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Equipment Discussions >> Refractors

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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: Astrojensen]
      #5521406 - 11/15/12 03:14 PM

Quote:

Quote:

The lens of an eye is just that, a lens, a "fish bowl", and just as a refractor's objective.

Sir, you dare question the design of the Maker?




There are actually at least one species of fish and, I believe, one of squid, who use reflectors in their eyes for collecting light. Their eyes strongly resemble Schmidt or maksutov cameras, have an extremely fast f/ratio and are very large, compared to the animal. These animals live deep in the sea, where there is basically no light, save for faint bioluminescence.


Clear skies!
Thomas, Denmark




Danged bottom-feeders get all the good equipment......


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: csrlice12]
      #5521411 - 11/15/12 03:17 PM


"...get a refractor"

I have one.

"...Get a Dob/Newt"

I have two of those.

"Want to see everything--get both."

I have both.

"Neither a reflector nor a refractor can do it all."

True.

"Neither is 'better' then the other..."

False.

"...just different."

True.

"They both have their advantages and disadvantages."

True.

Cheers,

Alan


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Kentuckystars
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Reged: 03/17/08

Loc: Northern Kentucky
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5521602 - 11/15/12 05:27 PM

If one design could do it all then it reasons that only one design is all we would have from which to choose. There would be no need for any variations in the telescope.

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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: Sky Muse]
      #5521614 - 11/15/12 05:35 PM

Quote:

Sir, you dare question the design of the Maker?




I had a private conversation with Her a while back. She said that after Newton arrived, he pointed out a few design errors and that given the chance, She would do it differently the next time.

Jon


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Kentuckystars
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Reged: 03/17/08

Loc: Northern Kentucky
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5521623 - 11/15/12 05:43 PM

Are apo's better than achro's? Of course they are, the apo is an improvement of the achro's design. The apo is a child of the achro, but mom and dad are still around too so he knows where he came from.

Will apo's ever replace achro's altogether? Maybe, but the cost is going to have to come down to that of an achro. And I don't mean within a few hundred dollars but the SAME. Then the need to market achro's will no longer exist. Until then achro's will remain an option on our astronomical table.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522186 - 11/16/12 12:58 AM

Jason,

The two immediate advantages an apochromat possesses over an achromat is that chromatic abberation is kept to a bare minimum, and the ability to keep it so while at least halving the traditional focal length, including the physical tube. An f/8 apochromat is perhaps only a little better than an f/16 achromat in the supression of spurious color, therefore an apochromat's primary strength is that it's more manageable, with a mount just capable of sustaining an f/8 being inadequate for an f/16, though not by much, but enough to neccessitate the purchase of a larger, more expensive mount.

However, I was never too troubled over the chromatic aberration exhibited by my 80mm f/11 achromat on brighter objects, for just having a refractor, regardless of variety, instills a pride of ownership as no other design dare approach, primarily Newtonians of at least twice a given refractor's objective diameter. To be fair, Maksutov-Newtonians being somewhat removed in that they're graced by a lens on the front for corrections of the original design.

Closer to being so honoured as the refractor are mirrored arrangements of the Cassegrain design, particularly those employing a lens, also corrective, that is, the Schmidt and Maksutov-Cassegrains, with the Questar being paramount.

There are many varieties of mirrored arrangements, from the Maksutov-Newtonian to the Ritchey-Chretien to the Tri-Schiefspiegler, so many in fact that it seems indicative of as many attempts to get it right. The Tri-Schiefspiegler did manage to eliminate that pesky secondary obstruction, but at what cost, as it's most inelegant in appearance, combined with a frightfully excessive focal ratio.

Happily, with the refractor, you have only two varieties, just two, though not nearly as differing as those within the mirrored arrangements, and a testament to imperfect man's near-perfection upon the first attempt, with "semi-apochromats" excluded for reasons obvious. Thus, whether a refractor be an achromat or an apochromat, one's sense of pride in ownership remains absolute and unaffected.

