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Achromats & Apochromats - Cutting Past The Hype

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#1 rsimpkins

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Posted 24 April 2014 - 02:43 PM

I was at a star party recently and I got to look through two refractors made by Explore Scientific. One was a ES127CF 5" f/7.5 ED triplet refractor. The other was a AR127 5" f/6.5 achromatic doublet refractor. Both were pointed at Jupiter in semi-*BLEEP* seeing that was keeping both scopes at lower powers. In the doublet I noticed a deep violet color fringing effect dancing around Jupiter that, quite frankly, was rather annoying (at least to me). The triplet was rock solid and looked great. Really, really great.

I've read that acromats are prone to this violet color effect, something that apochromats apparently fix. What's the actual non-BS take on this? If one wants to avoid the violet blues, do they have to step up to a more expensive triplet? Or is it possible to get no-color views through the "right" achromat in the "right" configuration?

I know the answer is probably really complex. I've read a few articles on this subject that break out all these complex graphs and complicated scientific explanations. I'm searching for a simpler "usually right" answer. If someone ponied up the extra $$$$ to get an apochromat, do they get views that generally avoid the violet effect?

It would really be lame to spend $2500+ on an apochromat only to find out that the purple star eater is still there. Might as well buy a much cheaper achromat and learn to live with it. Thoughts?

#2 GOLGO13

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Posted 24 April 2014 - 02:53 PM

You'd need a very long 5 inch achromat to avoid chromatic abberation.

Even the best achromats cannot eliminate all the CA. And it gets worse the larger the scope is. A 60mm achromat may be free of color if around F10+ ... a 5 inch one would need to be super long. There are charts for that.

Still, on other objects like DSOs, it's not as important visually. So that scope pointed on M13 may be just fine.

#3 Abhat

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Posted 24 April 2014 - 03:01 PM

There is no doubt APOs reduce fringing dramatically or might also eliminate it. Also slow achromats (F/8 and beyond) reduce fringing. I have heard that F/11 achromats virtually eliminate fringing and 4" versions are available for about $500. Using fringe killer or light yellow filter, the fringing can be brought under control. Whether that reduction is acceptable or not is up to individual and his or her taste for tolerance of such kind of fringing.

#4 Mark9473

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Posted 24 April 2014 - 03:08 PM

You'd need a very long 5 inch achromat to avoid chromatic abberation.

I wouldn't consider anything shorter than f/20 in that size. I had a 90 mm Vixen achromat that was f/14.4 and on Jupiter the purple haze still bothered me.


on other objects like DSOs, it's not as important visually. So that scope pointed on M13 may be just fine.

Yes, but... ;)
The apo will put more light into the airy disk and give tighter stars. The net effect on something like M13 is that a smaller apo will go as deep as a larger achro.

When I sold my 90 mm achromat I bought an 80 mm apo. I was a bit nervous about giving up light grasp and resolution. My first views on Jupiter and M13 immediately made it clear that I had in fact upgraded and not just downsized. Both these targets looked at least as good, if not better.

#5 jrcrilly

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Posted 24 April 2014 - 03:11 PM

Two rules of thumb for achromats. To bring the violet halos down to a level that one can ignore if sufficiently motivated - an F ratio about three times the aperture in inches or use very low magnifications (10X per inch or so). To bring the halos down to where they won't be noticed at reasonable magnifications - an F ratio of five times the aperture in inches. You can't say what F ratio will work unless you know the aperture.

ED doublets using FPL53 can do a pretty good job at an F ratio around two times the aperture in inches.

#6 EJN

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Posted 24 April 2014 - 03:12 PM

By the Sidgwick criterion a 5" achromat would need to be at least f/15. By the
Conrady criterion at least f/25.

#7 obin robinson

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Posted 24 April 2014 - 03:15 PM

It would really be lame to spend $2500+ on an apochromat only to find out that the purple star eater is still there. Might as well buy a much cheaper achromat and learn to live with it. Thoughts?


Try a minus violet filter. They work wonders. I was blown away by how much of a difference they make. I use one for imaging on my ST-80 and it really tames the violet down to apo-like levels. White stars look more white rather than a sharp blue/violet.

obin :)

#8 spencerj

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Posted 24 April 2014 - 03:24 PM

Try a minus violet filter. They work wonders. I was blown away by how much of a difference they make. I use one for imaging on my ST-80 and it really tames the violet down to apo-like levels. White stars look more white rather than a sharp blue/violet.

obin :)


The issue is that the purple fringe in the achro is unfocused light. Sure you can block that light with a filter so that you do not see it, but it is still unfocused light that is not making it to the final image. More focused light = a better final image. TANSTAFL

#9 REC

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Posted 24 April 2014 - 03:28 PM

Hmmm, I was just doing that last night. I had out my 80ED f7.5 600mm and my 102 f9.8, 1000mm scope for some general viewing as it was a nice night. On Jupiter with both scopes at 100x they where pretty close. A little CA for sure, but not really bad vs. the 80ED with the FPL53 glass element. I could see three belts pretty well in both. With a Baader moon and skyglow filter in the 102 achro, it was not bad.

