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).
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.
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.
Mark Parrish is a consummate craftsman who loves making astro accessories