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Does an APO always have three lenses?

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#1 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 12:36 PM

The reason I ask, is that all of those I've seen advertised do come with three except for the Meade APO's.

The Meade APO's have only two lenses. Is this a semi-APO?

The prices on these OTA's are also considerably less than a same-sized TMB, Tak, etc.

What say the experts?

#2 jrcrilly

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Posted 14 November 2003 - 12:52 PM

All right! This'll start a great discussion thread. :jump:

Most of the TV APO's are also doublets, like the Meade (at least in that respect). So's the new Orion 80mm APO. Both Meade and Orion claim to be doing it much more cheaply than the others. I've looked through the 5" Meade "APO" and it looked awfully nice.

Until Tele Vue proved APO performance was possible with a doublet, most folks considered a triplet to be necessary. You'll still find old definitions of the term which include the word triplet.

These days the formal definition is optical, having to do with how many different wavelengths of light are brought to a single point at focus. I'll let the optics gurus address that!

#3 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 12:57 PM

So it is not neccesary to bring all wavelengths to the same focal point using three lenses. Now it can be done with two?

Will the glass type have a bearing on the correction or is it the coatings, or combination of both?

If it can be done with a two lens sytem, why are some of a given aperture selling for three times the amount of another?

#4 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 12:57 PM

Don't forget Taks. Most Taks are doublets and are considered apos - although maybe not with the color correction of a Christen triplet.

Sam

#5 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 12:58 PM

So not as many as I would be led to believe are actually the old definition of an APO with three lenses.

#6 jrcrilly

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Posted 14 November 2003 - 01:06 PM

So not as many as I would be led to believe are actually the old definition of an APO with three lenses.


I think most of the less expensive (if that term can be used in this context) APO's are doublets. I'd guess the Tak TAO is a triplet but I'd have to look it up. I never got serious enough about one to research it! :roflmao:

#7 jrcrilly

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Posted 14 November 2003 - 01:09 PM

So it is not neccesary to bring all wavelengths to the same focal point using three lenses. Now it can be done with two?

Will the glass type have a bearing on the correction or is it the coatings, or combination of both?

If it can be done with a two lens sytem, why are some of a given aperture selling for three times the amount of another?


Well, it can be near enough to get away with. On paper, there'll still be some error with a doublet.

It's all in the glass choice and lens design. Purists will still insist on triplets and will pay whatever it takes to get one (listening, Charles?).

#8 jrcrilly

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Posted 14 November 2003 - 01:15 PM

Don't forget Taks. Most Taks are doublets and are considered apos - although maybe not with the color correction of a Christen triplet.

Sam


Hi, Sam.

Yes, it's probably fairly safe to say that all the $2000 4" APO's are doublets. Not all the $2000 4" scopes are APO's, though. Vixen gets that price for their neo-achromats. TV gets $4000 for their 4" Petzval which, although it has two doublets, is really only a doublet so far as color correction is concerned.

#9 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 01:22 PM

I think Tak has two triplet designs - the Tak TAO and Tak FCT150.

On paper, there'll still be some error with a doublet.



The same thing is applicable with triplets. I quote from Roland Christen "there will always be a bit of color error due to sphero-chromatism". It seems like there is a fanatical theme in the refractor world to try to reduce and eliminate c.a./ I wonder if there's the same fanaticism in the reflector world - maybe a search of a mirror figure as smooth as a baby's butt.

Sam


#10 jrcrilly

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Posted 14 November 2003 - 01:28 PM

I wonder if there's the same fanaticism in the reflector world - maybe a search of a mirror figure as smooth as a baby's butt.


Guilty. Reflector guys have two parameters to argue about, though. Mirror figure is one, and CO size (which relates to focal length) is the other. That's why I have a 12.5" reflector with an F/8 Swayze mirror that's too long to look through without a ladder. :roflmao:

#11 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 01:29 PM

I wonder if there's the same fanaticism in the reflector world - maybe a search of a mirror figure as smooth as a baby's butt.


I think that would be safe to assume.

#12 matt

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Posted 14 November 2003 - 02:02 PM

So it is not neccesary to bring all wavelengths to the same focal point using three lenses. Now it can be done with two?

Will the glass type have a bearing on the correction or is it the coatings, or combination of both?

If it can be done with a two lens sytem, why are some of a given aperture selling for three times the amount of another?

I think the Definition is that an APO brings three wavelentghs at the same focal point, the achromat only two.

You will have SOME color error because it's not an infinity of wavelengths, allowing for some hierarchy among the higher priced APOs (I must admit that once I could not see the difference between an FC100 and a Sky90, to the dismay of the FC100's owner). Hence the ortho-apo by Takahashi. Hence the two- or three-fold price difference for marginal image improvement (I hear high-priced APO owners gasping. But I'm not meaning they're wrong. I just almost wish I were able to tell the difference the way they do).

