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

Palomar 200 inch Figured to 2 millionths of an Inch or 1/10 Wave

  • Please log in to reply
70 replies to this topic

#1 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 14 August 2020 - 08:51 PM

I’m not sure where to post this, but someone recently said that the 200 inch mirror had a Strehl of like 0.333 because  it’s figure was so bad.  So since I was reading a book about it called “The Perfect Machine”, and It states in there that they stopped figuring the mirror when it was within 2 millionths of an inch of  a “perfect” parabola.  If I did my conversions right, for green light at 540 nm, a wavelength is 21.26 millionths of an inch.  So that would make it a 1/10 wave mirror.  Not too bad for a 16 and 2/3 foot diameter mirror.


  • doctordub and Pinbout like this

#2 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 14 August 2020 - 09:11 PM

I’m not sure where to post this, but someone recently said that the 200 inch mirror had a Strehl of like 0.333 because  it’s figure was so bad.  So since I was reading a book about it called “The Perfect Machine”, and It states in there that they stopped figuring the mirror when it was within 2 millionths of an inch of  a “perfect” parabola.  If I did my conversions right, for green light at 540 nm, a wavelength is 21.26 millionths of an inch.  So that would make it a 1/10 wave mirror.  Not too bad for a 16 and 2/3 foot diameter mirror.

As Jon noted in response:

"Well..

I calculate the diameter to the first minima to be 0.055 arcseconds.

 

With a Strehl of 0.3, that means 30% of the light is inside 1/20 of arcsecond.  I'd take a guess that it's well over 80% is inside 1/10 arcsecond.

 

I think I'd be happy with planetary scope that could do that..

 

Jon

Now that they're using adaptive optics, it's probably better than that...



#3 Stevegeo

Stevegeo

    Viking 1

  • *****
  • Posts: 651
  • Joined: 29 Jan 2019
  • Loc: Otisco ny.

Posted 14 August 2020 - 09:12 PM

I have that book as well and just got to the specs you mentioned . 

Corning glass being less then 100 miles as the crow flies , I have seen the other blank on display there . A massive piece of glass..

It's a wonder they could achieve such tolerances on something  that large.  A lot of work and MATH went into such a marvelous  instrument.  

I should be though the book soon.  Great reading .

Stevegeo. 


  • Cpk133 likes this

#4 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 14 August 2020 - 09:14 PM

I have that book as well and just got to the specs you mentioned . 

Corning glass being less then 100 miles as the crow flies , I have seen the other blank on display there . A massive piece of glass..

It's a wonder they could achieve such tolerances on something  that large.  A lot of work and MATH went into such a marvelous  instrument.  

I should be though the book soon.  Great reading .

Stevegeo. 

At the end they were using their thumbs to polish hills Down on the surface.  Amazing!


  • havasman likes this

#5 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 7,514
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 14 August 2020 - 09:19 PM

Yeah... I've read all the books. I would take that claim with a giant grain of salt, though. They had terrible issues with the magical mounting cell etc. etc. Even fish-scales pulling here and there on the back in an attempt to contort the actual operational wavefront to better. And... don't tell anyone else... but most ground-based observatory staff are ecstatic if operational first light, on a big scope... forms an impulse response / point-spread core --- that's within an arc-sec across. Yeah, the resolution of a decent five-inch scope. That's the reality of it.    Tom



#6 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 14 August 2020 - 09:30 PM

"in Figure 1, which compares Strehl ratios obtainable for the full aperture and sub-aperture cases for the 200-inch diameter Hale telescope at the Palomar Observatory, as a function of r o , the atmospheric seeing cell size. For typical r o 's of 30 -50 cm in the K band (2.2 µm), residual wavefront errors can be reduced from current levels of 200 -250 nm to under 100 nm, thus increasing Strehl ratios from about 40-70% to 88-96%. ..."

https://www.research..._fig1_252229954



#7 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 15 August 2020 - 09:28 AM

Yeah... I've read all the books. I would take that claim with a giant grain of salt, though. They had terrible issues with the magical mounting cell etc. etc. Even fish-scales pulling here and there on the back in an attempt to contort the actual operational wavefront to better. And... don't tell anyone else... but most ground-based observatory staff are ecstatic if operational first light, on a big scope... forms an impulse response / point-spread core --- that's within an arc-sec across. Yeah, the resolution of a decent five-inch scope. That's the reality of it.    Tom

