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gunfighter48
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: lamplight]
      #6487020 - 04/25/14 02:11 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.


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jgraham
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: gunfighter48]
      #6487216 - 04/25/14 07:29 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.

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Dave Lee
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: lamplight]
      #6487224 - 04/25/14 07:35 AM

Quote:



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?




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


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Tony Flanders
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: spencerj]
      #6487226 - 04/25/14 07:38 AM

Quote:

Quote:


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.


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Jon Isaacs
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: lamplight]
      #6487233 - 04/25/14 07:47 AM

Quote:

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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Mark MacKenzie
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Jon Isaacs]
      #6487276 - 04/25/14 08:19 AM

So here's my beginner first post....
What if I stop down that achromat? My 4" f9 shows some violet on Jupiter. What if I had a stop in front of the objective to make it a 3" f12? Or a 2.4" f15?
Bad idea or not? I haven't tried it yet.
Mark


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Vondragonnoggin
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Jon Isaacs]
      #6487278 - 04/25/14 08:20 AM

I don't know why these conversations don't go hand in hand, but there is always someone asking about which filters to buy for astronomy use and usual answers consist of UHC, OIII, H-Beta, or the like and everyone gets happy about the thought of getting a little MORE detail out of nebula viewing.

Mention achromat and filtering it out like with Longpass, or semi-apo, or minus V and suddenly people are worried and discussing how much LESS detail is seen.

Really. Make up your minds about it because you confuse people by emphasizing the lost information and exaggerating its effects. If you want the most information, it's simple - buy an apochromat or reflector.

If you want to get by on a budget with a doublet Fraunhofer design, then just put a filter on it if you find chromatic aberration objectionable or buy one with long enough focal length that the colors are all focused, or just view with it as is if you think it's alright.

There is generally too much exaggeration about detrimental effects when talking about achromats, yet consistently heavy filtering is recommended to get the most detail out of some objects. Even those with reflectors tend to like using contrast enhancers like the Baader Moon and Skyglow and will talk in a thread about it being a contrast enhancer rather than the typical detrimental loss of color information as described with an achromat.

In both instances of filtering, it is removing information. Just as heavy narrowband nebula filters remove a ton of visual spectrum information

People are okey dokey with one application and always whining about the other and it makes little sense to me.


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REC
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487300 - 04/25/14 08:39 AM

Just an FYI to add to this post. I used a Baader Moon & Skyglow filter on my C102gt and it really cleans up Jupiter and the moon. Also I use a nebula filter on it and it really provides a nice view of M42.

Also, the Magazine, Astronomy Technology Today just did a review of the 4 popular Baader Anti-Fringing filters and the one they picked wit the most pleasant view on Achro,s was the Contrast-Booster.


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Vondragonnoggin
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487312 - 04/25/14 08:44 AM

I really think to stop all the hand wringing, fretting , and general terror of lost information that these debates evoke, that people should educate themselves in the visual spectrum and typical object spectrum emission along with more detrimental spectrum emission such as sodium, mercury, and the like that we generally filter out if around sources that interfere with object lines of emission. Know what is being cut and what is not, what is irrelevant to the eyeball (human visual spectrum sensitivity) and what is not. What can be isolated and even in some cases amplified to get greater detail or to lose information purposefully with spectrum lines that are heavily overpowering less strong spectrum lines with good information.

A class in filtering on its own by experimentation is expensive to start with , but can be very useful and filters can be sold later to get some money back, but generally if one wants to research it, there is a ton of information about human visible spectrum and the varying degrees even that physiology of the sometimes unique nature can make people sensitive to.


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lamplight
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487341 - 04/25/14 09:03 AM

Isn't the goal different though? Gaining some contrast by removing some very specific areas of the spectrum vs achro concerns of gaining sharper focus , a brighter image, AND more potential contrast? The affected targets being totally different as well, it's kind of different issues yet from one perspective the same.. Filtering out some visible light. It seems the amount filtered out between the two issues is surely different amounts depending on target. Probably people get concerned about achro filtering light loss because we're talking about relatively bright/close objects and cutting out larger chunks of visible light that a better design wouldn't have to sacrifice. With the right nebulae and the right associated filter, the light loss is usually 100% acceptable and helpful, in the ranges affected. Right? That's just my interpretation of the facts subject To change due to information gained.

