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Old 17-08-2011, 10:57 PM
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alistairsam
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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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Old 19-08-2011, 11:06 AM
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ZeroID (Brent)
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You seem to be on the right track with it. I'd suggest you look at minimising your spider effect as well. My cheap 114 newt had a massive plastic moulded spider assembly which had 4mm wide 'vanes' and the central hub was larger than the diag by about 15mm. I replaced it all with a much smaller (23mm as originally about 40mm ) assembly and made a big diffence in both diffraction and reduced light obstruction.

Sometimes the simplest solutions yeild the best results.
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Old 24-08-2011, 01:06 PM
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alistairsam
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some pics with the difference in height between the two focusers.
its allowed me to move the primary back by about 30mm.

from what I've read, the primary reason for larger secondaries is because a few mm inward from most secondaries are not ideally usable.
did some eyeballing by keeping my eye at about where the dslr chip plane would be and there was roughly 7mm outside the reflection of the primary on the current 66mm secondary.

tried some calculations using mynewton software, results attached.
the 50mm secondary shows lower central obstruction but i don't know what the significance of the 100% FOV in degrees is (last line).
its 0.57deg for a 50mm secy and 2.04 deg for a 66mm secondary. what does this mean?
i've used 12mm as the desired 100% illumination FOV guessing that would cover the dslr sensor.

I also added wingnuts and two check nuts to the collimation bolts so it can be adjusted by hand. the wingnut is tightened all the way up and the two check nuts prevent it from loosening. works a treat.
i'll be using shorter bolts.
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