Image Size - What camera lens system should I use?

A simple formula can be employed to determine the size of the image on the negative based on the focal length (FL) of the lens used. The size of the image will be a function of the lens F and angular size of the object. To determine the image size, multiple the FL by the angular size in seconds for the object. Then divide by 200,000 to obtain the size of the image. The units used for the FL will determine the size units of the result. If the FL is measured in millimeters, the result will be millimeters.

Calculation of the image size of the Moon

Angular size (in seconds) * FL / 200,000= Image Size

The moon is 31 minutes of arc in apparent size or half a degree. Begin by multiplying that vale by 60 as in 60seconds per minute of arc to get a result of 1860 seconds. Divide that number by 200,000 to get the result 0.0093. Multiple the number by the FL of a lens system to obtain the resulting image size. Although only an approximation, it will be very close to the actual size you are able to obtain.

For reference, 35mm film has an approximate picture area of 15/16" x 1.4" or 24mm x 36mm. Getting within a few millimeters of the edge of the frame is not a good idea as there will be variable light levels hitting the film at that edge. The loss in lighting will vary from one camera configuration to another based on the length of the focusing tube for telescope-camera assemblies. Most telescopes are designed for viewing and not photography resulting in an effect called "vignetting". For clear consistent pictures of the eclipsed moon, you should use only about the middle 50% to 70%. Again, this will vary based on the length of the focus tube and adapter assembly. Experimentation will determine just how far you can go!

Considering the image size of the moon, a 1000mm lens will produce an image that is 9 mm in size. A 2000mm lens will result in an image size of 19 mm and a 2500mm will be 23 mm. Thus the maximum lens size to show the entire surface of the moon on the negative/slide is 2500mm. The recommended size in a good quality telescope/lens system is more like 1500mm.

To simplify the arithmetic, most people like to divide the FL by 100 to get a rough number of the size of the resulting image on film.

Thus very good images of the lunar eclipse can be obtained using standard telescopes such as a Meade ETX 90mm scope with T-adapter. The EFL of the ETX is about 1200mm meaning that the image size will be close to the optimum value. A Celestron 8" at f/10 has an EFL of 2032mm, a great image size that will be full of detail. A Meade 10" at f/10 will have an EFL of 2540mm, which is too large, and thus the f/6.3 focal reducer is recommended to produce an EFL of 1600mm.

Don't use too much magnification! The best views and images of the total lunar eclipse will include the entire moon. And the shadow falling across the craters is so subtle that photographs do not show it very well. Perhaps a digital camera or video camcorder attached to the telescope may be able to capture the images - if anyone tries this and is successful, it would be interesting to see and share here!

After determining what focal length and telescope-camera system you will be using, the next problem is to determine the exposure lengths required. During the eclipse, the Moon will darken as it goes into the umbra shadow of the Earth. The darkening of the moon will increase until it is total.