Thread: secondary size
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Old 17-08-2011, 10:57 PM
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alistairsam
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Location: Box Hill North, Vic
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secondary size

Hi,

I have an 8" F4 reflector OTA with the bintel 1:10 crayford focuser and the mirror has been moved up by almost 70mm for prime focus with a dslr.
the current secondary is 66mm.

I've bought an astrosytems phase 4 focuser from IIS primarily because its low profile.

my objective was to reduce the secondary obstruction by changing the secondary to 50mm. but to maintain full field illumination, i figured i could use a low profile focuser, move the primary back thereby avoiding vignetting.
is this assumption correct?

will there be a significant reduction in central obstruction between a 50mm and 66mm secondary?
the bintel crayford is 69mm from the ota while the astrosystems is only 34mm.

i've tried the newt software for calculating secondary size and illumination but wanted to check if there are any drawbacks to this?

I'm working on a focuser that's about 22mm so I can move the mirror further back. also reducing the amount the drawtube extends inside the ota when drawn in.

moving the primary back allows smaller springs, shorter collimation bolts, low profile focuser allows better balance with a dslr as torque is lower (gravitational).

i've seen so many people emphasizing bigger secondaries but what about obstruction? surely that matters as well?
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