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Takahashi FC-100 from Zen-Ugh

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#126 davidc135

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Posted 30 July 2022 - 02:14 PM

When I first received the lens I knife edge tested it in DPAC and saw its perfect freedom from false colour as well as its excellent correction. I also noticed handling the flint in certain lights that the front surface had an off centre circular patch around 2 inches across that reflected the light differently but which hadn't affected the DPAC test.

 

If this was the remains of the original (soft) coating it had been lost from much of the lens. Now, after some hours of pushing the lens over the pitch only a small central smudge can be seen if I use the lens to reflect light from a bulb. In this setup light is transmitted through R2 and then is reflected back from R1 except where the remains of the possible coating appear as a dark smudge.

 

David


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#127 Live_Steam_Mad

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Posted 30 July 2022 - 09:27 PM

LiveSteamMad

If you have access to the older, corroded FC100 as well as the more modern version it would be worthwhile doing a side by side to see just how much the haze affects images.

 

David

Alas, I do not have access to the older FC-100 with the peeling coating and corroded lens. It is merely information and pictures from an auction that I came across, that corroded scope is located in Japan. I only included one pic of the corroded lens above, the rest of the pix that followed with clear MC lens were from the seller of my own FC-100 scope (my friend in Japan bought it on my behalf) and pix sent by my friend of my scope.

 

I would find the comparison fascinating indeed. A pity that I don't have access to the corroded scope (unless I were to buy it myself). I would bet that the image would still be just as sharp as with mine, just with a little less contrast and slightly dimmer maybe?

 

Best Regards,

AG


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#128 Live_Steam_Mad

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Posted 30 July 2022 - 10:04 PM

Because the outer surface hasn't been damaged it looks like it was the conditions that existed between the lenses that caused the etching, not necessarily just the heat and humidity of the surrounding air. Perhaps condensed liquid water + 100% humidity is needed. I don't know if that was the case with LSM's example.

 

David

The corroded FC-100 scope's lens  that I showed only 1 pic of had lived in Japan it's whole life I think. My own FC-100 with MC clear lens in the other pix also lived in Japan it's whole life I believe. Unfortunately I don't know anything more about their history.

 

AG.


Edited by Live_Steam_Mad, 30 July 2022 - 10:04 PM.


#129 Live_Steam_Mad

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Posted 31 July 2022 - 12:00 PM

I found this on Google Books, included here is a small extract for educational purposes ;-

 

The book is from "Classic Telescopes: A Guide to Collecting, Restoring, and Using Telescopes" by Neil English, sounds like a good book to obtain.

 

It mentions that the optics were hand figured, and all completely uncoated, for the original FC-100 (unlike the later MultiCoated FC-100's). Interesting then that I have 2 of the hand made Takahashi scopes (my CN-212 optics are also hand made I believe, and ultra smooth, about 1/7 wave PV, same as every CN-212 it would appear, Airy labs tested a couple of them and found the same results, they were incredibly consistent (maybe the most consistent commercial optic ever made), which is why the CN-212 was so expensive, it was made as well as the Optician could make it, with no compromise, I like that.

 

Regards,

AG

Attached Thumbnails

  • Extract_from_Classic Telescopes_A Guide to Collecting, Restoring, and Using Telescopes by Neil English.jpg

Edited by Live_Steam_Mad, 31 July 2022 - 12:01 PM.

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#130 Live_Steam_Mad

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Posted 03 August 2022 - 10:22 PM

I've seen a few mentions around the internet of the mating glass being KzF2.

Telescopes Eyepieces Astrographs book by Smith, Ceragioli and Berry, says that the best mating glass for the best possible performance with Fluorite is Schott N-KzFS2, but that it stains (goes cloudy) from chemical attack when uncoated, and that even when coated, pin holes in the coating will eventually cause a cloudy lens, so for this reason the manufacturers use K7 instead (which has a 3 out of 4 on the scale for going bad from chemical attack due to weathering, so not great, but is considered to be a pretty stable glass).

