Building a Sun Finder

Aiming a telescope at the Sun is deceptively difficult. You can not use a optical finder for risk of eye damage. Unit power finders, like a Telrad, are of little use as you can not see the projected image. Telrads can also be damaged by sunlight. In a pinch you can simply use the shadow of the telescope, positioning for a minimum shadow. This at least gets you close.

Sun Finder
A Sun Finder mounted to a TeleVue 76mm 'scope
The best solution is to build a finder designed just for the Sun… A Sun Finder.

There are many plans for Sun Finders posted to the web. Most use a shadow or projected point of light. The version I built is no exception, using a pinhole to project a point of light on a translucent screen. The trick is to make such a device simple and accurate.

With simple metal working capability a Sun Finder like this one can be made from sheet metal, or machined from solid aluminum. I chose the latter as I had the capability. This design uses a pinhole that projects a similarly sized dot of light at the rear of the finder. The front face or the finder, through which the pinhole is drilled, creates a shadowed area for in which the projected dot can be seen.

Sun Finder
A Sun Finder in use with the projected dot visible
A longer distance from the pinhole to the screen will increase the sensitivity in aiming. In practice I have found that at least three inches is sufficient for most telescopes while keeping the device compact. Experimentation with the design can be entertaining and educational. No need to stick strictly with this design, just borrow the basic ideas, a lot of variations will work.

This design is based on an aluminum extrusion, a 3″ x 1.5″ channel. This save a good deal of machine work in creating the finder. As much of the machining is done along the length, a number of finders can be made at the same time. I made six finders from a seven inch scrap of extrusion out of the shop scrap pile.

Sun Finder Plans
Plans for the Sun Finder
The screen is made from a small piece of 1/8″ thick acrylic. Common 0.1″ thick material will work as well. One side is frosted with sandpaper to create a translucent screen. Use of a clear screen allows the solar dot to be seen from front or behind while aiming the telescope. The screen is simply secured with a glue, preferably RTV. The frosted side should be mounted towards the pinhole.

To keep the device simple there is no adjustment in aiming. If the finder is mounted reasonably well, the dot of light will be on the screen. The first time out it is necessary to first get the Sun in the field of view. You can then mark the position of the projected dot with a permanent pen (Sharpie or similar). After that aiming is simply a matter of positioning the dot on the mark. If the mark is made on the smooth side of the acrylic screen it can be easily erased and re-marked if necessary.

Done, a simple and reliable Sun Finder to work with just about any small telescope.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

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