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Equipment Discussions >> Cats & Casses

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
1/100 Wave Rooftop Scope, or Exuberant Marketing? new
      #5602577 - 01/03/13 09:14 AM

I'm considering applying for a job at a school with a rooftop observatory with a telescope that is, presumably, a Cassegrain. It clearly rates A++ for coolness, but would any skeptics or advocates care to rate its credibility?

http://www.clayobservatory.org/about/history/

Notable claimed details are:

(1) 1/100 wave mirror. Really? If such a mirror could be figured, could the scope as a whole be collimated to 1/100 wave?

(2) Amazing cooling system moves 10,000 cubic feet of air through the dome, keeping its interior temperature essentially matched to the air outside. All, presumably, without creating any turbulence or vibration to harm the view.

(3) Ductwork (in the floor?) carries all heat from the building below down five floors, then 300 feet away, always downwind to where, in rising, it can not harm the view.

(4) The five-floor-tall pier is wholly independent of the building, to isolate vibration, yet it is so mystically stiff as to not wiggle the view.

(5) The 25" aperture is well suited to the light pollution and seeing conditions just a few miles from coastal Boston, Massachusetts.

Other than that they may not want to hire me, nothing says I couldn't take the job, and procure better views of the sky by walking the ol' C8 to a far corner of the ball field. But, seriously, what do folks think of the claims made for this rooftop observatory?


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Sunspot
Postmaster
*****

Reged: 03/15/05

Loc: Surprise, AZ
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5602587 - 01/03/13 09:24 AM

If they have the money to do all of that, the job better pay very well.

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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5602615 - 01/03/13 09:46 AM

I doubt any aperture is 'well suited to the light pollution....' 25 inches or 2.5 inches. With appropriate filtration planetaries, globs and open clusters will be nicely seen. Faint fuzzies, not so much.

Will your job be to work with this telescope?

Dave


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Cotts]
      #5602695 - 01/03/13 10:34 AM

Needless to say, I am skeptical of the claims. My job would be to run summer programs for this school. I would not be hired as an astronomer, yet my background in astronomy may aid in securing an interview, because I would market programs involving this scope.

I am on an advisory board for a foundation that wanted to mount a telescope atop a 60-foot tower, to raise it above the trees. The board's engineer showed it could not be done, yet the foundation went ahead anyway.

Funny thing about this school is that for less money, they could have built a better free-standing observatory. But, that's leading the witness! Any other comments on their claims?


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Eddgie
Postmaster
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Reged: 02/01/06

Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5602758 - 01/03/13 11:15 AM

1/100th Wave mirror? I have my doubts. The amount of labor necessary to produce a mirror of this quality would be fantastically high.

There is this however. Some larger mirrors today are spin casted, and with this form of casting, it is indeed able to get amazingly good curves, but even the normal contraction and expansion from cooling will change the surface very slighlty (though not much). But 1/100th? I have my doubts.

And this.. It simply isn't necessary, and given the cost to achieve it, why would you do it?

For example, AP scopes even of many years ago were built to 1/57th. This will give a Strehl of about .987. This for all practical purposes is optically perfect. There is no discernable benefit to be had past this level of quality. It becomes academic.

I know of one telescope that was perhaps built to a better quality than 1/57th. It was a Ceragoli (I think that is the name) 8" f/9 triplett (which was almost given away in auction a few years ago, selling I think for less than $7500) This scope had optics that were within a whisker of being .99 (I think it was .989???), and as I recall, it took an enormous amount of touch-up work to get the Strehl this high, but of all the scopes that I have ever seen testing for, this scope was the most perfect of the most perfect. I have never seen a higher Strehl reported. for any other scope I have seen reports on.

Sometimes I still kick myself for not buying this scope, but I would have had to sell my C14, the mount, and my 6" APO to get it.

And again, my understanding is that it took a huge amount of effert ot get it that good.

As for the other claims, it sounds like someone did their homework.

A lot of what we attribute to atmosphereic seeing is really ground based thermal plumes, and a great deal of that is coming from the immediate environment. A rooftop, a driveway, a street, or even a hot patio cooling down... All of these result itn a termal plumes, and this is ofter far more damaging that the rest of the trip though the atmosphere.

And it looks like they have considered the damage that could be caused by the heat rising from the walls of the building itself and addressed them.