 

Attached image is a small excerpt for educational purposes.

 

Maybe Tak's FC-100 was originally mono-coated KzFS2, which would explain all the bad cloudy lenses from eventually failing coatings, and would explain why the cloudyness goes so deep requiring you to go back to fine grinding.

 

Later FC-100's with MultiCoating don't seem to suffer from cloudiness that I have ever seen, so either they must have created a very good coating, or else redesigned the lens to use K7 mating glass and MC.

 

Regards,

AG

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  • 1659582699246.JPEG

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#131 semlin

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Posted 04 August 2022 - 01:07 AM

Telescopes Eyepieces Astrographs book by Smith, Ceragioli and Berry, says that the best mating glass for the best possible performance with Fluorite is Schott N-KzFS2, but that it stains (goes cloudy) from chemical attack when uncoated, and that even when coated, pin holes in the coating will eventually cause a cloudy lens, so for this reason the manufacturers use K7 instead (which has a 3 out of 4 on the scale for going bad from chemical attack due to weathering, so not great, but is considered to be a pretty stable glass).

 

Attached image is a small excerpt for educational purposes.

 

Maybe Tak's FC-100 was originally mono-coated KzFS2, which would explain all the bad cloudy lenses from eventually failing coatings, and would explain why the cloudyness goes so deep requiring you to go back to fine grinding.

 

Later FC-100's with MultiCoating don't seem to suffer from cloudiness that I have ever seen, so either they must have created a very good coating, or else redesigned the lens to use K7 mating glass and MC.

 

Regards,

AG

that is an impressive find that would readily explain what we see.  



#132 Dave Trott

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Posted 04 August 2022 - 10:41 AM

Telescopes Eyepieces Astrographs book by Smith, Ceragioli and Berry, says that the best mating glass for the best possible performance with Fluorite is Schott N-KzFS2, but that it stains (goes cloudy) from chemical attack when uncoated, and that even when coated, pin holes in the coating will eventually cause a cloudy lens, so for this reason the manufacturers use K7 instead (which has a 3 out of 4 on the scale for going bad from chemical attack due to weathering, so not great, but is considered to be a pretty stable glass).

 

Attached image is a small excerpt for educational purposes.

 

Maybe Tak's FC-100 was originally mono-coated KzFS2, which would explain all the bad cloudy lenses from eventually failing coatings, and would explain why the cloudyness goes so deep requiring you to go back to fine grinding.

 

Later FC-100's with MultiCoating don't seem to suffer from cloudiness that I have ever seen, so either they must have created a very good coating, or else redesigned the lens to use K7 mating glass and MC.

 

Regards,

AG

At last we get an authoritative source on this subject. And it was waiting right on my bookshelf all this time!

 

Thank you for digging into this, Live_Steam_Mad!!!



#133 Eric P

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Posted 04 August 2022 - 12:12 PM

So if you fix the cloudy lens by grinding/polishing off the coating then I assume it will eventually go cloudy again since the fluorite is exposed?  



#134 davidc135

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Posted 04 August 2022 - 01:38 PM

So if you fix the cloudy lens by grinding/polishing off the coating then I assume it will eventually go cloudy again since the fluorite is exposed?  

Hopefully the sensitive glass surface can be protected with silica gel sachets or similar. The fact that the outer surface wasn't etched is reassuring.

 

David



#135 jkmccarthy

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Posted 04 August 2022 - 01:55 PM

Since David (davidc135) has both of Steve_M_M's lens elements and a precision spherometer, might I suggest he measure all four lens radii and provide those (along with best estimates of the center thicknesses) to someone (most likely David himself?) having the capability to perform a raytrace ?  We know the second lens element is CaF2, and with the optical prescription from David, it should be easy to tell by comparing raytrace results whether the first lens element in Steve's FC-100 objective is more likely to be Schott N-KzFS2 or Schott K7, given that (as noted in the text on the page that AG (Live_Steam_Mad) shared above, the difference in Abbe Number between KzFS2 and K7 (41 vs. 60.4) drives a K7/Fluorite doublet to have a stronger powers in each element (negative K7 and positive CaF2).  Using as-measured radii from a KzFS2/Fluorite doublet (if that's what Steve's FC-100 lenses indeed are) in a raytrace assuming K7 for the negative front element would show poor color correction in the raytrace, and so would using as-measured radii from a K7/Fluorite doublet in a raytrace assuming KzFS2 for the negative element.

