Buiding a 'focused' Krupa collimator to collimate the primary mirror

(klik hier voor de Nederlandse versie)
There are different tools to collimate the primary mirror of a Newtonian. Well known and used around the world are the cheshire eyepiece and the barlowed laser. Much less known and used is the Krupa collimator. Information about the 'rivised' or 'focused' Krupa collimator, the one I built, can be found in an article on 'IciInSpace', by Dave Nicholls. His explanation of how this collimator works and his building instructions are very clear, so I won't explain it here in detail. Most important thing is, that the lens (see below) needs to be one focal length away from the screen and, to the other side, one focal length away from your eye. What you see when looking into the Krupa is a red screen, with the dark back of a small led centered in it (see last photograph below). The lens magnifies what you see, so everything is very well visible. When collimation is OK, the primary mirror's center dot is centered around the dark looking led. If you build one, look for a lens of 25 - 50 mm focal length, or a bit longer if you can't find one in that range. If it's not achromatic: no problem. If the lens is convex-concave or plano-convex, the convex side ought to face away from your eye.

In the photograph below the parts of one of the 'Krupa's' I built are shown: the main tube, made of 35 mm outside diameter aluminium and turned to 1.25" inside diameter on my lathe, the spacers, made of 1.25" outside diameter PVC, the 1.25" diameter, 35 mm focal length lens, the transparant plastic window, with a hole for the 3 mm diameter, transparant 2000 MCD red led. The top of the led needs to be sanded off untill just above the light emitter and polished to be clear again. I was just making a mirror, so I polished a couple of minutes with cerium oxide. Look from the side while grinding the top away and take care not to damage the very thin lead to the chip. Sanding and polishing takes only five to ten minutes. Another important thing is blackening the led very well, exept for the sanded emitter side. That can be done with black paint. Just stick a small piece of tape on the emitter side of the led, dip the led in black paint and let it dry. This needs to be done a couple of times, to prevent light escaping -mostly from the rim- of the backside of the led. Look for light escaping from the back with a magnifier, because without a magnifier you easily mis it, while it can still be visible in use. I glued the led into the hole in the transparant window with some metal epoxy, but another kind of glue will also work. Total cost, including the lens is around $10,-.

The pictures below show the assembled collimator and a second one I made, with the main tube made of PVC and the spacers (not visible) made of aluminium tubing. The leds are 1.8 Volt and run on two 1.5 volt AA batteries, with a 100 ohm resistor between the batteries and the led. On the second photograph below, the 3V battery pack is shown.

The collimator works very well and very accurately. When the krupa showed excellent collimation, this was also the case in my cheshire. It may take a couple minutes of practicing, to hold ones eye in the correct position to see the red transparent screen, the dark back of the led and the dark ring-shaped shadow of the primary mirror spot. With the eye too close to or too far from the lens or not centered, the red screen and the shadow of the mirror's center spot are not (optimally) visible or you will see only darkness. The collimator also works well with a non-perforated center spot, but but the accuracy of the collimation can be seen a bit easier when the spot has a central hole.

Below is a photograph of the inside of a Krupa collimator, made by Nils Olof Carlin. The red screen, the black back of the led and the dark shadow of the center spot are clearly visible. Also visible are the wires from the led to the battery pack.

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Email: Jan van Gastel