Sun filter too small for new scope - no worries?

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Sun filter too small for new scope - no worries?

Postby Repairman » Fri Jan 14, 2011 6:05 pm

So you've bought a new scope but the expensive glass Solar filter you bought for your smaller scope is no longer big enough.
Adapt or fork out a fortune for another large one.

I had this problem but funds did not allow for a new large one.

I didn't want to destroy the new scopes lens cap so had to look around for a substitute cover; discovered a plant pot saucer was just the right size to fit smugly over the end of the telescope. Be careful as different makes of the same apparent size are different sizes at the inner part. Mine used a 7" saucer with an inner ring of 144mm diameter. I think this would fit most 127mm scopes. The original filter was bought for my 90mm refractor so it wasn't that much smaller.

Cut out a hole in the saucer just a little bit smaller than the diameter of the Solar filter mount and glue the filter to the top of the saucer with hot-melt glue and repeat on the other side of the saucer to give it extra strength. If you haven't got a hot-melt glue gun, time you had, they are indispensible in any workshop. Then cut a ring of thin black foam rubber and glue it to the inside of the saucer. This will stop any rubbing on the front of the telescope tube. Mask off the filter and foam ring and give it two coats of black acrylic paint (both sides of the saucer) and a good coat of clear acrylic lacquer. There are better paints for plastic but acrylic is the most readily available. Acrylic will scratch off the plastic but if you're careful it should last. Of course you don't need to paint the saucer, it's purely for appearance. If you can find a green one that will look better than the orange type.

I thought long and hard about how to retain the filter on the front of the scope; you certainly don't want it falling off whilst viewing; fatal for the eyes. In the end I went for the simplest option, elastic. Make two holes at opposite sides of the saucer and tie off the elastic with a double knot.

The fact that the filter is a little smaller than the aperture of the telescope is not that important; you don't need a large scope to view the Sun with a proper Solar filter.

Pics show both sides of the saucer and fitted in use. When fitting the filter make sure the elastic goes between the eyepiece mount and the focussing knob so there's no chance it will slip off.

Total cost of the project, just a few pounds; a lot cheaper than a new glass Solar filter.

Hope that helps someone.

Mike.

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Repairman
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