Quick test of a Optec NextGen 0.5 SCT reducer


Optec NextGen is a series of focal reducers / correctors for SCT telescopes. They come in three versions: 0.7, 0.5 and 0.33 reduction factor. I've got the 0.5x version supporting sensors up to 17,5 mm diagonal.

Description of the reducer

Optec states that it corrects for field curvature and spherical chromatism. There is no info if it corrects for coma (like in Starizona unit description)... however many correctors/reducers for SCTs doesn't describe what they actually do and correct. The NextGen looks like so:

Optec NextGen 0.5

From the telescope side we have a 2" barrel that can be put in a 2" focuser or 2" eyepiece holder. For C11/C14 Baader ClicLock could be handy (as it uses the 3" SCT thread without the standard SCT thread).

From the "camera" side there is no thread. You have to have a adapter plate from Optec, and they make many plates depending on your camera... but it's bit of an overkill. My reducer arrived with the simplest solution - low profile T-threaded plate.

The CCD to reducer distance is quite low - 60,5 mm and thus it's hard to use OAG + Wheel at once if they aren't integrated in the camera body. In my case NextGen to TruTek wheel and that to DSI III Pro with a low profile T-threaded mounting plate gave the optimal distance (a lot is lost on the threads, adapters and the Optec plate). Using TS filter drawer (15 mm) + TS-OAG 9mm + 360 deg rotatable/detachable T-extension (5,5 mm) could allow both OAG and filter changer at once (but not all filters work with such thin filter drawer).

Initial tests

As I don't have the ClickLock holder for my C11 I used JMI motocryford that hold the 2" reducer. Combined with TruTek manual filter wheel (1,25" filters) and the DSI III Pro I've made few basic tests to check / compare it performance vs standard f/6.3 (and f/3.3) SCT reducers.

Defocused star test with Optec NextGen 0.5x
Defocused star test
Star field test
Star field test

Two images above show tests made with Altair. Defocused test, although shows slight scope miscolimation doesn't show any noticeable problems in the corners (compared to this vignetting case). Second, focused image allows to check star shapes in the camera FOV - and also the shapes are for me very good (they aren't astrograph quality, but it's a strong reducer).

Optec NextGen 0.5x and CCDInspector
Optec NextGen 0.5x and CCDInspector - curvature

CCDInspector wasn't happy - not as many stars as he wanted but still I got some results. It showed the slight miscollimation, remaining field curvature and that the camera is slightly off axis (due to inaccurate DSI adapter). Top left corner is also affected by amp glow from the camera.

Optec NextGen - chromatic aberration test
Last test was to check if it adds noticeable chromatic aberration like my Hirsch f/6.3 reducer. Using Orion RGB filters and a Bahtinov mask I've checked if the spikes would move between filters (they don't without any reducer). As the test showed there is no eye-noticeable focus shift which is a very good result.
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