 

Until some "reverse engineering" analyses along these lines are performed, all we really have is "conjecture" --- however insightful and plausible and "convincing" it may sound.   Not knowing the specific melt indices for the actual front element in Steve's FC-100, and raytracing instead with nominal Schott catalog refractive indices, may not conclusively prove that the front element must be KzFS2 (or conversely K7) ... only that the available evidence shows the as-built optical design prescription is more consistent with it being KzFS2 rather than K7 (or conversely K7 rather than KsFS2).  But personally I think this would still represent a significant step forward in our understanding beyond the conjecture we have at present, valuable as this hypothesis certainly is in guiding our thinking forward toward finally understanding of the root cause of the haze.

 

        -- Jim

 

P.S.   Just now seeing David's reply to Eric P --- I thought it had already been established as fact (e.g., back in Steve_M_M's post #5 on page 1, and echo'd in a number of follow-up replies) that the biconvex rear Fluorite element is clear, and that the haze is confined to the second surface (R2) of the negative-powered front element.   So I don't understand "eventually go cloudy again since the fluorite is exposed?" --- given that the fluorite isn't the element having the problem in the first place.



#136 Terra Nova

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Posted 04 August 2022 - 02:16 PM

So I’m guessing that those of us with the mint green trim, MC optics have nothing to worry about? My 1997 FC-76 (thank you again Dave Trott) still looks pristine. It would break my heart if it spontaneously went to hell. I love that scope and take real good care of it.


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#137 jkmccarthy

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Posted 04 August 2022 - 03:41 PM

Because the outer surface hasn't been damaged it looks like it was the conditions that existed between the lenses that caused the etching, not necessarily just the heat and humidity of the surrounding air. Perhaps condensed liquid water + 100% humidity is needed. I don't know if that was the case with LSM's example.

 

David

But regarding the concern Eric P raised about the possibility of recurrence, I think David's observation + thoughts contributed on 30-Jul-2022 are quite significant -- the "exposed" front surface of the negative front element has not become hazy, only the second surface (separated from the front side of the fluorite element by the small airspace between the elements) degraded and became hazy....   Hmm....
 

EDIT:  In case it helps anyone trying to follow along, the arrangement of the lens elements within the objective in Steve_M_M's FC-100 is similar to example "c" at far right here, where in all three cases shown the optics convention is that (parallel) light is incident from the left, so the image formed by any of the three lens examples would be located somewhere off to the right of the doublet lens.  In a fluorite doublet, the positive element (thicker in the center than at the edges) is the one that's made of fluorite (crystalline CaF2), regardless of whether it is the front element (as in "a" [cemented, or oil-spaced] or "b" [air-spaced ... size of the airspace exaggerated for emphasis]) or the rear element (as in "c" [where the size of the airspace is likewise exaggerated]) :

 

Doublet-designs.png


Edited by jkmccarthy, 04 August 2022 - 04:02 PM.

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#138 Defenderslideguitar

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Posted 04 August 2022 - 03:50 PM

Interesting...................

 

We learn much here.....

 

So I wonder about the page posted in post 129 , thanks (Live S M),   It seems to throw all the original FC's  into the worrisome basket and states that it was not until the superlative FS series   etc.......

 

My original model FC -100  (Tak Green) is wonderful   so much so that I never should have sold it and just bought it back,,,

The original model FC -76 (Tak Green) is special.   Both scopes came originally from Koji btw, the FC -76 is the  Red  "F" lens   prior to the introduction of the FS series......so I am with Terra that    maybe some Gray trim models of the original FC series can have an issue, though some are just plain great,  I have not seen this issue with the Tak Green trim.................Finally,  the FS series, based on the one I got from Rolo, is top shelf....so we will not argue with superlative

 

Keep the discussion going thanks.....  

