Red Refractor

Testing

This is the experimental version of the scope utilising a lens unit believed to originate from a projector. The lens unit has a clear diameter of 72mm and is mounted in an 85 dia x 55 long alloy mount. There are no markings on the lens but focal length is approx 370mm suggesting a focal ratio of about f5. The test jig is built using 18mm MDF panels which were routed to the appropriate diameters. The panels are held at the appropriate optical spacing with threaded mild steel rods. The length can be varied by repositioning the nuts on the rods or by sliding the lens and eyepiece in the panels.

Initial tests reveal disappointingly strong chromatic aberration on bright objects like the Moon and Saturn although detail was encouraging at low powers (30-40x)

Completed Scope

I have now put the finishing touches to this small refractor seen below in “solar mode”. Although it was unlikely to become a high quality scope due to the poor optics, I thought it would be good to practice a few skills before spending any money tackling a better quality small refractor.

The lens assembly is held inside a length of PVC drain pipe by two MDF rings that are a good push fit inside. A single screw through the tube stops the parts slipping down too far.

The focuser end “plug” is again made from layers of MDF with pieces of scrap acrylic used for the side plates. The draw tube is a length of acrylic pipe sprayed black. I drilled out a piece of nylon (white) to fit inside the draw tube and accept 1.25” EPs. The rack and pinion is made from a nylon gear and a piece of corresponding toothed drive belt from an old photocopier. I cut a length of the belt and super glued it to the draw tube. The thumb wheels are ex-Meccano! Because the draw tube was pushed down a bit by the pressure from the pinion, I added a nylon wheel underneath (also from the photocopier) to resist any distortion. This all works quite smoothly and gives about 100mm travel.

The lining of the main tube is black crepe paper and the whole lot has had a quick coating of spray paint!

I tried it out with a Baader film filter and was able to resolve some sunspots. There were some interesting blue and yellow “flares” due to the poor objective quality but this doesn’t spoil the fun! At night, the scope resolves star fields quite well at low powers but brighter planets are problematic.