Solar Finder Scope: Free or Cheap 45 Minute Build

Overview

This solar finder scope works on any telescope with a tube. I show it attached to the Celestron Nextar 8 (CN8) but it can be used with any telescope with a simple adjustment to the elastic strap. The idea occurred to me while watching the Total Solar Eclipse on August 21, 2017. A fire fighter named Chris generously let me peer through his dedicated solar scope at a public viewing site we were both at. It had a very nice solar finder, using ground glass projection.

It was so convenient I decided to build a ground glass type solar finder of my own. With it, you can find the sun without sighting down the tube or otherwise risking your eyes. I used scrap I had on hand, but you can buy the parts for less that $10 at Home Depot. It takes about 45 minutes to make, start to finish.

Features of the Solar Finder

1. Crisp ground glass imaging;
2. Safe viewing for aligning a telescope with the sun;
3. Automatic alignment of solar finder with telescope tube. Since it is made from a tube, provided the cuts are straight, it will automatically align itself to the telescope tube thus eliminating irritating and time consuming finder alignment; and
4. Fast and easy attachment to telescope using bungee cord.

How this scope works

This scope works by projecting a pinpoint of light through a hole in the front sight onto the rear sight. By slewing the scope, the user can look at the rear site from almost any angle to align the bright pinpoint with a mark on the rear sight, and thus with the sun.

A Word of Caution

Looking at the sun without a proper solar filter is certain to damage your eyes. Don't do it. Never look directly at the sun and never point a telescope at the sun without a sun filter in place, it will burn out the optics in your telescope (this is no exaggeration).

To complete the build you will need:

Supplies:

1. Piece of PVC tube 2" or more in diameter; 12" or so in length.
2. Clear plastic sheet like Plexiglas, enough for two 2x2" square pieces - mine was 1/8" thick.
3. Bungee cord long enough to go around your telescope's main tube. My tube is 8" so I used a 24" flat bungee I had lying around.
4. Fine sandpaper (400 or 600 grit).
5. Epoxy or hot glue. I used hot glue only because I happened to be out of epoxy the night I decided to build this. If I had epoxy on hand, I would have used that because I believe it's stiffer.
6. Spray paint - I used flat black model paint, but any opaque paint will work.

Tools:

1. A saw. I used a radial arm saw which was ideal for this project. If you don't have a radial arm saw, you will need to control the angle and depth of cut to make perfectly perpendicular cuts. This can be done on a table saw or it can be done carefully with a hand saw, provided you use a guide to control the angle of the cuts.
2. Drill.
3. Hot glue gun if you use hot glue rather than epoxy.
4. A clamp. This project can be done without a clamp, but I used a clamp to hold the PVC tube in place while cutting it.

Cut the PVC tube in half lengthwise. The result of this step needs to be straight. Use a jig to hold the tube safely and securely in a fixed position so you can cut it in half. You can see the setup I used with a radial arm saw. Using this setup, cut the length of pipe part of the way down (I cut it about 3/4 of the length). Be careful, if you use this setup to avoid hitting the clamp with the saw. Depending on the length of your scope, you will have about 12" of half tube. Crosscut the tube into two pieces of half tube.

Next, cut two slots, one near each end of the half tube. To do this with a radial arm saw, raise the arm so that the saw blade leaves about 3/8" uncut. In my case, the saw cuts a kerf that just about exactly fits the width of the Plexiglas I had on hand. You may have to adjust the width of the slot. You want it as close to the width of the Plexiglas sights as possible.

Drill holes on both sides of the tube for the bungee cord. In my case the hole required was 3/8". Yours may vary.

Cut two 2" square pieces of Plexiglas.

Peel off any protective plastic film.

Sand the edges smooth being careful to remove any bur on the edges. A bur may interfere with mounting the sights in the slots in the PVC tube.

Drill a hole in the center of the front sight. Paint this sight on one side and allow the paint to dry before mounting it in the tube.

Take the rear sight (without the hole) and sand it on one side using fine sandpaper to produce a ground glass like surface for projection.

Glue the front and back sights in place. If the slots are not snug, shim them prior to gluing so the sights are perpendicular to the tube. Allow the glue to dry before proceeding.

Fasten the bungee cord to the tube. In my case the bungee had a fastener already attached which I removed from one end so it could be threaded through the hole in the tube. I tied this end in a knot and left the fastener on the other end. Depending on your circumstances, you may need to cut your bungee. The length of the bungee should be about 3X the width of your scope tube. My tube is 8" so I used a 24" bungee which was about perfect.

NOTE: make sure any knots or fasteners fit comfortably under the tube. If the knots or fasteners cause the finder scope tube to rise up off the telescope main tube, it may skew the alignment of the finder scope spoiling the automatic alignment feature of this design.

Mount the finder scope, a wide field of view eyepiece and a solar filter on the telescope. Slew the telescope so that the bright dot appears in the center of the rear sight. Viewing through the eyepiece, finish final adjustment to place the sun in the center of the eyepiece. Once the telescope is on center, place a black dot in the center of the bright dot formed by the sun on the rear sight. Alignment complete.

To place the sun in the center of your eyepiece in future viewing sessions, attach the solar finder to the telescope and slew the scope in the approximate direction of the sun. Once the bright dot appears on the rear sight, slew around until the black dot is centered in the bright dot and voila you're on target!