astrodon
Carpal Tunnel
Reged: 04/07/05
Posts: 2552
Loc: Portland, OR, USA
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I thought I read somewhere about replacing the Meade 16" f/4.5 4" secondary with a 3.1" or 3.5" secondary or something like that. I have the program Newt I think it is called which tells me whether the field is fully illuminated or not. How important is it to have a fully illuminated field? (I use this scope solely for visual use.)
Thanks!
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Eager to get the Cave 6" f/4 out with some 'high' (medium in this scope) power eyepieces!
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F.Meiresonne
Post Laureate
Reged: 12/22/03
Posts: 4136
Loc: Eeklo,Belgium
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For visual use i think it is not so important.I think before you noticed it it can drop to 40% at the edges.
How big is the sec of the meade? I have 18 inch obsession and even mine is only 3.1"
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Freddy
Obsession 18 inch #1638
Orion Optics 8 inch F/4.5 -1/8 wave optics -Vixen GP-E
22x85 Helios Apollo (=GO SS)
15x70 TS Marine (=Obie Ultra)
10x60 Helios Quantum 4(= Obie Mariner)
10x50 Fujinon FMT-SX
10x50,8x40 Helios Nature sport plus
Eyepieces in use :Pan 35,24,19, N13T6, Pentax 10, 7 XW, N9T6, TV2, BGO 12.5, 9 mm
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jgraham
Postmaster
Reged: 12/02/04
Posts: 11575
Loc: Dayton, Ohio
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Fully illuminated field aside, I don't think you'd notice any visual improvment replacing the secondary unless you were doing very high resolution work under very, very steady skies. Checking the fully illuminated field is a good idea if you're tempted to do this. The olde rule of thumb was to not go below a 50% illuminated field and I thought most commercial telescopes shot for a 70% illuminated field using a 1.25" eyepiece. If you're using 2" eyepieces be sure to take that into account.
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-John
The best advice on imaging I've ever been given... don't forget to look! 
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erik
telescope surgeon
Reged: 01/30/04
Posts: 24789
Loc: Hawaii
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I considered downsizing the 4" secondary in my 16", but after viewing through another 16" with a 3.1" secondary, I could see absolutely no difference. Even the 4" is only a 25% obstruction, which is reasonable, even for planetary viewing.
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-Erik Wilcox
Starstuff
Homebuilt 16" Truss Dob in Ohia Forest Observatory
(12' X 12' rolloff under Mag 7 skies)
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos
Homebuilt 80mm f/5 refractor
Mirador 60mm f/12 1960's refractor
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astrodon
Carpal Tunnel
Reged: 04/07/05
Posts: 2552
Loc: Portland, OR, USA
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Hmmm.. Interesting, thank you all for the input.
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Eager to get the Cave 6" f/4 out with some 'high' (medium in this scope) power eyepieces!
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Hal Pollner
Reged: 08/30/05
Posts: 6567
Loc: Southern California Desert
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YO ERIK:
For actual light-blocking, that 4" diagonal blocks only 6.25 percent of the incoming light to the 16" primary.
Here's my old 16" f/4.5 Meade Starfinder.
John Dobson actually looked through this telescope when he was the month-long guest of our Club, the High Desert Astronomical Society, in 1999.
He said the image was "very good"!
John was conducting a mirror-making class at the time.
Cheers...
HOWL
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