Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Caustic testers and other testers info need

mirror making
This topic has been archived. This means that you cannot reply to this topic.
98 replies to this topic

#76 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1558
  • Joined: 01 Oct 2007

Posted 15 October 2014 - 12:31 PM

A 75.5" f/2.2 mirror is unlike any other beast commonly encountered in the amateur world, and as such needs uncommon test methods, with an uncommon price tag. For a mirror that size and focal ratio, all traditional ATM tests fail. The Foucault zonal readings would not be at the coc of the zones but well inside of them; the caustic test was developed to compensate for that error.

 

OK, I'm calling BS on your above statement about Foucault.  You are wrong, and I am tired of people perpetuating the myth that the testing methods that I have used for years don't work.

 

A moving source knife-edge tester is a most accurate measure of slope with no bias.  There is no error in the geometry, and the readings are at the COC.  Foucault measures slope, not radius, after all.

 

Your statement above represents a misunderstanding of what the knife-edge actually measures, but don't feel bad - I have heard that same misunderstanding from other opticians, too.  Sadly that deprives them of a valuable measuring tool.

 

I used a pin stick and moving-source knife-edge to test the 50" f/2.2 hyperbola, and when later tested in autocollimation in combination with a mostly pre-figured RC secondary (which someone else made), the system looked reasonably good before I touched the secondary.  This proved that the primary was pretty darned close, and that the testing worked.



#77 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 8889
  • Joined: 02 Dec 2004

Posted 15 October 2014 - 12:33 PM

The other way that the mirror could be tested is with an oil flat and silvering a section of the 75" mirror. You can a buy silvering kits for under $100 that will allow you to silver about a 24" section of the mirror a couple of times. I know because I seen it done first hand on 25" mirror. So you get the figure as close as you can via all the other methods. Silver a 24" or larger off axis section of the mirror and test it via an oil flat. You need to silver the surface of the mirror to get enought light to do the test since oil flat reflects about as much like as uncoated glass. By other test methods you can determine if the optical figure is smooth, in the 24" silvered section you be able to see if the figure is over or under corrected. Get it to null and your going to be pretty darn close. The silver coating comes off in seconds with a mild acid solution and then you polish away. You can compare the double pass results to other methods and get a good estimate on how much and in which direction you need to do the correction. When the other test methods show you made the need correction you again silver the section and test again. Yes, it's not going to be easy and you'll need to build a structure to hold the mirror upside down and 15' or above the oil flat but it has the possibilities to get you were you need to go.

- Dave

#78 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22712
  • Joined: 22 Feb 2010

Posted 15 October 2014 - 01:09 PM

I thought Kevin said he has two 24" flats.

 

he could set it up like this.

 

75conic 2 24in flats.jpg



#79 kfrederick

kfrederick

    Soyuz

  • *****
  • topic starter
  • Posts: 3996
  • Joined: 01 Feb 2008

Posted 15 October 2014 - 03:11 PM

Yes I have Two of the 17x24s  Hextec  flats  /  Should use one to make a window chief /   Thanks for the info  . Why is the caustic  better than a knife edge ? 



#80 kfrederick

kfrederick

    Soyuz

  • *****
  • topic starter
  • Posts: 3996
  • Joined: 01 Feb 2008

Posted 15 October 2014 - 03:18 PM

 

A 75.5" f/2.2 mirror is unlike any other beast commonly encountered in the amateur world, and as such needs uncommon test methods, with an uncommon price tag. For a mirror that size and focal ratio, all traditional ATM tests fail. The Foucault zonal readings would not be at the coc of the zones but well inside of them; the caustic test was developed to compensate for that error.

 

OK, I'm calling BS on your above statement about Foucault.  You are wrong, and I am tired of people perpetuating the myth that the testing methods that I have used for years don't work.

 

A moving source knife-edge tester is a most accurate measure of slope with no bias.  There is no error in the geometry, and the readings are at the COC.  Foucault measures slope, not radius, after all.

 

Your statement above represents a misunderstanding of what the knife-edge actually measures, but don't feel bad - I have heard that same misunderstanding from other opticians, too.  Sadly that deprives them of a valuable measuring tool.

 

I used a pin stick and moving-source knife-edge to test the 50" f/2.2 hyperbola, and when later tested in autocollimation in combination with a mostly pre-figured RC secondary (which someone else made), the system looked reasonably good before I touched the secondary.  This proved that the primary was pretty darned close, and that the testing worked.