Cheers,

Alan


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5522198 - 11/16/12 01:37 AM

About apos here is a test on a famous model.
I leave you your own comments about.
http://www.airylab.fr/contenu/mesures/astro/rapport%202011-05002-public.pdf
On this site 4 units were tested 130 and 150 with the worst and the best.
Be aware of what is acquired actually and with regards to the investment.
In blue and sometimes red channels, frankly there is no value.
Stanislas-Jean


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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: Sky Muse]
      #5522340 - 11/16/12 07:07 AM

Quote:

However, I was never too troubled over the chromatic aberration exhibited by my 80mm f/11 achromat on brighter objects, for just having a refractor, regardless of variety, instills a pride of ownership as no other design dare approach, primarily Newtonians of at least twice a given refractor's objective diameter.




Oh well...

When it comes to pride of ownership, I take pride in scopes that I have built myself... I am much prouder of my 16 inch F/4.42 Dob than of my NP-101 or the 25 inch Obsession because the 16 inch was something that represents my efforts, not something I just bought.

Seriously, I have to wonder if you have actually done a side by side comparison between your 80mm F/11 achromat and decent 6 inch F/8 Newtonian...

Currently my collection includes three 80mm F/11s, all Japanese Meades, two of them Mizars. It also includes an RV-6... people compare RV-6s to 5 inch apos... no false color, small central obstruction, rapid cooling... The OTA only weighs 8 lbs...

When it comes to viewing the planets, splitting double stars, seeing faint DSOs... the Newtonian is so far superior there is really no contest. A more reasonable contest is between a good 80mm F/11 and a good 114mm F/8 Newtonian.

I enjoy a good refractor, I spend a lot of time looking through refractors but I don't make a fetish of it, I realize they are no more special than any good scope.

Jon "Not a SRF" Isaacs


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Kentuckystars
member
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Reged: 03/17/08

Loc: Northern Kentucky
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5522443 - 11/16/12 08:42 AM

Yeah, enough debate. Let's go out and use our scopes whatever design they may be. As long as we're all happy with the view then they have passed the test.

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Kentuckystars
member
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Reged: 03/17/08

Loc: Northern Kentucky
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522447 - 11/16/12 08:43 AM

The only test that matters.

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5522508 - 11/16/12 09:34 AM

My point being that a good achromat is not that far removed from a good apochromat so as to require moving heaven and earth to replace it. Contrast and sharpness are far more important than the occasional sighting of spurious colour.

Again, the Celestron XLT achromat would be a great choice for a refractor, if you're wanting a refractor.

Cheers,

Alan


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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]
      #5522517 - 11/16/12 09:40 AM

Quote:

About apos here is a test on a famous model.
I leave you your own comments about.
http://www.airylab.fr/contenu/mesures/astro/rapport%202011-05002-public.pdf
On this site 4 units were tested 130 and 150 with the worst and the best.
Be aware of what is acquired actually and with regards to the investment.
In blue and sometimes red channels, frankly there is no value.
Stanislas-Jean




Well I do not know anything about the company that did the test...
The linked test shows a Strehl that is rather low for such a premium refractor, was there a problem with the optic?
Was it checked with the manufacturer?
The red is already low at 0.88 while the blue at 0.68 is way too low.
I really do not know what the data of a single scope should proof.
As the owner of this exact scope I would want to have a word with the manufacturer... and I am sure he would do everything he can to resolve this.

For your comparison I give you this link to another AP telescope a 155EDT from the early 90's with a bit more information:
http://www.astro-foren.de/showthread.php?10849-APO-von-Astro-Physics-mit-Glas...

With a prism diagonal this telescope reaches a Strehl of 0.987 so you can extract as much "information" as you like and this across the complete visual spectrum.
The test proofs only one thing that the specific optic tested is excellent, it does not allow to draw conclusions regarding the quality of the next scope produced. Going with a premium manufacturer you can be sure that the quality differences from scope to scope are rather small.

But the question still stands how for example your ISTAR Achromat would do in the same kind of bench test. If perfectly figured it will show a very high strehl in the green light but by the nature of an Achro the red and blue wavelengths will have a much lower Strehl. This is a given fact.

