Solar filter for a small refractor

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Solar filter for a small refractor

Postby worcspaul » Tue Mar 10, 2015 4:19 pm

With the partial eclipse coming up on 20th March I thought I'd make a filter for my 'scope. I could've bought one but one of the right size never seemed to be in stock. Articles describing home-made filters usually make use of card and, while that was a possibility, I wanted something a bit more robust. That's when I hit upon the idea of using a CD/DVD "cake". The following pictures illustrate the stages of construction (though at the time of writing I haven't got a picture of the finished article as I'm waiting for paint to dry before final assembly. This technique should suit any scope with an outside diameter of about 110mm

Materials needed:

  • Empty 10 CD/DVD "cake"
  • 2 x protective discs. There's usually a black one at the bottom and a clear one at the top. I happened to use two black ones
  • Radius cutter
  • Invisible photograph mounts
  • Baader Planetarium AstroSolar Film
  • Sharp knife/scalpel
  • Calipers
  • Pair of Compasses
  • Off-cut of ply or MDF
  • Foam padding

I chose to use photograph mounts in order to avoid the possibility of glue getting where it wasn't wanted (on the surface of the film). It also makes disassembly easier should I ever need to replace the solar film.

Here's everything before I started. I'd already cut a hole on the "cake" lid to a mm or so less than the diameter of the scope's dew shield:

Image

Using the caliper I measured the diameter of the central hole in the protective discs. This turned out to be 15mm

I then drew a 15mm diameter circle on an off-cut of fibreboard and positioned one of the discs over the top:

Image

The disc was fixed to the board with screws and the radius cutter set to 10mm less than the radius of the disc. If using a larger aperture scope I'd recommend no less than 5mm less than the radius. This is to give enough room for the photo mounts (the screwdriver bit is there just to support the cutter while I took the photo):

Image

This was repeated for the second disc so I had two black rings between which the solar film would be sandwiched:

Image

Photograph mounts are stuck to each ring (the right hand ring split, hence the masking tape):

Image

A square of film was cut just larger than the rings and sandwiched between them (having first removed the backing paper!) with excess film being trimmed using a sharp knife:

Image

This assembly will be attached inside the lid (again with photo mounts) and 3 or 4 foam pads stuck around the inside to help keep the filter in place on the end of the scope.

Whilst I was at it, in true Blue Peter fashion I made a couple of filters for my 10x50 binos out of card from a cereal packet:

Image


STOP PRESS
(edit 19:26 10 Mar) Here's the (almost) finished item. I've found that the top of the "cake" isn't flat so I may have to use blu-tak to hold things in place


Image

And here it is perched on the end of the scope:

Image
Hope that provides some of you with a bit of inspiration.

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worcspaul
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Re: Solar filter for a small refractor

Postby jerryTheC » Sat Mar 14, 2015 2:00 pm

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