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#776 PawPaw

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Posted 08 March 2020 - 11:02 AM

This old eyepiece case came with a edmund lens I purchased years ago.  It came from the Netherlands.  Anyone recognize these eyepieces?

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#777 DAVIDG

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Posted 08 March 2020 - 11:15 AM

 The amount of bowing will be the same on both sides of focus when the grating is placed the same distance from the focal plane. 

   The reason why we use double pass autocollimation to test is that is a null test. What that means is when your telescope has excellent optics you will see straight Ronchi bands or if you use a knife edge  the optics will gray out and show no zones like a perfect spherical mirror does when you test it at the radius of curvature. So there nothing to measure to tell if you have good optics are bad. Straight = good, bowing of any amount means there is some level of errors  Also there is no critical spacing to set as with other tests. The result is that Double pass has very few sources of error. That means that the results you get have a very level of confidence in the results.

 With other test method you have to measure zones , set critical spacing and do calculation to determine the quality of the optics. All of those have errors that can and do lead to wrong results. The charts you posted shows bowing the Ronchi bands of what "parabola" may look like when tested  at the radius of curvature but you can't tell how good the parabola is just that might be something close to one. Wth double pass when the lines are straight you know it is the correct figure. 

    My guess is that you'll find the mirror is very much undercorrected  because the lines will bow inward  ))(( when you place the grating inside of focus.  So the figure is much closer to a sphere then a parabola.  Most likely the optics were ground and polished and never figured. 

  The good news is that you  have test method that will allow you determine when you have made it into a excellent parabola.   

 

 

                 - Dave     



#778 DAVIDG

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Posted 08 March 2020 - 11:17 AM

This old eyepiece case came with a edmund lens I purchased years ago.  It came from the Netherlands.  Anyone recognize these eyepieces?

 They look to be Jaegers.

 

                    - Dave 



#779 clamchip

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Posted 08 March 2020 - 11:40 AM

I tested this morning and worked on my photography.

Photos aren't much better and the photographer is probably

introducing some errors himself with the camera off axis.

By eye though its very clear, inside focus the lines bow out, but not much, as represented in the top photo.

Outside focus the lines are straight, bottom photo.

Robert

 

IMG_9631.jpg

IMG_9637.jpg


Edited by clamchip, 08 March 2020 - 11:49 AM.

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#780 clamchip

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Posted 08 March 2020 - 11:46 AM

 They look to be Jaegers.

 

                    - Dave 

I agree Jaegers.

I've never owned a  Jaegers eyepiece, I have seen their distinctive shape in pictures and 

and Jaegers catalogs.

 

Robert



#781 tim53

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Posted 08 March 2020 - 12:04 PM

I tested this morning and worked on my photography.

Photos aren't much better and the photographer is probably

introducing some errors himself with the camera off axis.

By eye though its very clear, inside focus the lines bow out, but not much, as represented in the top photo.

Outside focus the lines are straight, bottom photo.

Robert

 

attachicon.gifIMG_9631.jpg

attachicon.gifIMG_9637.jpg

It looks to me like the lines are bowing in one direction, not like a barrel.  I get this when I'm testing with my camera noticeably off axis from the light source.  This is worse when the light source is on axis and the camera is the only element off axis (to see past the light source).  A better arrangement would be to find on-axis with a laser collimator (you'd need a holographic one for a cassegrain), then offset the camera AND the light source by the same amount but opposite directions.  

 

That might not be a bad parabolic mirror, actually.


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#782 PawPaw

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Posted 08 March 2020 - 02:08 PM

I agree Jaegers.

I've never owned a  Jaegers eyepiece, I have seen their distinctive shape in pictures and 

and Jaegers catalogs.

 

Robert

Thanks.....I need to give them a cleaning and try them.



#783 DAVIDG

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Posted 08 March 2020 - 05:20 PM

 Both the pictures posted today and the one from last night all show a large amount of over correction if  the grating was inside of focus. I also see the lines starting  to bow inward around the area masked by the secondary indicating a hole in that area. Also note since your testing  6" of the 8" diameter of the mirror because the flat is 6" diameter  you are  seeing only about 50% of the total area, so the error is most likely larger.   Having testing many optics, the bowing I see is not small but pretty large, especially for a fast f-ratio one like this one. I estimate your at 1/2 wave or worse. 

  As I have said many times 1/2 wave don't produce an image that is fuzzy mess at low to medium power so it fools amateurs into believing their optics are much better than they really are. When the image breaks down at high power they blame it on the seeing, eyepiece or many other reasons. 

  Time for some pitch and polish if you want a great image.

 

                     - Dave 



#784 clamchip

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Posted 08 March 2020 - 06:28 PM

Thanks for the help Dave.

Now that the mirror's problem in known maybe we can see just how bad it is

with a Foucault test.

I built a Foucault tester and haven't used it yet.

I'll need to add a micrometer for a parabola.

Its a bit of a antique, the plans are from 'All About Telescopes' Sam Brown and

the Edmund library.

Robert 

 

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Edited by clamchip, 08 March 2020 - 06:40 PM.

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#785 clamchip

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Posted 08 March 2020 - 07:20 PM

Something I may have forgot to mention is I added the Newtonian focus

as a quick way to see if parabolic.

When I bought this scope from the Doc it had no focusers,cass or newt.

The telescope was built in the Yerkes observatory shops with the help

of the Yerkes optician, and since the primary is aluminized it's finished.

I just can't imagine it has such gross errors, unless it is a design we are

not looking at because of my newt focus throwing us into thinking classical

cass.

I wonder if the primary could be hyperbolic if it's a over corrected parabola?

This would be a Richey-Chretien wouldn't it. The most difficult of all the cass's

to make but the best.

The secondary if far from being finished, no knowing what was intended with

it as far as telescope design and the Doc can't remember.

 

Robert


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#786 DAVIDG

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Posted 09 March 2020 - 09:04 AM

 If the primary is hyperbolic that would make it a RC type  Cassegrain with the need for the secondary to also be hyperbolic.  Measure some zones and see were it comes out. Just be sure you understand that there are many sources of errors in zone measurements and to get accurate results those errors need to be accounted for. If not, the results are like many many  optics I test were they are stated to be 1/10 wave but in reality are 1/2 wave.  This is why I use null test methods vs zone reading since tests like double pass have far few sources of error hence one has much more confidence in the results.

 

                 - Dave 


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#787 clamchip

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Posted 09 March 2020 - 10:07 AM

Its interesting the Yerkes 41 inch RC mirror was being made by a graduate student in the 

Yerkes workshop the same time as this mirror in 1968. He died unexpectedly and Tinsley

finished the mirror.

 

Robert



#788 DAVIDG

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Posted 09 March 2020 - 12:57 PM

If you measure the radius on the primary and that of the  secondary I can determine the exact conic values. From there we can determine what the zone reads should be when you measure them using the Foucault test.  The radius on  the primary you can measure with your Foucault tester as explained in your "All About Telescope Book". The radius on the secondary would require a calibrated spherometer.  

 I hope Tinsley did a better job then what they did on the 24" at Mt Cuba and the 32" at the Virginia Tech  and BYU.  They were pretty bad. Back in '85 I refigured the 24" at Mt Cuba, It had about a 2 wave figure to start.  Here is a  picture of M33 taken a few weeks ago with the 24" at Mt Cuba with the optics I refigured when I was a kid. 

 

                  - Dave 

 

m33color_120s.jpg


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#789 clamchip

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Posted 10 March 2020 - 10:12 AM

I do know the focal length of the primary from the Newtonian focus: 43 inches.

Radius of primary: 86 inches. Diameter of primary: 7-5/8"

The secondary is going to be more difficult without removing it from it's cell.

If I remove it from the cell I can mike the edge and center, or the secondary

may be clear enough I can measure the convex radius from the back.

This morning I came up with another plan.

