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Equipment Discussions >> Refractors

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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5529540 - 11/20/12 07:56 AM

Frédéric, thank you for the additional information regarding your testing process and the resulting information.

So it was an Astreya quadruplet at f7... seems like the cell has its problems holding collimation. It is not the first time I heard about readjustments on the lens level.

Fantastic collection of tests, just what Jim was looking for:
http://www.airylab.fr/AirylabUS/index.php/optical-metrology/astronomy-tests-r...

best regards
Chris


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: chonum]
      #5529660 - 11/20/12 09:30 AM

Hi Frederic,

Thanks for chiming in. I'll second Chris, your site is well done, with some very interesting information for us amateurs. But some things are still not clear to me.

Quote:

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.




That is quite unusual way to put it. "Wavefront" is geometric category, while "phase" is related to the wave (oscillation) itself. Kind of, don't stick together. Wavefront error is usually expressed either directly, in geometric units (like micron, nm, etc.) or relatively, in units of wave. Phase error, on the other side, is usually expressed in units of full phase, i.e. 2Pi radians for a full wave, either as "variance" (standard phase deviation squared) or "standard deviation", with the latter being the phase analog to the RMS (geometric) wavefront error (e.g. 0.1x2Pi radians phase error is equivalent to 0.1 wave RMS wavefront error).

Quote:

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.





Any apo objective will show significant spherochromatism if pushed far (fast) enough (try to scale the TOA down to f/6). Air-spaced objectives are generally better in controlling aberrations; they do tend to produce somewhat less spherochromatism too, but the difference usually isn't really significant.

Quote:

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




There's not much use of such graph, considering that faster apos typically have mixed 3rd and 5th order spherical (for instance, the AP 155 has mostly 5th order in the red, mixed 3rd/5th in the green - which means that the 3rd alone is nominally significantly larger than the combined error - and mainly 3rd order only in the blue).

I also still wonder how the longitudinal aberration plots are obtained.

Vla


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chonum
member


Reged: 09/02/08

Loc: France
Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5529807 - 11/20/12 10:51 AM

Hi Vla,

WFE is in µm (or nm) indeed. Nevertheless it is a phase map should you convert it to fringes using the lambda value. We prefer to use nm anyway as you don't need to take into account the wavelength.
But when i need to use diffraction, or even spot diagram to get the real scale then the lambda is obvioulsy required.

I don't agree that much about the TOA. Of course it's not very fast and it would show a less flat 3rd order SA curve at F/6. Nevertheless at 7.7 native the SA is flat and almost null. The big difference with a legacy triplet is that you can still use the reducer down to 5.8 and be almost spherochromatism free (at the expense of the field coverage but there's still plenty for most CCD sensors).
A guys i know use it in the CAK line for solar imaging and it's maybe the only refractor to be able to get there and stay clean.

In the case of the AP155, 5th and 7th order SA are quite the same over the spectrum. Only one AP130 shows some evolution of its high orders SA.

But it's true that sometime high orders can compensate partially the 3rd order, Mak are good example of this.

LSA is calculated by projection of the normal from the wavefront to the axis using tangent of the derivated zernike development 3, 5 and 7. That's just dull trigonometry, i was surprised not to find this somewhere in an optical receipe book. After doing it i recognize it's just a PITA to do
We are supposed to make an article about that in a book to come.


Frédéric.


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wh48gs
Carpal Tunnel
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Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: chonum]
      #5530202 - 11/20/12 01:36 PM Attachment (19 downloads)

Frédéric,

Quote:

I don't agree that much about the TOA. Of course it's not very fast and it would show a less flat 3rd order SA curve at F/6.




Then we pretty much agree, except that I'd say wouldn't be nowhere near flat anymore.

Quote:

In the case of the AP155, 5th and 7th order SA are quite the same over the spectrum. Only one AP130 shows some evolution of its high orders SA.




That is not what the plots - including your own - show. It is very obvious that the 3rd and 5th order (transverse aberration; 4th and 6th order on the wavefront) are quite a bit mixed up (7th order is negligible). I've made a quick model of the AP 155 EDF triplet, with FPL53 and two crowns (Ohara equivalents to Schott's BK7 and K5), giving very similar plot 0.4-1 micron as the published one. It shows 5th order (changing longitudinally with the 4th power of zonal height) dominating in the blue, and 3rd order, to somewhat lesser degree, dominating in the red. The green is nearly perfectly balanced between the 5th and 3d, which means that each alone has nearly six times greater nominal RMS (of opposite signs).

Any time strongly curved surfaces generate significant 5th order spherical - rather common case with spo doublets, fast triplets and Maks - it has to bi minimized by balancing it with the 3rd order.

Quote:

LSA is calculated by projection of the normal from the wavefront to the axis using tangent of the derivated zernike development 3, 5 and 7.




