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Construction of a reflex sight The popularity of unit power finders made me wonder if it was possible to construct one myself. The principle of these finders is relatively simple. A reticule or point like source is projected as a parallel beam. The eye sees this reticule as projected onto the night sky. The same principle is used in HUD in airplanes. Align the reticule with your telescope and you've got yourself a unit power finder. In true ATM spirit, I constructed the finder with very basic materials: some plywood, a little PVC plumbing and some basic electronic components (LED, resistor, ...). The only component that might require a little effort to find is a little magnifying lens with a diameter of 25 mm or more. I found an old small binocular with 30 mm lenses and a focal length of about 75 mm (This is perhaps a bit short). Another good source are the ubiquitous yet useless 6x30 finder scopes. Mostly they are made with an achromatic lens with a focal length of about 110 mm. The general principle is shown in the picture below. A more detailed construction plan is provided as an AutoCad drawing. BOM: PVC pipe diameter 50mm PVC sleeve F/F 40mm PVC pipe 40mm Magnifying lens 30mm Window glass Mirror glass Threaded rod M4 Wing nut M4 Bolt M4 Standard 5mm LED Resistor 470 Ohm Potmeter 4,7 kOhm + switch 9V battery 20 cm (length depends on focal length of the lens) 1 7 cm 1 58mm x 80mm x 2mm Ellipse 40mm x 57mm 25 cm 4 1 1 1 1 1 Total cost price: less than 10€ and a lot of building fun (the original Telrad© costs approximately 90€). The PVC sleeve fits neatly into the 50mm PVC pipe and will be used to hold the led and the reticule. In a first attempt the reticule is replaced by a pinhole. This pinhole was made by piercing a piece of folded aluminium foil. Later a true reticule will be made. The position of this holder can be adjusted in order to be able to focus the pinpoint to infinity, in other words: the distance between the reticule and the lens equals the focal length of the lens. With 3 wing nuts the finder can be aligned with the telescope. By turning these nuts the tilt of the mirror is changed, causing the reticule to move in the finder window. A precise alignment is hereby possible. In order to be able to locate faint objects the intensity of the led can be changed with the potentiometer. For the moment I can see a couple of drawbacks in the design: A double reflection may be visible in the finder window due to the thickness of the glass. By minimizing this thickness, this problem is reduced (I wonder how the commercial types solve this problem). Probably a little heater and dew shield may be required in order to keep the finder window dew free. But only by weather with a great humidity the lack hereof causes a problem. On the other hand, since the viewing window isn't an optical element, one can simply wipe it dry with a cloth. Due to the aberrations (mainly spherical I think) of the lens there will be a parallax error. For the moment I have no absolute values for this error. A lens with a longer focus will reduce this error at the cost of a larger finder. Instead of the plain mirror glass, which is in fact a Mangin mirror (a back reflecting mirror), an optical flat, front reflective mirror will be better. In order to replace the LED or the reticule one must remove the potmeter. But since the LED and reticule last probably a lifetime... All of these flaws are nothing compared to the ease of use of a reflex sight. It's not meant to be a replacement for the more common finders, but it sure is a valuable addition to my telescope. Just one precaution: never let the sun hit the lens, because the reticule would be ruined and it would probably set your whole reflex sight on fire! Step-by-Step construction: 1) Create a pinhole or a reticule and fix it in the 40mm sleeve. 2) Fix the LED and make the necessary connections as shown in the following diagram. 3) Drill a 4mm hole in the sleeve to accommodate a M4 bolt. The holder should now look like this. 4) Take a piece of 40 mm diameter pipe with a length of 70 mm. One end makes an angle of 45° with the axis. 5) In the straight edge put a piece of 12mm plywood. The centre of a hole cut with a 40mm hole saw is perfect to do this. Drill three holes in this piece of plywood so that they make an equilateral triangle (120° apart) and fix three lengths xxx mm of threaded rod (M4) in them. 6) Glue the elliptical mirror to the chamfered end. The mirror holder looks like this. 7) Take a 50mm PVC plug and drill 3 holes 120° apart. 8) Heat one end of a 250mm length PVC pipe (diameter 50mm) and put in the plug. 9) As close as possible to the end of the 50mm pipe, make a hole approximately 40mm square. 10) Cut the plywood according to this plans and glue them to the pipe. 11) Attach the lens in the hole and fix the window glass to the plywood. The result looks like this. 12) Put the reticule holder in the pipe and turn on the LED. Find the position of best focus and mark this position. 13) Make a groove 4mm wide along the axis of the 50mm pipe and centred on the previously made mark. 14) Assemble the finder. 15) Make a hole for the potentiometer. 16) To attach the finder to the scope, fix 2 pipe clips to the OTA. 17) The final result!