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Equipment Discussions >> Refractors

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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5526083 - 11/18/12 12:54 PM Attachment (35 downloads)

Stanislas,

Quote:

These are bulletins but what are your conclusions about them and the OTA behind? excellence, lemon, can be improved, etc...




If we just take the data as presented, the unit is visually close to "true apo" technically, and probably there if the criteria is a min 0.95 photopic polychromatic Strehl. Photographically, it probably doesn't exceed 1/4 wave p-v in the red r-line (706nm) and might be even within in the violet g-line (436nm), which would make it a "true apo".

But, the problem is that the test isn't convincing. The graph implies the possible design minimum of less than 0.003 wave RMS wavefront error of nearly balanced secondary spherical in the optimized wavelength (with the actual error about 50% larger, or less than 1/200 wave RMS). I know of no glass combination that would exhibit nearly as low spherical aberration in this fast triplet. If you look at the wavefront map, the actual spherical is no more than 1/3 of the RMS wavefront error, which is clearly mainly the wavefront tilt (i.e. resulting from minor alignment errors in test setup). Correcting for the tilt leaves in roughly this magnitude of error (0.02-0.03 p-v) mainly at the edge. In other words, this unit would be practically perfect in the e-line. At f/6.3? Hmmm...

The tester doesn't specify the glasses and, since AP did not publish them, the question is how did they get the data to obtain this plot? It might be that they obtained it from AP (note that the graph indicates different values than test, especially in the blue line). AP itself is very shy in this respect, only stating that the focal shift from g(436nm) to r(706nm) is less than +/- 0.006%.

The French graph, on the other hand, shows paraxial foci shift of +/- 0.1mm, i.e. +/- 0.012% from 0.473nm to 0.633nm already. So, my guess is as good as anyone's, as to what +/- 0.006% should imply. It sure looks too good to be true, but it's just another useless figure.

As a comparison, here's such a triplet made with FPL53 (nearly certainly the central element in the AP) and Schott's ZKN7, the best mach for it out of the available glasses. There is noticeably more spherochromatism, in the optimized wavelength, due to the strongly curved R2/R3, but the error is still only 0.018 wave RMS (graph in the middle is for the three tested wavelengths, for comparison, and at right is with the conventional five lines). In the actual unit, it would be hard to get below 0.03 wave, or 1/10 wave p-v of lower order spherical. Technically, this objective is not a "true apo", having the g-line error larger than 1/2 wave p-v (0.7 wave). However, it is more than compensated by a smaller error in the red, so it could be considered a true apo overall. Its polychromatic photopic Strehl is 0.965, which does make it true apo visually. It could be slightly bettered by optimization, but it wouldn't be of practical importance.

Sure, the scope could be using special melt for its matching glass, but it would make it very expensive. The conclusion is: we just don't have the data needed to asses specific level of correction, or to compare most of these instruments.

Vla

Edit: added values for the RMS in wavelengths in the text box top left, so that they are directly comparable with the values given with the graphs

Edited by wh48gs (11/19/12 10:30 AM)


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5526103 - 11/18/12 01:03 PM

Hi Chris,

Quote:

You should ask him why he pulled out the test result for the AP 130EDF that is optimized for CCD work...




I think I know the answer: it is always interesting to try something new

Vla


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5526253 - 11/18/12 02:32 PM

When all is said and done, I think it's also a question of your skies and seeing...

I live in what was a white/pink zone. New LED street lighting has improved matters somewhat - especially when they are dimmed by 50% at 11.00 pm.

My 8" SCT and RFT still show a bright background but my Carton 100mm F13 shows me fantastic contrasty views with a black background.

Why pay out for an APO or buy a light bucket?

For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526258 - 11/18/12 02:37 PM

What'd it cost you to mount the Carton?

Regards,

Jim


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5526272 - 11/18/12 02:46 PM

Quote:

What'd it cost you to mount the Carton?

