Download Construction of a unit power finder

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!