1 camera with OAG.
While the short (10 mm)
CS-to-1.25" barrel adapter can be threaded into
internal CS-mount thread, the large-diameter M42 adapter interferes
with screws fixing the camera in the OAG guider port. This is why C1
variants with CS-only adapter are still supplied.
C1 camera with T-thread (M42 × 0.75) adapter is still
compatible with CS-mount CCTV lenses
The new T-mount/CS-mount adapter and classic CS-mount
adapters are not user replaceable. It is necessary to specify the
adapter upon order. So, if you plan to use C1 on OAG, order camera
with CS-mount only.
If the adapter has to be changed later, it
is necessary to send the camera to manufacturer.