Regards,

Jim




Uses the same Autostar modded EQ-5 as the SCT...

Total cost of mount so far 150 Uk pounds (including a Criterion "Golden Pyramid" tripod - rock solid...).


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5526481 - 11/18/12 05:01 PM

So at the conclusions you wrote, the bulletin doesnot give enough data for expecting the OTA figure in use on the sky (under excellent conditions).
But the question was through the 4 data results regarding 2 units 130mm and 2 units 150mm same fabricator the great results difference between the 2 for each aperture. This is something disturbing somewhere.
That is making some questions about the QC follow-up or simply this is the standard offered.
The investigation inspection method could be discussed but the same was on the 4 units.
Stanislas-Jean


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526629 - 11/18/12 06:59 PM

What's your saddle height, Andy?

Recently I swapped my CG5 from a 48" pier to a 60" pier for my ~95mm, ~f/15.7 Antares. I was getting irritated having to drop the observing seat to the lowest usable rung to view near zenith. The extra foot took the eyepiece from ~2-feet to ~3-feet off the pitch at zenith, but it also added to the post-focus settling time. I do use VSPs, as I find the setup almost unusable on hardscape without them, and they add leveling travel for the pier feet.

I don't have a picture of my solution yet, but here's a picture illustrating the problem:



Regards,

Jim


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John Huntley
scholastic sledgehammer
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Reged: 07/16/06

Loc: South West England
Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5526740 - 11/18/12 07:59 PM

Quote:

....For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)




Was that one of the Skylight kits Andy ?

I thought they cost a lot more than that !


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5527913 - 11/19/12 01:08 PM Attachment (14 downloads)

Quote:

But the question was through the 4 data results regarding 2 units 130mm and 2 units 150mm same fabricator the great results difference between the 2 for each aperture.




I wasn't aware of the other tests. But looking at it, there's not much difference between the two f/6.3 units. One is nearly perfect in the optimized wavelength, the other has about 1/10 wave p-v added to the optimized wavelength (under-correction as shown). This is quite possible consequence of suboptimal spacing, and it somewhat improved the other two lines (change of correction in the optimized wavelength always affect correction of the other wavelengths as well). The Strehls remain relatively close.

Still, I'm puzzled by the near-zero minimum possible for the error in the optimized wavelength that the graphs implied. The other two wavelengths do show quite a bit of spherochromatism (including strong secondary spherical in the blue line), even more than the FPL53/ZKN7 triplet I posted earlier. It does not agree with how spherochromatism normally behaves. Another thing is that the foci are reversed to the common scenario, with the blue (paraxial) focusing farther away. That must be some really odd glass combination, in addition to having correction in the optimized wavelength that just cannot be explained with the available glasses.

The focal shift is about 0.2mm in unit 1 and 0.23mm in unit 2, for about 0.025% (somewhat more if extended to the g-line, 436nm). That's more than twice the AP's "less than +/-0.006%" figure. Still don't know how the plots are obtained; measuring focal location for several zones at each wavelength, and interpolating? That can be sort of tricky way to calculate wavefront errors. The RMS for the optimized wavelength's s.a. implied by the graph is about equal to the RMS error shown with the wavefront map - it would imply that nearly entire aberration is spherical, yet the map shows some rather irregular shape, with the dominant tilt component (similarly to unit 1). And so on.

Those things don't add up, but there is not enough information to confirm or deny. Which leaves you two choices: either believe the units are correctly presented, or don't.

