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Apochromatic or Achromatic does it really matter?

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#1 Dubliner

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Posted 07 May 2012 - 02:10 PM

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? :o

#2 Thomas Karpf

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Posted 07 May 2012 - 02:57 PM

What sort of 'mono' photos are you planning on taking?

Imagine the light of a single star refracted through a lens. Let's consider three very specific wavelengths (deep red, green, and deep violet). The scope is focused so the green is in focus.

With an achromat, you would see a green dot in the middle, and a single ring around the star composed of red and violet. Focus on the red, and you have two concentric rings around the red dot, first the green and then the violet.

With an apochromat, those all three colors focus on that single dot.

If you image using three specific filters, you need to make sure that you focus correctly for EACH wavelength one at a time. If you shoot it all at the same time on a 'black and white' sensor, you would see rings with the achromat, and none with the apochromat.

Does that make sense???

#3 hfjacinto

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Posted 07 May 2012 - 03:01 PM

Tom pretty much states it in the above.

#4 gillmj24

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Posted 07 May 2012 - 03:06 PM

$2000 scopes are generally corrected to a much higher level for other aberrations than $200 scopes, so you are paying for more performance in other aspects, not just less color.

#5 Dubliner

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Posted 07 May 2012 - 04:08 PM

Thanks all,
I was only speaking about single shot mono cameras - without filters.
So if I have got this straight then those colours red , green, violet are not focused exactly in achromat,that is why we see colour fringes. If I take a mono photo these colour fringes are not removed by virtue of the fact I am shooting in b&w...the image will not be perfectly sharp by still having this fringing albeit now in monochrome.
Ok, maybe I was just trying to console myself with a cheaper scope!

#6 spaulmac

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Posted 07 May 2012 - 04:13 PM

A mono camera still "sees" all colors, but only records the b&w component of each color. As per Thomas' explanation, even a mono imager in an achromat would record a fuzzy image.

One instance I am aware of in which an achromatic lens might provide an acceptably sharp image is if some form of narrow band filtration were used before the imager. This can be the case for solar telescopes, such as Hydrogen Alpha models, which could use less well corrected objectives since they only have to focus a very narrow band of light.

#7 Jon Isaacs

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Posted 07 May 2012 - 04:27 PM

Ok, maybe I was just trying to console myself with a cheaper scope!



You asked a good question... for visual use, achromats can provide very nice views and certainly offer a good value for your money. Apochromats provide about as perfect a view as is possible for those willing to pay for the perfection.

Jon

#8 spaulmac

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Posted 07 May 2012 - 04:39 PM

I'll second Jon's statement on the value of achromats for visual use. Until recently, I was still quite happily using as my primary scope the same Borg achromat I purchased 10 years ago.

#9 watcher

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Posted 07 May 2012 - 10:29 PM

I have seen some VERY nice photos shown at CN taken with Orion ST80's. if the optics are decent, it will just take more time and effort but it can be done.

#10 Al Miller

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Posted 08 May 2012 - 08:39 AM

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 did some experiments with yellow and green filters recently (I intend to do more IF I ever get the time!). I found the Baader 495nm pass filter (Yellow) particularly useful even with color (OSC) imaging. It cuts the blue/violet halos around brighter stars and sharpens the image while not cutting the blues from the object much at all. On monochrome images stars are considerable sharper, and resolved. Check out the link to my previous post on this as it contains a comparative image of M42 and a UV scan of the filter showing the passed spectrum. This filter was used on my Orion ST-120 that produces a significant halo if no filter is used. Of course your $200 scope wont become a $2000+ scope but, it will take images of very good quality (and save you some $$$). If anything, it's a good start to imaging while you save for a higher quality scope.

Yellow Filters

#11 jrbarnett

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Posted 08 May 2012 - 09:42 AM

Excellent question. As with all excellent questions, the answer is "it depends". More precisely it depends on you, your preferences and tolerances, and the kinds of targets you like to observe most.

In general, an apochromat will show less false color than an achromat. Note, however, that precisely what constitutes an "apochromat" isn't settled, and many scopes that are dubbed "apochromatic" by their makers or branders, nonetheless show false color under certain circumstances (much like an achromat does).

Here are a few rules of thumb that may help you figure out if an achromat would be "good enough" for your preferences and tolerances:

First, if you plan on imaging with the scope, even if your eye doesn't see false color (defocused light of particular wavelengths haloing brighter targets) the more stringent electronic eye of the CCD chip likely will. Imagers typically rank freedom from false color high on their "wish list" for a refractor.

