Dartmouth Balloon Observatory

The Big Lens Experiment

Introduction

Early attempts to use an inexpensive video camera to image stars failed to produce images of anything except stars brighter than about second magnitude. In order to improve this performance, I removed the original lens from the video camera and replaced it with the lens from a 35mm camera. The increased light gathering power from the larger lens brought out stars down to about 8th magnitude. The video camera is a model SK2081 from an unknown manufacturer. Its spec sheet claims that it uses a SONY 1/3 inch CCD image sensor with 510x492 effective pixels. The device is very compact--25.2 mm square by 21.8 mm deep, and weighs about 30g. The supplied lens is 3.6mm focal length at f/2. I purchased the camera from All Electronics as their part number VC-28, for a mere $24.

Lens Mounting

The new lens is a Helios 44-2, salvaged from a Zenit-E 35mm camera. The lens has a focal length of 58mm, maximum aperture of f/2, and Pentax-type screw mounting. After removing the original lens from the video camera, exposing the naked CCD, the video camera slides into the end of a 50mm length of 1 1/2 inch PVC tubing. The lens attaches to the other end of the tubing with two split rings, a paper shim, and a hose clamp. The inside of the tube is lined with black paper. Sliding the camera unit in the tube allows for rough focusing, with fine focus using the built-in focus adjustment on the lens.

The result is a video camera with a field of view of about 4.5 degrees and a clear aperture of 29mm.

Test Results

To capture video I used a "USB Instant DVD" video capture and MPEG encoder device and its companion "CapWiz" software manufactured by ADS. I also monitored the video on a 9-inch black-and-white TV monitor. With the camera on a tripod and pointed at Aquila I could see stars down to about mag 8. The still below is a snapshot of the monitor screen. The bright star int the upper center is Altair (α Aquilae, mag 1.0). The bright star in the lower left is Tarazed (γ Aquilae, mag 3.0). The relatively bright star in the uppr right is ξ Aquilae (mag 5). Next in brightness, at bottom center, is ο Aquilae (mag 5). The faintest clearly visible stars appear to be about mag 8.

One great concern for balloon observations is that the vehicle sways and spins. This lens arrangement, however, gathers enough light that the stars are bright enough to remain clearly visible even with a moving camera. If the embedded player below does not work, or if you just want a bigger picture, try The Biglens Movie (wmv) or The BigLens Movie (mpg).