Moravian instruments, Inc., source: http://www.gxccd.com/art?id=512&lang=409, printed: 24.7.2017 20:41:06
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|Interfacing G3 and G4 cameras with 3" coma-correctors||Updated: 25.5.2017|
|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 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.
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.