Afocal coupling tests

Part 2: Daytime, 80mm f/6 refractor, four eyepieces

For these tests I used my "ZenithStar 80 Short" 80mm f/6 refractor. This is similar to the regular ZenithStar 80, but is 65mm or so shorter to allow for use with binoviewers. To use the scope without binoviewers or a diagonal I had to use two extension tubes (54mm and 80mm length) and could still only just reach focus on this near target. Focusing at infinity should be easier. I also used a Baader IR/UV filter to reduce any camera sensitivity at these wavelengths, as they would not be correctly focused by the refractor.

I took two pictures with each eyepiece: one at the camera's widest zoom setting and one at the narrowest. 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.


SuperView 40mm, zoomed out SuperView 40mm, zoomed in
SuperView 40mm, with a 28-T step ring
This eyepiece is a 1¼" 40mm Plössl, so its apparent field of view is about 44°. Coupling it to a camera with more than 50° field of view produces very obvious vignetting! Using the camera's 4x zoom gets the second picture. This has the same magnification as the unzoomed Pentax picture below, yet looks a lot better. Perhaps I was lucky with the focus on this one.


Hyperion 17mm, zoomed out Hyperion 17mm, zoomed in
Baader Hyperion 17mm, with a 28-43mm step ring
Apart from the minor vignetting, this is a very pleasing result. The brown colour cast I saw when photographing the moon is not noticeable. The chromatic aberration visible on the zoomed in shot is probably due to the scope. At 4x zoom the overall magnification is about 113x.


MaxView 14mm, zoomed out MaxView 14mm, zoomed in
ScopeTronix MaxView 14mm, directly connected
Once again, this purpose made digiscoping eyepiece is the only one to show no vignetting when the camera is at its widest zoom setting. However, the very corners show too much lateral colour. The same true field can be better captured with a longer focal length eyepiece and a bit of camera zoom.


XW 10mm, zoomed out XW 10mm, zoomed in
Pentax XW 10mm, with a 28-43mm step ring
These pictures are a bit disappointing. There appears to be less contrast than with the other eyepieces, which is not what one expects from a Pentax! More investigation is needed.


My conclusion from these tests is that the best results are obtained with the camera zoomed in to its narrowest field of view. This is not very surprising - the camera's lens and the eyepiece will both have fewer aberrations near the centre of the field. So I've ordered a 2" 50mm T-threaded eyepiece to get some wider fields.

Part 1: The Moon, 9¼" SCT, three eyepieces
Part 3: Widefield views, DCL-52 vs Rini