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Equipment Discussions >> Refractors

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maknewtnut
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Reged: 10/08/06

Re: Apochromatic or Achromatic does it really matter? new [Re: Jared]
      #5216438 - 05/11/12 12:11 AM

Jared and others...if one or a few choose to bring up tube length as an issue, let's discuss that. Take a good look at the pic of the 10" f/11. Can you even see the ground? No.

Any person considering a refractor NEEDS to consider focal length. Why? Because a vast majority of amateur astronomers will never even consider the sort of mount that is required to utilize these telescopes.

I know this is the refractor forum, and that the discussion revolves around the strengths and weakness of achromats and apochromats. Still, when looking at what is required to utilize a 10" f/11 refractor, all of a sudden a 10" Newtonian seems far more appealing....and no chromatic abberation to boot.

CA is an attribute of considerable contention to many. Juptier is not purple, and therefore should not appear awash in purple through the eyepiece to many. At 250mm, f/11 is far too fast to avoid considerable levels of CA. At that aperture one might have to get to f/18-20 or slower to even play on the same field as a 10" triplet at f/9.

Is tube length an issue. Absolutely!


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tmbuser6
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Re: Apochromatic or Achromatic does it really matter? new [Re: maknewtnut]
      #5216472 - 05/11/12 12:42 AM

"At that aperture one might have to get to f/18-20 or slower to even play on the same field as a 10" triplet at f/9."

You are being generous Mark. To equal the correction of a 10" triplet at f9, an achromat would have to be f/40 or a bit longer. I've used a 13" Clark refractor and a 10" f/12 D&G extensively to be bought for a local public observatory I spearheaded for the community years ago. FINE instruments, GRAND views. But views of bright objects like the Moon or Jupiter were still awash in violet haze like there was a black light on inside the tube. That is simply wasted light not given to the image and smears the contrast from being even better than what it is. Still, an apo of that size is unspeakably expensive. Sometimes one must resign to inevitability and give in to using a Wratten #11 or something. One can do a lot, lot worse, and few things in this life are as good as viewing through BIG GLASS.

Wayne


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Jon Isaacs
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Re: Apochromatic or Achromatic does it really matter? new [Re: maknewtnut]
      #5216616 - 05/11/12 06:45 AM

Quote:

I know this is the refractor forum, and that the discussion revolves around the strengths and weakness of achromats and apochromats. Still, when looking at what is required to utilize a 10" f/11 refractor, all of a sudden a 10" Newtonian seems far more appealing....and no chromatic abberation to boot.




Or a 12.5 inch or a 16 inch Newtonian on a Dob mount... that 110inch focal length requires a serious mount.

Jon


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plyscope
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Reged: 11/23/06

Loc: Perth, West Australia
Re: Apochromatic or Achromatic does it really matter? new [Re: Jon Isaacs]
      #5216649 - 05/11/12 07:41 AM

This is an interesting review by Phil Barker of New Zealand comparing a 102mm f9 fluorite doublet with a 5" f15 achromat. Two fine telescopes with different pros and cons.

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skyjim
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Re: Apochromatic or Achromatic does it really matter? new [Re: plyscope]
      #5216767 - 05/11/12 09:30 AM

I read threw this post and its like the same old story that goes round and round by the same old guys here on which is better and why, then the experts chime in and give you the brutal details of why achro just dont cut the bar. I forgot the newt lovers chime in as well. Some here have allot invested in there heard of scopes, have had many samples pass threw there hands, some happy with what the now own and some never happy then there are the die hard achro guys that from what I can see are ATM'ers that place many hours and time into there prise larger achro, hey these guy I have to give credit twards cause instead of wipping out a credit card they have year of hard work going into there scope and to me thats a labor of love for this hobby. Then there's the vendors that chime in, I thought they had there own forum but I could be wrong. It seems to me like there are some here that are just plain insulting in there delivery of replys and you guys know who you are but the bottom line is whatever one has for a scope if they use it and like its veiws they should be happy and remember this is only a hobby and the guys who take this hobby to a semi pro level dont even post in most of these forums, they use what they have or can afford and do there work, most of those folks dont even care which is better cause there not scopeaholicks like some on these forums. What realy matters is that you enjoy the veiws even if there is some CA and good well figured achro can give up some pleasing veiws, maybe not 100% perfect but if you have deep pockets that 99% can be gotten from the APO camp and there are a whole lot of scopes in between.