 

 That is what I think Mike . Thanks  



#81 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 15 October 2014 - 03:54 PM

 

 

OK, I'm calling BS on your above statement about Foucault.  You are wrong, and I am tired of people perpetuating the myth that the testing methods that I have used for years don't work. A moving source knife-edge tester is a most accurate measure of slope with no bias.  There is no error in the geometry, and the readings are at the COC.  Foucault measures slope, not radius, after all. -- Mike Lockwood

 

Wow, Mike, why such harsh words? I did not invent this, and no one here has questioned the quality of your mirrors. What I wrote down as far as why it was deemed by the professional community that the Foucault test was not sufficiently accurate for large and fast mirrors (i.e. the results being inside the zonal center of curvature), and the reason why the Platzeck-Gavioloa caustic test was devised, is affirmed by  Platzeck and Gaviola,  by Irvin H. Schroader,  by Malacara, and countless others. Specifically, Schroader writes (my emphasis added):


"The final clincher by way of experiment would be to cut two hole in a mask about 3/4 inch in diameter, each with its center say 2/3 of the distance from center to edge of the mirror along the horizontal diameter. Set the knife-edge so that (as nearly as can be judged) the two sides of the zone darken evenly and simultaneously. Now remove the mask and it will be easy to see that the shadows on the mirror move in the same direction as the knife-edge across both sides of the zone previously exposed by the two holes; that is,  the knife-edge is inside the center of curvature of these two areas of the mirror surface."  Irvin H. Schroader, ATM Book III (1953), pp 431-432)

 

I think you'll agree that what Schroader described is the standard test using mask cutouts to test concave astronomicla mirrors on a daily basis. I think you'll also agree that he is saying this method places the knife edge inside the zonal center of curvature, and that the drawing below explains why this is so.

 

http://imagebank.osa.org/getImage.xqy?img=OG0kcC5sYXJnZSxhby0zMS0yMi00MzQzLWcwMDE

 

It is also worth noting that Pletazek and Gaviola published photographic evidence for this in JOSA, Volume 29 (1939), pages 484-500.

 

If you think this represents a fundamental misunderstanding in general, please explain.

 

regards,
Mladen


Edited by MKV, 15 October 2014 - 06:21 PM.


#82 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1558
  • Joined: 01 Oct 2007

Posted 15 October 2014 - 07:02 PM

Wow, Mike, why such harsh words?

...

I think you'll agree that what Schroader described is the standard test using mask cutouts to test concave astronomicla mirrors on a daily basis. I think you'll also agree that he is saying this method places the knife edge inside the zonal center of curvature, and that the drawing below explains why this is so.

...

If you think this represents a fundamental misunderstanding in general, please explain.

 

I was blunt, not harsh.

 

I discussed a moving-source tester.  Caustic is done with a fixed-source, as the diagram that you link to shows, and that geometry is different.

 

Though not at the ROC of the zone, when the source and knife lie at the same location, one can still measure where rays cross on the optical axis with very good accuracy.  In this case the knife acts simply as a spatial filter to locate the crossover.

 

Thus, as I said above, the knife edge measures slope, not ROC.



#83 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 16 October 2014 - 04:44 AM

 

 

A moving source knife-edge tester is a most accurate measure of slope with no bias.  There is no error in the geometry, and the readings are at the COC.  Foucault measures slope, not radius, after all.

...

I used a pin stick and moving-source knife-edge to test the 50" f/2.2 hyperbola, and when later tested in autocollimation in combination with a mostly pre-figured RC secondary (which someone else made), the system looked reasonably good before I touched the secondary.  This proved that the primary was pretty darned close, and that the testing worked.

 

I am not aware that amateurs use the "Wire Test" (described in Telescope Making #6)

In brief, you use a slit/pinhole and a wire, moving it along as in Foucault and reading the zonal radius of its circular shadow against a pinstick.

It will show the foot-of-normal for the longitudinal position, much like Foucault does, and the curve evaluated as Foucault (Sixtests if fixed source - if moving, less critical). I have no suitable mirror available, but simulations (Diffract, but it is limited in size) seem to show it is good for large, fast mirrors (the wire shadow tends to float out and be hard to read for inner zones).

