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Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3077
Loc: Bradenton, FL
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Quote:
...Below is my chart showing light cutoff for the 1.3" secondary I used in my 6" f-6 with 7" tube and 5.5" distance to focal plane. Very good to about 3/8" fov.
Off-Axis Illum. Light Loss
0.00 in 100.0% 0.00 mag
0.10 in 100.0% 0.00 mag
0.20 in 100.0% 0.00 mag
0.30 in 95.12% 0.05 mag
0.40 in 84.77% 0.17 mag
0.50 in 73.25% 0.33 mag
0.60 in 61.50% 0.52 mag
Did you mean very good to about 3/8-inch radius fov?
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Simoes Pedro
member
Reged: 02/03/09
Posts: 16
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As a rule of thumb use the inverse of the focal ratio.
If your scope is f6 then the obstruction could be 1/6.
A f4 mirror would have a 25% obstruction.
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Yes Vic, that's exactly what I meant.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
The stock obstruction is 50mm. How do I find out the smallest usable secondary size for visual? For future replacement. I dont know how to "read" the numbers in "Newt".
CJ
Hi CJ:
Getting the numbers and understanding their implications, as you have surmised, are two different things. I'd recommend popping over to my web site and reading "Sizing Up the Newtonian Secondary" to get some insight into the understanding side of the equation.
As for getting the numbers, I personally don't think Newt is the best tool for the job. Others in this thread have suggested better calculators. But in the end, you're going to have to make some compromises (there are only so many sizes to choose from) -- the trick is knowing what they are and understanding their meaning.
Regards,
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Maybe we ought to ask what the scope would be used for primarily- planetary, or general purpose viewing. (?) It would likely make a difference as to the demands of secondary size.
Mark
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So many projects, so little time!
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Doug Reilly
super member
Reged: 07/29/08
Posts: 158
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I really like Mike Lockwood's discussion and table of secondary sizes. He lists mirrors by aperture and focal ratio, secondary to focal plane height, and illuminated field. For planetary, you can choose the .25", for wider field/general use, the .5" illuminated field.
http://www.loptics.com/ATM/diagonals.html
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I keep a blog on astronomy, astronomy outreach, and other related topics at punkastronomy.com Nerd is punk!
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Keithdrengen
sage
Reged: 02/20/08
Posts: 254
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Quote:
Maybe we ought to ask what the scope would be used for primarily- planetary, or general purpose viewing. (?) It would likely make a difference as to the demands of secondary size.
Mark
It will be used for the moon below at low to 100x and just gazing at starfields, the milkyway and such. Quite casual I guess.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Multi-purpose. Try to get a field of near half inch diameter that's 100% illuminated. You'll have to make a choice between 1.3 and 1.52 sizes. If you can get a really low height focuser, I'd go for the 1.3.
M.
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So many projects, so little time!
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