Vla


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5528048 - 11/19/12 02:06 PM

Here is the principle of the lab test method (unfortunately in french only)
http://www.airylab.com/contenu/incertitudes%20v1.05.pdf
those following in english
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/science-and-industry
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/astronomy
They trace the optics in the 3 color channels and results are raw data not interpolated I think.
They had been followed by refrerence laboratories and own reference optics to follow their calibrations and references. The pdf booklet gives an example of comparison results between different labs and them.
For more info it should be interresting to contact them directly people knows english (in spite of the website in french). Hope this is an help.
Now results when read does not imply confidence, my feeling, even researching reasons why such status.
Please compare with the following test:
http://airylab.com/contenu/mesures/astro/Astreya/rapport%202011-50001-a.pdf
That is excellent after realignement of the lenses by the lab.
Stanislas-Jean


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csrlice12
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5528093 - 11/19/12 02:33 PM

I suppose the real question, is did they ever go out of style as they're still widely manufactured and sold.......

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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5528259 - 11/19/12 03:51 PM

Inconsistencies cannot be explained by a testing process setup, only by possible errors in it and/or the presentation. As for the quadruplet apo (Astreya 152mm f/7) it has unusual correction mode, more like Maksutov corrector. Color error is lower than in the AP 130 f/6.3 but, then, it wouldn't be vs. an f/7 lens. The RMS for the three wavelengths (0.473, 0.543, 0.635 micron) are just 0.023, 0.022 and 0.021 micron, or 0.049, 0.041 and 0.031 in units of the wave, respectively. However, the error in the green is larger than for triplet designs, more than double that in the FPL53/ZKN7 f/6.3 triplet, and another more than a double vs. AP's f/6.3 as tested (although the graph suggests it could be further reduced by 10-20% by correcting a slight alignment error), and could be still significantly larger in an f/6.3.

The plots are similar to that of Gauss doublet achromat, except, of course that the colors are brought together. So the objective could be a doublet consisting of two strongly curved achromatised menisci, each using low dispersion glass. A bit too fast (too much spherochromatism) for comfort, considering that the actual units will likely have more spherical in the optimized wavelength (which, with this design, means in all other wavelengths as well)) than the design minimum.

Vla


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rwiederrich
Goldfinger
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Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5528560 - 11/19/12 06:44 PM

Quote:

Now that is a "Great Refractor"!




And the views are stunning as well......covered in a fantastic sage green.....

Thanks


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rwiederrich
Goldfinger
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Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5528565 - 11/19/12 06:47 PM

Quote:

A "great refractor" is a refractor with aperture that is on par with a medium-sized amateur Newtonian reflector.

- Jim




10"..right up my alley.


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5529217 - 11/20/12 12:33 AM

Quote:

As for the quadruplet apo (Astreya 152mm f/7) it has unusual correction mode, more like Maksutov corrector. Color error is lower than in the AP 130 f/6.3 but, then, it wouldn't be vs. an f/7 lens.




Hi Vla

Are you sure the tested tube is using the Astreya quadruplet?
The missalignment would be an indication since there have been some reports of instabilities of the lens cells in these optics in the past...
A&M (Now Officina Stellare) from Italy also manufactured tubes using the LZOS triplet but this would be either 152mm f6 or f8?
All a bit confusing.

best regards
Chris


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5529292 - 11/20/12 01:46 AM

Yes it is a triplet (according my informations).
Now regarding the rms it is always a matter as the scaling of the measures being too coarse the results are from my opinion just indicative.
If we push the investigation on this website as there are numerous tests about different designs it should be interresting to make the comparison performance.
On an absolute value the data expressed by the buletins can be discussed, but the main topic remains the comparative evaluation between tests using the same measure means (it is an automatic machine i presume whare longitudinal flaws are collected.
Thanks Vlad for trying to interprete them accordingly.
We can argue for the actuality of a PTV, the surface figure quantitied by the polynome but the overall "shape" remains regarding each design and this is the point where I wanted to go the comparison of the scope quantified.
All the OTA also I presume are lambda scopes not selected, brought by owners that authorised the data publication for knowledge and who wanted to get a characterisation.
Facing this, being just a buyer, I am wandering a lot for a pertinent selection.
Stanislas-Jean


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5529370 - 11/20/12 04:31 AM

Quote:

What's your saddle height, Andy?

