lineman_16735
3.14159265
Reged: 12/04/04
Posts: 1623
Loc: At home
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Detailed instructions would be great as I am a dummy.
Thanks,
Chris 
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MMICKELS
Aluminum Knight
Reged: 01/20/04
Posts: 25192
Loc: The Land of Shake and Bake
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As another dummy, what is ROC?
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Mark
"The only thing wrong with immortality is that it tends to go on forever."
Herb Caen
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Vic Menard
Carpal Tunnel
Reged: 07/21/04
Posts: 2681
Loc: Bradenton, FL
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Radius of curvature can be checked in a number of ways.
If the primary mirror is mounted in an OTA, you need to find the focal length of the mirror. You can usually do this by pointing the scope at the Moon and, placing a piece of paper flat on top of the empty focuser, focus the image of the Moon. Now you need to measure the distance of the face of the primary mirror to the center of the diagonal, and the center of the diagonal to the focus position. Add the two measurements (which is the focal length of the primary mirror) and double them to get the ROC.
If the mirror is not mounted, you can use a flashlight to find the ROC. Point the flashlight towards the face of the primary mirror and, holding a piece of paper parallel to the face of the flashlight, back away until you see the image of the flashlight come into focus on the paper. Measure this distance to the face of the primary mirror and you have the ROC. Multiply ROC by 1/2 to get the focal length.
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Lawrence Sayre
Post Laureate
Reged: 10/16/04
Posts: 3706
Loc: N.E. Ohio
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Hold a flashlight along side your head at eye level (height wise, and also light source to eyeball parallel), and point it at the horizontally mounted mirror. Keep your eye directly on the axis of the mirror (which is often harder to do than to say). Then back up (or if too far back already, move forward) until the image of the reflected light literally "explodes" to blindingly full mirror width. This is the ROC point of the mirror.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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tishovlin
Carpal Tunnel
Reged: 11/11/03
Posts: 1813
Loc: Springfield, Pa
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Perhaps Chris already knows the focal length, in which case the radius of curvature is simply focal length X 2.
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Tim S
C6-RGT
13.1" Truss Dob
Osypowski dual-axis Eq. Platform
15x70 Obies
NP-101
Gibraltar tripod/mount
Losmandy G-11 w/Gemini
I'm headin for Galt's Gulch
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jack savard
sage
Reged: 05/26/04
Posts: 476
Loc: quebec canada
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the real question is maybe
why asking for roc
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nobody made it I AM INTEREST
46,49 N 71,14 O Québec canada 2\L100 2\D100 0\S100
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lineman_16735
3.14159265
Reged: 12/04/04
Posts: 1623
Loc: At home
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I ask beacause I am on 120 grit with my first mirror. I just wanted to check ROC to verify my saggitta. I am using a feeler gauge to measure and I noticed my straight edge was not so straight. I would like to get as close as possible before moving on to another grit. BTW 6" blank shooting for F/8.
Thanks folks,
Chris
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MMICKELS
Aluminum Knight
Reged: 01/20/04
Posts: 25192
Loc: The Land of Shake and Bake
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I have some precision ground O1 tool steel if you need a better straight edge. I'll make a donation of it if you want me to.
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Mark
"The only thing wrong with immortality is that it tends to go on forever."
Herb Caen
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lineman_16735
3.14159265
Reged: 12/04/04
Posts: 1623
Loc: At home
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That would be great, man thank you.
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Chris
AP 900
TSA 102
SBIG ST-2000XM
Astrodons
Pyxis
http://home.dejazzd.com/prowatt/
"quidquid Latine dictum sit altum videtur"
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MMICKELS
Aluminum Knight
Reged: 01/20/04
Posts: 25192
Loc: The Land of Shake and Bake
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PM me with an address. I'll try to ship it today. How about 1/2" X 1/16 x ? how long do you need it? The material is left over from parts I made for James and the Giant Peach.
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Mark
"The only thing wrong with immortality is that it tends to go on forever."
Herb Caen
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lineman_16735
3.14159265
Reged: 12/04/04
Posts: 1623
Loc: At home
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bright light better, and should I just use water to give the blank a reflective surface?
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Chris
AP 900
TSA 102
SBIG ST-2000XM
Astrodons
Pyxis
http://home.dejazzd.com/prowatt/
"quidquid Latine dictum sit altum videtur"
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tishovlin
Carpal Tunnel
Reged: 11/11/03
Posts: 1813
Loc: Springfield, Pa
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Just for grins, I came up with
(r squared/2S) + S/2 = ROC
r= mirror radius
S= Sagitta
ROC= Radius of curvature
Does anybody know what the book says?
Is that right??
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Tim S
C6-RGT
13.1" Truss Dob
Osypowski dual-axis Eq. Platform
15x70 Obies
NP-101
Gibraltar tripod/mount
Losmandy G-11 w/Gemini
I'm headin for Galt's Gulch
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Vic Menard
Carpal Tunnel
Reged: 07/21/04
Posts: 2681
Loc: Bradenton, FL
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Water will work fine (some old timers used glycerine--but then you have to clean it off!) You can also use the Sun and stick trick. Wet the mirror, hold a stick next to it, walk up to the eve of your house (or other shady flat spot about 4 feet away for your 6 f/8) and focus the Sun on the shady underside of the eve. Measure the distance with the stick (the focal length) and multiply by 2 for RC.
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