I can’t speak for everyone, but my criteria for evaluating a wide-field imaging scope are:
- Fast optics: since the goal is typically to capture large, diffuse, and dim objects (usually nebulae), focal ratio is key.
- Quality optics: stars should be sharp to the corners, it should be truly apochromatic, and the field is evenly illuminated.
- A good focuser: the zone of critical focus is very narrow at fast focal ratios, so any focus shift is unacceptable.
The William Optics Star 71 recently caught my eye as a replacement for my old Borg 77. I signed up for the pre-order, and Agena AstroProducts delivered my scope this week. The WO Star 71 has a stated focal ration of f/4.9 and a focal length of about 354 mm* (though this is not explicitly stated anywhere I can find), which nicely frames many large showpiece objects on common sensors. For an APS-C sized sensor, the field of view is 3.6 x 2.4 degrees. For a 4/3 format sensor like the KAF-8300, the field of view is about 2.9 x 2.2 degrees. I plan to use it mostly with the ST-8300, and this field of view nicely frames a lot of narrowband targets I’m after this year.
(* Based on the field of view calculated by astrometry.net (3.61 degrees) and Canon’s stated 22.3 mm sensor measurement on the long side for their APS-C sensor, I calculated a focal length of 354 mm, which leads to a focal ratio of 4.98 if the aperture is truly 71 mm. If they fudged a bit, and it’s 72 mm, the ratio would be 4.91, which would match the stated 4.9.)
The evening I got the scope, it was actually clear. Now, it was a lousy night for imaging because it was over 80 degrees, the winds were gusting, and high humidity made for terrible transparency. Worse, the waning gibbous moon would only allow for about two hours of dark skies. So it was a terrible night for imaging… but perfect for taking a few test shots.