Many optical designs of coma-correctors are available for
2 inch standard. Such correctors are
sufficient for sensors used in G2 series of cameras (maybe also for
smaller sensors in G3 cameras, but this depends on the telescope
f/ratio and also on the amount of vignetting we are willing to
tolerate). When a fast mirror is used together with G3 camera equipped
with large sensor, 3 inch corrector is a
necessity. In the case of G4 cameras using of 3 inch corrector is absolute minimum. Currently only
two large 3 in correctors are available
on the market — the Wynne design made by ASA and
the updated Type II Paracorr BIG made by TeleVue.
Both Wynne and Paracorr offer 3 inch
barrel, but the attachment to the camera is designed proprietary. We
designed adapters specially for these two correctors to allow usage of
G3 and G4 cameras with fast Newtonians to leverage all the potential
hidden in these optical setups.
3" Paracorr BIG adapter
3 inch Paracorr BIG coma corrector
prolongs the primary mirror focal length by a factor of approx.
1.17×. This could
seem like a disadvantage, as users typically prefer shorter focal
length (faster optics) to be able to accumulate more light on the
detector area. But prolonging of the focal length brings one
significant advantage — the corrector and
camera have to be moved farther from the primary mirror to achieve
focus. This also moves the corrector entry pupil, which is the
main source of vignetting, to the narrower portion of the light
cone created by the primary mirror and thus causes significantly
smaller vignetting. This means the Paracorr can be used also with
large-sensor G4 cameras with acceptable vignetting.
The Paracorr mechanical construction is also rather
camera-friendly. The flange with 3.4 inch, 24 TPI
thread on the outer edge can be just unscrewed as it only adds
unnecessary diameter and disables usage of OAG etc. Then the
corrector optical tube is just a 3 inch barrel, ending with metric M73.5 × 0.75 inner thread. The G3/G4 camera Paracorr
adapters uses this thread.
Standard 2" Paracorr with T-thread compared to 3"
Paracorr BIG (left), G4 camera with EFW, OAG, G1 and Paracorr
BIG corrector (right)
There are two variants of the Paracorr adapter for
The first short one is designed to be used with
camera using OAG on the External Filter Wheel. It screws into
M73.5 × 0.75 thread on the Paracorr
and to M68 × 1 thread on the G3/G4
OAG. Thankfully, the Paracorr is designed to provide enough back
focal distance, so there is enough room for the adapter, OAG and
External Filter Wheel.
The long adapter variant is intended for usage
without OAG, but with the External filter wheel. It is also
possible to use it without External filter wheel and with G3
camera with Internal Filter Wheel with proper M68 × 1 adapter distance extensions.
3" Wynne adapter
Wynne corrector shortens the primary mirror focal length by a
factor of 0.95×.
This leads to faster resulting optics, but brings a necessity to
move the corrector entry pupil closer to the primary mirror, which
increases vignetting, because the pupil cuts the light cone when
it is wider. Still, faster resulting optics is attractive for G3
cameras with smaller sensors compared to G4 cameras.
From the mechanical point of view the ASA 3" Wynne corrector is
everything but well thought out. There is a flange on the
corrector back side, exceeding the 3 inch diameter. What's more, corrector should be
attached by 6 screws from the camera side of the flange. It is not
clear to us how the designer imagined attaching of the camera,
especially when we take into account relative short back focal
distance of this corrector.
G4 camera with Wynne adapter attached to OAG. Distance
adapter is used instead of External Filter Wheel on this
Either way, we offer three variants of Wynne
The first one is designed for G4 cameras with External
Filter Wheel and OAG. The M68 × 1
ring must be removed from OAG and the first part of the adapter
must be attached instead. The second part of the adapter then
must be attached to the Wynne corrector flange. Then these two
parts can be connected using another 6 screws.
The second Wynne corrector adapter variant is designed
for G4 cameras with External Filter Wheel, but without OAG. The
necessity to attach the Wynne corrector with screws from the
camera side resulted into design, which requires M68 × 1 outer thread adapter on the G4 camera
head. Then the corrector adapter has to be attached to the Wynne
corrector with 6 screws and threaded to the M68 × 1 adapter on the camera head.
The third Wynne corrector adapter is intended for G3
cameras with External Filter Wheel, but without OAG. It is
simply attached by 4 screws to filter wheel housing and by 6
screws to Wynne optical tube.