 

I believe it is easy to try, if you have a mirror for it, and a Foucault tester with a not too wide slit (the "wire" can be a piece of clear plastic, with a line drawn with a scalpel - easy to try varying the width). The symmetry of the shadow would, I believe, make reading at least as precise as Foucault.  Mike, did you ever try it, or feel like trying? It ought to be by far the easiest test to setup of all discussed here...

 

Nils Olof



#84 kfrederick

kfrederick

    Soyuz

  • *****
  • topic starter
  • Posts: 3996
  • Joined: 01 Feb 2008

Posted 16 October 2014 - 09:22 AM

Testing by hanging the mirror upside down over a oiled flat is Not going to happen . The ton of glass will bend and not work . The Moving light knife edge Or wire test  Is all I understand :)  .If the caustic light is fixed not sure how good that is/   Other tests need other optics and there errors and alignment compound the problems .The Foucault  or wire test I can get direct readings of any zone a real number/ Only Error being My ability to read the zones good enough.  And working at a RC of 330 inches a power full test.  Mike Lockwood has test a 50 f2.2  Hyperbolic this way :waytogo:    So his posts are backed up by what he has done. . The Foucault should get me close . Thanks for the great info     



#85 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 16 October 2014 - 10:20 AM

 

I was blunt, not harsh. -- Mike Lockwood

 

Harsh or blunt, depends how one takes it, but certainly uncalled for because my statement simply relayed what optical science recognized long ago:  that the Foucault zonal test is not as accurate for large and fast mirrors as the caustic test is. Which is precisely the reason Platzeck and Gaviola saw the need to developed and introduce that test in 1938. I would hardly call that "BS", or misunderstanding" as you suggest. No one says the Foucault test is "wrong"; just that  the caustic test is more accurate.

 

Mladen


Edited by MKV, 16 October 2014 - 11:17 AM.


#86 Pierre Lemay

Pierre Lemay

    Apollo

  • *****
  • Posts: 1330
  • Joined: 30 Jan 2008

Posted 16 October 2014 - 12:24 PM

MYSTERY SOLVED!

 

Had the opportunity at the metrology lab this morning to measure in 3 dimensions the accuracy of my caustic tester to a resolution of 0.000 01" in all directions. You will remember earlier in this thread that I had found an unexplainable 15% discrepancy with the Foucault results.  Here is a picture of the caustic tester ready to be measured on the Mitutoyo three axis tester:

 

Measuring Caustic Tester.jpg

 

To measure both the absolute displacements and orthogonality of the two axis, I replaced the slit light source with a small cylinder I machined yesterday. The three axis tester would find and use the center of this cylinder to accurately monitor and measure actual movement of the carriage that normally carries the Foucault illuminated slit or Caustic wire, depending on test.

 

The good news is the tester is very accurately built. The angle between the X and Y axis were measured at 90:04:32 so the perpendicularity error is only 4'32". The accuracy of the longitudinal measurement (optical axis) is also very accurate. When a movement of 3.000" was applied to the carriage using the digital scale, the three axis machine detected a movement of 3.000 13", an error of a little more than 0.0001". It shows the digital scale is mounted quite parrallel to the longitudinal linear bearing. This is more than good enough for both Foucault and Caustic.

 

The perpendicularity of the large drum micrometer axis of rotation was also checked in relation to the tester's longitudinal axis. In one plane the error was 19`33" and in the other plane 3`34" for a compound error of 19`13". Not bad and certainly not a problem.

 

And now for the bad news.

 

The large drum micrometer which must read to 0.0001" accuracy and which I thought good is really bad ! On a distance of 0.5000" it measured 0.50137", that's a little more than 0.001" of innacuracy over only 0.5 inches. I had that micrometer measured years ago and it was better than 0.0001", I was dumbfounded! When testing my 20 inch, it would mean my measurements for the outer zones would have been more than 0.0005" off. No Wonder I was not seeing the same result with the Foucault. The only explanation I can think of why the 3 inch diameter micrometer is no longer accurate may be due to an accident on my part. I dropped the micrometer on the floor years ago, dammaging the flange that attaches to the drum. The threads to attach the micrometer were dammaged and I had to machine them out and mount the micrometer another way. I can only suppose I dammaged it more than I had initialy thought.

 

So I need to get another 0.0001" micrometer.  Any suggestions on where to purchase one without breaking the bank? (the Mitutoyos are nice but at about $300, very pricey).