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]
      #5522617 - 11/16/12 10:39 AM

The lab is independant, works for industry in europe and makes tests for amateur owners and professionals.
The results are not oriented.
Frankly the strehl got in G channel is excellent and will give superbe views classically in IL. Now if you are working in R channel this is not bad but we expect somthing better. In blue channel too, but refractors are degraded in that fields even apos.
The balance results between investment and prestation is too abnormal for getting such: unproper for venus works (for the unit tested, a C8 will do same).
Now what for correcting this, change some lenses.
Actually what is the cdc of that OTA, to get high contrast views in IL. At the spectrum edges we may have a good model, an average also.
A newtonian of 150-180mm will do better on the edges of the spectrum.
Now the achromat 150 owned here is PTV 7 with the zygo test, R channel, still good in G channel (by roddier test) degraded in B channel (so un-proper for venus works).
We know the limits, but are others knowing their OTA.
Reasons for waiving on other designs.
The website has other OTA tests on other designs and we have also surprises.
What is abnormal when delivering such expensive OTA this is the fact of absence of test results as given on that site with the buy.
Stanislas-Jean


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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: Sky Muse]
      #5522909 - 11/16/12 01:18 PM

Quote:

Contrast and sharpness are far more important than the occasional sighting of spurious colour.




Indeed... contrast (true contrast) and sharpness are both primarily functions of aperture and quality.... one might argue about sharpness but in my view, a scope that can resolve a 0.5 arc-second double is sharper than a scope that cannot...

Jon


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ThomasWos
super member


Reged: 02/10/07

Loc: Bellingham WA
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5522999 - 11/16/12 02:29 PM

I don't have any optical training, but I do have some sound engineering training. When certain frequencies of sound within the strereo image are disturbed, the stereo image becomes smeared to some degree.
When more colors from an object come to focus, that implies to me a truer image. Hence the real advantage of an apochromat to me.

Tom


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: Kentuckystars]
      #5523389 - 11/16/12 07:56 PM

That would be 100% correct if all designs cost the same to execute on the same scale.

There is one single "best" telescope design for visual observing, and if money were no object, it would be the only design any of us used.

Problem is, money is an object, which opens the doors to compromise designs (compromise performance per inch in favor of low dollars per inch).

Never take a diversity of designs for equality of those designs in absolute terms. More often design diversity is all about achieving certain characteristics for a price at which a better all-around design could not achieve those characteristics.

The other thing is this. That "best design" is not static. It will change with advances in technology. Today it's probably a mirrored or catadioptric design with adaptive optics. None of us amateurs have those. Tomorrow, I suspect it will be adaptable liquid lenses.

- Jim


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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]
      #5523402 - 11/16/12 08:02 PM

Quote:


The balance results between investment and prestation is too abnormal for getting such: unproper for venus works (for the unit tested, a C8 will do same).





I fully agree with this sentence. Something is not OK with this telescope and should be checked with the manufacturer. In this league you can expect better results. The test you showed us is rather the exception than the rule for high quality APO's.

You have to be aware that the same can happen if you buy an Achro or Newton. If the tolerances and quality controls are off you end up with a lemon. This has nothing to do with the type of telescope. Most manufacturers do not send you an interferogram with the optics.

If you discover such a problem, I would advise to have it checked by an independent lab and then go with the results to the manufacturer.
This is also the reason you find more "bad" results in the scopes tested, if the quality is initially OK the scope will most likely not end on the optic bench for analysis.

best regards
Chris


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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? *DELETED* new [Re: Mark Harry]
      #5523412 - 11/16/12 08:08 PM

Post deleted by KWB

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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]
      #5523479 - 11/16/12 09:23 PM

Talking about achros versus apos:

http://www.cloudynights.com/item.php?item_id=1281

It always come back to that "law of diminishing returns"


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: Jon Isaacs]
      #5523521 - 11/16/12 09:53 PM

"...was something that represents my efforts, not something I just bought."

Yes, I know the feeling, as I've made things from scratch...

http://public.fotki.com/MississippiAL/miniatures/miniatures.html

"...between your 80mm F/11 achromat and decent 6 inch F/8 Newtonian."

I no longer have the achromat, as I gave it to a relative after I bought my apochromat in 2003. However, if you have said reflector to send, I'll compare it to my apochromat and give an objective opinion.

"Currently my collection includes three 80mm F/11s, all Japanese Meades, two of them Mizars."

I want one! ...and the best of the three, in your esteemed estimation, of course.

"It also includes an RV-6... people compare RV-6s to 5 inch apos...the Newtonian is so far superior."

I'm a bit skeptical, naturally, but if you say so I'll take your word, however, "superior" save that it exhibits reflections rather than direct views.

"...but I don't make a fetish of it..."

...not of refractors in any case.

"I realize they are no more special than any good scope."

If you say so.

Alan "Not a NC, ST or a MQ"

Although, I could be a MQ...


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