Sone of your comments make little sense.  “... the resolution of a a decent 5 inch...”.  Resolution of a telescope is always calculated based solely on the diameter of the objective.  I have a 16 inch Dob with a Zambuto mirror.  It’s resolution is 0.29 arcseconds.  The formula for calculating it has NO input for seeing.  As stated above, if the figure of the 200 inch mirror is within 2 millionths of an inch, it’s a 1/10 wave mirror.  The resolution would thus be 0.02 arcseconds.

 

The “... grain of salt, ...”  comment is also purjorative.  You weren’t there, and the author of the book has done a wonderful job of documenting his references for the information presented.  His references, by Chapter, are in the back.
 

If you read the book, you would also know they did all of the grinding and polishing of the 200 inch mirror with the the “lever devices” embedded in the back of the mirror to correct for the sagging of the disk in different orientations.  So the “... fish scales... “ comment, is out of line as well.

 

I will never understand people who belittle the great achievements that America and its people have made.


Edited by SandyHouTex, 15 August 2020 - 09:33 AM.

  • Mike W likes this

#8 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 86,825
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 August 2020 - 10:41 AM

Resolution of a telescope is always calculated based solely on the diameter of the objective.  I have a 16 inch Dob with a Zambuto mirror.  It’s resolution is 0.29 arcseconds.

 

That's the Dawes limit. 

 

The Dawes limit represents a 5% reduction in brightness between the centers of the two overlapping disks. 

 

Tom pointed to a 1 arc-second point spread function. This is real world. 

 

The diameter to the first minimum would seem to be a better approximation of the point spread function, that's 2.2" for a 5 inch, 0.68" for a 16 inch..

 

Jon



#9 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 7,514
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 15 August 2020 - 10:56 AM

I've been ~on the inside, looking out~ for so many decades. I celebrate our accomplishments, but (am admittedly notorious) for pointing out the difficulties, dangers, failures, politics, and press-release overstatements. That worked out well and made for a great career! At any given moment, most high-tech machines are imperfect, and not living up to their full potential... and many never have. Same exact statement for our amateur gear that we so prize. Nothing wrong with that; nothing wrong with admitting that. Indeed, our leading-edge creations most often have a laundry list of issues to be addressed or tuned-up. Get inside any perfect machine, and you'll find a lota duct tape, chewing gum, rubber bands, and tie-wraps. And even that is to be celebrated, as accomplishments of the creators. Tom


  • doctordub, Pierre Lemay, ianatcn and 1 other like this

#10 MitchAlsup

MitchAlsup

    Aurora

  • -----
  • Posts: 4,615
  • Joined: 31 Aug 2009

Posted 15 August 2020 - 11:27 AM

Sone of your comments make little sense.  “... the resolution of a a decent 5 inch...”.  Resolution of a telescope is always calculated based solely on the diameter of the objective.  I have a 16 inch Dob with a Zambuto mirror.  It’s resolution is 0.29 arcseconds.  The formula for calculating it has NO input for seeing.  As stated above, if the figure of the 200 inch mirror is within 2 millionths of an inch, it’s a 1/10 wave mirror.  The resolution would thus be 0.02 arcseconds.

Would you be able to resolve 0.29 arc seconds in crappy seeing--NO.

 

Big telescopes are always (ALWAYS) operating in what we amateurs would call crappy seeing, there simply are no place on earth where on can take a 5m or 10m light column and deliver it from space to the first surface of the telescope without more than a full wave of path difference. As Texeraux explains in "How to Make a Telescope" section XV-4 page 302.

 

Until adaptive optics came around in the mid 1990s resolution of large telescopes were entirely dependent on seeing.



#11 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 15 August 2020 - 11:44 AM

That's the Dawes limit. 

 

The Dawes limit represents a 5% reduction in brightness between the centers of the two overlapping disks. 

 

Tom pointed to a 1 arc-second point spread function. This is real world. 

 

The diameter to the first minimum would seem to be a better approximation of the point spread function, that's 2.2" for a 5 inch, 0.68" for a 16 inch..