So, while I have my doubts that 1/100th is possible in a mirror this size, that is the only thing that I would have any question on.


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Eddgie]
      #5602775 - 01/03/13 11:23 AM

Have you ascertained if this figure is a P/V or RMS value? The distinction is most significant.

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: GlennLeDrew]
      #5602786 - 01/03/13 11:32 AM

Looks like a DFM:

images of DFM telescopes

I've got to get myself on in to work now, but if you're interested in more details on the specs of that particular scope, I'd contact them.

-Tim.


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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: tim53]
      #5602902 - 01/03/13 12:27 PM

Quote:

Looks like a DFM:

-Tim.




Good eye, Tim.
Clay Observatory is indeed listed as a DFM client on their site.

dan k.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: HunterofPhotons]
      #5602926 - 01/03/13 12:40 PM

What's a "DFM?"

I posted this in Cats and Cassess because of the scope itself. Another thread described the implausibility of building a 1/10 wave C8, even if the mirror itself were 1/10 wave. The difficulties aligning the optics were just too great. The current scope is not a C8, but apparently C8's big cousin. Wouldn't the same issues apply?

And, about the 60-foot pier: Homework completed or not, could such a tall pier really be adequately steady? Wouldn't the sway so far from the ground blur the images?


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John Boudreau
scholastic sledgehammer
*****

Reged: 04/06/08

Loc: Saugus, MA
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5602973 - 01/03/13 01:06 PM

Here's a shot many may recall seeing in the media from that very scope at the Clay Center a few years ago:

http://apod.nasa.gov/apod/ap070628.html

The paragraph below is from the Clay Center website @
http://www.clayobservatory.org/facility/telescopes/

Quote:

0.64-m (25-inch) f/9.6 Ritchey-Chrétien OTA
Our unique, high-resolution primary and secondary mirrors are made from zero-expansion-coefficient substrate. They were custom-figured in 2002 for the Clay Center Observatory by world-class opticians at Brashear LP in Pittsburgh, Pennsylvania. Taking more than one year to figure and polish, they provide diffraction-limited performance and a wide, flat field. The full-thickness Zerodur primary is an f/2.8 hyperboloid figured to 1/90 wave RMS at 632.8 nm. The Astrositall hyperboloid secondary has a surface figure of 1/100 wave RMS at 632.8 nm. The optical tube features computer-controlled aperture doors, as well as two sets of primary and secondary baffles. Users can select either a 2-inch-diameter unvignetted field with minimal central obstruction or a 4-inch-diameter unvignetted field with a slightly larger central obstruction. The combination of superb optics and good local seeing enables the 25-inch telescope to produce sharper images than telescopes several times its size.




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tim53
Postmaster
*****

Reged: 12/17/04

Loc: Highland Park, CA
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5602977 - 01/03/13 01:07 PM

DFM builds professional telescopes. They've been advertising in the back of S&T forever: http://www.dfmengineering.com/products.html

-Tim.


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hottr6
Carpal Tunnel
*****

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5603152 - 01/03/13 02:44 PM

Quote:

Funny thing about this school is that for less money, they could have built a better free-standing observatory.



One of the reasons quoted by school regents for NOT having a free-standing observatory is security.... not for the telescope, but the students.


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Jared
Postmaster
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5603838 - 01/03/13 10:50 PM

Quote:

What's a "DFM?"

I posted this in Cats and Cassess because of the scope itself. Another thread described the implausibility of building a 1/10 wave C8, even if the mirror itself were 1/10 wave. The difficulties aligning the optics were just too great. The current scope is not a C8, but apparently C8's big cousin. Wouldn't the same issues apply?

And, about the 60-foot pier: Homework completed or not, could such a tall pier really be adequately steady? Wouldn't the sway so far from the ground blur the images?




I believe that thread was referring to 1/10 wave PTV, while the claim in this thread is certainly RMS and is measured in the red portion of the spectrum, so would be equivalent to 1/85th wave RMS.

This is certainly possible, but would be quite expensive in a 25" scope. As far as the other thread goes, keep in mind that a C8 has a moveable primary while this scope likely does not. This matters in terms of maintaining collimation.


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DesertRat
Postmaster
*****

Reged: 06/18/06

Loc: Valley of the Sun
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? [Re: Jared]
      #5603870 - 01/03/13 11:14 PM

Relation between PTV error levels and RMS figures is fairly complex. It would depend not only how the error is distributed but the mix of the aberrations. Assuming a purely spherical aberration at 1/10wv PTV yields approx 1/34wv RMS.

