Now we are going to package these components into a device you can actually use.

In addition to the electronics mentioned, you also need a project box with integral PCB guides and a ‘star mask’.

The latter comprises a precision pinhole in ultra-thin foil and mounted in a 25mm diameter aluminium disc to provide a well-defined and bright ‘star’ source.

We can determine the ‘star’ size using basic trigonometry and applying the Dawes’ limit values usually to be found in the specifications for your telescope.

For a 4- to 5-inch reflector this works out at 13µm over a test range of 6.6m; for comparison, the thickness of a human hair is 17µm.

Precision pinholes are typically (but not exclusively) available in 5µm increments, so a 10µm size will give good resolution to an intense spot of light, such as given out by a bright white LED, at the set distance specified when directed through its microscopic hole.

Mark out and drill the end faces of the enclosure for the ‘star’ aperture, and the switch and indicator LED holes.

If required you can also drill the base of the enclosure to take a standard camera tripod attachment. Stripboards will form the basis of our permanent, soldered circuits.

It is important when cutting and filing board to always wear a mask to prevent dust inhalation.

Cut the stripboards with a junior hacksaw to fit width-wise between the integral guides, copper sides facing in.

Pilot drill the stripboard that will hold the ‘star’ through the ‘star’ aperture in the enclosure box, then remove the board and open the bezel hole in steps to full size.

Install the toggle switch and shape the rear board around it, maintaining two full tracks above to act as positive and negative rails.

Cover the top of the switch casing with insulating tape to isolate it from the copper strip.

Drill a hole in the enclosure lid for the potentiometer.

Mark the ‘HI’ and ‘LO’ (maximum and zero LED intensity) positions on the outside.

Solder the potentiometer leads to the terminal lugs.

Check this by connecting multimeter leads as follows: positive to the output (red wired) terminal and negative to the input (centre yellow wired) terminal. With the meter set to ohms turn the shaft between extremes. ‘HI’ should indicate 0Ω and ‘LO’ 4.7kΩ.

Remove the stripboards from the enclosure, then complete basic assembly by fitting the ‘star’ – the LED and bezel combination to the front board using insulating washers.

Insert a short length of wire insulation through the top centre hole in the rear board.

Feed the indicator LED anode through this and the cathode through the hole below.

Continue to make solder connections, mimicking the breadboard layout from earlier.

Replace the boards in the enclosure, routing and tying the wiring back to clear the battery partition.

Proceed with the pinhole mount installation as described in the documents available at the link above.

To use the star simulator, position it at 6-7m distance from the telescope and at the same height as its optical axis. Switch the unit on and adjust the intensity of the simulated star.

With the aid of a webcam and laptop, obtain the view of the star using the telescope’s focuser.

Defocus slightly to check for skewed images such as in the topmost of the images above.

Make small adjustments to the collimation while monitoring the view on the laptop screen, until the out of focus rings are concentric and collimation is achieved, as seen in the lower of the images above.

Kim Clark

How to build a star simulator