#87 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1558
  • Joined: 01 Oct 2007

Posted 16 October 2014 - 12:49 PM

No one says the Foucault test is "wrong"; just that  the caustic test is more accurate.

 
Uh, you need to re-read your own post.  YOU said it was "wrong" in your eariler post - see quote of your statement below:
 

The Foucault zonal readings would not be at the coc of the zones but well inside of them; the caustic test was developed to compensate for that error.

 

You said that Foucault readings are in error, and in effect systematically "wrong".  That's why I responded to it originally, because that statement is incorrect.  If the readings are taken properly, there is no error, since there is no error in the geometry.

 

Now, one might not be able to read Foucault quite as accurately, but random errors in measurement are quite different than an error in the geometry.  There is even a formula to correct for the geometry of a fixed light source/moving knife for fast mirrors.


Edited by Mike Lockwood, 16 October 2014 - 01:00 PM.


#88 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 16 October 2014 - 02:14 PM

My take on this:

With both tests, you get a diffraction peak to measure. With Caustic (=the test), you read it at its sharpest at or very near the caustic (or more exactly, where the light source is focused). With Foucault, you read it on the optical axis, some distance from the best focus (larger, the farther out the zone) - thus, you need to keep the zones down in width to get the "depth of focus" large enough to include the optical axis, and symmetric enough to give even illumination of the aperture. Thereby you restrict the aperture width compared to Caustic, and work with wider diffraction peaks. Mladen calls this an "error", I understand what is (or should be) meant, but I rather see it as a "compromise", well suited to moderate mirrors. For very large, fast mirrors, the number of contiguous zones may well be embarrasingly large. As I understand it, he does not claim that the readings are wrong,

The width of the diffraction peak also limits other tests such as the pentaprism - I believe an unexpanded laser beam would not be readable, to a meaningful precision, on return.

BTW with modern light sources, I see no reason why you can't make a Caustic tester with moving source.

 

Nils Olof


Edited by Nils Olof Carlin, 16 October 2014 - 02:19 PM.


#89 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 16 October 2014 - 03:33 PM

 

 

So I need to get another 0.0001" micrometer.  Any suggestions on where to purchase one without breaking the bank? (the Mitutoyos are nice but at about $300, very pricey).

 

Here is a Nikon for $99.

 

http://www.ebay.com/...=item35ca67c1a7

 

Mladen



#90 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 16 October 2014 - 11:01 PM

 

 

YOU said it was "wrong" in your eariler post...You said that Foucault readings are in error, and in effect systematically "wrong" -- MIke Lockwood

 

I never used the word "wrong". That is  too broad of a brush as far as I am concerned. I was very specific. The readings are in error because the crossover positions of the zonal Foucault test are not those of the COC  of the zones (as assumed), but actually inside the COC. Irvin Schroader calls it "the knife-edge error" (ATM, III, p. 443). If it isn't, what it is? He even suggests how to test for it: 

 

"Now remove the mask and it will be easy to see that the shadows on the mirror move in the same direction as the knife-edge across both sides of the zone previously exposed by the two holes; that is, the knife-edge is inside the center of curvature of these two areas of the mirror surface"..." (p. 432)

 

In other words, if the zonal test with the mask cutouts showed both zones to darken evenly and simultaneously, it suggests that the knife-edge was at the COC of the zone, but when the mask is removed it's obvious that it's inside the zone. If a test suggests one thing and it turns out it's another thing, what do we call that if not an error

 

The problem with the zonal testing using masks with cutouts seems to have a strong subjective element as well, which would tend to vary form person to person.  Again, Schroader (my emphasis in bold):

 

"Although  each side a and b  of a masked zone is very nearly spherical, the centers of curvature A and B (Figure 2 and 3) of these spheres are

separate from each other and do not lie on the optical axis. In performing the Foucault test, then, we placed the knife-edge at the

crossover point, (that is, inside the actual centers of curvature) and then struggled to force the shadows to behave as though it were at the

center of curvature of both a and b simultaneously; a little like trying to force a nut when the thread is crossed." (p. 433)

 

E.H. Linfoot, states that the accuracy of the Foucault zonal test is (again, my emphasis in bold) "limited only by the observer's limited power of brightness-discriminaction." (Monthly Notices of the Royal Astronomical Society, Vol. 108, p.428). On the other hand, I think it is generally agreed that the subjective bias is largely removed in the caustic test.

