Artificial Star

I read a Sky and Telescope article by Richard Berry describing how to build an artificial star for collimation. I borrowed the basic optical setup from the article but used found items to actually construct the unit. Here are pictures.

 The 'star' is made out a about 30" of 2" cpvc pipe painted black on the inside. The old focuser assembly from my Coulter Odessey, a condensor lens, an old 7mm ortho ocular, and a 'white' LED.

The LED is painfully bright, at less than 20ma supply current, but doesn't get warm. The original article used an incandescent high intensity lamp and focused an image of it's filament on the focal plane of the ocular. The LED is easier on batteries.

In this version it is the ~5mm dia diffused face of the LED that is projected to the focal plane of the ocular.

You can see the pinpoint of light formed here, it may not seem bright but keep in mind the light just above it is from the southern sky in mid afternoon.

The 'star' is a realistic blue white color and when viewed at night appears to be about as bright as Jupiter at opposition.

This shows how the eyepiece is inserted into the tubing 'backward'. It takes the light focused on it's focal plane and makes it very nearly collimated.

The image of the LED appears to be about 100x further away than it would without the ocular.

Normally to subtend 1 sec of arc a 5mm dia target would need to be about 1km distant. This can be achieved in about 10m with the setup above.

I have a 60m free path across the back yard over which to use it.

Here is the light projected by the condensor as it appears at the eyepiece.

Here is the cardboard mask that stops down the light from the LED before it is projected by the condensor. The pipe coupling allows the entire assembly to press fit together. Collimation is done simply by flexing the pipe till the projected spot falls in the correct spot with maximum brightness.

Here is the 'stack' formed by the cardboard aperture stop and condensor lens.

This shows the two halves of the assembly pulled apart and allows the LED to be viewed directly. It is incredibly bright.

This is the artificial star at 60m through the main scope. This is a 60sec exposure with my Genesis camera. It saturates in 500msec I will need to add an adjustable current supply to allow dimming. In the original you can read the print on the tripod head. It's only 10 arcsec tall!!! This image was autofocused by focusmax see the Servo Focuser page

I have discovered that collimating on a real star is often fairly easy to do so I don't use this unit very often. It does work very well though. I have collimated on real stars using a b/w security camera. The the diffraction rings enlarged to 6"-8" dia on a monitor it is really easy to get very good collimation.

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