Fun With



More Images...

Here's something to try with a new digital camera and some old lenses...

Plugging the video out from the digital camera into a monitor is a great convenience when aligning the setup!

Design Limitations and Design Details

This system is limited to photographing the moon, unfortunately. I believe this is due primarily to two factors: aperture and chromatic aberration.

The Olympus C730 viewfinder uses a small electronic LCD display. Because alignment must be done via this viewfinder, the visibility of objects through the viewfinder establishes practical limits. The apparent brightness of objects in the viewfinder is determined in part by the (external) lens aperture. With the best possible camera and lens settings, planets such as Saturn and Jupiter are still a bit small as they become too dim to sight on. (Note that the exposure time for the camera can be manually increased, but the exposure time associated with the viewfinder during alignment appears fixed and is relatively brief. This limits the effective sensitivity during alignment operations.)

Chromatic aberration is also a limiting factor. This is a lens characteristic that results in false coloring that is noticeable at the edges of objects. The image of Saturn below illustrates the limitations of the system.

The overall elliptical shape of Saturn is apparent but the magnification is insufficient to reveal the rings. (This image is a small cropped region, 1/400 the area of the original). Also note the false coloring with the red and green, due to chromatic aberration of the lens. This image was acquired with a magnification of ~80x, with 10x from the C730 optical zoom, and an additional 8x due to the external lens apparatus.

The external lenses consist of a 200mm telephoto zoom lens (aperture 1.875", 1:4.5) together with a 2x teleconverter extender which together act as the 'objective' lens. A 50mm lens (1:2.8 aperture) then acts as the 'eyepiece' lens. These lenses are typical for 35mm photographic cameras - not professional grade. The 'objective' and 'eyepiece' lenses are mounted back-to-back. Mounts are made using lens back caps (dust protectors). Holes were cut in the back caps and plumbing adapters were found to form the whole lens mount. The back caps snugly fit into 'ABS cleanout bodies' which were connected by a nipple, permitting the lens separation to be adjusted during assembly. The lenses are Pentax bayonet type.

In operation, the zoom adjustment, lens apertures, and focus rings on all the (external and internal) lenses can be adjusted. This provides nice flexibility. Also the camera has a video out which is quite useful. The video out is fed into a small TV (via RF modulator). This displays the camera view during all pointing and alignment operations, and eliminates awkward viewing positions when using the viewfinder.

The 'fine' positioning is accomplished with 1/4-20 all thread. This seems adequate given ~18" radius (about 1/6 of a degree change in pitch/yaw, for a full turn of the adjustment). The pitch hinge is the type used for cabinets, and seems acceptable. The yaw bearing is a 4" turntable that would be used for a lazy susan. This is a weaker point of the design. It has a bit too much play, but is tolerable.

Images of the moon were stored as grayscale format, to eliminate the appearance of the chromatic aberration. This eliminates the false coloring, but these areas remain somewhat blurry compared to other parts of the image.

Latest idea: As images are typically resized to smaller dimensions for posting on the web or for storage, I am now reducing the amount of magnification in the digital camera. This seems to be the weaker link, optically (sorry Olympus). Images seem a bit crisper and the chromatic aberation is somewhat reduced. See images of the lunar eclipse on May 15, 2003.