 

Perhaps, as Nils Olof suggested, I should have perhaps  euphemistically called it a "compromise" rather than an error. It's important also to stress that no matter what we call it, it is not always significant, especially for smaller and slower mirrors. The main point is that the caustic test is more accurate then the Foucault test, which is why it was introduced to begin with, and which makes it "better", especially for bigger and faster mirrors.  

 

regards,

Mladen


Edited by MKV, 16 October 2014 - 11:06 PM.


#91 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 17 October 2014 - 06:36 AM

Mladen wrote

"In other words, if the zonal test with the mask cutouts showed both zones to darken evenly and simultaneously, it suggests that the knife-edge was at the COC of the zone, but when the mask is removed it's obvious that it's inside the zone. If a test suggests one thing and it turns out it's another thing, what do we call that if not an error?"

" I think you'll also agree that he is saying this method places the knife edge inside the zonal center of curvature..."

Of course it does - noone has disputed it (I hope), only its relevance to testing.

The information we want from either test is the relative slope of the zone to measure, by locating the normal to the "middle" of the zone in question. This normal will intersect both the caustic and the optical axis (the latter a bit nearer the mirror), and we might choose to locate either point of intersection by doing Caustic and Foucault respectively. We all agree that it can be located at the caustic, but Mladen claims it is an error locating this normal by its intersection with the optical axis (or rather the line of symmetry), Mike claims that it is not an error. It is indeed not an error. The precision of either test is another matter.

Seen from a point near the axis of symmetry, the zonal curve for an ideal parabola will give an optical distance that can be described as a 4th order polynomial of the distance from center. Moving along the optical axis, you can (in Foucault) find the point where the first order term is nulled, and by locating the caustic, you can also have the second order term canceled, but the higher order terms still remain - it is still not a sphere, even if closer.
In either case, what you have to go by is a diffraction peak representing the image of the small light source, as formed by the zonal aperture. As long as the distance between the axis and caustic is small, and/or the zone is narrow, the peaks at either place are not dissimilar - the side peaks may be more pronounced at the axis than at the caustic, but the width of the peak is much the same, determined by the width of the zonal opening. This is the domain of Foucault testing of moderate (amateur) size mirrors - where the Caustic has nothing to offer worth the extra trouble.

As mirrors grow larger/faster, the widths of the zones for Foucault must be successively narrower away from center, and as Caustic offers the possibility to use wider zones, thus narrower peaks allowing more precise location. This is how the Caustic test offers better precision.

Schroader's analogy "a little like trying to force a nut when the thread is crossed" is a strawman argument - a fairer one would be trying to get a sharp portrait with a camera of fixed focus. This can be achieved to reasonable result by stopping down, and having the subject not too close.
Nils Olof

#92 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2227
  • Joined: 26 Jul 2004

Posted 17 October 2014 - 06:39 AM

The knife edge reading of Foucault is amazingly simple and clever, however limited the authorities consider it. The two diffraction patterns are cut in half (approximately), and then Fourier back-transformed to some weird images of the zonal openings, but as long as the peaks are acceptably symmetric, of equal total intensity. This is convenient, but not necessarily the optimum in precision. I see no reason why the diffraction peaks could not be located like in Caustic even if more complex. One method that I believe would work is picking up the peaks on a camera chip - the center of the peak could be determined to much less than a pixel. The downside is that the phase information is sacrificed, and you would have to shade all openings but the one measured (to avoid damaging interference). For Caustic, it would have the immense advantage of dispensing with the critical lateral movement of the stage - the image scale of the camera chip known.

The simplified Caustic test that Schroader describes (p.435, last paragraph) has later been named the "Poor Man's Caustic" test. He writes (p.436): "This displacement is just three times as large as that measured in Foucault's zonal test. Thus the displacements measured in the [here described, simplified] caustic test may be divided by 3 and then compared to the old familiar formula r^2/R"
While this is true for a perfect parabola, it is not true for an imperfect one, and realizing this led to 1) the loss of popularity for this test and 2) a correct (AFAIK) algorithm for evaluation by Jim Burrows. This test estimates the curvature of the zone in question, unlike Caustic and Foucault that meaure the slope, and thus might be of value as an independent test, still easy to implementwith a KE tester. But it can be quirky if the mirror is zoney.
Nils Olof

#93 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 17 October 2014 - 09:14 AM

 

 

