A Simple Star Testing Telescope

Designed and built by Jerry Oltion

Featured in the April, 2009 issue of Sky & Telescope Magazine

Kathy and I and a bunch of our friends get together on Sundays for what we call our "Knitty Gritty" session. The women knit and the guys grind mirrors. When we got to the point where several of our mirrors were nearing completion, we started thinking about how to star test them all, and rather than build a different scope for each mirror I decided to build a single platform that would hold any mirror and let us position the secondary and focuser at the right distance for anything we were likely to grind. I wound up with the ugly but serviceable scope you see here. You place your primary on the bottom board, where it's supported by two pegs on the lower edge and three cork pads on the back. The secondary and focuser slide up and down the main rail (a 2 x 4) to the right height, and the secondary can slide in and out so it's centered over the mirror. You collimate by twisting the secondary, twisting the focuser board, and with the screws sticking out of the mirror board. (They push on the backing board to change the angle of the mirror.) The whole works is designed to point at one target: Polaris. Polaris is bright enough to star test on, and it stays put long enough to let you examine your mirror's diffraction pattern without constantly nudging the scope. That simplifies the design considerably: I just prop it on a stick of the right length to hold the front of the scope aimed at Polaris. (The stick is a 1 x 4 mounted on the main rail with a hinge so it won't slip and so it will fold up for carrying.) The 2 x 4 was a little wobbly on its own, so I added a cross-brace -- another 1 x 2 mounted diagonally from the opposite end of the bottom board to the top of the rail.

Here's the primary mirror support from the other side. You can see that the 2 x 4 rail is screwed on, but it's also glued to make it stiffer.

Here's the primary mount from above. Note the pegs that the mirror rests on and the cork pads supporting it on back. Also notice the holes drilled for a different-sized mirror. (The pegs should fit snugly into the holes.) The idea is to keep the edge of the mirror a fixed distance from the right edge of the board so the focuser is always as close to the incoming light path as possible. I also tried to keep the center of the mirror on the same line for as many diameters of mirror as possible so I could keep the focuser at a standard distance from the 2 x 4 rail. (If it gets too close, the focuser bumps into the rail -- see below.) With mirrors larger than 10", the center creeps upward as well as outward to keep the bottom of the mirror from hanging off the bottom of the board. You can see where I've marked the center of each size mirror I'm likely to use. The lower edge of the mirror can't stick out below the board because the cross-brace would cut across the light path. (The brace looks vertical in this picture, but that's just a trick of perspective. It actually tilts over to the right and joins the main 2 x 4, as you can see in the other pictures.) The collimation screws are just 1/4 x 20 bolts with rubber hose over the ends so I can turn them with my fingers. I tapped threads straight into the board rather than using T-nuts, and the threads seem to be holding up fine. They push on the board below, which lets you tilt the mirror in whatever direction you need for collimation. I've got 4 collimation screws here because a central screw on top would get in the way of a 16" mirror. It's not hard to get used to 4 rather than 3, and in fact I like being able to tilt the mirror at 90-degree angles rather than 120-degree angles. It would be easy enough to design it with only three screws, though, if you want.

Here's the focuser board. You can see here and in the photo below how it's held on with a simple C-clamp. I've drawn lines to help me align it over various diameter mirrors. Note the series of holes that let me move the secondary spider (a shelf bracket) upward to allow centering larger secondaries in the focuser's view. (Yes, the holes to the right of the nuts are mistakes.) As noted below, you can collimate the secondary in one plane by simply rotating the whole focuser board around the C-clamp.

This is the spider vane, made out of a shelf bracket. The secondary can slide outward along it to account for different size primaries. I've marked the center distances for various mirrors so placement is easy. The spider vane wiggles quite a bit. I thought that was going to cause a lot of trouble, but it damps down pretty quickly and it doesn't start up again when you're operating the focuser, so that turned out not to be a problem.

This is the clamp that holds the secondary to the shelf bracket. It's just a piece of thick metal strap bent into an "L" and screwed into the wooden secondary support, with 1/4 x 20 threads tapped into the clamp side. I use a wrench to tighten this rather than trusting finger-tightness. I don't want the secondary falling on anything! (You can see the chip on the lower edge where it did once.) The secondary mount has collimation screws, but I've learned that the simplest method is to twist the whole works. The clamp bolt lets the secondary move toward and away from the focuser just enough to let you collimate in that plane, and the C-clamp lets you twist the entire focuser board in the other plane. It's a fine enough adjustment that there's really no need for the adjustment screws.

A Few Other Design Considerations:

I built the base on mine too small, so it's a little unstable. That cost me a secondary mirror one night when I lifted off the primary to take it back inside and the scope tipped over onto the pavement. It's probably a good idea to make your base wider or put some outriggers on it so the scope can't tip over.

You might want to put some kind of failsafe catch on the secondary so it won't fall off if the clamp bolt loosens. A string going over the top would provide some insurance.

I keep meaning to mark a scale of inches on the side of the main rail so I don't have to use a measuring tape to figure out where to clamp the focuser board every time I test a new mirror, but I haven't gotten around to it yet.

How to use it

Here's the steps I go through when I star test a new mirror:

- Set the primary mirror outside somewhere safe to cool.

- Insert the pegs into the appropriate holes for the mirror's diameter.

- Place the cork pads somewhere near 70% of the mirror's diameter, and spaced roughly 120 degrees apart. (I use "Tak 'n Stick" poster mounting clay under them so I can peel them off and stick them down again and again.)

- Adjust the secondary to the right distance out from the focuser.

- Clamp the focuser the appropriate distance up the side rail from the primary. (Subtract the secondary-to-focuser distance from the primary's focal length. Aim for putting your focal plane at the top of the focuser in the middle of your focuser's range of travel. Remember the thickness of your primary when you do your measurement.)

- Aim the 2 x 4 rail at Polaris. A simple finder mounted on the rail helps, but it'll only get you close because the aim is never exactly the same from one mirror to the next (or even from one collimation attempt to the next). Remember to lift the prop while you're moving the scope from side to side so it remains vertical in the side-to-side plane, and remember to keep the bottom of the prop OUTSIDE of vertical in the front-to-back plane. If you pull it too far toward the primary, the whole scope could tip forward and fall over.

- Place the primary on the base. (You didn't do this earlier because you don't want to drop a C-clamp or the secondary on it!)

- Collimate. I mark the center of the primary and use a laser collimator, but a cheshire works fine, too. (A center dot on the primary is still useful.)

- Use a wide-field eyepiece to find and center Polaris. If Polaris is way out of focus, you might have to reposition the focuser board and recollimate.

- Give your mirror time to cool if it hasn't already! (This is vital. A mirror becomes more parabolized while it's cooling, so it will look more parabolized at first than it really is.)

- Put in a high-power eyepiece (about 25x per inch of mirror diameter) and see what the image looks like.

I won't go into the details of star testing here. Check out Harold Richard Suiter's Star Testing Astronomical Telescopes for the definitive word, or any of several good web sites on the subject. But the basic idea is that you want your focused image to be sharp, and you want your outside-of-focus image to be the same as your inside-of-focus image. (The secondary shadow shows up at about the same distance inside as out, and the diffraction rings look similar, with no rings brighter than others.)

That's basically it. I hope you find this design useful.

-Jerry Oltion

How to contact me

I'd love to hear from people who are interested in this scope design. Please feel free to email me at the address on the right. (Sorry you can't click on it or copy and paste it; it's a graphic file to thwart spambots that search the internet for addresses to send junk mail to.) I have no idea how much mail this idea will generate, so I can't guarantee a response, but I'll do my best to answer everyone who writes with a genuine question or comment about the design.