[amsat-bb] Re: DIY: Parabolic mirror

[Joe]

Yes,, BUT,,

it may be good enough for a low sreq radio signal,  but in this casting 
they still had to make the reflecting surface the same old way,  
grinding with grits, to even out the surface,,  then polish it,  then 
paraboloze it.

all the spin casting does it take the "Rough Grinding" portion out of 
the process, since 90%+ of the concave shape is raedy.

But as is, it's still no where near as accurate as it needs to be for 
optical work.

Radio, maybe,, optical  nope.

mvm wrote:

> Joe,
>
> ??
>
> As you surely can see this is an example of rotation casting. In this 
> case of a large mirror.
>
> Joe schreef:
>
>> And your point is?
>>
>> mvm wrote:
>>
>>> http://medusa.as.arizona.edu/lbto/tech/mirror.htm
>>>
>>> Greg D. schreef:
>>>  
>>>
>>>> Best "form" is probably not to have one.  Wasn't there a post 
>>>> recently talking about making a mirror by spinning casting resin in 
>>>> a shallow round dish?  Perhaps a good use for that old record (*) 
>>>> player gathering dust in the garage...
>>>>
>>>> Greg  KO6TH
>>>>
>>>> (*) an ancient mechanical instrument for storing music in an analog 
>>>> form
>>>>
>>>>
>>>> ----------------------------------------
>>>>      
>>>>
>>>>> Date: Thu, 14 Aug 2008 15:59:26 -0400
>>>>> From: ka2pbt at arrl.net
>>>>> To: amsat-bb at amsat.org
>>>>> Subject: [amsat-bb] Re: DIY: Parabolic mirror
>>>>>
>>>>> agree ..... what would be a good form to start with rather than 
>>>>> the bottom
>>>>> of some random water tank??
>>>>>
>>>>> Assuming you had a proper form ... I think the construction 
>>>>> process has
>>>>> merit.
>>>>>
>>>>> The next step would be to coat the fiberglass shell with something 
>>>>> metallic
>>>>> ... foil ... screen ????
>>>>>
>>>>>
>>>>>
>>>>> On Thu, Aug 14, 2008 at 2:04 PM, Franklin Antonio wrote:
>>>>>
>>>>>          
>>>>>
>>>>>> At 07:34 AM 8/14/2008, Joe wrote:
>>>>>>
>>>>>>              
>>>>>>
>>>>>>> Too bad it's not parabolic,  and probably far from even spherical.
>>>>>>>
>>>>>>>                   
>>>>>>
>>>>>> Precisely.  Apparently the author thinks "parabolic" just means 
>>>>>> some kinda
>>>>>> symmetrical concave surface.
>>>>>>
>>>>>> This guy obviously isn't an antenna guy.
>>>>>>
>>>>>> I've found Make magazine to be a crock in general.  Too bad.
>>>>>>
>>>>>>
>>>>>>
>>>>>>               
>>>>>
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>>>>
>>>> _________________________________________________________________
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>>>>       
>>>
>>>
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>>>   
>>
>
>
>