 

Jon

“Real world” depends on where the telescope is used.  So if you have a 200mm APO in New England, it will only have the resolution of a 4.5 inch scope due to the “jet stream”.  If someone up there had a telescope like that, would he say, “It’s a beautiful scope  but it only has the resolution of a 4.5 inch.”  No he wouldn’t.  

 

Here in in Houston where I’m at, I routinely can operate my scope at the “Dawe’s” limit because I’m surrounded by atmosphere stabilizing water.  Galveston Bay on the East, and the Gulf of Mexico on the South.  When someone asks the resolution of my 16 inch, my reply is simply, “0.29 arcseconds”.  I’d say the same if I was under the “Jet Stream”.


  • Bonco2 and RLK1 like this

#12 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 15 August 2020 - 11:47 AM

Would you be able to resolve 0.29 arc seconds in crappy seeing--NO.

 

Big telescopes are always (ALWAYS) operating in what we amateurs would call crappy seeing, there simply are no place on earth where on can take a 5m or 10m light column and deliver it from space to the first surface of the telescope without more than a full wave of path difference. As Texeraux explains in "How to Make a Telescope" section XV-4 page 302.

 

Until adaptive optics came around in the mid 1990s resolution of large telescopes were entirely dependent on seeing.

See my comment above.  I, almost always, have a steady atmosphere and can operate my scopes at the theoretical limit.

 

Besides, this post is about how “optically” perfect the 200 inch mirror was figured, not how it operates through the atmosphere on Palomar mountain based on the “seeing”.



#13 yeldahtron

yeldahtron

    Sputnik

  • -----
  • Posts: 45
  • Joined: 16 Jan 2014
  • Loc: Rideau Ferry, ON

Posted 15 August 2020 - 09:47 PM

Yeah. I think I'll stick with TOMDEY. Much more interested in expert advice from a practitioner of the art.


  • Pierre Lemay and ianatcn like this

#14 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 16 August 2020 - 01:10 AM

See my comment above.  I, almost always, have a steady atmosphere and can operate my scopes at the theoretical limit.

 

Besides, this post is about how “optically” perfect the 200 inch mirror was figured, not how it operates through the atmosphere on Palomar mountain based on the “seeing”.

Agreed, and I'd note that major observatories are generally situated on select mountain tops, such as those in Chile or equivalent, that can take advantage of subarc second seeing frequently throughout the year.

Additionally, what I've read and cited in an above post in this thread and elsewhere attest to the optical quality of the mirror and the necessary steps to insure it. From the Cal Tech archives:

http://calteches.lib...7/1/Palomar.pdf

Additional info on final adjustments:

http://adsabs.harvar...PASP...62...91B

Commentary on the mirror:

"The 200 inch telescope remained the world’s most powerful telescope for 44 years, an extraordinary period for a single instrument to hold that distinction in a vibrant field of science. “Most powerful” is arguable in the second half of that period—with dramatic improvements in instruments at all telescopes, better testing technology, and evolving performance standards—but in any case the 200 inch telescope remained the largest telescope with good image quality. The reason was that no one could figure out how to make a larger mirror that would hold its shape to the accuracy required. The telescope should add very little blurring to the best images delivered by the atmosphere—around 0.3 arcsecond—and the implied surface accuracy is around 100 nm rms. The mirror has to hold this shape accuracy against changing gravity loads as it sweeps across the sky, wind up to around 10 m/s, and changing air temperature in the open dome."

https://www.spiedigi...4601.full?SSO=1


  • SandyHouTex and Cpk133 like this

#15 Augustus

Augustus

    Fly Me To The Moon

  • *****
  • Posts: 9,952
  • Joined: 26 Dec 2015
  • Loc: Stamford, Connecticut

Posted 16 August 2020 - 01:12 AM

“Real world” depends on where the telescope is used. So if you have a 200mm APO in New England, it will only have the resolution of a 4.5 inch scope due to the “jet stream”. If someone up there had a telescope like that, would he say, “It’s a beautiful scope but it only has the resolution of a 4.5 inch.” No he wouldn’t.