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BigC
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Re: Apochromatic or Achromatic does it really matter? new [Re: skyjim]
      #5217006 - 05/11/12 12:03 PM

Good summation.

I think that second to looking at the sky,beating dead horses is a main interest.

Actually it is a toss-up with some if bragging about their gear isn't more appealing than using it.

Edited by BigC (05/11/12 12:05 PM)


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avarakin
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Re: Apochromatic or Achromatic does it really matter? new [Re: Dubliner]
      #5222739 - 05/14/12 08:48 PM

Quote:

Hello all,
Don't know if I'm in the right place for this question but I'll plough ahead regardless!
I know the basic differences between these lens types and you get what you pay for as as Apochromatic are multiples of the cost of an Achro scope.. But one of the main benifits
of Apros is their ability to eliminate (almost) false colour. So when it comes to imaging as most photos I see are in mono then surely there is no false colour and thus no need for such expensive optics? A 200$ scope would be as good as a $2000 scope? I speak only here of mono photos.
I must be misunderstanding something here and their is a big hole my reasoning?




Going back to original question:
Most of the time "mono" photos you see on Astrobin or Astrophotogallery are taken in Hydrogen Alpha (Ha) wavelength using narrowband Ha filters. For such images achro or apo does not matter because light hitting the sensor has the same wavelength, so one can focus even achro precisely and get perfect star shapes.

It is also possible to take several narrow band images of same object using other wavelengths, like same Ha and OIII and SII and combine them into a false color image. In this case focus will also be perfect because each wavelength can be focused separately.

One can also take a real mono picture with a mono CCD and in this case APO will produce a better image than Acro, but I dont think this technique is very popular.

So answer is: it depends on what you want to do.
I personally like the narrowband techniques I described above because they allow to cut through light pollution, so in this case apo vs achro does not matter.

As far as general statement: "200$ scope would be as good as a $2000 scope?" - it is possible if $200 scope is a reflector or Schmidt Newtonian, although I guess this is a wrong forum to post such an opinion Of course $2000 scope will have much better mechanical components(especially focuser) and fit and finish, but optically they would be very similar.

Alex


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brianb11213
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Re: Apochromatic or Achromatic does it really matter? new [Re: avarakin]
      #5223078 - 05/15/12 05:46 AM

Quote:

Most of the time "mono" photos you see on Astrobin or Astrophotogallery are taken in Hydrogen Alpha (Ha) wavelength using narrowband Ha filters. For such images achro or apo does not matter because light hitting the sensor has the same wavelength, so one can focus even achro precisely and get perfect star shapes.



A singlet lens would do just fine for monochromatic imaging. You'd have to refocus when switching filters whereas with a parfocal set & a reflecting scope, or a good apo, you might get away without.

Of course, with extra surfaces (four instead of two for a air spaced achromat, six in an air spaced or oil filled apochromatic triplet) there is opportunity for a designer to correct aberrations other than chromatic rather better than can be done with a singlet lens, and usually there is a compromise between complete correction of chromatic & spherical aberration ("on axis") with off axis aberrations (coma, astigmatism & field curvature). Every optical system is a compromise to some extent, & extra surfaces introduce more light scatter, making a very simple optical system critical for some applications e.g. a Lyot coronagraph despite the efficiency of modern lens coatings.

So far as ordinary visual astronomy is concerned, an achromatic doublet will be adequate, provided that the focal length is kept reasonably long. A triplet apo design allows a shorter focal length with colour correction as good or better than an doublet with a longer focal length (even when using special glass) & is consequently easier to handle as well as having a larger field.