 

Learning much from the discussion though. 


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#139 davidc135

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Posted 04 August 2022 - 03:53 PM

 

 

P.S.   Just now seeing David's reply to Eric P --- I thought it had already been established as fact (e.g., back in Steve_M_M's post #5 on page 1, and echo'd in a number of follow-up replies) that the biconvex rear Fluorite element is clear, and that the haze is confined to the second surface (R2) of the negative-powered front element.   So I don't understand "eventually go cloudy again since the fluorite is exposed?" --- given that the fluorite isn't the element having the problem in the first place.

Yes, the fluorite is perfect, it's just R2 and whilst I haven't looked at R1 very closely, damage is certainly not apparent.  David


Edited by davidc135, 04 August 2022 - 04:05 PM.

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#140 Marcus Roman

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Posted 04 August 2022 - 03:59 PM

Telescopes Eyepieces Astrographs book by Smith, Ceragioli and Berry, says that the best mating glass for the best possible performance with Fluorite is Schott N-KzFS2, but that it stains (goes cloudy) from chemical attack when uncoated, and that even when coated, pin holes in the coating will eventually cause a cloudy lens, so for this reason the manufacturers use K7 instead (which has a 3 out of 4 on the scale for going bad from chemical attack due to weathering, so not great, but is considered to be a pretty stable glass).

 

Attached image is a small excerpt for educational purposes.

 

Maybe Tak's FC-100 was originally mono-coated KzFS2, which would explain all the bad cloudy lenses from eventually failing coatings, and would explain why the cloudyness goes so deep requiring you to go back to fine grinding.

 

Later FC-100's with MultiCoating don't seem to suffer from cloudiness that I have ever seen, so either they must have created a very good coating, or else redesigned the lens to use K7 mating glass and MC.

 

Regards,

AG

Thanks, this is very interesting and to me unknown info! Interesting to see what Vixen declared about its flint in the Polaris FL advertisement (see attached): KzF5 as best match for their Fluorite crown.

Attached Thumbnails

  • DE533B20-62BB-4841-8723-B1BE0D447B41.jpeg

Edited by Marcus Roman, 04 August 2022 - 04:00 PM.

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#141 davidc135

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Posted 04 August 2022 - 04:04 PM

Since David (davidc135) has both of Steve_M_M's lens elements and a precision spherometer, might I suggest he measure all four lens radii and provide those (along with best estimates of the center thicknesses) to someone (most likely David himself?) having the capability to perform a raytrace ?  We know the second lens element is CaF2, and with the optical prescription from David, it should be easy to tell by comparing raytrace results whether the first lens element in Steve's FC-100 objective is more likely to be Schott N-KzFS2 or Schott K7, given that (as noted in the text on the page that AG (Live_Steam_Mad) shared above, the difference in Abbe Number between KzFS2 and K7 (41 vs. 60.4) drives a K7/Fluorite doublet to have a stronger powers in each element (negative K7 and positive CaF2).  Using as-measured radii from a KzFS2/Fluorite doublet (if that's what Steve's FC-100 lenses indeed are) in a raytrace assuming K7 for the negative front element would show poor color correction in the raytrace, and so would using as-measured radii from a K7/Fluorite doublet in a raytrace assuming KzFS2 for the negative element.

 

Until some "reverse engineering" analyses along these lines are performed, all we really have is "conjecture" --- however insightful and plausible and "convincing" it may sound.   Not knowing the specific melt indices for the actual front element in Steve's FC-100, and raytracing instead with nominal Schott catalog refractive indices, may not conclusively prove that the front element must be KzFS2 (or conversely K7) ... only that the available evidence shows the as-built optical design prescription is more consistent with it being KzFS2 rather than K7 (or conversely K7 rather than KsFS2).  But personally I think this would still represent a significant step forward in our understanding beyond the conjecture we have at present, valuable as this hypothesis certainly is in guiding our thinking forward toward finally understanding of the root cause of the haze.