Here in in Houston where I’m at, I routinely can operate my scope at the “Dawe’s” limit because I’m surrounded by atmosphere stabilizing water. Galveston Bay on the East, and the Gulf of Mexico on the South. When someone asks the resolution of my 16 inch, my reply is simply, “0.29 arcseconds”. I’d say the same if I was under the “Jet Stream”.


The low quality seeing in New England is dramatically overstated. If you can get near a body of water especially it’s often quite good. I bagged the Encke gap and Galileo Regio last night from NW CT....
  • TOMDEY likes this

#16 Oberon

Oberon

    Gemini

  • -----
  • Posts: 3,427
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 16 August 2020 - 05:24 AM

The Hale 5m is a tremendous success, but there is a reason why several 4m class telescopes that followed it did not use Pyrex, but used more exotic glass ceramics like Cervit. I know the AAT 3.9m was regarded as a finer telescope, better engineered and with better optics (capable of putting more light into a pixel than the Hale), but then 25 years later so it should be. Hale pushed the boundaries then but lessons were learned, techniques refined and improvements applied.


  • Jim Davenport, Neptune, Pierre Lemay and 4 others like this

#17 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 7,514
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 16 August 2020 - 07:34 AM

The low quality seeing in New England is dramatically overstated. If you can get near a body of water especially it’s often quite good. I bagged the Encke gap and Galileo Regio last night from NW CT....

Yes! I see the Enke Gap... almost routinely, here from Springwater, N.Y. We have elevation over local topology and green vegetation surrounding the observatory, which is also well-ventilated and above ground. I believe all of those contribute to the good seeing.    Tom

Attached Thumbnails

  • 11 24-Foot dome Tom and assistant Igora.jpg

  • paul, germana1, ianatcn and 4 others like this

#18 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 16 August 2020 - 12:11 PM

Yes! I see the Enke Gap... almost routinely, here from Springwater, N.Y. We have elevation over local topology and green vegetation surrounding the observatory, which is also well-ventilated and above ground. I believe all of those contribute to the good seeing.    Tom

That’s a really nice set-up you have there, but for me, when I retire I’m headed west where all of the big observatories are, and no NOT california.  Probably in west Texas near the McDonald Observatory, or if I want to pay state and local taxes (not likely) Kitt Peak.


  • SteveV likes this

#19 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 16 August 2020 - 12:17 PM

The 200"is far from being outdated, despite what you might think after reading a comment or two in this thread, because of the inclusion of adaptive optics into the system. As noted in the Sky & Tel article:

"Combining AO with "lucky imaging," the Palomar team has in some cases achieved an angular resolution near the 200-inch telescope's theoretical limit. That's about twice as sharp as the best images from Hubble, because the Hale telescope has a mirror twice as big across, and resolving power scales directly with aperture diameter once you remove the atmosphere from the equation."

https://skyandtelesc...g-the-200-inch/


Edited by RLK1, 16 August 2020 - 12:26 PM.

  • doctordub, Jim Davenport, SteveV and 5 others like this

#20 MitchAlsup

MitchAlsup

    Aurora

  • -----
  • Posts: 4,615
  • Joined: 31 Aug 2009

Posted 16 August 2020 - 04:50 PM

See my comment above.  I, almost always, have a steady atmosphere and can operate my scopes at the theoretical limit.

 

Besides, this post is about how “optically” perfect the 200 inch mirror was figured, not how it operates through the atmosphere on Palomar mountain based on the “seeing”.

I remember some time ago when EAO was looking for a site for some new big telescopes.

They sent an amateur out with a 10" scope to measure start widths and after a couple of years observing in Chile he suggested Cero Pranal and claimed it has 0.4 arc second seeing for 10 minutes at a time. This is the mountain where four 8 metre telescopes are placed.

 

Based on my reading of Texeraux and other (professional) authors, there is basically no where on earth that puts up 0.3 arc-second nights "all the time".

 

Now, I live in central Texas, and I got the kinds of seeing where I could use all the resolution of my C11 about 3-5 times a year when observing from my drive way when observing 100+ times a year. I could use 90% of its resolution more-or-less most of the time. Yes, we have way better than average seeing down here, especially when the high pressure dome builds into the summer time heat.