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mikey cee
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Loc: bellevue ne.
Re: Apochromatic or Achromatic does it really matter? new [Re: maknewtnut]
      #5223310 - 05/15/12 10:33 AM Attachment (47 downloads)

Mark you are dead on about needing a big mount. But as far as objectionable CA goes you couldn't be further from the truth. My lens must have one heck of a figure plus the R30 rating I've been told by Istar actually performs closer to a 35% reduction in CA. Sure there is CA there but nowhere near what I was expecting. I'd bet if I put a cardboard 8" aperture stop on there it would perform so well I wouldn't believe my eyes. Mike

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buddyjesus
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Reged: 07/07/10

Loc: Davison, Michigan
Re: Apochromatic or Achromatic does it really matter? new [Re: mikey cee]
      #5223444 - 05/15/12 12:12 PM

beautiful scope mikey

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bobhen
professor emeritus


Reged: 06/25/05

Re: Apochromatic or Achromatic does it really matter? new [Re: mikey cee]
      #5223650 - 05/15/12 02:27 PM

Quote:

Mark you are dead on about needing a big mount. But as far as objectionable CA goes you couldn't be further from the truth. My lens must have one heck of a figure plus the R30 rating I've been told by Istar actually performs closer to a 35% reduction in CA. Sure there is CA there but nowhere near what I was expecting. I'd bet if I put a cardboard 8" aperture stop on there it would perform so well I wouldn't believe my eyes. Mike




The quality of the optical figure has nothing to do with CA.

There is also a post on the Yahoo refractor forum from a noted optical design (not Roland) where he states that Istar objectives are purchased from DKD in China and that R30 does not reflect a 30% reduction in CA. But that these lenses are designed to just shift the correction curve towards the Blue end of the spectrum. And that some improvement will be seen in the blue but there will be a worsening in the red-orange part of the spectrum. In other words, the amount of CA is the same its just shifted to another part of the spectrum.

And, that for planetary work, it is actually better to shift the correction towards the red end of the spectrum.

Nothing wrong with a nicely figured achromat but Question: if these lenses cost more than a standard achromat are they really worth the increase in cost over a standard design if the actual CA is the same?

Bob


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Apochromatic or Achromatic does it really matter? new [Re: bobhen]
      #5223701 - 05/15/12 02:55 PM

"In other words, the amount of CA is the same its just shifted to another part of the spectrum"...where your eye is a lot less sensitive anyway.

A scope that reduces the amount of false color you can actually see is worth something. Apochromats aren't apochromatic. There isn't a refractor in existence that crosses every single visible wavelength at a single point. It's all a question of degree. How far apart are the crossings, and what wavelengths cross at or near the same focal point? "Apochromat" and "achromat" are just labels. What matters is what your eye or your CCD camera *sees*. I'd say the R30 strategy has as much promise and value as the correction strategy employed in the Takahashi TOAs. It's not a new strategy, by the way. The recent crop of 102mm f/7 doublets from WO, Orion, Stellarvue and others, also shifted color correction so blue and green crossed near the same point making red the odd man out.

Regards,

Jim


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bobhen
professor emeritus


Reged: 06/25/05

Re: Apochromatic or Achromatic does it really matter? new [Re: jrbarnett]
      #5223771 - 05/15/12 03:32 PM

Quote:

"In other words, the amount of CA is the same its just shifted to another part of the spectrum"...where your eye is a lot less sensitive anyway.

A scope that reduces the amount of false color you can actually see is worth something. Apochromats aren't apochromatic. There isn't a refractor in existence that crosses every single visible wavelength at a single point. It's all a question of degree. How far apart are the crossings, and what wavelengths cross at or near the same focal point? "Apochromat" and "achromat" are just labels. What matters is what your eye or your CCD camera *sees*. I'd say the R30 strategy has as much promise and value as the correction strategy employed in the Takahashi TOAs. It's not a new strategy, by the way. The recent crop of 102mm f/7 doublets from WO, Orion, Stellarvue and others, also shifted color correction so blue and green crossed near the same point making red the odd man out.

Regards,

Jim




Whether you can see CA is somewhat irrelevant, its the lost information in the out of focus part of the spectrum that never reaches the eye that is important - whether you see that lost information as false out-of-focus-color or not.

So - if CA is the same in these lenses as standard Achromatic lenses visible, not visible, in the red, or in the blue would not the damage to the image in the eyepiece be the same?

Bob


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BillP
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Loc: Vienna, VA
Re: Apochromatic or Achromatic does it really matter? new [Re: jrbarnett]
      #5223778 - 05/15/12 03:36 PM

Quote:

It's not a new strategy, by the way.




I believe I read that Zeiss used to do this also. So a very old strategy.

