I occasionally use eyepiece projection for achieving larger scale. I use an eyepiece projection adapter sold by Orion. They are relatively inexpensive. The major drawback to eyepiece projection, unless you use a specialty eyepiece, is field curvature (where only the center of the field is crisp while focus drops off rapidly away from the center).

To calculate image scale and focal ratio use the following formulas (all measurements in millimeters):

1. Projection focal ratio (PFR) = Telescope focal ratio (FR) x magnification (M)

2. M = Eyepiece (EP) to film plane distance - EP focal length (FL)

EP FL

For example: Let's calculate the magnification if we used a 20mm eyepiece projection device (assuming that we are using an f/7 scope).

M = 80mm - 20mm

20mm

M = 60mm

20mm

M = 3 power

To calculate what projection f-ratio the scope is working at:

PFR = 7 x 3

PFR = 21

The shots I have taken using the 10.5 mm TeleVue Plossl results in a focal ratio of 45 with the 6-inch, f/7 scope. For f/28 shots, I used a 17.5mm TeleVue Plossl.

I have also utilized an AP Barlow (2x) which effectively doubles the magnification (or making the 6-inch f/7 scope work at f/14). On photos, such as the Straight Wall and Eratosthenes, I used a TeleVue 5x Powermate--a neat alternative to eyepiece projection, allowing me to work a 130mm f/6 scope at f/30. However, there is still the problem of field curvature.

To the next article
Back to the Basic Guide list
Back to Ann's Lunar home page