 

Back to Hale, The mirror was generally regarded as a 2 wave mirror most of its life, then in the early 2000s they rebuilt the flotation system {new bearings, grease, ...} and at last word was working better than it ever had. Whether this surface deformation came from the figure on the glass or from friction in the flotation system is anyone guess. With 200 inches of aperture and most observers using the spectrographs, 2-waves is more than good enough. The wave error comes form the book "First Light" Richard Preston, the rebuild of the flotation system was discussed here a bit over a decade ago. 


  • Neptune likes this

#21 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 16 August 2020 - 05:37 PM

I remember some time ago when EAO was looking for a site for some new big telescopes.

They sent an amateur out with a 10" scope to measure start widths and after a couple of years observing in Chile he suggested Cero Pranal and claimed it has 0.4 arc second seeing for 10 minutes at a time. This is the mountain where four 8 metre telescopes are placed.

 

Based on my reading of Texeraux and other (professional) authors, there is basically no where on earth that puts up 0.3 arc-second nights "all the time".

 

Now, I live in central Texas, and I got the kinds of seeing where I could use all the resolution of my C11 about 3-5 times a year when observing from my drive way when observing 100+ times a year. I could use 90% of its resolution more-or-less most of the time. Yes, we have way better than average seeing down here, especially when the high pressure dome builds into the summer time heat.

 

Back to Hale, The mirror was generally regarded as a 2 wave mirror most of its life, then in the early 2000s they rebuilt the flotation system {new bearings, grease, ...} and at last word was working better than it ever had. Whether this surface deformation came from the figure on the glass or from friction in the flotation system is anyone guess. With 200 inches of aperture and most observers using the spectrographs, 2-waves is more than good enough. The wave error comes form the book "First Light" Richard Preston, the rebuild of the flotation system was discussed here a bit over a decade ago. 

Taking a quick look at "First Light" by Richard Preston, I couldn't find a reference to wave error but assuming it's in there, it's not clear where he may have got that figure. Preston is not an astronomer and he writes on a variety of subjects. "First Light"was published in 1987 so it's not a current reference and that 2 wave error is in conflict with that noted in references cited in other posts in this thread and so is his description that that parabola was merely acceptable or good enough or words to that effect. As noted in another reference in this thread during the construction of the mirror:

""It is this accuracy we are going to get with the 200-

inch mirror that will make the Hale Telescope 'payoff'
for it is on nights of 'good seeing' that it will do its
best work. It is then that we will be able to get out to
the billion light-years for which it was designed. There
may be no more than twenty such 'good seeing' nights
in a year. When they occur we expect to be ready to
take advantage of them. _ Weare shooting at a maximum, not just a 'good enough' accuracy. We have
better than that already."


  • Markovich likes this

#22 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 529
  • Joined: 19 Apr 2020

Posted 16 August 2020 - 06:29 PM

From the "History of Palomar", the engineering goal is stated to be, "to achieve a final surface accurate to within 2/1,000,000ths of an inch of a perfect paraboloid." By 1941, the parabolization was stated to be, "90% complete". Post WW2: "Early in 1947, John Anderson, an optician, a physicist, and executive director of the Observatory Council, declared that the surface was within the engineering tolerances required of a true paraboloid. After several additional months of testing and fine figuring, in October 1947 Anderson declared the mirror finished." At installation: "At this point it was discovered that John Anderson’s optimism had been premature. In fact, the edges of the mirror did not sag as predicted and work on the mirror would need to continue for a year and a half. In addition, a series of adjustments were made to remove a slight astigmatism and correct other issues with tracking system." 

https://www.astro.ca...ut/history.html

So it appears they got the required surface accuracy on the mirror itself and had to make additional modifications of the mirror cell supports and tracking system over the years. At the end of the day, "The Hale's 200 in (510 cm) mirror was near the technological limit of a primary mirror made of a single rigid piece of glass.[22][23] Using a monolithic mirror much larger than the 5-meter Hale or 6-meter BTA-6 is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight as the telescope is rotated to different positions,[24][25] changing the precision shape of the surface, which must be accurate to within 2 millionths of an inch (50 nm). Modern telescopes over 9 meters use a different mirror design to solve this problem, with either a single thin flexible mirror or a cluster of smaller segmented mirrors, whose shape is continuously adjusted by a computer-controlled active optics system using actuators built into the mirror support cell."

https://en.wikipedia.../Hale_Telescope


  • Jim Davenport, SandyHouTex and Woj2007 like this

#23 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 16 August 2020 - 09:21 PM

I remember some time ago when EAO was looking for a site for some new big telescopes.