Over at M35 star cluster, both scopes showed nice sharp stars and the CA was not much of an issue. Same thing on M42 and the larger aperture did show it a little better.

No real big comparisons, as I said, just out for some casual viewing. I use both of these scopes as my Grab n Go when I don't want to set up my larger scopes.

#10 Abhat

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Posted 24 April 2014 - 03:36 PM

The issue is that the purple fringe in the achro is unfocused light. Sure you can block that light with a filter so that you do not see it, but it is still unfocused light that is not making it to the final image. More focused light = a better final image. TANSTAFL


Does it mean that a 5 inch Achromat with Semi APO filter will work like a 4" APO after assuming 20% loss of light blocked by the filter? Or the image quality would still be bad as that spectrum of light is lost?

5" Achro is substantially cheaper than 4" APO.

#11 EJN

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Posted 24 April 2014 - 03:47 PM

It would really be lame to spend $2500+ on an apochromat only to find out that the purple star eater is still there. Might as well buy a much cheaper achromat and learn to live with it. Thoughts?


My thought is just get a Newtonian, you can get much more aperture for
the money and zero color. If you get a premium mirror like Zambuto it
will eat a smaller apo.

#12 StarStuff1

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Posted 24 April 2014 - 03:53 PM

Years ago I had a very nice 4.5-in f/12.5 refractor with a Fraunhoffer (sp?) achro objective. I was very happy with it until one night a guy set up next to me at a star party with a "small" 102mm f/9 scope. It blew me away with the contrast and color purity on Saturn. A few months later I had a copy of that same APO, a Celestron C/102f (fluorite). The achro was sold.

APOs cost less today that in the past. My C102/f tube alone cost nearly $2500 new. I paid about $1550 used. Today a good APO in that size can be had for less than that.

Still, achros can be great values today. As mentioned, filters can help if color fringing is objectionable. They can be stopped down for really bright objects such as the Moon and bright planets. What would Galileo have given for a modern 4-in f/10 achro? :cool:

#13 RussL

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Posted 24 April 2014 - 03:55 PM

I get better images in my 80ED doublet (f7.5) than in my 120 achro (f5) at the same mag with the same eyepiece. CA is much more reduced. That's on bright objects. On dim object either scope performs well.

#14 David Pavlich

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Posted 24 April 2014 - 04:22 PM

I have a 4" Stellarvue f11 achromat and with a Baader fringe killer, Jupiter looks very nice!

David

#15 Jon Isaacs

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Posted 24 April 2014 - 04:23 PM

The issue is that the purple fringe in the achro is unfocused light. Sure you can block that light with a filter so that you do not see it, but it is still unfocused light that is not making it to the final image. More focused light = a better final image. TANSTAFL


Does it mean that a 5 inch Achromat with Semi APO filter will work like a 4" APO after assuming 20% loss of light blocked by the filter? Or the image quality would still be bad as that spectrum of light is lost?

5" Achro is substantially cheaper than 4" APO.


The short answer to your question is that no, a filter will not magically turn a 5 inch achromat into an 4 inch apochromat. The issue is defocused light, that the different colors do not come to focus at the same place. A filter can help make the image more aesthetically pleasing by removing some of the defocused light but in my experience the views are not comparable to crisp, clean views that good quality ED doublet or triplet provides.

In general, one can say that the slower the focal ratio, the better the color correction and the smaller the aperture, the better color correction. A small long focal ratio achromat, say a 60mm F/13, there is very little false color. On the other hand, as the original poster discovered, a 127mm F/6.5 achromat shows a considerable amount of false color.

One rule of thumb people use is called the Chromatic Ratio. The CR is equal to the focal length divided by the aperture as measured in inches. As John Crilly mentioned, a CR of three is viable at higher magnifications, a CR of 5 is almost apo like.

The two scopes mentioned above, the 60mm F/13 and the 127mm F/6.5 have CR's of 5.6 and 1.3. As one would expect, the 60mm is almost color free, the 127mm is quite colorful.

I have several refractors, two that make an interesting comparison are TeleVue NP-101, it's a 4 inch F/5.4 apo that has 4 elements and is corrected for field curvature as false color, coma and astigmatism. New, one of these will set you back close to $4000. The other is an Orion 100mm F/6 achromat, for what it is, for what it cost new, (about $200), it's a good, solid telescope.

