Discovery for Celestron FirstScope
by Mike Otis
by Mike Otis
Experiment Gone Fantastic!
Change your telescope's world with a simple Fresnel card. Turn your telescope into a very fast f/.5 system!
Over the years the flat plastic Fresnel lens about the size of a credit card has significantly improved and now forms much better grade optical images. The card is designed to be used as a low cost flat magnifier and conveniently carried in a wallet like a credit card,
I bought several flat Fresnel lenses for experimentation in the lab. I noticed a long focal length of several inches and the last night out with the Celestron FirstScope, I held one up to the rack and pinion and pointed the telescope at the Moon. I was surprised to see a very tiny Moon, as if the Fresnel lens was acting like a massive focal reducer.
I set up a rig to measure the focal length of the Fresnel and got 6 to 8-inches. The Celestron FirstScope has 2.99-inches aperture with a focal length of 11.8-inches at f3.95. Using the upper limit of the Fresnel focal length, I got f/3.95/8-inches = f/.5 and a magnification of 11.8/8=1.5x. Technically, the flat Fresnel performs like an 8-inch focal length eyepiece giving 1.5 power. After learning the very bright tiny dot in the telescope was the Full Moon and not a reflection, I realized the lens was acting like a spectacular focal reducer. Having a Celestron FirstScope converted to a very fast f/.5 system is like having an all new telescope, very fast lens, or a Schmidt Camera.
Aberrations
System aberrations include small effects of flexure - deviations from a plane surface, and achromatic aberration due to using the single plastic element. When taking images for processing, the blue end of the spectrum is easily subtracted or filtered out.
Fresnel Lenses - Tips & Techniques
I also discovered the opposite effect of the focal reducer is possible. Make a 2x or greater Barlow by stacking two or more credit card lenses and observe the overall magnification increases.
Extra Experiments
* Try making a telescope using the Fresnel lens as the primary objective
and stack two for the eyepiece
* Record the magnification increases when stacking cards for a Barlow effect
* Prop one up on the telescope tube for use as a finder card
* Place over the hand controller to enlarge the display
