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Interfacing G3 and G4 cameras with 3" coma-correctors
 Fast Newtonian telescopes provide excellent price/performace ratio. They offer greatest aperture for the money, because their optical design if very simple, elegant and efficient. When a Newtonian telescope is build from high quality component (Pyrex mirrors, carbon tube, robust mirror holders and focuser, ...), they are hard to beat by any other optical design. The only problem remains — parabolic mirror creates perfect image on the optical axis only, aberration called coma distorts the image as the field of view increases. Fortunately advanced refracting correctors eliminate this aberration and help to make Newtonian the perfect wide-field instrument.

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 G3/G4 cameras:

  • 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 image.

Either way, we offer three variants of Wynne adapter:

  • 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.

    Remark:

    Please note the Wynne cannot be used with G3-OAG simply because of too big outer diameter of the corrector flange, which prohibits placing of the guiding camera close enough to the optical axis.

  • 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.

 
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