FWIW, by looking at this chart, the human eye is very much more sensitive to blue+violet, compared to just red. Jupiter, Venus, Mercury, Moon, Uranus, Neptune should all be better visually I would think when the spectrum is shifted for better correction towards the blue. Mars and Saturn being the only ones where advantageous for a shift towards the red I would think (and maybe just Mars as Saturn is stronger in yellow-orange). I had a red-shifted achromat and this is how it played out for me. Never had a blue-shifted one though.


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ValeryD
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Re: Apochromatic or Achromatic does it really matter? new [Re: BillP]
      #5224079 - 05/15/12 07:37 PM

Quote:

the human eye is very much more sensitive to blue+violet, compared to just red. Jupiter, Venus, Mercury, Moon, Uranus, Neptune should all be better visually I would think when the spectrum is shifted for better correction towards the blue. Mars and Saturn being the only ones where advantageous for a shift towards the red I would think (and maybe just Mars as Saturn is stronger in yellow-orange). I had a red-shifted achromat and this is how it played out for me. Never had a blue-shifted one though.




This is true ONLY at VERY low illumination, when an eye can't really see colors - night vision.
When observing planets, they create at the eye's retina almost the same illumination level, as a day light. So, the color sensitivity curve is about the same as in a day light. And in this case better to have a red shifted achromat. AS Zeaiss semi-apos are not only semi-apo, they also somewhat red shifted - much better for planets.

Also, Saturn and Mars both are much more valuable for most planetary observers, than Venus, Mercury, Uranus, Neptune
together.

In any case, the color correction shift in achromat is an easiest deal - two radii change. Material and labor cost are ecaxtly the same as in any another achromat.


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MRNUTTY
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Re: Apochromatic or Achromatic does it really matter? new [Re: jrbarnett]
      #5224245 - 05/15/12 10:01 PM

Quote:

"the recent crop of 102mm f/7 doublets from WO, Orion, Stellarvue and others, also shifted color correction so blue and green crossed near the same point making red the odd man out.




Beautiful, I'm almost entirely blind to red. Finally it comes as an advantage.


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mikey cee
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Re: Apochromatic or Achromatic does it really matter? new [Re: ValeryD]
      #5224254 - 05/15/12 10:07 PM

Valery....What you say is exactly how my correspondance with Istar's Master Optician Zdenek went. As long as it "looks" less to me at a fraction of an apo I could care less. I'm as happy as a clam and that's really all that matters. To each his own.

Edited by mikey cee (05/16/12 02:10 AM)


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skyjim
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Re: Apochromatic or Achromatic does it really matter? new [Re: mikey cee]
      #5224417 - 05/16/12 12:01 AM

Mike, you are right on, but what I dont think Valery realizes is your scope started as an ATM project, you didnt buy it all together, yes some comapny made the lens but your scope is a labor of love and an excellent one at that but the APO Gods dont seem to understand that. Your scope looks and sounds excellent and I have seen most of your posts as you have built this scope, my hats off to you, excellent job!
Jim


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BillP
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Re: Apochromatic or Achromatic does it really matter? new [Re: ValeryD]
      #5225163 - 05/16/12 01:33 PM

Quote:

Quote:

the human eye is very much more sensitive to blue+violet, compared to just red. ...




This is true ONLY at VERY low illumination, when an eye can't really see colors - night vision...




Think you are mistaken. With Photopic vision (day adapted) the eye is more sensitive to violet+blue (410-500nm) as my original reference chart shows. With Scotopic vision (dark adapted) the eye has essentially no sensitivity to red (655-700nm). Here is a chart of both vision types overlayed - link. So just look at the amount of area under the two ranges (violet+blue = 410-500nm and red = 655-700nm). In either vision type there is more area under the violet-blue than red.

Edited by BillP (05/16/12 01:39 PM)


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PhilCo126
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Re: Apochromatic or Achromatic does it really matter? new [Re: BillP]
      #5269750 - 06/13/12 03:14 PM

Visually there's some difference between an Apochromat or Achromat refractor... putting scopes next to each other under the same conditions will show this in practice!
Going for an Apochromat makes sense if You're considering to try Your hand at some AP ... which most of us do ... sooner or later


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