They sent an amateur out with a 10" scope to measure start widths and after a couple of years observing in Chile he suggested Cero Pranal and claimed it has 0.4 arc second seeing for 10 minutes at a time. This is the mountain where four 8 metre telescopes are placed.

 

Based on my reading of Texeraux and other (professional) authors, there is basically no where on earth that puts up 0.3 arc-second nights "all the time".

 

Now, I live in central Texas, and I got the kinds of seeing where I could use all the resolution of my C11 about 3-5 times a year when observing from my drive way when observing 100+ times a year. I could use 90% of its resolution more-or-less most of the time. Yes, we have way better than average seeing down here, especially when the high pressure dome builds into the summer time heat.

 

Back to Hale, The mirror was generally regarded as a 2 wave mirror most of its life, then in the early 2000s they rebuilt the flotation system {new bearings, grease, ...} and at last word was working better than it ever had. Whether this surface deformation came from the figure on the glass or from friction in the flotation system is anyone guess. With 200 inches of aperture and most observers using the spectrographs, 2-waves is more than good enough. The wave error comes form the book "First Light" Richard Preston, the rebuild of the flotation system was discussed here a bit over a decade ago. 

I don’t live near Austin, so your seeing isn’t what I get with water on my East and South.  So I don’t get your point.

 

If your saying you know what kind of seeing I get where I live, when you live in Austin, well, you really don’t have a clue, regardless of how many books you’ve read.



#24 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,017
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 16 August 2020 - 09:30 PM

Taking a quick look at "First Light" by Richard Preston, I couldn't find a reference to wave error but assuming it's in there, it's not clear where he may have got that figure. Preston is not an astronomer and he writes on a variety of subjects. "First Light"was published in 1987 so it's not a current reference and that 2 wave error is in conflict with that noted in references cited in other posts in this thread and so is his description that that parabola was merely acceptable or good enough or words to that effect. As noted in another reference in this thread during the construction of the mirror:

""It is this accuracy we are going to get with the 200-

inch mirror that will make the Hale Telescope 'payoff'
for it is on nights of 'good seeing' that it will do its
best work. It is then that we will be able to get out to
the billion light-years for which it was designed. There
may be no more than twenty such 'good seeing' nights
in a year. When they occur we expect to be ready to
take advantage of them. _ Weare shooting at a maximum, not just a 'good enough' accuracy. We have
better than that already."

The 2 millionths of an inch is specified in Ronald Florence’s book, “The Perfect Machine”, published in 1995.



#25 ed_turco

ed_turco

    Mercury-Atlas

  • *****
  • Posts: 2,710
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 18 August 2020 - 01:52 PM

I was a winner in the National Science Fair in Seattle, 1962.  I won two awards, one of which was an all expense trip to the Wright-Patterson Airforce Base.  We were  not told this but a small number of us were housed in the Bachelor Officers Quarters #16, where all the Mercury, Gemini and Apollo astronauts did their sleeping.  Who knows; I may have slept in Neil Armstrong's bed!  We were never told about this but found this out later, and when I did, I had the most shivering spine of all time!

 

But to the topic.  I met some  very big folks who were studying new optical systems or the Armed Services.  There I was allowed to actually join in the discussions and ask questions (If I dared). 

 

Somehow, the Palomar telescope came up and I found out that the 200 inch accuracy was only 1 or 2 waves.  I found this hard to believe and my questions revealed the following:

 

It was said by the early planners that the photographs taken would be so spectacular that the accuracy of the optics would never come up.

 

I used this same idea when I produced  my first 12.5" f/4 reflector in 1964.  I barely knew what I was doing; it, like Palomar, was only 1 wave at best. But at low powers, all viewers thought the images were stunning!.  No one asked about that other thing.

 

What an adventure this was for someone in the eleventh grade.  The World's Fair was really grand too.

 

Ironically, I never fulfilled my promise.


Edited by ed_turco, 18 August 2020 - 01:55 PM.

  • paul and BradFran like this


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