Finderscope Mod Last revised: 06/22/02 Astronomy

 Light Block Disc 

The Problem

The bane of urban amateur astronomers is, of course, stray light.  Light from streetlamps, passing cars, house lights, display lighting, etc., makes it hard to stay dark-adapted and focused on the celestial target of interest.  So we must make adjustments in our observing location and behind-the-tube setup.  This means keeping the eye close to the back-body of a SCT and/or adjusting the position of the diagonal.  The veteran urban amateur will find themselves doing these things unconsciously.  What I find most annoying is lifting my eye up to a straight-through finder and being exposed to the light-glare I'd been avoiding behind the main eyepiece.  

The Solution

Light Block on a C5++

Figure 1.  Disc on C5+ finder.

The directions here are to build a finderscope "light-block disc" to attenuate the problem.  The disc slips onto the finder and forms a light shield against oncoming light.  Figure 1 indicates the light-block with a red arrow.  For scope transportation it can be taken-off and stored flat.  Construction is very inexpensive and doesn't require much precision to build.  Consider the directions here as a prototype and build the light block that suits your finderscope.  For instance the circular model pictured matches the telescope aesthetically, but a rectangular one or one that drops-down and merges more with the main tube may be better for you.  


  • A sheet of black board at least 2-mm (1/16") thick and large enough to cut a 10-cm (4") diameter circle from.  You'll find this at an art-supply store -- usually black on one side and white on the other.  Do look around for substitute material first: make sure it is rigid enough not to flex, easily cut by scissor or hobby-blade and folds without cracking.
  • Compass or template to draw circles.
  • Tool that can cut the material in a straight line or circular curve: scissor or hobby-knife/single-edge razor blade.
  • Ruler for measuring.  
  • A straightedge for drawing lines and guiding a blade if you are not using a scissor.
  • Rubber band for removable installation or tape for permanent installation.
  • Clear glue (optional).

Measure the Finder

The tube diameter of the back of the finderscope must be measured.  Take the end-cap off and measure the tube with a ruler, micrometer or test the diameter with a circle template (from that long-ago -- pre-computer -- drafting or art class).  The 6x30 LER that came with the C5+ has a 24-mm (15/16") diameter.  Remember we're measuring the thin part where you put your eye -- not the tube that holds the finder objective.

Mark the Material
figure 2 -- the disc
Figure 2. Disc Overview

  1. Mark the material on its light colored side if it has one.

  2. First, use the ruler to set the compass points 5-cm (2") apart.  Use this setting to draw the outer 10-cm (4") disc. 

  3. The diameter of the center hole must account for material thickness.  Board is too thick to form a sharp crease like folded writing paper, so the fold must begin further "back"-- you'll see when you get to that part.  Set the compass points to the radius (half the measured diameter) of the finderscope measurement plus the thickness of the material.  For a 6x30 LER (12-mm radius) and 2-mm material this adds up to 14-mm.  Re-center the compass and draw the center hole. 
    figure 3 -- inner circle
    Figure 3. Center Hole Detail

  4. Draw one more circle with a radius 1/3 that of the center hole -- about 5-mm in our example.  The material within will be cut out of the center hole.

  5. Using the straightedge draw 24 radial lines through  the center hole from one side to the other.  An easy method is to draw crossed perpendicular lines and then bisect those to form "8-slices" (of pizza!) -- evenly distributed every 45-degrees.  By "eye-ball" divide each slice into three slices by drawing two evenly spaced lines within.  Draw them from circle edge, straight though the radius, across to the opposite side.

At this point the markings on the center hole should match those in figure 3.

Cut and Fold

  1. Cut out the outer disc.
  2. Cut out the small circle inside the center hole -- where the radii meet.  Here is where a hobby-blade or small scissor would work best.
Now you have something that looks like a scaled-down CD-ROM disc.
  1. Cut along the radial lines.  Use a scissor or blade.  Start cutting from the middle -- where the small circle was cut out-- and continue to the center hole boundary.
  2. Flip the material so the black side-- the side you'll be seeing when mounted on the finder -- is up.  Peal back each radially cut "pie" slice.  I found it best to pick up the material and push from below with the thumb against the base of each segment.  You'll find that the material does not bend along the line of the center hole but within it -- that's why the center hole was drawn slightly bigger than the finder diameter.

Mount the Light Block on the Finder and Secure it

Slip the light block onto the finder from the back.  Make sure that the black side of the disc is facing the back.  It should slip on snuggly with the "pie" slices trailing. 

For permanent installation wrap a piece of tape around the "pie" slices.  Use clear tape or black tape to match the finder color.  If you plan on packing-up and transporting the scope use a rubber band instead of tape.  That way the disc can be slipped off and the "pie" slices folded back inward for flat storage inside an equipment case.


If you are satisfied with the finished product and have a few hours before putting it to use: rub some clear glue along the edge of the outer disc.  This will keep humidity from being absorbed and delaminating the board.


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