Second, in any refractor that is going to display visual false color, it will do so to a larger extent at higher rather than lower magnification. If you spend a lot of time at higher magnification (e.g., you spend a lot of time observing the planets and the Moon), you will likely see more false color than if you spend more time at lower magnification.

Third, for a given aperture, irrespective of design (apochromat or achromat) the propensity for false color drops as focal ratio grows. A 4" f/5 FPL-51 "apochromatic" triplet would be more likely to show false color than a 4" f/9 FPL-51 "apochromatic" triplet, all else being equal.

Fourth, false color is generally more apparent on brighter targets than dimmer. Planets, the Moon and second magnitude or brighter stars are much more likely to show false color than dimmer stars and DSOs.

Fifth, not everyone perceives color in the same way. I have a 105mm f/14.4 achromat that, to my eye, shows a very large amount of false color at magnifications above ~110x on brighter targets. Others who own this same model profess that it is visually color free. It's important to calibrate your own eyes' color sensitivity (by trying different refractors and experiencing false color first hand).

The right answer for me (but maybe not for you) is that quality is king. I would choose the refractor with the superior optical figure over the one with a more average quality figure, irrespective of whether the better quality optic was apochromatic or achromatic by design. So for me, apochromatic vs. achromatic matters only a little bit in the grand scheme of things.

Regards,

Jim

#12 clintwhitman

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Posted 08 May 2012 - 10:15 AM

Very well put Jim,
It seems my eyes like to see color problems more than most. The other thing I have seen over the years. Objectives that are not collimated correctly and focusers that are not square can produce false color issues, especially in large diameter refractors of either type.
Clint

#13 BigC

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Posted 08 May 2012 - 10:31 AM

In my casual experiments this past week with common filters from Celestron and Zhumell whilst viewing with a Meade 102 f7.7 achromat ,I found the darker filters regardless of color seemed to sharpen the Moon a bit compared with paler filters .I touched up focus every time the EP and filter was swapped.Sans filters i see a bright yellow or purple fringe at the edge of the Moon at higher powers although don't notice false colors on the face.The Moon may not be the best test subject since one can filter the image much more than the dimmer light from a typical DSO people want to image.

Hopefully soon I will find time to try imaging with my DSI Pro and color filters and achro.

#14 frolinmod

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Posted 08 May 2012 - 11:10 AM

When using an achromat,

The problem if you're using a B&W camera without color filters is that you're recording all colors at once, each color will focus differently and you'll get blobs for stars.

If you're using color filters, then each one will have a different focus position and need to be focused differently than the others. You then combine them and everything is in focus. Well, not quite! Each of the color filters does have some overlap with the others, so each one will still catch some unfocused light. And then there is the luminance filter which is (almost!) the same as no filter at all above. Once again, blobs for stars.

When using an APO, all the colors focus closer together (or very close) and you get tighter stars. That's why you want an APO for imaging.

#15 aa6ww

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Posted 08 May 2012 - 03:22 PM

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? :o


I've got plenty of refractors for a variety of difference uses and moods and sky conditions. My overall favorite however is my Celestron Omni 1500 F/5. Favorite in that I find myself wanting to take it out more and more, more so than even my largest Refractor or SCT's, even on dark nights where a larger scope would give me more deep space performance. The Scope is about the same length as my TSA-102, and easily mounts on my GP-DX, so despite it being so short, it really packs a big wallop on performance. These latest generations out of China have no spherical aberration at all, so star patterns are tac sharp right out to the edges, even with the latest generation of 100 deg field of view Ethos and ES eyepieces, and my 41 Pan.
It packs considerably more punch than my TOA-130 for most what I view, and does a nice job even on bright objects like Venus and Jupiter with an MV filter as well as splits stars well. Globulars and open clusters are beautiful, especially under dark clear sky's, and even a friend I observe with, with his NP-127 is impressed how how sharp stars are throughout the entire field of view.
There are times when I want more aperture, or more planetary performance, but I have those options also, with different scopes,. For crisp planetary view, my TOA naturally wallops the Achro Doublet, on just about every deep space object I can think of, aperture still rules and the 6" Achro is my preferred scope to bring out. Its also excellent with my SM-90 Cornoado Filter with my BF-15.
Pound for pound my 6" F/5 beats out my other scopes as my favorite scope this season.

