Diagonal Size For A Simple Newtonian

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JayTee United States of America
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Diagonal Size For A Simple Newtonian

#1

Post by JayTee »


Hi all,

I want to upgrade the 1.25" focuser on my 8" f/7.5 Newtonian to a 2" focuser. My question is, will my current diagonal mirror minor axis size (38mm) be adequate for a 2" eyepiece?

Does anyone know the correct minor axis size given the focal length (60" or 1524mm) and the use of a 2-inch eyepiece? My "L" distance (focal plane to diagonal) is 10.25" (260mm)

Thanks,
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°

Searching the skies since 1966. "I never met a scope I didn't want to keep."

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notFritzArgelander
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Re: Diagonal Size For A Simple Newtonian

#2

Post by notFritzArgelander »


JayTee wrote: Fri Jul 09, 2021 1:16 am Hi all,

I want to upgrade the 1.25" focuser on my 8" f/7.5 Newtonian to a 2" focuser. My question is, will my current diagonal minor axis size (38mm) be adequate for a 2" eyepiece?

Does anyone know the correct minor axis size given the focal length (60" or 1524mm) and the use of a 2-inch eyepiece? My "L" distance (focal plane to diagonal) is 10.25" (260mm)

Thanks,
Looks OK to me but there are are several different criteria. Berry and Dakin say it's a touch small. Efberg and Peters and Pike say it's OK.

Here's an online calculator that I've found handy.....

http://www.catseyecollimation.com/designie5.html
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: Diagonal Size For A Simple Newtonian

#3

Post by MistrBadgr »


I would think it would depend on what you want to do. Your current secondary ratio is 19%, which is going to give you about as good resolution as anyone can ask for from a reflector doing planetary work. Where it will be limited is with wide angle viewing. If you are trying for a wider TFOV, then the outer portions of the field, compared to the field you are currently seeing, might appear to be darker than you want. If so, then a larger secondary mirror can illuminate the outer field better for a possible tradeoff of a little of your resolution. Going to the next larger secondary mirror size would make your scope a little more general purpose, while the setup you have leans more toward lunar/planetary/double star work.
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
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Re: Diagonal Size For A Simple Newtonian

#4

Post by notFritzArgelander »


The nice thing about the calculator link I posted is that it produces a table of % illumination in the focal plane. When I enter 7.5 for the f ratio and 10.5 for the distance of the focal plane from the center line I get only 34% illumination pat 2" diameter. For planetary and doubles it's ok at that. I'd want a bigger diagonal for dso work though. A 2.14 diagonal gives 65% illumination.

Sometimes I wish it was easy to swap different diagonals for different targets.....
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: Diagonal Size For A Simple Newtonian

#5

Post by mikemarotta »


Looks OK to me but there are are several different criteria. Berry and Dakin say it's a touch small. Efberg and Peters and Pike say it's OK.
You don't make it easy.

PETERS
Peters, William T., and Robert Pike, "The Size of the Newtonian Diagonal," Gleanings for ATM's, Sky and Telescope Magazine , March 1977, pp. 220-223.

EDBERG
-“How to Determine the Size of Diagonals, Draw Tubes, and Dew Caps,” by Stephen Edberg, Telescope Making No. 18 (Winter 1982/83)

DAKIN
Dakin 1962 R.K. Dakin, "Placing and Aligning the Newtonian Diagonal," Sky & Telescope (Gleanings for ATMs), pp. 368-369, Dec. 1962. erratum p. 114, Feb. 1963.

Peters reference found by way of "Practical Calculations for Designing a Newtonian Telescop"e Jeff Beish (Rev. 07 February 2019)
http://www.alpo-astronomy.org/jbeish/Ne ... Mirror.pdf

And Richard Berry ...
Berry Books 12.jpg

Here's an online calculator that I've found handy.....
http://www.catseyecollimation.com/designie5.html
Same but different
https://www.catseyecollimation.com/designns5.html
---------------------------------------
Michael E. Marotta
Astro-Tech 115 mm APO Refractor Explore Scientific 102 mm f/6.47 Refractor Explore Scientific 102 mm f/9.8 Refractor Bresser 8-inch Newtonian Reflector Plössls from 40 to 6 mm Nagler Series-1 7mm. nonMeade 14 mm. Mounts: Celestron AVX, Explore Twilight I Alt-Az, Explore EXOS German Equatorial
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Re: Diagonal Size For A Simple Newtonian

#6

Post by JayTee »


Thank you for all the great info. I am going to handle this as a two stage process. I'll upgrade the focuser and then if the image looks to be clipped or darker around the edges I will then consider getting a new diagonal which also requires getting a new spider.

Thanks again,
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°

Searching the skies since 1966. "I never met a scope I didn't want to keep."

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Re: Diagonal Size For A Simple Newtonian

#7

Post by MistrBadgr »


If you decide you need a secondary mirror change and are not familiar with 1800destiny.com, you might check them out. Hans, the owner, is good to work with. I like his curved vane spiders and secondary mirrors.
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
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Re: Diagonal Size For A Simple Newtonian

#8

Post by JayTee »


Along with the helpful link that nFA provided (thanks), I have used that in the past but I find the multitude of inputs to be a bit confusing so I found another old bookmark from Stellafane that is a lot easier to use. For now, it seems that the diagonal currently installed should be sufficient. We'll see.

https://stellafane.org/tm/newt-web/newt-web.html

Cheers
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°

Searching the skies since 1966. "I never met a scope I didn't want to keep."

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Re: Diagonal Size For A Simple Newtonian

#9

Post by notFritzArgelander »


JayTee wrote: Fri Jul 09, 2021 8:08 pm Along with the helpful link that nFA provided (thanks), I have used that in the past but I find the multitude of inputs to be a bit confusing so I found another old bookmark from Stellafane that is a lot easier to use. For now, it seems that the diagonal currently installed should be sufficient. We'll see.

https://stellafane.org/tm/newt-web/newt-web.html

Cheers
That's also a good online calculator.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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