What doesn't fit in is that the plots show smooth spherical WF deviation (thinking of AP 130 f/6.3 test), while the wavefront map shows that other - mainly random - forms of deformations dominate.

Do you plan on English edition for the book?

Vla


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VNA
member


Reged: 11/13/09

Loc: 37.893 N, 122.124 W
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5530832 - 11/20/12 07:00 PM

Quote:

The problem with the "law of diminishing returns" is that, as applied to telescopes, it is entirely subjective.

For example, observer A and B hate false color. Observer B also hates reflectors. Observer C dislikes small aperture and also dislikes false color. Observer D doesn't mind false color doesn't mind reflectors, but would like to save as cash as possible. The four are on budgets of $1000 They are asked to consider four scopes; a 12" f/5 Dob for $1000, a 6" f/8 achromat for $500, a 4" f/15 achromat for $800 and a 3" f/6 apochromatic triplet for $1000 (all real life scopes, available today).

Let's apply the law of diminishing returns to these three observers. Observer A hates false color. His personal cost-benefit analysis says that he could go either 3" f/6 APO or 12" f/5 Dob, avoid dreaded false color, and get what he perceives to be "value" with either scope for his money the extra he spends to avoid his dislike (false color) is a small price to pay; in fact it is a necessary price for him to pay to be satisfied. Observer B has only one choice, because for him, reflectors don't cut it, and he goes with the 3" APO. For him, it is the only choice that makes sense given his preferences. Observer C, too, has only one choice. 3" won't cut it, so he goes for the $1000 12-incher. Observer D is tolerant of false color, and above all values economy. He could go foe either the 6" f/8, get more aperture and false color, but also pocket $500 or the 4" f/15 and get some, but a bit less false color, but less aperture and have $300 less left over. He goes for aperture and $500, and buys the 6" f/8. Each of these observers has applied a version of the law of diminishing returns (given the limited scope pool in the example) and come up with a different answer.

A "law" that, when applied, results in different answers, isn't all that useful as an analytical tool.

- Jim




Was thinking more simply in terms of equal diameter only:

A 6 inch refractor apochromatic versus a 6 inch refractor achromatic

The price will be 6 times or more for equal aperture the only variable will be that "chromatic aberration" that can be corrected for little money.


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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: VNA]
      #5530910 - 11/20/12 07:44 PM

Quote:


Was thinking more simply in terms of equal diameter only:

A 6 inch refractor apochromatic versus a 6 inch refractor achromatic

The price will be 6 times or more for equal aperture the only variable will be that "chromatic aberration" that can be corrected for little money.




OK, I'll bite... How can the chromatic aberration of a 6 inch achromat be corrected for very little money?

The least expensive way I know of to correct the CA is the no longer available Chromacorr and that was not totally satisfactory. One can try to eliminate the defocused colors with a filter but that is not correcting the color, to correct the color, it must be refocused so it ends up where it belongs, in focus with the rest of the colors.

There are virtues and limitations to just about every telescope ever made... it really makes no sense to argue over personal preferences as which compromise makes the most sense. The important thing is just to recognize the virtues and limitations, try to make the choices that best an individuals needs and desires and move on.

Last night I had my 100mm F/6 achromat out in the back yard. The seeing was so so but it was giving me a nice clean split of the double-double. Later I tried to split delta Cygni but I just couldn't get it. I pulled out my 80mm F/7 FD and was able to get a nice clean split.

The right scope for the job...

Jon Isaacs


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5530975 - 11/20/12 08:24 PM

Well Jon I was wondering about the same point....

How to correct the chromatic aberration for little money.
If there is many people would love to know.

best regards
Chris


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Deep13
Carpal Tunnel
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Reged: 01/25/05

Loc: NE Ohio
Re: Are achromats enjoying a come back? new [Re: rocketsteve]
      #5531019 - 11/20/12 08:51 PM

Achromats are enjoying a never-went-away.

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5531440 - 11/21/12 01:42 AM

By the use of narrow band filters, for example the tricolor set filters.
If you are interested by some studies (visually)
calcium K: for venus still enough transparency otherwise the B filter tricolor during day is well enough,
RGB: all planets,
baader continuum: all planets,
O3: venus ashen light,
Halpha all planets in the bandwidth and for venus ashen light,
Now a simple synta will fail in some field as the presence of spherochromatism, but well designed doublets can do more than honestly.
This is not to collect pretty views but data.
Stanislas-Jean


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5531483 - 11/21/12 03:06 AM

Well yes, narrow band filters may do the trick in some cases....
E.g. some fantastic solar imaging is done with Achromats filtered accordingly.

However for the full visual spectrum that our eye can see on planets, this is not the solution.
Since I like pretty pictures.

