LAB EXERCISES WITH A CCD CAMERA

by

Laurie Ann Holle, Russell Plummer, David C. Schlereth and Mark G. Strobel

(Harald Schenk - Advisor)

INTRODUCTION

In previous years, amateur astronomers had to rely on photographic film in order to obtain celestial photographs with a telescope. Today, the same images can be obtained electronically. Simple point-and-shoot digital cameras can be used to get images of the Moon and the planets. The advantage with digital cameras is that results are visible without a lengthy wait to get the film developed. This first section is an attempt to explain the procedure in taking simple astro-photos.

PROCEDURES

The easiest way to get an image using a digital camera is to point it directly against the eyepiece. This method is called AFOCAL PHOTOGRAPHY. The following formula can be used to calculate what the resulting EQUIVALENT FOCAL LENGTH of the afocal telescope/camera lens combination will yield. Note that for our experiment we used an 8" Celestron with a focal length of 2000 mm. It contained an eyepiece of 25 mm focal length. The camera had a lens of 39 mm focal length. The formula is:

Effective Focal Length + (F1/F2) x F3 = 3120 mm

Where F1 = telescope focal length (2000 mm)

           F2 = eyepiece focal length (25 mm)

           F3 = camera lens focal length (39 mm)

The effective FOCAL RATIO of an afocally-coupled camera and telescope system can be determined by dividing the system's focal length as determined above by the telescope's aperture. In the case of the above, the sysem's 3120 mm focal length is divided by the telescope's 200 mm aperture to yield an f/15.6 photographic system.

RESULTS

DISCUSSION

Using afocal photography requires the lens of the eyepiece to be in close contact with the camera lens. Many eyepieces do not permit this close contact. As a result, an eyepiece is required that has a long eye relief. We found that if the telescope is focused with the eyepiece, and the camera then held against that eyepiece, that the image would be in focus.

By replacing the digital camera with a CCD camera, you can obtain better images at longer exposures. We used a Starlight XPress 716 CCD camera for our initial experiments.

The camera sends the image to a laptop. It is a simple matter to store each image in a folder for later analysis. The exposure can be directly controlled on the screen. The first problem is to center the target in the field of view. We use a Meade beam-splitter. It allows us to find the object with a regular eyepiece. By flipping a mirror out of the way, the beam of light is then directed on the CCD array. The next problem is to get a proper focus. We found that a focus with the eyepiece is NOT the same as a focus with the camera.

EARLY RESULTS

         Images are displayed on the laptop monitor.                      Jupiter                          Saturn                                  Mars