I enjoy them both and both provide decent quality, enjoyable views. At low magnifications, the better quality optics and flat field of view mean the NP-101 does provide sharper, cleaner views but the biggest difference is at high magnifications. The false color of the 4 inch achromat with it's CR of 1.6 limits it ability to resolve close double stars as well as show crisp, contrasty views of the planets. In my experience, filters make the image somewhat more aesthetically pleasing but do not greatly improve the resolution or the contrast.

There is a lot to know about the color correction in refractors, the different ED glasses, the focal ratios and apertures and how it all plays out. If that 100mm refractor was F/15, things would have been different, much less false color but it would have been about 60 inches long..

The bottom line is that Rsimpkins had it right, he saw what he saw and that is what one would expect to see.

Jon Isaacs

#16 Mr. Mike

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Posted 24 April 2014 - 05:07 PM

What about high-end doublets like the Takahashi Flourite model? Is that a color-free scope? It had better be for the price!

My SV110 does show some violet fringing on the planets but tight focus calms it way down and for whatever reason I get nothing color-wise on the moon. As for DSOs like star clusters & galaxies? I see no issues with a good quality doublet.

Realistically, it costs a TON of money for what is essentially violet color reduction on planets & perhaps slightly better performance on DSOs but Id question how much. At our next star party, Im gonna try and look through a high end APO(of similar aperture & eyepiece) at some bright globular and see how much better it really is than my SV110mm.

I think the rule of thumb for this hobby is: As you spend more, the differences between the quality of the products and views gets smaller and smaller. Like I said - you pay dearly for that last small percentage of color correction & small contrast bump. Lets realize though that a cheap, faster achromat is going to be pretty *BLEEP*.

Either way - I WILL have a Takahashi triplet someday. :D

#17 Widespread

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Posted 24 April 2014 - 05:12 PM

I don't know about 5 inch versus 4 inch, but my 4 inch F/9.8 achro can show just a bit more than my 90mm F/7 apo on most targets. Yes, the color is less pristine than the apo, but there is a tiny bit more detail, and the lesser field curvature and long FL make for pleasing views and easy focusing.

That's a $60 scope beating a $1300 scope (barely). But when it comes to mechanicals and the joy of usage, the SV wins hands down. On a manual altaz, it's very easy to reach the slow motion controls with the apo, and it's small and light enough to ride easily on my little SLT GoTo mount.

#18 Dave Lee

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Posted 24 April 2014 - 05:22 PM

The issue is that the purple fringe in the achro is unfocused light. Sure you can block that light with a filter so that you do not see it, but it is still unfocused light that is not making it to the final image. More focused light = a better final image. TANSTAFL


Does it mean that a 5 inch Achromat with Semi APO filter will work like a 4" APO after assuming 20% loss of light blocked by the filter? Or the image quality would still be bad as that spectrum of light is lost?

5" Achro is substantially cheaper than 4" APO.


Two things going on here (although they are really the same thing).

1) The color fringing that was reported and found objectionable. You can filter this out (for the most part).

2) All those 'fringey colors' that created a halo are 'visual information' that has been lost, even when you filter out the fringing. This information is unfocused and unviewable. Or put another way if Jupiter's Great Red Spot was the Great Purple Spot it would be invisible (pretty much) through a achromat with a typical Semi-Apo filter.

#2 is where the APO's of the world generate views that are inherently better than achromats at manageable focal lengths. But this does not mean that the achromats of the world can't generate very good viewing experiences.

dave

#19 lamplight

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Posted 24 April 2014 - 11:01 PM

when i was looking to upgrade my achromat refractor i basically just read lots and lots of user reviews keeping my eyes peeled for color issues in the reviews.. i asked the same question here as lots of people do, and i frankly didn't understand much of the really technical discussion that this thread doesn't have :)

in my case i came across some reviews of a certain series of scopes made by orion (and another manufacturer i think, but basically the same scope/glass) that was getting great reviews as an extremely well corrected doublet ED with the good glass (FPL53). based on price thats what i went with.. as far as CA it was a HUGE upgrade.. very little color on the brighter objects.. i haven't used it in a while so i can't give a good description as others here have on specific objects, which should be helpful as thats exactly what it sounds like you were looking for.... so.. "cutting past the hype" is pretty simple: how do YOU enjoy the views? if you can get to a local astro club chances are there are always some refractors to look through. from what i can see there are some good doublets out there that are VERY close to triplet APO's as far as color goes. i understand "very" might be subjective depending on the visual acuity of the viewer.. anyhow, mines pretty fast too (120mm f/7.5), so its quite surprising to me. like i said, i have no idea how its done, but i can see a HUGE difference over your average achro :lol:

i do understand Jon's and others' descriptions of how the different wavelengths of light come to focus at different points in a refractor.. as i recall there are three (?) colors that come to focus at different points, so if the purple is one of them, would that equate to 1/3 (or whatever the equivalent is) loss of light gathering if using a filter to remove it? :shocked:

#20 lamplight

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Posted 24 April 2014 - 11:12 PM

heres a monster thread to confuse the &%$# out of you on doublet vs triplet APO's. i don't mean to complicate the issue especially as i just mentioned how confusing it all is even to me.. but i wanted to point out one thing that has stuck out to me in my research: there are some smoking good doublets out there. they're almost as expensive as some triplets for good ones it seems. You can thank me later for the added confusion :roflmao:

#21 gunfighter48

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Posted 25 April 2014 - 01:11 AM

A tale of two refractors.

My C102GT on Jupiter showed purple fringe around the planet. There was also purple in the bands and polar caps of Jupiter. On the moon there was slight purple and sometimes greenish/yellow fringe around the moon depending on the eyepiece used.

Using my ES AR152 on Jupiter there was slight purple fringe around Jupiter but NO color on the planet. Using it on the moon there was less purple fringing than the C102GT. There was also some greenish/yellow fringing depending on the eyepiece used. The fringing is better controlled in the AR152 than in the Celestron C102GT.

According to all that I've read and been told, the fringing should have been worse in the AR152. But the opposite is true. Theory is fine but often runs into trouble in the real world. Some of us are not bothered or just don't see the fringing that other folks see. The fringing in both of my refractors is not a deal breaker for me. The C102 shows some spectacular views of clusters! I haven't had the chance to look at clusters in the AR152 yet.

#22 jgraham

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Posted 25 April 2014 - 06:29 AM

I own several refractors from my little 40mm Copyscope all the way up to my biggo LXD75 AR-6 and the degree to which I notice the purple fringing varies quite a bit and not just between scopes, but between objects and sky conditions. Most of the time I don't notice it at all or that it is so slight it doesn't bother me. If it does, I'll install a Baader semi-Apo filter. No, it doesn't turn an achro into a Apo, but it does a nice job of reducing the purple fringinging and also serves as an effective light pollution filter. A few nights ago we had a rare evening of exceptional seeing and I just happened to have my AR-6 out for observing double stars. I was not using my semi-Apo filter and I has my best view of Mars ever and there wasn't even a hint of purple fringing even at 290x. When I take pictures with my AR-6 the fringing is always there and the filter is a big help. Visually, it comes'n goes.

#23 Dave Lee

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Posted 25 April 2014 - 06:35 AM


SNIP

as i recall there are three (?) colors that come to focus at different points, so if the purple is one of them, would that equate to 1/3 (or whatever the equivalent is) loss of light gathering if using a filter to remove it? :shocked:


In theory it could. But in practice most likely not.

1) Not all 'targets are the same' in terms of their color content. This is particularly true for targets such as emission nebula.

2) The eye is far less sensitive to deep red and violet (the unfocused colors in most achromats these days) vs. the red to blue-green range that is typically focused.

So in practice, no.

dave

#24 Tony Flanders

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Posted 25 April 2014 - 06:38 AM


Try a minus violet filter. They work wonders. I was blown away by how much of a difference they make. I use one for imaging on my ST-80 and it really tames the violet down to apo-like levels. White stars look more white rather than a sharp blue/violet.

obin :)


The issue is that the purple fringe in the achro is unfocused light. Sure you can block that light with a filter so that you do not see it, but it is still unfocused light that is not making it to the final image. More focused light = a better final image. TANSTAFL


I don't entirely agree. The violet light is really not good for much -- the image is simply better without it.

The real problem isn't the violet -- the most obvious manifestation of chromatic aberration -- but all the other colors that do continue to make it through a minus-V filter. They continue to focus at different points, blurring the image.

If you don't mind viewing the Moon in false color, an aggressive color filter will reduce the blur even more. But as everyone says, you still can't turn a 5-inch f/6.5 achromat into a 5-inch f/6.5 apochromat -- or a 5-inch f/20 achromat -- using filters.

#25 Jon Isaacs

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Posted 25 April 2014 - 06:47 AM

as i recall there are three (?) colors that come to focus at different points, so if the purple is one of them, would that equate to 1/3 (or whatever the equivalent is) loss of light gathering if using a filter to remove it?



The purple is not a primary color. Purple is a combination of the defocused red and violet light, the ends of the spectrum.

While one might think of light as being composed of separate colors, it is in fact a continuous spectrum so if one eliminates part of the spectrum that is most bothersome there still remains unfocused light.

Using single color filters is possible but one is potentially eliminating details.

Newtonians and reflectors are free of chromatic aberration but has issues of their own.

Jon






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