[T]he width of the peak is much the same, determined by the width of the zonal opening. This is the domain of Foucault testing of moderate (amateur) size mirrors - where the Caustic has nothing to offer worth the extra trouble. As mirrors grow larger/faster, the widths of the zones for Foucault must be successively narrower away from center, and as Caustic offers the possibility to use wider zones, thus narrower peaks allowing more precise location. This is how the Caustic test offers better precision. -- Nils Olof Carlin

 

Which begs the question what is the "cutoff" aperture and/or focal ratio when it's no longer a worthless extra trouble? Kevin started this thread precisely to find out what different tests can do for his 75.5 inch f/2.2 mirror.  Considering that the practical number of zones is about 11 or so (even though some claim 19 or 20),  so, perhaps, the zonal Foucault test mask may not be his number one choice,  given that for his mirror he may need as many as 47 or so zones with cutout widths way past the f/300 limit set by E. H. Linfoot. On the other hand, the Everest "pin stick" method may prove to be more useful.

 

Mladen


Edited by MKV, 17 October 2014 - 11:46 AM.


#94 MKV

MKV

    Cosmos

  • *****
  • Posts: 7959
  • Joined: 20 Jan 2011

Posted 17 October 2014 - 09:21 AM

 

Mladen claims it is an error locating this normal by its intersection with the optical axis (or rather the line of symmetry)  -- Nils Olof Carlin

 

I am simply presenting what Schroader unambiguously calls "the knife-edge error", i.e. the fact that the axial measurement of the crossover of the zonal apertures is not the true COC of that zone.

 

regards,

Mladen



#95 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9414
  • Joined: 03 Jun 2005

Posted 18 October 2014 - 12:24 AM

The large drum micrometer which must read to 0.0001" accuracy and which I thought good is really bad ! On a distance of 0.5000" it measured 0.50137", that's a little more than 0.001" of innacuracy over only 0.5 inches. I had that micrometer measured years ago and it was better than 0.0001", I was dumbfounded! When testing my 20 inch, it would mean my measurements for the outer zones would have been more than 0.0005" off. No Wonder I was not seeing the same result with the Foucault. The only explanation I can think of why the 3 inch diameter micrometer is no longer accurate may be due to an accident on my part. I dropped the micrometer on the floor years ago, dammaging the flange that attaches to the drum. The threads to attach the micrometer were dammaged and I had to machine them out and mount the micrometer another way. I can only suppose I dammaged it more than I had initialy thought.

So I need to get another 0.0001" micrometer.  Any suggestions on where to purchase one without breaking the bank? (the Mitutoyos are nice but at about $300, very pricey).

 

Well, there ya go.  I recommend the Mitutoyos without reservation.  If you shop around on eBay a bit you can likely come up with one in excellent condition for about half that price.  The one I showed (I have a few) was about that price and brand new in box. 

 

Best,

Mark



#96 steveastrouk

steveastrouk

    Viking 1

  • *****
  • Vendors
  • Posts: 949
  • Joined: 01 Aug 2013

Posted 18 October 2014 - 12:14 PM

I've built high-spec measurement stuff using glass scale DROs with 1um precision and probably 2-3um accuracy  for less than a good micrometer head - specifically because I needed the travel,  assuming the temperatures are stable. Last time I bought a 70mm head, it cost me around 100GBP (150 USD). A 2 axis display was 125 GBP



#97 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22712
  • Joined: 22 Feb 2010

Posted 18 October 2014 - 05:37 PM

 

Testing by hanging the mirror upside down over a oiled flat is Not going to happen

 

 

but Why not? :grin:

 

you could always use karo syrup.

 

gumby optics.jpg


Edited by Pinbout, 18 October 2014 - 05:38 PM.


#98 hbastro

hbastro

    Vostok 1

  • -----
  • Posts: 155
  • Joined: 25 Apr 2013

Posted 18 October 2014 - 07:16 PM

Danny, That is a great test setup! :lol:  :lol:  :lol:

Dave



#99 kfrederick

kfrederick

    Soyuz

  • *****
  • topic starter
  • Posts: 3996
  • Joined: 01 Feb 2008

Posted 19 October 2014 - 06:52 PM

 

 

Testing by hanging the mirror upside down over a oiled flat is Not going to happen

 

 

but Why not? :grin:

 

you could always use karo syrup.

 

attachicon.gifgumby optics.jpg

 

Is that a hard hat?  Just a ton of glass over head . Funny he looks like some one I know 




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics