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Compare the image quality difference between John Drapers 1840 5 inch Newtonian to a 5 inch Newtonian made today?

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#1 JerryStellar

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Posted 20 June 2022 - 06:47 PM

Hello…. This question is not a astrophotography question… I was reading about the first astrophoto obtained which was a 20 minute exposure of the moon using a 5 inch Newtonian…in 1840!   I was wondering what  the visual quality difference between a high quality 5 inch Newtonian available today compared to the one used by John Draper back in 1840?

  . I was having fun utilizing my extremely limited scope knowledge speculating how different the two views would be if we could compare the two scopes side by side today…

  any thoughts?

  Jerry 



#2 Kokatha man

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Posted 20 June 2022 - 07:30 PM

Regardless of the scope used - & btw Draper was not the first person to photograph the Moon, he's credited with the first to photograph the Full Moon - his daguerreotype was in fact a horrible image compared to almost any of today's outcomes using much smaller scopes, even from novices.....but of course at the time was a bit of a triumph for both him & photography to a certain extent! ;)

 

As to the quality of his scope compared to today, one can find some pretty woeful 5" Newts being made today...but unless his was a very exceptional one I would suspect a quality unit today would probably leave it for dead! ;)



#3 Tony Flanders

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Posted 20 June 2022 - 08:20 PM

Hello…. This question is not a astrophotography question… I was reading about the first astrophoto obtained which was a 20 minute exposure of the moon using a 5 inch Newtonian…in 1840!   I was wondering what  the visual quality difference between a high quality 5 inch Newtonian available today compared to the one used by John Draper back in 1840?


There's no way to know. Undoubtedly the image was dimmer, because no mirror made prior to the invention of metal-on-glass coatings had very good reflectivity. But in terms of sharpness there's no reason to think that modern reflectors are any sharper than the best reflectors in the 1840s. It's helpful to know that old-fashioned speculum mirrors had to be polished frequently, and every time they were polished the figure changed a little. No doubt highly experienced mirror-makers like the Herschels maintained excellent figures on their mirrors. And no doubt they were better sometimes than others.


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#4 Jon Isaacs

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Posted 23 June 2022 - 06:03 AM

There's no way to know. Undoubtedly the image was dimmer, because no mirror made prior to the invention of metal-on-glass coatings had very good reflectivity. But in terms of sharpness there's no reason to think that modern reflectors are any sharper than the best reflectors in the 1840s. It's helpful to know that old-fashioned speculum mirrors had to be polished frequently, and every time they were polished the figure changed a little. No doubt highly experienced mirror-makers like the Herschels maintained excellent figures on their mirrors. And no doubt they were better sometimes than others.

 

The Foucault test was not invented until 1858.  I believe that was the first method of testing a mirror that allowed for systematic measurement of the mirror and provided guidance in how to correct the errors.   Given the difficulties they were facing, I just don't see how mirrors from that era could really compare to today's glass mirrors.

 

Jon


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#5 Augustus

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Posted 23 June 2022 - 11:51 AM

The Foucault test was not invented until 1858.  I believe that was the first method of testing a mirror that allowed for systematic measurement of the mirror and provided guidance in how to correct the errors.   Given the difficulties they were facing, I just don't see how mirrors from that era could really compare to today's glass mirrors.

 

Jon

John Hadley invented the idea of using the star test for checking figure and parabolization. Herschel, the Drapers and others used the star test to make their mirrors, and we know that Hadley and Herschel were excellent at making optics. 



#6 Jon Isaacs

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Posted 23 June 2022 - 12:05 PM

John Hadley invented the idea of using the star test for checking figure and parabolization. Herschel, the Drapers and others used the star test to make their mirrors, and we know that Hadley and Herschel were excellent at making optics. 

 

Nonetheless, the Foucault test was a major step forward.  Given the fact that we are talking 180 years later, it seems that actual testing of their optics is not possible since they would need to be polished.

 

Jon


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#7 havasman

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Posted 23 June 2022 - 12:16 PM

...if we could compare the two scopes side by side today…

  any thoughts?

  Jerry 

His would need re-coating. And then it wouldn't be the same.



#8 Augustus

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Posted 23 June 2022 - 01:08 PM

Nonetheless, the Foucault test was a major step forward.  Given the fact that we are talking 180 years later, it seems that actual testing of their optics is not possible since they would need to be polished.

 

Jon

There was a thread a while back that mentioned someone dug up Herschel's mirrors in the 1960s and tested them, and they were quite good.



#9 ausastronomer

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Posted 23 June 2022 - 09:15 PM

John Hadley invented the idea of using the star test for checking figure and parabolization. Herschel, the Drapers and others used the star test to make their mirrors, and we know that Hadley and Herschel were excellent at making optics. 

 

There was a thread a while back that mentioned someone dug up Herschel's mirrors in the 1960s and tested them, and they were quite good.

 

Seriously ?

 

In their day they probably were, but modern technology, materials and processes would tend to indicate that they don't really compare to what we have today.  The same applies to eyepieces and the like, modern glass types, coatings and designs means that todays eyepieces are a quantum leap ahead of what we had 50 years ago when I started out.  I wouldn't even want to think about comparing a Nikon NAV HW, A Pentax XW or a Delos with eyepieces from 150 years ago.

 

The P51 Mustang was a great plane in its day.  Probably just about the best of that era, once they stuck the Pommie engine in it, and it could perform at altitude with a lot more speed.  The F15 was a whole lot better.  The F22 is a whole lot better again, to the extent that in a simulated exercise 1 x F22 took out 5 x F15's.  How long would it take an F22 to disintegrate 5 x P51's ?

 

Cheers 


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#10 KWB

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Posted 23 June 2022 - 10:12 PM

 

The P51 Mustang was a great plane in its day.  Probably just about the best of that era, once they stuck the Pommie engine in it, and it could perform at altitude with a lot more speed.  The F15 was a whole lot better.  The F22 is a whole lot better again, to the extent that in a simulated exercise 1 x F22 took out 5 x F15's.  How long would it take an F22 to disintegrate 5 x P51's ?

 

Cheers 

waytogo.gif

 

I think an even greater game changing aspect of the P-51 was it's range. It could escort the B-17 bombers all the way to Berlin and back. It was said that once Goering witnessed that fact he truly knew the war was lost. This aircraft was so good that it's status as a combat aircraft lingered into the jet era. During the Korean War, it was designated the F-51. 



#11 Augustus

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Posted 23 June 2022 - 10:39 PM

Seriously ?

 

In their day they probably were, but modern technology, materials and processes would tend to indicate that they don't really compare to what we have today.  The same applies to eyepieces and the like, modern glass types, coatings and designs means that todays eyepieces are a quantum leap ahead of what we had 50 years ago when I started out.  I wouldn't even want to think about comparing a Nikon NAV HW, A Pentax XW or a Delos with eyepieces from 150 years ago.

 

The P51 Mustang was a great plane in its day.  Probably just about the best of that era, once they stuck the Pommie engine in it, and it could perform at altitude with a lot more speed.  The F15 was a whole lot better.  The F22 is a whole lot better again, to the extent that in a simulated exercise 1 x F22 took out 5 x F15's.  How long would it take an F22 to disintegrate 5 x P51's ?

 

Cheers 

I'm referring to the figure of the primary mirrors, nothing more. But obviously Herschel's mirrors sucked because of a fighter jet analogy.


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#12 ausastronomer

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Posted 24 June 2022 - 12:11 AM

I'm referring to the figure of the primary mirrors, nothing more. But obviously Herschel's mirrors sucked because of a fighter jet analogy.

 

Forget about the fighter jet analogy.  I can't think of 1 single thing in optics from 150 years ago that remotely compares to the quality of what we have today.  Telescopes of whatever design, eyepieces, binoculars, riflescopes, cameras, camera lenses, microscopes.  It's a different world today.  Do you think Hershcel had access to the quality of mirror grinding abrasives available today in terms of quality control, consistency and uniform particle size?  To the quality of mirror substrates that could be polished to the same level of smoothness, as todays substrates?  I can just see Herschel polishing his speculum mirror to the same scratch / dig ratio as Carl Zambuto polishing a quartz blank.


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#13 Augustus

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Posted 24 June 2022 - 12:37 AM

Do you think Hershcel had access to the quality of mirror grinding abrasives available today in terms of quality control, consistency and uniform particle size?

Yes.

 

I'm curious... have you made your own mirror before? 

Plossls and Kellners were invented in the mid-1800s.


Edited by Augustus, 24 June 2022 - 12:41 AM.


#14 ausastronomer

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Posted 24 June 2022 - 01:31 AM

Yes.

 

I'm curious... have you made your own mirror before? 

Plossls and Kellners were invented in the mid-1800s.

 

Yes I have.  As a matter of fact I made my first mirror, a 6"/F8  47 years ago.  Do you think an 1800's plossl is the equal of a  modern production Televue Plossl ?



#15 dave brock

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Posted 24 June 2022 - 01:55 AM

All Augustus initially said was he read somewhere that Herschel's mirrors were quite good.
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#16 Jon Isaacs

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Posted 24 June 2022 - 07:41 AM

All Augustus initially said was he read somewhere that Herschel's mirrors were quite good.

 

I am still wondering how one would determine that.  In a modern test, I have to think the speculum surface would be seriously corroded.  

 

Jon


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#17 dave brock

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Posted 24 June 2022 - 02:25 PM

We know at least his 6.2" was capable of picking that Uranus is not a star. I don't know what power he used for that observation.
Weren't the mirrors that were tested boxed up. They might have just been dull as opposed to corroded.
It makes you wonder though, he sold a lot of scopes. Did the owners send the mirrors back when they tarnished for repolishing.
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#18 areyoukiddingme

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Posted 24 June 2022 - 10:46 PM

My guess is that Herschel's best mirror would not have been far behind a typical mirror today. It would be interesting to find out what the actual reflectivity was.

 

But I bet his eyepieces sucked. I'd expect very narrow field, but directly on axis, maybe not so bad.

 

Apparently they used sand for abrasive, and obviously the sieve is not a new invention:

 

https://www.youtube....l=ScienceMuseum


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#19 Kokatha man

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Posted 24 June 2022 - 10:55 PM

...I'll stick with what I said in my original response (Post #2) & leave it to you fellas to continue! grin.gif Herschell the observer was pretty special, great skies notwithstanding; his own speculum mirrors were pretty decent comparatively but apart from his disappointment about today's light-polluted skies, I'm sure he'd drool a bit also about some of the decent, new-fangled glass mirrors etc of today! lol.gif


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#20 areyoukiddingme

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Posted 24 June 2022 - 10:57 PM

And a quick bit of googling says speculum max reflectivity at 6500 angstroms of about 75%.

 

A modern aluminum mirror does about 90% . . . but I am not going to do the arithmetic on figuring out angstroms to nm . . .

 

So I'm going to guess about 20-30% less reflectivity. Not bad.

 

I'm also going to guess uncoated eyepieces. . . even still, I have to think that the cutting edge back in the day can figure speculum beyond 1/4 wave, and have > 50% ultimate transmission to eyepiece.

 

So the 6" would behave as a modern ~4.5"

 

https://iopscience.i...1/24/9/308/meta


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#21 ButterFly

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Posted 25 June 2022 - 02:13 AM

He prefered off-axis because the reflectivity was so poor. No need for a secondary. His eyepieces didn't have the benefit of modern day coatings, but his mirrors had the benefit of ... Caroline.
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#22 Jon Isaacs

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Posted 26 June 2022 - 05:06 PM

And a quick bit of googling says speculum max reflectivity at 6500 angstroms of about 75%.

 

A modern aluminum mirror does about 90% . . . but I am not going to do the arithmetic on figuring out angstroms to nm . . .

 

So I'm going to guess about 20-30% less reflectivity. Not bad.

 

I'm also going to guess uncoated eyepieces. . . even still, I have to think that the cutting edge back in the day can figure speculum beyond 1/4 wave, and have > 50% ultimate transmission to eyepiece.

 

So the 6" would behave as a modern ~4.5"

 

https://iopscience.i...1/24/9/308/meta

 

That data was for modern electrodeposited speculum with 45% tin.

 

"The reflectivities of electrodeposited speculum (a copper-tin alloy containing preferably 45% of tin) and of speculum prepared by evaporation in vacuo have been determined over the range 4500-6500 A. The reflectivity of the freshly polished electrodeposited alloy containing 45% of tin, which is the ideal composition from the viewpoint of maximum resistance to tarnishing, varies from 63% at 4500 A. to 75% at 6500 A."

 

According to Wikipedia, the common alloy in use had a tin content of about 31%.

1 nm = 10 angstroms.  Transmission = 63% in the blue, 75% in the deep red.

 

Visually, deep red is of little use and even modern coatings it lost 10% in 6 months.

 

I'd figure something closer to 60% at best based on polishing rather than electrodepositing, material differences, tarnishing. That would be 45% transmission compared to two 90% mirrors.

 

But this thread is about the figure.. testing a 180 year old mirror would require polishing and that would require refiguring.

 

Jon



#23 Kokatha man

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Posted 26 June 2022 - 10:53 PM

...couldn't resist throwing this into a thread like this! lol.gif Did Herschel actually observe (over time) Uranus' rings..?!? I imagine this has some relevance to the quality of his own mirrors if one entertains the notion... wink.gif

 

Once dismissed out of hand, there are quite a few aspects of supporting evidence, but I'll let those interested do the referencing...

 

Me? - I'm non-committal! grin.gif



#24 JerryStellar

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Posted 28 June 2022 - 12:23 PM

I am amazingly grateful for all your input. I feel as if I just read a book on comparing old bs new glass!
Jerry😉

#25 Peter Natscher

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Posted 01 July 2022 - 07:42 PM

I am still wondering how one would determine that.  In a modern test, I have to think the speculum surface would be seriously corroded.  

 

Jon

Especially in the damp English climate.
 




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