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David PavlichAdministrator
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Mark MacKenzie]
      #6487351 - 04/25/14 09:10 AM

Quote:

So here's my beginner first post....
What if I stop down that achromat? My 4" f9 shows some violet on Jupiter. What if I had a stop in front of the objective to make it a 3" f12? Or a 2.4" f15?
Bad idea or not? I haven't tried it yet.
Mark




Welcome to Cloudy Nights! Interesting question!

David


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spencerj
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487361 - 04/25/14 09:14 AM

Quote:


I don't know why these conversations don't go hand in hand, but there is always someone asking about which filters to buy for astronomy use and usual answers consist of UHC, OIII, H-Beta, or the like and everyone gets happy about the thought of getting a little MORE detail out of nebula viewing.

Mention achromat and filtering it out like with Longpass, or semi-apo, or minus V and suddenly people are worried and discussing how much LESS detail is seen.

Really. Make up your minds about it because you confuse people by emphasizing the lost information and exaggerating its effects. If you want the most information, it's simple - buy an apochromat or reflector.





Very few things in this existence are black and white. To pass the time we talk about the various shades of gray. If you talk about anything long enough, the subtle differences become exaggerated.

Acrhomat refractors can be spectacular visual instruments. They can also be dogs. The quality of the telescope design and the optics matter. Filtering has its place. Using a specialty filter to tease out fine detail in an object is a reasonable thing to do--even though you are blocking out other information.

If every topic had exactly one right answer, then the forums would have a lot fewer posts.


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jrcrillyAdministrator
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Mark MacKenzie]
      #6487390 - 04/25/14 09:27 AM

Quote:

So here's my beginner first post....
What if I stop down that achromat? My 4" f9 shows some violet on Jupiter. What if I had a stop in front of the objective to make it a 3" f12? Or a 2.4" f15?
Bad idea or not? I haven't tried it yet.
Mark




Sure. That's a time-honored method of taming false color in fast achromats. You lose liught gathering and resolution, but you gain color control.


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Jon_Doh
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: David Pavlich]
      #6487396 - 04/25/14 09:32 AM

Not all achros are equal from my experience. Every one of those Celestron 102's that I've looked through have had a huge amount of CA. But the ES achros vary from scope to scope. On Jupiter the AR 152 that I viewed it through showed a thin dark band of purple. You really had to be looking for it to see it because it blended in with the dark background of the sky. The planet showed no CA. Same with the moon. Bright stars and M13 were fine. To me the stars focused tightly and showed nice color.

So, I would look through an achro like this one and think all the negative things about acrhos are over blown. Then I would look through one of those Celestron achros and the CA was so bad it almost blinded me.


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howard929
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: spencerj]
      #6487398 - 04/25/14 09:33 AM

I feel that David K. says it best in his tag line that goes something like: If your not having fun with this hobby, your brain is broken and it's causing you to do it all wrong... or words to that effect.

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Vondragonnoggin
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: lamplight]
      #6487407 - 04/25/14 09:38 AM

Quote:

Isn't the goal different though? Gaining some contrast by removing some very specific areas of the spectrum vs achro concerns of gaining sharper focus , a brighter image, AND more potential contrast? The affected targets being totally different as well, it's kind of different issues yet from one perspective the same.. Filtering out some visible light. It seems the amount filtered out between the two issues is surely different amounts depending on target. Probably people get concerned about achro filtering light loss because we're talking about relatively bright/close objects and cutting out larger chunks of visible light that a better design wouldn't have to sacrifice. With the right nebulae and the right associated filter, the light loss is usually 100% acceptable and helpful, in the ranges affected. Right? That's just my interpretation of the facts subject To change due to information gained.




That's because the focus usually gets immediately steered to planetary observations (brightest objects). As I said, examine the filter curves of the contrast boosting Moon and Skyglow filter realizing its a popular filter even in reflectors for teasing out details in planetary observations. It also has some heavy cuts in various ares of the visible and IR spectrum.

Pound for pound and inch for inch, if getting the most information is your goal, the reflector remains top dog in larger apertures. Central obstructions cause slight loss in contrast so in smaller sizes, budget aside, the apochromat is probably the best bet for maximum visual information with less contrast loss. Bigger sizes tend to be less easily mounted, handled, afforded by average consumer, etc. so reflectors make sense.

The achromat offers the general populace, access to unobstructed views but with caveat that it doesn't concentrate all colors to focus equally.

My achromat is not a planetary instrument personally. I like it for DSO's and less bright targets at lower powers. I like my reflector for planetary, but with the Baader Moon and Skyglow filter enhancing contrast (and losing some visual information too).

My point was really that someone new to getting all this information, might worry unnecessarily about loss of info and think they either must get a reflector or an apochromat to get a decent view. The best idea is to just take a look if possible, through others scopes and try to get a sampling of reflector, achromat, and apochromat, rather than reading through a lot of subjective information on detrimental effects and causing worry as it gets built up to some bogeyman levels in threads like these.

Filtering is good in my opinion. In a lot of cases it can entirely enhance the viewing experience. One can save for highest quality views in a refractor or a reflector (such as a premium optic mirror), but during that period of saving, just get out and enjoy what you can use. The information can sometimes be subtle in what is lost at filtering but huge in the thread talking about it.

Good luck to all and enjoy the scopes you have and save for ones you want.



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Mark Costello
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487411 - 04/25/14 09:39 AM

I had my achro rig out last night for a little over an hour, spending 30 minutes apiece on two targets. The last one was M44. The session with it included a main sketch at 27X, then adding extra details at 49X, 69X, and 118X. The interesting thing is that with each hike in power, more stars came into view. The hike from 27X to 49X brought the most stars, about 15 extra stars, into view.

But it was the first object that gets closer to the discussion into the forum. The first object was Jupiter. I guess the "achro-is-dead-don't-waste-your-money-on-one-because-it's-good-only-for-stuff-like-M44" bunch would say that was a waste, but what the hey, the ES AR127 is my only scope. And it didn't look like a waste to me. The 5 minute view at 118X showed some hints of thin banding in both polar regions so up we went to 236X to do the sketching. That showed some interesting shadings and whorls in the two main tropical bands (the winds there must make our hurricane look like soft breezes) and yes, there appeared some whitish strips inside the polar caps: one in the south cap and a couple in the north cap. Again, some interesting wind patterns evidently are in there.

My experience with Jupiter and the moon is pretty much as gunfighter48's with his ES AR152. There is a deep and soft purple haze around Jupiter but the shadings on the disk are cream and brownish gray, and not so yellow. There is a thin purple halo over the rim of the moon's image but when I'm observing the lunar landscapes head-on, I just don't see the evidence of chromatic aberration.

If I was starting over again now, m-a-b-y-e I would have gotten a smaller ED refractor. But except for participating in threads like this (I occasionally do but not often), I don't think about it. After all, my 5" achro is here and doing everything I ask of it. I don't have the urge to replace it with an ED. Oh, I'm hoping to get a genuine apo some day to supplement my refractor, the kind that works with mirrors.

Edited by Mark Costello (04/25/14 09:40 AM)


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Dave Lee
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487440 - 04/25/14 09:55 AM

Quote:

I don't know why these conversations don't go hand in hand, but there is always someone asking about which filters to buy for astronomy use and usual answers consist of UHC, OIII, H-Beta, or the like and everyone gets happy about the thought of getting a little MORE detail out of nebula viewing.

Mention achromat and filtering it out like with Longpass, or semi-apo, or minus V and suddenly people are worried and discussing how much LESS detail is seen.

Really. Make up your minds about it because you confuse people by emphasizing the lost information and exaggerating its effects. If you want the most information, it's simple - buy an apochromat or reflector.

If you want to get by on a budget with a doublet Fraunhofer design, then just put a filter on it if you find chromatic aberration objectionable or buy one with long enough focal length that the colors are all focused, or just view with it as is if you think it's alright.

There is generally too much exaggeration about detrimental effects when talking about achromats, yet consistently heavy filtering is recommended to get the most detail out of some objects. Even those with reflectors tend to like using contrast enhancers like the Baader Moon and Skyglow and will talk in a thread about it being a contrast enhancer rather than the typical detrimental loss of color information as described with an achromat.

In both instances of filtering, it is removing information. Just as heavy narrowband nebula filters remove a ton of visual spectrum information

People are okey dokey with one application and always whining about the other and it makes little sense to me.




While I don't totally disagree with the quoted above, the 'longpass/Apo filter' application vs. UHC/OIII/etc applications really are quite different.

In one (UHC/OIII/etc) the 'information of interest' is very specifically in a couple of quite narrow bands. Excluding 'the rest of it' is for almost every application FOR THESE OBJECTS perceived as a good thing (note that I did say almost.

In the achromat case in general, the issue is WAY/WAY more broad. IMHO they are not equivalent things - not unrelated but really not the same. Sometimes the 'color free viewing' of whatever benefits from some kind of filtering and sometimes not. In my mind this is not the same as dealing with chromatic aberrations across the board.

But I would agree that the information available across the internet on achromat vs. APO (pick your APO definition now) issue is confusing and not always accurate.

dave


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Tony Flanders
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Mark MacKenzie]
      #6487472 - 04/25/14 10:13 AM

Quote:

So here's my beginner first post....
What if I stop down that achromat? My 4" f9 shows some violet on Jupiter. What if I had a stop in front of the objective to make it a 3" f12? Or a 2.4" f15?




Stopping down the objective will greatly reduce the false color -- and also greatly reduce the amount of detail you can see. It may be worth it if your goal is strictly aesthetic. It's definitely not worth it if your goal is to see as much planetary detail as possible.


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Tony Flanders
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6487492 - 04/25/14 10:25 AM

Quote:

I don't know why these conversations don't go hand in hand, but there is always someone asking about which filters to buy for astronomy use and usual answers consist of UHC, OIII, H-Beta, or the like and everyone gets happy about the thought of getting a little MORE detail out of nebula viewing.

Mention achromat and filtering it out like with Longpass, or semi-apo, or minus V and suddenly people are worried and discussing how much LESS detail is seen.

Really. Make up your minds about it because you confuse people by emphasizing the lost information and exaggerating its effects. If you want the most information, it's simple - buy an apochromat or reflector.




Sorry, but there's an excellent reason that the two discussions don't go hand-in-hand. It's because they have nothing -- zip zero -- in common.

Filtering to reduce chromatic aberration with a minus-V is a matter of compensating for an equipment defect. It does not reduce detail -- in fact, it increases it. However, it does not increase it as much as using an inherently color-free instrument in the first place.

Mind you, all of this is secondary to aperture. My 4-inch f/6 achromat shows hideous color fringes around planets, but it still shows a lot more planetary detail than a 60-mm APO. Likewise, the main reason to use reflectors isn't that they're color-free (though they are), but because they have much bigger apertures than refractors of similar cost.

But even when using reflectors, serious planetary observers often use color filters. This isn't to compensate for any defect in the instrument, but rather to eliminate light coming from the object itself. If you want to view the Great Red Spot, eliminating the blue light will increase the signal-to-noise ratio and improve the image. This is a case where less is more for a strictly scientific, information-theoretical reason.

Likewise with nebula filters. When you want to observe an emission nebula that glows primarily in the O III bands, eliminating other wavelengths improves the signal-to-noise ratio and allows you to see more detail.


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