 

        -- Jim

 

Unfortunately my spherometer can only measure flats and concave surfaces and even then it's only to detect small differences in depth to a high accuracy. It might manage the shallower R1 and R4 but would need to be remade to measure R3 which is around f0.7.

 

I would be a bit anxious about scratching the valuable fluorite surfaces with the plain steel ball bearings.

 

David


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#142 jkmccarthy

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Posted 04 August 2022 - 05:00 PM

Unfortunately my spherometer can only measure flats and concave surfaces and even then it's only to detect small differences in depth to a high accuracy. It might manage the shallower R1 and R4 but would need to be remade to measure R3 which is around f0.7.

 

I would be a bit anxious about scratching the valuable fluorite surfaces with the plain steel ball bearings.

 

David

Thanks David for pointing out these practical difficulties.

 

Setting the spherometer aside, might it instead be possible to measure the radius R2 (once your re-polishing is complete) via reflection off that surface, by locating its center of curvature?

 

For purposes of distinguishing between glass types for the negative element (based on the individual element powers needed for color correction with CaF2, as a function of the Abbe Number of the glass used for the negative element), I wonder if one could then just assume R3 = R2 (as in reality these are unlikely to be too dissimilar, even if they are unequal), such that given this estimate of R3 and measuring the focal length for the fluorite element alone would let one solve for R4, and having thus deduced values for R2, R3, and R4, knowing the focal length of the assembled doublet would let one solve for R1 (for an assumed refractive index of the negative first element).  From here, the check via raytrace (or just hand-calc of L.Ch.A between the F and C foci ?) to assess overall color correction for various assumed mating glass types might then enable one to draw empirical+analytic conclusions on whether the [thusly approximated] lens radii are consistent, or else inconsistent, with the mating glass type being KzFS2, KzF5, K7, etc. ?

 

Granted there may be some slight amount of 'circular logic' involved since the derived value for R1 will differ for each assumed mating glass, but because the Abbe values (dispersions) of the candidate mating glass types vary much more than do their refractive indices, so I'm thinking that this empirical+analytic 'test' of candidate mating glass materials still ought to be perceptive in showing which mating glass type(s) can be ruled out as inconsistent with the measured R2 and the measured diopter power of the fluorite element, no ?

 

Food for thought, anyway ...

 

Cheers,

 

      -- Jim

 


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#143 davidc135

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Posted 04 August 2022 - 06:41 PM

I have some numbers that I'll post tomorrow.  David


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#144 pancho61

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Posted 05 August 2022 - 03:55 AM

Hi everybody,

 

I have found this information in Carton brand catalog ( It is only optical part catalog) from 1989, you can see three optical formula who has been used in this period with combining elements.

 

- Classic achromat

- Triplet scope

- Fluorite

 

That's really interesting as it seems probably Vixen and Takahashi was not the inventor of this designs maybe it was Carton.

 

 

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  • 331844763_catalogueoptique1989_page-0001(1).jpg.67928dcd4d4f2047303ecaf48dceae70 (Copier).jpg

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#145 Marcus Roman

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Posted 05 August 2022 - 09:58 AM

Hi everybody,

 

I have found this information in Carton brand catalog ( It is only optical part catalog) from 1989, you can see three optical formula who has been used in this period with combining elements.

 

- Classic achromat

- Triplet scope

- Fluorite

 

That's really interesting as it seems probably Vixen and Takahashi was not the inventor of this designs maybe it was Carton.

Yes interesting indeed! Seems they also adopted Flint KzF5 and a Crown Fluorite as Vixen…


Edited by Marcus Roman, 05 August 2022 - 09:58 AM.

