Queensmessenger
newbie
Reged: 10/31/09
Posts: 1
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Hi Just experimenting at this stage but has anyone built a reflector using a standard magnifying say 3X) shaving mirror from local drug store?chemists shop or other high st. shop. I would love to try this but feel that any gring accuracy would not be suitable. Perhaps these types of mirrors are not even ground but simply cast.
Thanks in anticipation of any answers
Doug
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grendel
sage
Reged: 04/12/09
Posts: 241
Loc: Canterbury, Kent, UK
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I know someone who did that, for cloudy nights, to look at his star charts across the room.
Grendel
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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I would think that the worst attempt to make a mirror by the most inexperienced ATM would be 100 times better optically than a shaving or makeup mirror. You wouldn't see anything worth wild through a shaving mirror made into a telescope.
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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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Mirzam
super member
Reged: 04/01/08
Posts: 118
Loc: Lovettsville, VA
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Your question prompted me to take a look at my wife's make-up mirror. It appears to be about an f/1 or maybe even f/0.5.
If you wanted to make a newtonian you would need to have about a 70% obstruction. Sounds problematic to say the least.
Jim
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Zamboni
sage
Reged: 01/03/05
Posts: 256
Loc: Arizona
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Not to mention that a shaving mirror would probably be a second-surface mirror. Useless for telescope building.
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-Tristan Schwartz-
My Equipment:
6 inch Orion DSE dob
4 inch Galileo Newtonian
Schwartz Observatory Homepage
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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Shaving mirrors are very inaccurate, they are made by sagging window glass into a graphite mold in a kiln, the figure can be anything from a near sphere to a twentieth order grunge curve. They are also second surface mirrors, that is, they are silvered on the back so that the light has to pass twice through the thickness of the mirror. NO you do not use shaving mirrors as a primary, Forget it! 
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pogobbler
member
Reged: 09/30/08
Posts: 30
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I would say don't bother trying if it'll take much of any money to do so. But if you have the necessarily materials at hand, I'd say go for it just to put this sort of question to rest. I've heard this come up before with the same sort of responses... but I don't recall anyone ever actually trying it to make sure it'd suck as bad as the consensus is. Sometimes, someone has to step up and actually do it... maybe we can get the Mythbusters to try it, they're brave enough to try an experiment even when they're 99.99% sure of how it'll turn out.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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As a myth buster myself who will try anything ( remember the Hypochromat dispute, see back threads,the pundits averred that it was useless, yet the hypos I am building now are very good.) I will have a go and see if a shaving mirror can work in some way, but let me add, a shaving mirror is only about F:2,and this presents difficuties of access to the focus. I have an F:2 paraboloid, and I succeeded in getting at the focus by using a plain glass sheet set at 45 degrees to reflect the ray cone to the side, there was light attenuation of course but the images were bright enuff for all that, and a 40mm eyepiece yealded quite sharp images, so I built a short stubby scope and used it as a low power "look about" you just gotta give it a go.
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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The best one I found is a 5 1/2" f/4. For a make-up mirror this one is quite spherical but for a telescope it's awful, I bet the Moon looks blurry even stopped down to 2". You can check the curve by looking at your own eye, getting it as big as you need to see how off the surface is from a sphere, usually these mirrors are way more curved towards the middle. Photo of 5 1/2" mirror reflecting my striped shirt. Maybe they used to be ground and better surface.
Edited by greenglass (11/02/09 12:45 PM)
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David L
sage
Reged: 11/12/07
Posts: 278
Loc: Lee County Iowa
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I have thought about making such a telescope to show people why telescope mirrors cost as much as they do.
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16" Lightbridge, 10" F4.5 Homemade with Orion optics, 6" F6.5 Antares 1529 refractor, 6" F8 Homemade with Orion optics, 100mm F5.6 finderscope, 22X100 Oberwerk binocs, 8.5X44 Swift Audubon binocs, Orion Atlas mount, equatorial platform, various tripods and altaz mounts
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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Nice one! I set up a shaving mirror with a large diagonal to access the focus. The mirror is 5.25 inches dia and average focal length 15 inches. I tried it with a 40mm plossl and got a hopeless blur, I then tried a 140mm focus bino OG as an eyepiece, and the image of a roofline is curly and very bad. The figure of the mirror is toroidal. The set up is as the diagram. No, shaving mirrors are hopeless, even for very low power use.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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I took a piccie of a tree 50m away, using the 140mm bino OG as an eyepiece, you can see how bad it is.
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1250
Loc: Ottawa, Ontario, Canada
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I think my shaving mirror could crudely be called an oblate spheroid. The radius of curvature is noticeably smaller toward the edge. I wonder if I should bring it to the shop and set it up for a Ronchi test. I'm sure I won't have to get close to the null (as if there would be just one!  ) before seeing wobbly lines!
However, as to the question at hand... Many years ago (1980-ish) I did indeed cobble up a 'scope' using a shaving mirror. At the fullest aperture I could actually utilize it was abominable (it couldn't work at full aperture because of both the too-small secondary and the HUGE exit pupil). But when stopped down to 1/2 aperture or less, it wasn't completely excreble. And the fact that it was a second surface optic didn't have a deleterius effect because of the thinness of the glass and the necessarily low power.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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Has anybody tried a stainless steel shaving mirror, these are first surface. If you reduce the aperture either by a stop, or by using a very low power eyepiece letting the iris of the eye do the stopping, then you could get some reasonable results. It could be used as an RFT of sorts!?
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johnnyha
Pooh-Bah
Reged: 11/12/06
Posts: 1092
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I have used a primary as a shaving mirror, does that count?? 
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Johnny
Spicewood, TX
Sherman Oaks, CA
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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1233
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What about searchlight/klieglight(?) mirrors? Some of these are made of some super-reflective metal (I forget which), but the curvature is crazy.
Jess Tauber
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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Johnny you could do a helluva lot of shaving with a 36 inch. searchlight mirrors are accurately made, I read somewhere once that a professional used a 60 inch searchlight mirror for photoelectric work on stars, they are very fast and coma would be a problem if a reasonable FOV was required. Small searchlight mirrors are sometimes second surface Mangin mirrors, I use an old F:1 mangin for beveling mirror edges as a grinding tool so they have an indirect use.
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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1233
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What about use as an astrograph?
Jess Tauber
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 729
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Quote:
The best one I found is a 5 1/2" f/4. For a make-up mirror this one is quite spherical but for a telescope it's awful, I bet the Moon looks blurry even stopped down to 2". You can check the curve by looking at your own eye, getting it as big as you need to see how off the surface is from a sphere, usually these mirrors are way more curved towards the middle. Photo of 5 1/2" mirror reflecting my striped shirt. Maybe they used to be ground and better surface.
This looks like my Optical Craftsmen mirror!!!!! See the thread, with foucault test, in "Classic telescopes"!!! Greg N
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"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 370
Loc: UK
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I don't think that it would be any good as an astrograph, even at very low powers the images are atrocious, I used a 140mm bino OG as an eyepiece! for my experiment, and you still could not get an image; this eyepiece has an exit pupil of 40mm diameter, and my eye has 2mm ep, so I was only seeing 1/20th of the aperture at any one time; that is, 1/4 inch aperture seen, "reading" the mirror figure by eyeball showed that the mirror was deeper in the middle. than the edge.
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