Build a clip-on finderscope illuminator

Build our red light device for your telescope to help you pick out your crosshairs on bright targets.

completed finder HEADER

If you have ever tried planetary imaging or high-powered visual observing, you will appreciate how important it is to be able to accurately point your telescope so that your target falls within a very narrow field of view.

Aligning, say, a planet precisely in the crosshairs of your finderscope is the key, but this can be tricky when the target is bright because it is hard to pick out the crosshairs.

In this How To we show you how to make an illuminator for a regular finderscope that makes its background glow red, against which the crosshairs and planet will show up clearly.

Tools – Soldering iron, wire cutters/strippers, craft knife, steel rule, craft mat, hot-melt glue gun.

Materials – Three mini LEDs, three resistors (330Ω), small quantity of thin wire, battery case (2x AA with a built-in on/off switch), flexible plastic sheet (an A4 stationary wallet or packaging).

Sundries – Solder, electrical tape, hot glue sticks.

Finish – Spray paint to match finder (optional).

tools and equipment

Our design does not require any permanent modifications and can be adapted to suit finders of different sizes.

To avoid the need for sourcing any tubing of any specific size, our design uses a simple rolled-band method and the judicious use of a hot-melt glue gun.

In this example we’ll be using strips of plastic cut from a stationery folder, but you could also use any other clean plastic packaging material – just take care when cutting it, as you’ll need a sharp knife.

If you are feeling ingenious you could adapt this aspect of the design to work with any other materials you have in your scrap box.

The 1.8mm LEDs that provide the red light are glued to a band of thin plastic around the inside of the finderscope’s dew shield.

This doesn’t greatly reduce the aperture, so it is not detrimental when locating bright objects.

Protective resistors for each LED reduce the current to a suitable level, and three LEDs produce a soft background glow.

The light level could be reduced further if required by adding more resistors.

Easy wiring

You glue the resistors to a second band around the outside of the finderscope with a pair of wires leading back to a small battery and switch case.

You may wish to locate your resistors closer to the battery case to reduce the overall size of the illuminator.

This would require four wires between the case and the finder (one positive wire for each LED and one common negative), so some old telephone cable might suffice.

Diagrams for both variants are available to download from the link at the top of this article.

Soldering is the most reliable and neatest joining technique, but you could use automotive-type crimp connectors as an alternative.

The layout is quite straightforward but you must be sure to orient the LEDs correctly or they won’t work.

The positive leg of each is only slightly longer than the negative, so identify each before you begin bending things around!

Solder a resistor to each positive leg then join the resistors together using short pieces of wire.

Join one of these wires to the positive wire coming up from the battery case.

Cover up the exposed ‘positive parts’ with insulating tape, then use short wires to join all the negative legs in a chain, and finally link this chaing to the negative wire of the battery case.

We used plastic strips to cover the LEDs on the inside of the shield, and the resistors and wiring on the outside.

Use glue to fill all the gaps between the components and to provide a diffusing layer over the LED lenses.

Once the glue is set, the whole unit becomes rigid and robust enough to be slipped off and on again as required.

We chose to add a ring of stiff card to tidy up the front and sprayed everything in black paint to match the finderscope.

Velcro pads can be used to hold the battery case onto the side of your scope or mount when you’re not using the illuminator.

Although the design is primarily for brighter targets, we found it helpful to flick the illumination on and off whilst aiming at fainter areas of sky for a reassuring confirmation of the crosshairs’ position.


Step 1

1 cutting strips

Use a craft knife and a metal rule to cut some strips of thin flexible plastic approximately 12-15mm wide to make bands for the inside of your finder’s dew shield and around 20mm wide for the outside.

We used red plastic to show up in the photos.

Step 2

2 glueing LEDs

Test fit the inner band inside the finder’s dew shield and mark where it overlaps.

Space the LEDs along it equally and use a hot melt glue gun to stick them in place.

Make sure the LEDs are all orientated the same way – the long leg is positive.

Step 3

3 soldering resistors

Insert the first band and glue the overlapping join.

Make a wider band to fit around the outside of the finder.

Fold the positive legs of the LEDs over and solder each to a resistor.

Glue each resistor to the outer band to keep them still while soldering.

Step 4

4 joining wires

Solder short pieces of insulated wire to join the free ends of the resistors in a chain.

Solder a longer wire to link one end of this chain to the red output from the battery case.

Fold over the negative LED legs create a chain as above, then join one end to the case’s black output.

Step 5

5 card ring

Insert some batteries and check it all works.

Add secondary inner and outer bands to cover all the elements and fill all the spaces with hot glue.

Take care not to stick it to the finder.

Add a diffusing layer of glue over the lenses of the LEDs.

Step 6

6 filling with glue

You can add an optional ring to neaten up the outer end.

We used thick card and painted the whole accessory with black spray paint to match the finder and offer some protection in damp conditions.

A Velcro pad on the battery box holds it on the scope.


Mark Parrish is a consummate craftsman who loves making astro accessories