Re: ATM aargh! maybe

[Mark Holm <mdholm@telerama.com>]

With his permission, I am posting a short exchange between Andrew Melville and 
myself on this topic.  I think Andrew's questions and my reply may be helpful to 
beginners, or those wavering on whether to try mirror making.


First, Andrew's message in Re: ATM arrgh! maybe

> Mark
> 
>   I dont have a problem with you venting like that!! It just scares me!!! I have

> just finished with 120 grit on my first mirror and am building the Foucault

> apparatus. If you were already much better than 1/4 wave why did you keep going?

> Isnt 1/4 the theoretical best?
> 
> Andrew
> 
> 

Now, my reply,

> 
> 
>> It just scares me!!!
> 
> 
> 
> Don't be too scared.  Figuring is the most tricky part, but not impossible.  You 
> need to work carefully, and patiently with testing between each polishing 
> session, and as you approach your goal, make the polishing sessions shorter and 
> the testing more critical.  If I had been going hard instead of very slowly, I 
> really would have made a mess.  As it is, I think I can probably recover without 
> too much trouble.
> 
> I use Sixtests to reduce the Foucault data.  It is a great program.  One flaw is 
> that it doesn't print out it's plots.  I use a freeware screen capture program 
> called ScreenHunter to grab the plots.  Then I paste them into MS Paint, edit 
> down the sides of the image to save ink, and print them out.  That way I have a 
> graphical record of each step in the process.  This is a huge, huge improvement 
> over my previous mirror making attempts which were in the pre PC days.  Figuring 
> is still tricky, but, much of the black art is removed by programs like Sixtests 
> and Figure.
> 
> 
>> If you were already much better than 1/4 wave why did you keep going? Isnt 1/4
>> the theoretical best?
> 
> The 1/4 wave value comes from a statement made in the 19th century by an English
> physicist, Lord Raleigh.  He said that if the path lengths of the waves arriving
> at the final focus differ by less than 1/4 wave, that one will not be able to
> tell the difference between that image and a perfect one.
> 
> 
> 1. The statement refers to the final image, since light traverses the path to 
> the mirror twice, once before reflection and once after, defects in the mirror 
> surface are multiplied by 2.  Thus the 1/4 wave rule becomes, for mirrors, the 
> 1/8 wave rule, but everbody still calls it the 1/4 wave rule.
> 
> 2. Since Lord Raleigh, both quantitative theoretical work and careful 
> observation by a number of well qualified astronomers, both amateur and 
> professional, and opticians, both amateur and professional has shown that Lord 
> Raleigh was not quite correct.  When observing fine detail, under good 
> conditions, one can see improvements in the image from optics having better than 
> 1/4 wave images.  Most of the improvement comes from the first factor of two, so 
> that changing the rule to 1/8 wave at the image, or 1/16 wave at the mirror gets 
> one the largest part of the additional benefit.
> 
> 3. The Foucault test and the data reduction program are not perfect.  It is not 
> possible to perfectly characterize the mirror surface using, in my case, only 8 
> data points.  To be safe, I set my goal a bit under the 1/8 wave standard so 
> that errors remaining undetected would be likely to be less than 1/8 wave at the 
> image.  Hence 1/10 wave at the image, or 1/20 wave at the mirror.
> 
> 4. I am intrigued by the level of detection one can get with careful application 
> of the Foucault test and a program such as Sixtests.  I have had features as 
> small as 10nm (1/50 wave) reliably show up in test after test, and have been 
> able to monitor changes in them that were in accord with what I expected from 
> the figuring strokes.  With somewhat more uncertainty, I think I have been able 
> to detect changes as small as 5nm.  I was/am trying to get the P-V error under 
> 25nm partly just to see if I can do it.  (My Foucault tester is just about the 
> most minimal one possible to get this level of performance.  I have to be extra 
> careful not to disturb it while making measurements, because the thing is rather 
> flimsy.  One wrong move, and a whole set of readings is trash.)
> 
> You don't have to be this crazy with it.  A lot of enjoyment can be had with a 
> mirror that just makes the 1/4 wave limit, or even a bit worse.  The degradation 
> in the image quality is smooth, though more rapid as the wave errors increase. 
> You can see a lot of neat stuff with a 1/4 wave image, or even with one that is 
> slightly worse.  I can tell you from experience that a lot of pits remaining 
> from an incomplete polish, or trouble in fine grinding, will degrade the image 
> more than 1/4 wave figuring errors.  The degradation is different.  Pits scatter 
> light all over the image, figuring errors scatter it nearby.  With pits the 
> whole image seems hazy and the background not as dark as it could be, with 
> figuring errors, bright stars don't look as sharp as they could, and fine lunar 
> and planetary detail is degraded somewhat.
> 
> Have fun with your mirror making.  It is challenging, but certainly not impossible.
> 
> P.S.  Your message seems to be private and not copied to the ATM list.  Would it 
> be OK for me to post your message and my reply to the list.  There are a lot of 
> beginners out there who may benefit from both.
> 
> Mark Holm
> mdholm@telerama.com
> 
> 
> 
> 
>