If you can give me a approx radius of what the secondary should be I can

go through my achromat elements and see if I can find a close sphere to

do some tests as if this were a classic cass, at least see if it will focus.

Robert

 

post-50896-0-46767900-1543440693_thumb.jpg  


Edited by clamchip, 10 March 2020 - 10:14 AM.

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#790 Terra Nova

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Posted 10 March 2020 - 10:23 AM

Perhaps this project needs it’s own thread? Seems it would be a good idea if these posts could be moved into one. I think it a topic where interest would make it deservingly so.


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#791 Terra Nova

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Posted 10 March 2020 - 10:34 AM

Inquiring minds want to know!

 

https://www.cbsnews....ve-mail-amazon/



#792 DAVIDG

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Posted 10 March 2020 - 10:55 AM

 You need the exact radius of curvature to at least a 1/16" or better on  both the primary and secondary.  So when you say the primary has a radius to 86" is that 86.00" or just 86" plus or minus who knows what  This is an example of were the errors in making Foucault readings starts to come in and why many "1/10" wave optics when correctly tested turn out to be  1/2 a wave. For a 6" f/8 that is discussed in Sam Brown's excellent book "All About Telescopes" there is fairly large error window. This is why a 6" f/8 is  a great place to start and why if you get it close it most likely under 1/4 wave. Hence the reputation of the great images they give. 

 When your doing Foucault zone measurements the reading are calculated using the formula of r^2 / R  were little r is the zone radius on the mirror surface and  big R is the radius of curvature.  You said the clear aperture of the primary is 7.625" so the radius of the surface of the mirror is 3.8125" and the radius of curvature is 86" . So the Foucault reading from the center zone to the edge ie the total correction should  0.1690"  time a factor to make it correct for a Hyperbola vs a Parabola. The error allowed in that reading to have it fall within a 1/4 wave is only about plus or minus  0.002"  ie  That is if all the other measurements are dead on the money, no  errors but that is not the real world.  Say the actual radius is not 86.000" but is off by 1/16" and it is really 86.125" Now the total correction should be 0.1687"  but because of the error in the measurement  that you didn't know about you shot for 0.1690" and got that reading. Your starting creep closer to the 1/4 wave  limit. add more errors to the readings like errors from how the tester is made to errors in how you judged were the zone grayed out and before you know it,  the  confidence level in your measurement is outside the 1/4 wave limit !   This is why  making zone reading can easily lead to the wrong results without people knowing it.  So the bottom line is that we need much  more accurate measurements of the radii to calculate the ideal zone reading for hyperbola figure on the primary or your going to take some reading and they many be "close" and then you'll believe you have an excellent figured mirror when in reality it is not. 

   So when you use a test like douible pass your not measuring anything. You can SEE the errors  and if the mirror doesn't gray all over or give dead straight Ronchi band , then you can SEE it is not perfect. 

 

                       - Dave 


Edited by DAVIDG, 10 March 2020 - 12:27 PM.

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#793 DAVIDG

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Posted 10 March 2020 - 01:33 PM

 I calculated some values based on guesses. So this are just a starting points. I assumed that the design is an RC. ie hyperbolic primary and secondary, that total focal length is F/15 so 120" and that the back focal length behind the primary is 10". Again these are guesses. The primary would need to be hyperbolic with a conic of -1.12373. A perfect parabola has a conic of -1.000 so you take the Foucault reads for the 30, 70 and 90% zones and multiple them by 1.12373 to get the values for THIS hyperbola.  There are infinite number of hyperbolas so you need to calculate the valves based on the EXACT design.

   So as an example  you have the total radius of the surface of primary mirror to be  3.8125" and say you want to find the value that you measure at the 90% zone on the surface of the mirror. Take 90% of total radius and that is  3.43125". square that =11.77347 divide by the radius of curvature of 86, that equal  0.1369" . The 0.1369"  is the value measured from the center zone  to the light comes to focus  from the 90% zone on the mirror if it was a perfect parabola but since we want a hyperbola which is deeper we multiple that value by the conic value for THIS hyperbola ie   1.12373  x 0.1369 =  0.15383"   So you zero the Foucault tester on the center  zone and move the knife edge backwards until you see the two hole in the Foucault mask  at the 90% so even levels or grayness. That value if should be 0.15383". 

