Stellafane 'Little Man' - click for Stellafane Home Page TG Page Header

2-Inch Gerrish Solar Projection Telescope

Built by Jim Daley of of New Hampshire
2001-Jun-25

If you have a garden shed or a south facing home window then a nice little solar observatory is but a few pleasant atm hours away. This little observing gem is based on the Gerrish Polar Mount. I have been observing sun spots in white light with this arrangement for many years and can tell you that no other solar projector design has pleased me so much. The Gerrish mount places a refracting telescope's optical axis coincident with the polar axis about which the tube rotates carrying the eyepiece etc. around with it. Just a ways in front of the aperture is an optical flat that reflects the sun into the objective. The flat is attached to the tube thus is carried around giving right ascension pointing. The flat is adjustable on an axis perpendicular to the polar motion for declination setting. If you view the annotated photographs below a good idea can be had as to how it all works.

This Solar Gerrish is built into the south wall of my backyard observatory and projects the image upward along the polar axis to a hinged screen which is swung down from it's stored position in the rafters. Because the observing room is dark you sit inside the camera and enjoy incredibly contrasty images, just like the Porter Turret solar projection arrangement. The wind can be howling but the image stays vibration free due to the stiff nature of the Gerrish type and the small area exposed to wind loading. In the biggest puffs one observes the image shift slightly as the building flexes but no harmful image jitter is ever seen. I track the sun by gently twisting the tube in RA on it's Teflon, "V" block supported, bearing pads. The screen is two feet in diameter and the image size used most of the time is around 14-inches so the time between adjustments is long enough for good views. Image (field) rotation moves the north point around the screen as the day progresses, however, the drift direction lets the observer determine east and north easily. With a clear aperture of only 2-inches you would think the resolution a bit low but in actual practice it seems to be a good all-round size for this sort of scope. Only rarely is the seeing so good that a bigger aperture could prove useful. In the best seeing, surface mottling and linear penumbra features can be detected and faculae are beautifully defined. To vary the image size the eyepiece is changed for one of another focal length just as in any projection set-up.

Lately, a digital camera has been used to record the projected image and a sample photo is included here. One problem that shows up when photographing is the specular reflection off the screen showing up in the photo because, to get a fairly circular image, you need to position the camera (close beside the eyepiece) as normal as possible to the screen. This is avoided visually because the viewing angle is a somewhat to the side. I may try an old idea which employs a cast dental stone screen.

The instrument can be used in a direct view manner with appropriate, solar, aperture filters. Viewing this way is quite comfortable as you are seated and look down the polar axis. At night, the views of moon and planets are pleasing and provides something to do while the CCD Schmidt is cranking out a long exposure. After observing, the scope is lifted off the "V" block and put into a specially made storage box and then the wall port is closed with a down swinging door. Construction wise, the photos in this article should give the TN enough information, keeping in mind that every installation will reflect the builders site and optical "junk box". I should mention that the flat (a very high quality 2.25-inch elliptical diagonal in my version) is used at a large angle of incidence and must be really good to avoid astigmatism. Try to get 1/10 wave or better flatness. The mid-winter sun is so low that the aperture is not quite filled in my instrument. This does not seem to hurt the image too much, however, a flat with a long dimension about 1.8 times the aperture is preferred in any Gerrish system. As to the telescope: many nice inexpensive refractors are available in the 60mm size. Even some of the dept. store "500 power" jobs (swap tables are often piled high with them) sometimes have pretty good objectives and with a high quality eyepiece substituted, your on your way to a neat Solar Observatory!

For another version of a polar mounted solar projection telescope see: Sky & Telescope, September,1963 Gleanings For ATM's "A Polar Telescope For Visual Solar Work" by B. C. Parmenter.

Photo
Photo
Photo
Photo

Additional photos are available on Jim's page in the Astro Image Gallery.

Back to the Home-Made Telescope Gallery