Recently I swapped my CG5 from a 48" pier to a 60" pier for my ~95mm, ~f/15.7 Antares. I was getting irritated having to drop the observing seat to the lowest usable rung to view near zenith. The extra foot took the eyepiece from ~2-feet to ~3-feet off the pitch at zenith, but it also added to the post-focus settling time. I do use VSPs, as I find the setup almost unusable on hardscape without them, and they add leveling travel for the pier feet.

I don't have a picture of my solution yet, but here's a picture illustrating the problem:



Regards,

Jim




Saddle height is at my eye level (I'm 6'2").

The Criterion tripod legs have a good height range and it's very solid.

Yeah - eyepiece gets low at zenith but I usually target planets and moon with this scope.


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Andy Taylor
Twisted, but in a Good Way
*****

Reged: 09/24/08

Loc: Epsom - UK
Re: Are achromats enjoying a come back? new [Re: John Huntley]
      #5529375 - 11/20/12 04:39 AM Attachment (12 downloads)

Quote:

Quote:

....For me, far more bang per buck with the Carton.

(Cost me 250 Uk pounds as a self build)




Was that one of the Skylight kits Andy ?

I thought they cost a lot more than that !




No, I bought the objective and built the rest.

If you look closely at the pic you can see that I've been a bit crafty. It shares my single speed crayford focuser with my 8" SCT.

Just screws on the back...


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chonum
member


Reged: 09/02/08

Loc: France
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5529377 - 11/20/12 04:41 AM

Hi guys,

I'm the guy who tested those instruments at Airylab.

The lab uses a Shack Hartmann wavefront analyser from Imagine Optic with three laser sources (DPSS 473nm, HENE 543nn, controlled LED 635nm), all being fiberized through a 4µm fiber core and therefore spatialy monomode.
Tests are performed against a 530mm ceramic flat mirror from REOSC (the that built VLT mirrors).

The analyser gives pure wavefront phase error in µm. Those values are not related to the wavelength as it is the case for interferometry. We need to take into account the wavelength only when processing the wavefront to get the MTF, the spot diagrams, the PSF and Strelh ratio.
Note that the wavefront is given WITHOUT any Zernike polynomial fit, and avoid any error and uncertainty due to this fit.
Then only a fit is done to get some information about the implied error terms splitting the error into legacy aberration terms (astig, SA, Coma, Trefoil...).

The spherochromatism (LSA) is calculated based on the Zernike fit for the 3, 5 and 7th order of SA. This is not done the the analyser, but in another code we made. It usually fits quite well manufacturer's data that is issued from Zemax. Except that it is reversed.

Spherochromatism in SC and triplet depends on the F/D ratio, and the aperture.
The classical triplet (lenses in contact or very close) loose one degree of freedom for the designer cannot use the lens spacing. Therefore those designs show spherochromatism, that is inevitable.
The only instruments that don't show any (besides reflectors) are Mak and special triplet designs such as the TOA cooke.

Here is a graph of the 3rd order SA over wavelength for three refractors :


Does that make the AP a bad refractor ? No for the polishing is excellent (the best telescope we tested on one wavelength was an AP130) and mecanic as well, not to mention a very large corrected FOV using the 4" flattener that just a few telescopes are able to achieve for imaging.

BTW The 152 was indeed the Astreya quadruplet version, prior to the triplet.

Frédéric.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Are achromats enjoying a come back? new [Re: Andy Taylor]
      #5529431 - 11/20/12 06:19 AM

Quote:


My 8" SCT and RFT still show a bright background but my Carton 100mm F13 shows me fantastic contrasty views with a black background.




At that same exit pupils? Sky brightness is a function of exit pupil... at 200x, my 60mm is pretty dark, the 25 inch is quite bright even under dark skies. With a 1mm exit pupil, both are similarly dark though the magnifications are quite different.

Jon


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