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]
      #5531486 - 11/21/12 03:14 AM

This is always a matter of what everybody is aiming, data collection or pretty views with a bank account.
Stanislas-Jean


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astroneil
Pooh-Bah
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531662 - 11/21/12 08:21 AM

Quote:

Quote:



Last night I had my 100mm F/6 achromat out in the back yard. The seeing was so so but it was giving me a nice clean split of the double-double. Later I tried to split delta Cygni but I just couldn't get it. I pulled out my 80mm F/7 FD and was able to get a nice clean split.

The right scope for the job...

Jon Isaacs




Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).

Delta Cygni is fairly easy in both my 80mm achromats (f/9 and f/11).

I used to own a 100mm f/6.5 achromat and my records show that this system was also easy to split from my less than idyllic vantage with this instrument.

http://www.cloudynights.com/item.php?item_id=1515

Elephants never forget.

Cheers,

Nelly.

Edited by astroneil (11/21/12 08:24 AM)


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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: stanislas-jean]
      #5531703 - 11/21/12 08:44 AM

Quote:

By the use of narrow band filters, for example the tricolor set filters.




Filters do not correct chromatic aberration, the eliminate the offending colors...


Jon


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5531711 - 11/21/12 08:47 AM

Quote:

This is always a matter of what everybody is aiming, data collection or pretty views with a bank account.
Stanislas-Jean




If one is collecting data, the obvious solution is a reflector.. no filters needed. In fact for viewing the planets, the obvious solution is almost always a reflector, particularly if one is trying to do it economically.

Jon Isaacs


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Jon Isaacs
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Re: Are achromats enjoying a come back? new [Re: astroneil]
      #5531728 - 11/21/12 08:57 AM

Quote:


Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).




I am very appreciative of the wonderful climate and the often excellent seeing. But this does not mean the seeing is always great and in this case, with Delta Cygni low over a warm roof, the seeing was not so hot...

I might have gotten it with one of my 80mm F/11's, certainly it is easier in the 80mm F/7 apo. Last night I had out my NP-101, it really pulls the faint companions out of the bad seeing. Delta Cygni was an easy split. Rigel was barely above the horizon and very unstable, flashing colors, and I was able to pick out the companion at 60x... Almost like a Newtonian.

I don't know about other fast achromats but in my experience, there is enough chromatic aberration and general muck in the ones I have used that apochromats with their superior color correction and most likely the superior optics certainly are better performers on doubles, particularly when the seeing is sketchy.

Jon Isaacs


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531736 - 11/21/12 09:03 AM

See the us alpo site for the use of filters, this is not for the improving contrast research (the strehl anyway is).
Sure you know this.
Stanislas-Jean


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astroneil
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5531743 - 11/21/12 09:10 AM

Quote:

Quote:


Congratulations Jon! You must be beside yourself for having finally split Delta Cygni with an 80mm 'scope (having posted negative findings in this aperture class several times in the past even in your excellent climate).




I am very appreciative of the wonderful climate and the often excellent seeing. But this does not mean the seeing is always great and in this case, with Delta Cygni low over a warm roof, the seeing was not so hot...

I might have gotten it with one of my 80mm F/11's, certainly it is easier in the 80mm F/7 apo. Last night I had out my NP-101, it really pulls the faint companions out of the bad seeing. Delta Cygni was an easy split. Rigel was barely above the horizon and very unstable, flashing colors, and I was able to pick out the companion at 60x... Almost like a Newtonian.

I don't know about other fast achromats but in my experience, there is enough chromatic aberration and general muck in the ones I have used that apochromats with their superior color correction and most likely the superior optics certainly are better performers on doubles, particularly when the seeing is sketchy.

Jon Isaacs




I find this all very convenient.


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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: astroneil]
      #5531839 - 11/21/12 10:05 AM

Quote:


I find this all very convenient.




Indeed, apos are very convenient.

To get that same sort of views in an achromat, it would need to be very long and even then the color correction would not be up to the reflector-like performance of the NP-101...

The real convenience the NP-101 is the ability to provide both a 4.5 degree field that is flat and the near perfect high power views all in a compact package.

Last weekend, I had great fun with the 100mm F/6 achromat out under dark skies, I can enjoy just about any telescope around and it does a good job. But as much fun as I had, pulling out the NP-101 reminded me of just how perfect it is...

Jon


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csrlice12
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Loc: Denver, CO
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5532037 - 11/21/12 11:31 AM

Well, I just bought one yesterday. The Omni 102XLT. Couldn't afford an APO. To be truthful, Acromats never went away. I have no doubt if someone looked at sales stats, Acromats probably far outsell Apochromats. If nothing else, due to the costs.

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Joe Bergeron
Vendor - Space Art


Reged: 11/10/03

Loc: Upstate NY
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5532727 - 11/21/12 05:47 PM

Jon, don't tell me you are a member of the worldwide anti-achromat conspiracy. Your contrary views suggest as much.

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