greju
member
Reged: 10/13/05
Posts: 15
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Hi, Just getting into ATM and picked up a mirror last weekend. Have looked and looked but cannot find an equation to help me out.The mirror looks to be for a NEWT and is one and a quarter inches thick. Also looks to be new. It was in a very sturdy looking aluminum mirror cell and it all sold for a very reasonable price. The only thing etched on the side of the mirror is 56.5. It is a 8" mirror. What, if anything, can I deduce from these two numbers? Thanks
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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sounds like you have a f 7
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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I agree--56.5 is probably the focal length in inches. For an 8" diameter mirror that would come close to f/7.
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3455
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Odds are the 56.5 is the focal length of the mirror- which you can confirm by taking it outside and holding it in the sunlight against your house- and letting the sunlight reflect up into the eave. Move the mirror back and forth until you get the smallest, focused spot. I'm betting that will happen at a distance of 56.5" under the eave. To find out the f/ration of the mirror, the formula would be 56.5/8- which would make it approximately an f/7 mirror. Also, this would tell you that the distance from the face of the mirror to the secondary- and then to the eyepiece would be APPROXIMATELY 56.5". There are other factors that enter into this last bit of information, such as tube diameter and focuser height.
What you don't know is the quality of the mirror- and wether it's parabolic- or spherical. Do you live near any ATM'ers?? Did you get the mirror from a commercial site? eBay? Frequently the source can reveal more about the mirror...Good luck- perhaps some others can provide you more insight.
edit: Boy, bunch of quick answers- at least you can be fairly certain it's an f/7 mirror! 
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
"When your work speaks for itself- Don't Interrupt" -Gamble Rogers
Edited by Wes James (10/08/09 01:49 PM)
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Darenwh
Pooh-Bah
Reged: 05/11/06
Posts: 1224
Loc: Covington, GA
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I wouldn't use the house for this. Use an ant hill....
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Daren
Covington, GA
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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or make a foucault tester
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10" f/5 dob unf.
7x50 Tasco binos
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Quote:
or make a foucault tester
Not a bad idea. If you're just making one to have a general look at the figure and not make any measurements, you could probably cobble one together out of scrap in no time.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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jg3
sage
Reged: 05/27/07
Posts: 329
Loc: near Auburn, CA
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Don't even take it out to the sunlight! It's a reckless stunt. The space of intensely focused sunlight is invisible until it hits something, even at wide off-angles. Don't let that something hit be someone's eye.
Better is to focus the moon or very-distant lights onto a scrap of paper. Or approximately verify that possible focal length by looking at it, then backing up gradually as your face gets bigger, then your eye gets huge, until your eye or some part of your face is a total blur. This distance from your eye to the mirror is the radius of curvature, half of which is the focal length.
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3455
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Quote:
Don't even take it out to the sunlight! It's a reckless stunt.
Perhaps I shouldn't have suggested that... however it's been around a long time. It's in Sam Brown's "All About Telescopes", though there they are referring to an uncoated mirror- while grinding it- to test it. However, if you consider that a reckless stunt, you probably shouldn't be handling something made of glass in the first place.  Or be using power tools. YES you have to be careful... just like using a green laser pointer- an electric drill- or driving a car. But this is the kind of thinking that will lead to rubber hammers and plastic screwdrivers out of fear someone might hurt themselves.
Wes
Edited by Wes James (10/08/09 07:20 PM)
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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Is the mirror coated? If so, it might need recoating. Take it out of the cell, and after you scorch the paint under your eaves checking the focal length  look through the back of the mirror for pin holes in the coating.
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3455
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Quote:
and after you scorch the paint under your eaves checking the focal length
O.K., Ross- O.K.... so perhaps I was wrong. I wouldn't think you're going to burn your house down just focusing it there long enough to check the focal length... Beat me up... 
If I saw smoke starting to pour from the eaves, I'd probably turn the mirror away from the eaves.
But that's just me...
Wes
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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My 18" scorched some poor guy's hat at Stellafane after he opened the lid to photograph the mirror box. He didn't seem to notice, and I didn't point it out to him.
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3455
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greju
member
Reged: 10/13/05
Posts: 15
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Thnks fr al the rplys. Realy suny in Mpls tday and I wnt out to try the tst undr the eaves bfore Al the wrnings. I thnk I got a bug or smthing in my Eye dring the test thogh. Eye Relly hrts and blrd vision! All kidding aside, really, thanks for all the quick and informative ( to me anyways ) info. Didn't mean to start such a lively discussion! And I think we can all agree, SAFETY is job 1 in all our endeavors!
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Quote:
My 18" scorched some poor guy's hat at Stellafane after he opened the lid to photograph the mirror box. He didn't seem to notice, and I didn't point it out to him.
 Yeah I wouldn't either.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
My house.
DRAACO
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