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John,
A really simple trick I learnt for checking 2ndry size is when sighting down the drawtube with a film canister etc is that the Primary should just fit inside the circle of the 2ndry. If you cannot see all of the Primary in the 2ndry then the 2ndry is too small.
Even if the 2ndry is only a tiny bit too small quite a bit of light from the primary will travel back out of the tube passed the 2ndry, and the passing light can illuminate the underside of the spider which in turn will be visible in the primary. Yuk!
EDIT: And don't forget that the larger the 2ndry, the less the light gathering capacity on the primary.
Last edited by ballaratdragons; 25-06-2005 at 02:05 AM.
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