Afocal coupling tests

Part 3: Widefield views, DCL-52 vs Rini

This is a simple comparison of two 2" widefield eyepieces designed for afocal camera attachment. They are the William Optics DCL-52 (75mm focal length, 52mm camera thread) and the Rini 50mm (with T-thread). As in part 2, I used my "ZenithStar 80 Short" 80mm f/6 refractor. Both these eyepieces require about 35mm more "out focus" than normal eyepieces, and I could not bring them to focus on my terrestrial target without using both of my extensions (54mm and 80mm) and a 2" diagonal (approx 100mm path length). Using the diagonal introduces two problems: the image is reversed L-R (trivial) and dust on its mirror is visible as blobs on some of these images. Hopefully I'll be able to reach focus on astronomical targets without the diagonal or yet another extension tube.

Another characteristic of these two eyepieces is their very long eye relief. This makes them hard to use visually, but is a great advantage for afocal photography if your camera has a recessed front lens. The Coolpix 4500 is better than most in this respect, and to avoid vignetting I had to use threaded spacers to increase the distance from eyepiece to camera.

I took three pictures with each eyepiece: one at the camera's widest zoom setting, one at the widest setting that shows almost no vignetting and one at maximum zoom. Click on any image to see a larger version. No processing has been performed on the images other than rescaling to fit on a web page.


DCL-52, zoomed out DCL-52, slight vignetting DCL-52, zoomed in
DCL-52, with a 52mm threaded 22mm extension, a 42mm-52mm step ring, a T-42mm adapter and a 28mm-T step ring
I'm very pleased with this result. There is no vignetting over a zoom range from about 2.3x to 4x. The eyepiece has the high quality fit and finish I've come to expect from William Optics and it appears to perform well.


Rini, zoomed out Rini, slight vignetting Rini, zoomed in
Rini 50, with a T threaded 6mm extension and a 28mm-T step ring
This eyepiece is more disappointing. Both of these eyepieces should show the maximum possible field of view of a 2" eyepiece, yet the zoomed out picture clearly shows less than does the DCL-52, albeit at a larger magnification. However, the usable zoom range is 1.7x to 4x, which is a good result.


What I want from these eyepieces is the maximum field of view the telescope they're used with can provide. Both do well on this, but the DCL-52 has the edge. (I have very recently purchased a DSLR, and afocal projection with the Coolpix 4500 and either of these eyepieces gives a wider field than the DSLR captures at prime focus!)

The DCL-52 also has more eye relief, which could be an advantage with some cameras. I have never seen any suggestion that it is suitable for use with Nikon's Coolpix range, yet with suitable adapters and extensions it seems to work well.

One feature missing from both of these is some means of varying the distance from eyepiece to camera lens. This distance is critical, and adjusting it with extension tubes is not satisfactory. The "SuperView 40" tested in part 2 has this, as does a recently announced 2" eyepiece from Teleskop-Service.

Part 1: The Moon, 9¼" SCT, three eyepieces
Part 2: Daytime, 80mm f/6 refractor, four eyepieces