...Ralph

#16 Fomalhaut

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Posted 09 May 2012 - 04:36 AM

We tested a 150/750 Skywatcher Achro under 6.5m skies against my fluorite-triplet (below) and the Skywatcher was not only (brutally) outdone on Jupiter as target, but interestingly also (distinctably) on M13. Yes, the globular seemed twice as bright in the Skywatcher, especially when both instruments being slightly defocused, but in focus and at 70x - 90x the stars where so much less pinpoint in the achro than in the Apo that the latter one showed better definition (=> more perceptible single stars) also on this low-contrast-object.

By the way that's a thing many observers seem not to be aware of: GCs behave as low-contrast objects when it comes down to discern feeble stars at visual threshold within the contrast-lowering haze generated by a GC's many other stars below visual threshold...

The big relative advantage of an apo versus an achro: virtually each photon goes where it belongs to, thus not haunting the image by contrast-lowering haze.

IMO, a decent short focus-refractor is quite acceptable at low magnifications (exit-pupil ~4mm and above), but when it comes down to resolution on low-contrast detail, even a 30% smaller apo will best it under critical scrutiny, and not just on planets, but also on certain DSOs.

Chris

#17 Starhawk

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Posted 09 May 2012 - 08:40 AM

Yes, it matters. 130 mm with f/6.3 and no color with a sharp image is a planetary thru DSO observing, one shot color imaging, all in one dream scope. But that requires APO prescriptions.

-Rich

#18 oldmapman

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Posted 09 May 2012 - 08:48 AM

YES

#19 skyjim

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Posted 09 May 2012 - 03:53 PM

THats the wrong achro to test against, that skywatcher is like a color machine, somethiong like a good C6R or the AT152/CT152 would be a better sample of an achro to do that test with, these scope are way better than the Skywatcher 150 F5.
Jim

#20 ken hubal

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Posted 09 May 2012 - 04:17 PM

Having owned a large number of apo's and achros, a well corrected long focus achro is well suited for serious visual planetary observing. For visual work, an apo is overkill.

#21 coutleef

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Posted 09 May 2012 - 04:26 PM

Having owned a large number of apo's and achros, a well corrected long focus achro is well suited for serious visual planetary observing. For visual work, an apo is overkill.


i like using a fast apo for visual as it allows me to have large FOV and wide views a long achro could not give as well as nice high power views.

#22 gnowellsct

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Posted 09 May 2012 - 04:30 PM

Incidentally the choice is not either-or. The intermediate state between apo and achro is ED doublet. These exist in various formulations, including the FS tak series (FS102, FS128, FS152).

I definitely think that the color correction of an ED doublet is worth the price. In fact, am thinking about getting ED binoculars, but sometimes I just wait for the feeling to go away.

Greg N

#23 Refractor6

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Posted 09 May 2012 - 04:36 PM

Having owned a large number of apo's and achros, a well corrected long focus achro is well suited for serious visual planetary observing. For visual work, an apo is overkill.


i like using a fast apo for visual as it allows me to have large FOV and wide views a long achro could not give as well as nice high power views.



Bingo!...we have a winner!

Due to light pollution conditions around my place having a 120 f/7.5 apo for viewing the "brighter" targets at high powers at night or for going down to low powers for daytime use on a shorter tube easy to view through rock solid package has been a real pleasure.

#24 skyjim

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Posted 09 May 2012 - 05:04 PM

Agree a 6" F12 achro like the D&G would be real nice but I jjust dont have the mount for that scope as of yet, I went threw a few F8 and under achros to finally find one that had what I would consider decent optics, the current scope seems to hit it and its an F5.9, slighly better correction than the CR150 I had back 10 years ago and the CA is way better controlled. I still want the 6" F12 but maybe someday but have been more than pleased with the AT152, even on lunar the amount of CA on the limb is not all that bad, craters and mountain shadows are black and white, no blueish haylows and thats without any filters in line plus saturn has looked real well. It takes a slight back seat to my 7" mac but not by allot and on deep sky wide feilds its stunning. I have had a few apo's and ED doublets up to 5" but the 6" class is just a different veiw IMO.
Jim

#25 ken hubal

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Posted 09 May 2012 - 05:56 PM

I've found that deep sky objects appear with better contrast in a long achro than in the much overhyped apo! The ONLY reason for an apo is to help keep the tube length manageable while maintaining decent color correction. Depth of focus which is important for close binary and planetary observation is sorely lacking in the fast apos. :grin:






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