A 3D Printed Finder ‘Scope

I wanted a smaller, lighter finder ‘scope for the Astrola, and I wanted right angle to stop straining my neck!

3D model of the 40mm finder ready for printing
3D model of the 40mm finder designed in OpenSCAD

I have been using a big Orion 9x60mm finder on the Astrola, the only telescopic finder ‘scope I have among the Telrads. It is nice, with a bright image that shows many faint fuzzies. It is also huge, and very heavy, it is also straight through, requiring one to crane your neck around to view through it on the Newtonian ‘scope.

A small, lightweight finder was in order, a perfect project for a newly arrived 3D printer.

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A Sun Finder

A few years back I posted the design of a Sun finder. With the eclipse looming it is time to highlight that post again. Aiming your telescope or camera at the Sun can be a nuisance. A problem with a really simple solution.

Sun Finder
A Sun finder mounted to a TeleVue 76mm ‘scope
The idea is simple… A pinhole that casts a small dot of light on a target. Line up the dot on the target and the Sun should be neatly in your field of view. Having used these devices many times, it really is that easy, Sun in view in seconds.

Sun Finder
A sun finder in use with the projected dot visible
My version is a custom machined part made from aluminum and plexiglass. In a crush a similar item can be made from cardboard and tape with a pair of scissors, and probably function just as well.

I would suggest light cardboard, the type used in a cereal box, and some masking tape that will peel clean off your telescope or camera. Just line up the telescope once and mark the dot of light’s position with a pen… Done.

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.