Newtonian Reflector Cooler

Parts Required

92mm 12v fan (eBay 5.56)
92mm fan finger guard (eBay 1.78)
12v fan speed controller (eBay 4.13)
3mm Black acrylic A3 sheet (eBay 5.20)
2mtr 12v cigarette lighter extension lead (eBay 3.88)
Sticky Velcro (eBay - 2.39)

12v in-line switch - Optional (eBay - 1.95)

Why cool a Newtonian?

The resolution of a Newtonian telescope will improve by forcing ambient airflow over the primary mirror, usually with one or more small fans. The wave front of light will be deformed as it passes through a very subtle temperature difference near the face of the primary mirror that is known as 'The Boundary Layer' and it's effects are significant. For most locations on Earth, even a small Newtonian's primary mirror will not track the falling night time temperature closely enough unless fans are used.

The fan needs to do three things :-

1. Cool the primary
2. Remove the boundary layer from the front of the primary.
3. Remove tube currents.

A rear mounted fan with a baffle does all three jobs simultaneously (providing you primary mirror is vented as below).

It can be quite interesting to turn the fan off when the mirror appears fully cooled - the views go softer after about 15 seconds - that's the boundary layer reforming.

Bottom of SkyWatcher 200P Newtonian

Build Process

1. My SkyWatcher 200P has a rear diameter of 218mm (inside the external lip). Ensure you leave adequate room and drill a 3mm pilot hole in the A3 sheet of acrylic. (mine was 109mm from one edge along a center line).

Pilot hole

2. Place a block of wood under the acrylic and put a nail (3mm preferably) through the pilot hole into the wood. Using a marker pen and some wire or string set the radius and mark the circumference of the size of the baffle reqired.

Draw the circumference

3. Using a protractor and a ruler mark three lines from the centre at 120 each, these will be for the collmation bolt cutouts.

120 radials

4. Using a 38mm hole cutter cut three circles out of the acrylic where the lines from the centre point meet the outer circumference.

Bolt cut-outs

5. With a fine toothed blade in a jigsaw cut along the outer circumference line.

Cut blank

After cut

6. Test the fit and trim if needed.

Blank on the 200P

7. Using a 89 mm hole cutter and using the pilot hole as a guide cut a hole out of the centre of the acrylic.

Fan aperture

8. Snip off the cigarette socket from the extension lead and strip the ends, then snip the power connector off of the speed controller. Connect the fan and the speed controller up to a 12v supply and ensure everything is working ok. The yellow wire on the power connector is not required and can be cut short.

Power cable Fan speed controller 92mm fan

9. Drill 4 5mm holes in the acrylic to mount the fan and check it aligns ok.

Test fan for fit

10. Drill additional holes for the fan, power and speed controller cables to pass through.

Additional holes

11. Remove the film from both sides of the acrylic and pass the power cable through the hole and tie a knot in it so it wont pull through.

Pass power through

12. Snip the other cables to length and pass them through their corresponding holes.

Pass cables through

13. Remove the backing on the sticky pad on the speed controller and stick it to the acrylic.

Attach speed controller

14. Strip the ends on the cables and twist the corresponding cables together and then solder.

Join wires

15. Cover the soldered ends with shrink tube and shrink for a tight fit.

Cover wires

16. Give both sides of the fan assembly a wipe before attaching to the scope as you don't want to blow any dust into it.

Clean fan assembly

17. Stick some sticky Velcro to the base of the scope and to the fan assembly (don't stick too many as it my be difficult to remove the fan assembly from the scope).

Sticky Velcro Velcro on fan assembly

Velcro on 200P

18. The finished fan assembly running and cooling the scope.

The final fitting Running Fan

I have ordered a switch to add into the power cable in case I want to turn the fan off for any reason with out disconnecting it from the power supply.

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Copyright 2014 Andy Hutcheon