     I hope you can see that to  have accurate values we need the exact radius on the primary and the exact radius on the secondary since that will lock down all the other parameters. So it is not just a matter of taking some Foucault zone reads based on some guess and say they "seem to be right". You have to have accurate measurements to start or again what you assume is "1/10" wave optics is nowhere close to that in reality.

 

                  - Dave 



#794 clamchip

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Posted 10 March 2020 - 02:09 PM

I should probably read up on zonal Foucault testing.

I have three books in my library that cover the subject very well.

'All About Telescopes' Sam Brown, is easy to read and understand with

his wonderful illustrations.

'How to Make a Telescope' Jean Texereau. Excellent classic.

And 'Build Your Own Telescope' Richard Berry. I like Richard's tester, I

may or probably will need a new tester with a micrometer carriage for

the knife.

These tools and the knowledge to use them will be valuable for evaluating

the old and even new telescopes.

I like the DPAC but its more of a 'pass-fail' test, I'd like to know how far off

and where the problem lies.

 

Robert


Edited by clamchip, 10 March 2020 - 02:16 PM.


#795 DAVIDG

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Posted 10 March 2020 - 03:15 PM

 Foucault measurement is learned skill. It is like learning to play a musical instrument but there is one  big difference that few amateurs understand. That is you need to confirm you are doing the whole method completely correct. With music you can compare how you play vs what it should sound like  You can hear that you screwed up when playing Stairway to Heaven.  So just like in playing music Foucault testing requires measuring a reference of known value and see that you can get the correct results. If not you can being doing the measurement wrong and never know it. It could be an error in misunderstanding how it is done, and or an unknown error in the test equipment and/or a random error like making a math mistake in calculated the values   I have seen all of those many times and by people that having being doing the measurements for years but wrong for those years.

   Getting the same results is not the same as getting the correct results. The Hubble mirror is great example of this. They only believed the results from the one test method and that had an error in the construction of the tester. They had great precision but very poor accuracy. Most amateur mirror makers don't understand the difference so they  enter data into a computer programs that tell them they made a great mirror but have no idea of the errors in their measurements and if they are doing the total method correctly. 

 

 

                       - Dave 



#796 PawPaw

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Posted 13 March 2020 - 07:20 PM

Need some help in identifying the manufacturer of this erecting prism.  I have a hunch but would like others input first. 

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Edited by PawPaw, 13 March 2020 - 09:43 PM.

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#797 clamchip

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Posted 21 March 2020 - 12:27 PM

I've been reading up on testing Cassegrain telescopes.

This is a great book on the subject covering not just the SCT but all telescope designs.

I'm sure this book is still available ether here, Astromart, or the Author.

Robert

 

Scan0007.jpg

Scan0009.jpg


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#798 Kasmos

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Posted 26 March 2020 - 04:40 AM

I had a pleasent little surprise last night.

 

After doing some work on the Mizar I opened a box of old Sky & Telescope clippings I had cut out about 10-15 years ago.

Back then I cut out interesting articles and photos in an effort to make space, but later regreted.

I still have a box of survivors, but these 4 issues were in the box of clippings so I had no idea they had survived.

S&T-4.jpg

Another good thing, quite a few cool ads were saved and are fun to look back on.

S&T-Ads.jpg


Edited by Kasmos, 26 March 2020 - 06:21 PM.

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#799 Terra Nova

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Posted 29 March 2020 - 03:11 PM

Check out this uber-cool giant howitzer of a telescope. 
 

http://www.astrosurf...or_20181113.pdf


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#800 clamchip

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Posted 29 March 2020 - 03:57 PM

Check out this uber-cool giant howitzer of a telescope. 
 

http://www.astrosurf...or_20181113.pdf

Incredible!

 

Robert


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