Afocal coupling tests

Part 1: The Moon, 9¼" SCT, three eyepieces

This is an attempt to compare three widefield eyepieces when used for afocal coupling photography with my Nikon Coolpix 4500 camera. All three pictures were taken on 11th December 2005 at around 19:00 GMT (which is also local time!). The camera's zoom was at the widest setting — 7.8mm focal length. The aperture was f/2.6 ie 3mm: comfortably larger then the exit pupil of all three eyepieces. The telescope used is a Celestron 9¼" SCT mounted on a Sky-Watcher EQ-6.

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.


with 17mm Baader Hyperion Baader Hyperion 17mm, with a 28-43mm step ring
1/125 sec

The most striking thing about this picture, compared to the other two, is the brown tinge. The Hyperion eyepieces are believed to be derived from the Vixen Lanthanum range, which has a reputation for a brown colour cast.

This combination of eyepiece, step ring and camera does show some vignetting, particularly visible at the lower right corner. The assymetry of the vignetting is slightly worrying. I don't yet know the cause of this, but maybe my step ring is not truly concentric.


with 14mm ScopeTronix MaxView ScopeTronix MaxView 14mm, directly connected
1/125 sec

This eyepiece is the only one of the three that is designed with "digiscoping" in mind. It screws directly into the Coolpix camera's filter thread with no need for a step ring. This gets the camera lens very close to the eyepiece's eye lens and results in no visible vignetting.

The ScopeTronix adapter is not without its faults. It has some lateral colour, as can be seen on the ridge at the right of the picture, about half way down.


with 10mm Nikon XW Pentax XW 10mm, with a 28-43mm step ring
1/60 sec

Subjectively, I think the Pentax shows the most contrast of the three, but it is hard to compare when each eyepiece produces a different magnification. It has slightly more vignetting than the Hyperion, and still with some assymetry.


To conclude, all three of these eyepieces are usable for afocal coupling photography, but only the ScopeTronix allows the camera's widest angle zoom setting to be used. The vignetting with the other two is not necessarily a problem though - the camera lens probably has most distortion at its widest zoom setting, and most eyepieces are not as sharp at the edge of field as at the centre. Zooming the camera in a bit should reduce these problems.

The Pentax and Baader eyepieces both require a step ring, which can be hard to find. I did extensive web searching and eventually found one at USCamera.com. I think step rings are intended to be used to mount filters on a camera, where some eccentricity would not be a problem. Finding a ring that holds the camera exactly on the eyepiece's optical axis may be a problem.

Part 2: Daytime, 80mm f/6 refractor, four eyepieces
Part 3: Widefield views, DCL-52 vs Rini