If you add the contribution from both mirror surfaces using the figures the Clay Center Observatory published it probably comes out better than 1/34wv RMS. They say 'diffraction limited' but a better term might be aperture limited. A great scope no doubt.

Glenn


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Keith Howlett
Pooh-Bah
*****

Reged: 03/06/07

Loc: Northumberland, UK
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5604046 - 01/04/13 03:31 AM

Hi Joe,

Good luck with the job!

It looks like an excellent facility with some interesting research and education programs, I'm sure you could have a lot of fun there.

The 25 inch looks like a very impressive scope - let us know what you find out about the mirror specs and the thermal properties of the rooftop dome when you start!

Cheers,

Keith


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Dan McConaughy
sage


Reged: 11/11/06

Loc: LA
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Keith Howlett]
      #5605551 - 01/04/13 08:41 PM

Why do they discuss the accuracy of each element? Isn't the total system performance more important? Star Instruments gives an analysis of the combined system (double pass auto-collimation interferometric analysis) on their R-C optics sets. If the mirrors are made independently and not tested together, the combined system performance could be much less.

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cavefrog
Carpal Tunnel
*****

Reged: 11/11/08

Loc: loozyanna
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Keith Howlett]
      #5605765 - 01/04/13 11:30 PM

Quote:

Hi Joe,

Good luck with the job!

It looks like an excellent facility with some interesting research and education programs, I'm sure you could have a lot of fun there.

The 25 inch looks like a very impressive scope - let us know what you find out about the mirror specs and the thermal properties of the rooftop dome when you start!

Cheers,

Keith




There's some positive thinking for ya! :>)


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ryderc1
sage
*****

Reged: 04/15/06

Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Joe Cepleur]
      #5606001 - 01/05/13 04:11 AM

I can't speak to the specific wavefront numbers for the optics but I've been to that observatory several times in the course of buying/selling personal astronomy items with one of the school's employees. During my first visit I was given a tour of the observatory and Science Center and to say the facility is state of the art is an understatement. As I recall being told, the optical set of the scope in question purportedly cost upwards of $200,000. And yes, the pier supporting the scope/observatory is a massive multi-story structure extending through the entire height of the building and deep into the ground.

As I said, I can't quote the exact spec's of the optics of the RC scope but I can attest to the quality of the Takahashi FSQ 106 that rides atop the RC scope like a fly on an elephant---it used to be mine and is an impressive performer -:)

I'm sure you'd be most impressed and awed by the observatory and Science Center, its equipment, and background/experience of its staff.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: GlennLeDrew]
      #5606071 - 01/05/13 07:06 AM

Quote:

Have you ascertained if this figure is a P/V or RMS value? The distinction is most significant.




I know there is a difference, but do not yet understand it myself. Lots to learn here!

These forums need data detectors. When your iPhone's Web browser displays a telephone number, it recognizes it as such, and offers to link it to your contacts. Cloudy Nights should link technical terms to a glossary and discussions of the terms in question. This could be turned on or off at the member's discretion.



Actually, I have seen this observatory. I attended a function at this school six or seven years ago. The top floor of the building has a library with glass walls looking into the observatory. It was awe-inspiring. All this for a school serving little tots through high school. This observatory is some donor's idea of an upgraded Zeiss Telementor!

Special thanks to all wishing me well with the job. I'm sure I could have fun here, even if, with the local seeing, that Takahashi guide scope often gave a better image than the 25" marvel. Or, think what fun it would be to stop down a six-billion-dollar telescope with a quick cardboard mask to improve the view! Not that I'd complain on nights when the air was truly steady.

Edited by Joe Cepleur (01/05/13 07:24 AM)


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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: 1/100 Wave Rooftop Scope, or Exuberant Marketing? new [Re: Dan McConaughy]
      #5606186 - 01/05/13 09:30 AM

Quote:

Why do they discuss the accuracy of each element? Isn't the total system performance more important? Star Instruments gives an analysis of the combined system (double pass auto-collimation interferometric analysis) on their R-C optics sets. If the mirrors are made independently and not tested together, the combined system performance could be much less.




The original Hubble Telescope knows all about this.....

Dave


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