As for glass type and coating, for all I've read and heard, glass type is the only factor for color correction itself, coatings having the part of light transmission and reflections reduction.

#13 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 02:08 PM

Thanks for your replies to this point. I'd like to hear more on the matter. I've never really studied optics. I'd rather just buy something that has been highly recommended and leave the optics to the designers, but I'm always interested in learning something new.

#14 Ron B[ee]

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Posted 14 November 2003 - 03:13 PM

The quick answer is no; two lenses are also APO: Tak FS102, TV-102, Vixen 102-FL/102ED, etc. But the triplets and Petzval (like NP-101) are "better" APO and can have shorter focal ratio but alas cost more :bawling:.

For a more complete answer, please take a look at this.
http://www.tmboptica...d.asp?cat_id=32

Ron B[ee]



#15 Ron B[ee]

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Posted 14 November 2003 - 03:16 PM

Until Tele Vue proved APO performance was possible with a doublet, most folks considered a triplet to be necessary. You'll still find old definitions of the term which include the word triplet.


Though I truly love what you said :flower: about Tele Vue, Takahashi was selling the FS102 doublet flourite APO (and even earlier with the FC100) long before Tele Vue doublet APO ;).

Ron the 4-inch Tall Evangelist B[ee]


#16 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 03:59 PM

I wonder if there's the same fanaticism in the reflector world - maybe a search of a mirror figure as smooth as a baby's butt.


Oh they need to be much smoother than that! In fact I hear some companies are starting to use babies' butts in their mirror grinding. However, there have been some complaints of diaper rash:-\

#17 jrcrilly

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Posted 14 November 2003 - 04:06 PM

Though I truly love what you said :flower: about Tele Vue, Takahashi was selling the FS102 doublet flourite APO (and even earlier with the FC100) long before Tele Vue doublet APO ;).


Hi, Ron.

That must have been before I was capable of even wondering about a 4" telescope for that kind of money! :foreheadslap:


#18 Bill Grass

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Posted 14 November 2003 - 04:32 PM

Oh they need to be much smoother than that! In fact I hear some companies are starting to use babies' butts in their mirror grinding. However, there have been some complaints of diaper rash:-\


:lol: :roflmao:

#19 matt

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Posted 14 November 2003 - 04:51 PM

Thanks for your replies to this point. I'd like to hear more on the matter. I've never really studied optics. I'd rather just buy something that has been highly recommended and leave the optics to the designers, but I'm always interested in learning something new.


The "articles" section of CN contains some helpful articles for someone getting into this, and if you like it after that you will love Thomas Back's (of TMB fame) page at http://voltaire.csun.edu/tmb/tmb.html :cool:

#20 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 05:12 PM

Thanks.

#21 Ron B[ee]

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Posted 14 November 2003 - 05:48 PM

That must have been before I was capable of even wondering about a 4" telescope for that kind of money! :foreheadslap:


Well John, at least you had a great store of experience back then :bow:. The FC100 and FS102 came before I got into this hobby. Believe it or not, 2-1/2 years ago when I entered, I wanted only a Meade or Celestron and all these strange sounding brands (Takahashi, Astro-Physics, Tele Vue, Vixen, Starmaster, MAG1 (sound like hot wheel), etc.) must have been the fallouts from Tasco, i.e., all department store telescopes off shoot :question:. I even convinced myself when I first saw how Tele Vue tooted the 70mm Proto to astronomers; afterall, it's only 10mm bigger than the Tasco :foreheadslap:!

Ron B[ee]


#22 Ron B[ee]

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Posted 14 November 2003 - 05:53 PM

Thanks for your replies to this point. I'd like to hear more on the matter. I've never really studied optics. I'd rather just buy something that has been highly recommended and leave the optics to the designers, but I'm always interested in learning something new.


Well then, if I maybe so bold as to assign you a brain-beating homework assignment for the weekend ;)?
http://alice.as.arizona.edu/~rogerc/
...to then be followed up by Rutten's book :bawling:.

Ron B[ee]



#23 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 06:52 PM

Well now, that should keep me busy for awhile. Thanks.

It looks very interesting.

#24 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 07:04 PM

That ought to make for some interesting reading some rainy afternoon! :sleepy:

Seriously, thanks for the link, I will actually read it some rainy afternoon (probably tomorrow!)

#25 Guest_**DONOTDELETE**_*

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Posted 14 November 2003 - 07:11 PM

Coincidentally, that's what it's supposed to do here all weekend.


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