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#146 DAVIDG

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Posted 05 August 2022 - 10:54 AM

 The glass is clearly water reactive and the coating fail on R2 allowing to the surface to be attack. When you have water sensitive glass you don't use an aqueous based polishing solution since as soon as the surface is polished it can immediately react with the water and hazy over.  So it never polishes  It is similar to when you machine certain metals like aluminum since the  fresh surface hasn't oxidize over and is very chemical reactive. If you use the wrong cutting fluid it can react and smoke and even start a fire. 

    You have similar situation with freshly polished glass surface. So what is done is that the polishing solution is not water based but glycerin is used instead. CaF2  and NaCl  surfaces are polished this way. One a side note Brashear in the late 1800's figure out how to make NaCl prisms for spectroscopic work by using kerosene instead of water in his polishing solution since NaCl dissolves very nicely in water.     

   So I believe this maybe why when the surface was first polished on pitch and using cerium oxide with most likely water  that there didn't seem to be an improvement. 

 

                     - Dave 


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#147 davidc135

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Posted 05 August 2022 - 03:16 PM

I gingerly took a spherometer reading of R4 as well as R1 and R2 and my (imperial) figures are:

 

R1 = 12.12 ins cx

R2 = 6.063       cv

R3 is weaker than R2 by around the thickness of a sheet of typing paper (0.1mm)  cx

R4 = 152.39 (nearly plane) cx

 

Edge thickness of flint = 11mm approx

                           fluorite   5mm

 

Hopefully no mistakes.

 

Maybe someone can help me with one question: I probably have made R2 deeper by perhaps 5 waves than the original and so will have over-corrected the colour fractionally. If it is relevant would the airgap need to be increased or reduced to counter this chromatic over-correction and would there be a corresponding effect on the SA? At the moment I have guessed the airgap at the edge to be 0.2mm.

 

David


Edited by davidc135, 05 August 2022 - 03:28 PM.

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#148 davidc135

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Posted 05 August 2022 - 06:51 PM

At a glance the flint looks clear although there are still plenty of pits, especially near the edge. 51/2 hours polishing.

 

P8011932.JPG

 

I decided to start figuring. Here's a knife edge pic in DPAC with the ke from the left.

 

P8031936 (2).JPG

 

It looks pretty bad with an outer raised zone and turned down edge. But measuring the knife edge travel between nulls for the different zones showed that the error was modest, a bit over 1/4 wave pv. There was no colour seen visually.

 

David


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#149 davidc135

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Posted 05 August 2022 - 07:12 PM

To begin with, very short strokes cut down the error but progress faltered and so I switched to a small polisher and the machine.

 

P8051939 (2).JPG

 

The action is fast but there's a risk of cutting record grooves into the glass. And sure enough.

 

P8051941 (2).JPG

 

In an f8 system with DPAC and a stationary light source 2.25mm knife edge travel corresponds to 1/4 waves of error. For most of the aperture ke travel is only the thickness of an ink line. But at the cost of some grooves, unfortunately.

 

I'll try to smooth them down but from experience, they tend to hang around. I think that they are off by 1/6th to 1/4 wave.

 

Again, visually I see no colour in these tests.

 

David


Edited by davidc135, 06 August 2022 - 02:12 AM.

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#150 davidc135

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Posted 08 August 2022 - 03:19 AM

After a little more work with the small polisher I put in an hour and a quarter of very short strokes with the 4'' lap which softened the grooves although they are still obvious. 

 

A couple of regrets. One is that I should have taken a record of the ke test of the original objective which would have told me what marks are down to the test setup and which are due to my work.

And another time I'd rethink the local polisher to give a smoother result.

 

P8081950 (2).JPG

 

I think that a high centre (focusing short) and a turned down outer zone is creeping in. The actual edge is turned up but that would be hidden by the cell. All this polishing is slowly reducing the leftover pits.

 

I will do a little more and then return the objective to Steve. It'll be interesting to see how it performs.

 

David


Edited by davidc135, 08 August 2022 - 03:32 AM.

  • Steve_M_M, Live_Steam_Mad, Bomber Bob and 3 others like this


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