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Chromatic Aberration and Strehl

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#1 Steve Allison

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Posted 20 February 2019 - 05:25 PM

Dumb question time.

 

Does the presence of chromatic aberration in a lens lower its Strehl rating.

 

Say you have achromat that has absolutely perfect spherical correction and the best color correction possible for its crown and flint design. Would it have a lower Strehl than an apo with the same level of spherical correction?

 

I read a post recently to the effect that an early Clark lens was so perfectly corrected for spherical aberration that it was beyond the 1/40th wave detection limit of the testing apparatus. No Strehl rating was given, however.

 

Anyone know?

 

Thanks.

 

Steve


Edited by Steve Allison, 20 February 2019 - 05:25 PM.

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#2 sg6

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Posted 20 February 2019 - 06:16 PM

Cannot answer the question but I would have assumed that the Strehl Ratio was defined at a single wavelength, and then CA would be not applicable.

 

You would get wave front errors but no chromatic aspect.

 

The Cook lens I would question the claims ultimately as in optics it is a bit like Heisenberg you cannot have everything. If the SA was measured at Green what was it at Red and at Blue. As the wavelengths would have different refractive indices then for the wavelengths the SA would be minimal at one but exist at the others. They just refract through the glass differently. So at some wavelengths SA has to exist.



#3 TOMDEY

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Posted 20 February 2019 - 06:19 PM

Actually, a very good point! And, of course, chromatic would/will/does functionally lower the Strehl. PS to all, including myself: The Strehl Ratio is the absolute (aka non-normalized) ratio of the central peak to what it would have been, in the entire absence of aberrations. [tacit in that computation is that the pupil is uniformly-illuminated, unobstructed circular]. So it kinda just plops out of the wavefront mapping (analytic or measured).

 

But, in actual practice/use we would/should be interested in the actual in-use ratio, relative to ideal... so things like chromatic, central obstruction, entire scope build, atmospherics... will lower the as-used ratio, typically by a lot!

 

Anyway, if it is that impact of chromatic that you are interested in, one would compute that, taking into account the spectral weightings across the used spectrum. So, the chromatics of the scope, filter, detector, (eye or camera) will appear in there. And, quite frankly, that is why you will rarely see such published.

 

Strehl is almost always just a single number that pops out of the wavefront analysis software; take it with a grain of salt!   Tom


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#4 Eddgie

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Posted 20 February 2019 - 07:42 PM

The refactor forum likes to demonize secondary obstructions, but totally ignores chormatic aberration and spherochromatism. 

 

 

In a refractor, there are two kinds of Strehl.  The first kind is manufactued Strehl. This is a test of the quality of manufacture.  If the scope has a zone or turned edge or surface roughness or whatever, this will show in the Strehl test which is done usually at one wavelength.

 

Note that when reflectors are tested, they give the Strehl ratio without the effects of the obstruction.  If effects of the secondary obstruction were included, the Strehl of a perfect reflector with a 30% obstruction would only be about .8.  But this figure does not tell how good the quality of the wavefront is, and this is what the bench test is used to describe, and Strehl is they way it is summarized.  

 

When refractors are tested, they only look at the curves/wavefront, and don't report the error caused by CA or spherochromatism. Typically the will report Strehl in different colors individually, but they do not compute the final polychromatic Strehl number in exactly the same way they omit the obstruction in a reflector.  The bench Strehl is really only looking at the quality of the manufacter and not the characteristics of the design. 

 

The kind of Strehl that is used to tell you how much light is lost to color defocus and spherochromatism is called Polychormatic Strehl. This is calculated by looking at the amount of light that is not in perfect focus for each different color.

 

Here is a great page that shows how different disigns perform on Polychromatic Strhel.  See table 12:

 

https://telescope-op...po_examples.htm

 

This page shows how different poly-Strehls would compare in contrast for different 6" and 4" scopes, and also shows how some Achromats would compare to different size obstructions. See figure 74:

 

https://telescope-op...romatic_psf.htm

 

 

(And it was a very good question.  Again, the refractor forum is very quick to rattle on about secondary obstruction, but totally ignores the loss of energy caused by spherochromatism and chromatic aberration.  Since very few manufacturers actually disclose polychromatic Strehl, most people don't really have a clue as to how much contrast is being lost in their refractors.)


Edited by Eddgie, 20 February 2019 - 07:50 PM.

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#5 Jim Waters

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Posted 20 February 2019 - 07:53 PM

Eddgie - Thanks for posting.  I learned something today. 


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#6 SeattleScott

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Posted 20 February 2019 - 08:25 PM

But remember as mentioned polychromatic Strehl is variable depending on circumstances. CA is not an issue for H alpha solar observing, narrow band nebulae observing, etc. And minus violet filters remove much of the CA increasing the contrast, at the expense of a dimmer image and a yellow cast.

The type of viewing makes a big difference. I don’t care what the polychromatic Strehl of my AR6 is, it still bests my 8” Newt on Double Cluster or Pleiades. And they have the same FOV so it isn’t an issue of just framing them better. Stars just look nicer at low to medium power in the big achro. But the Newt gets its revenge on high power lunar viewing.

Scott
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#7 TOMDEY

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Posted 20 February 2019 - 09:23 PM

Yes indeed to eddgie's wisdom up there. And, adding just a bit here... If you are doing visual or color camera... your affective will see the entire impact of chromatics and spherochromatism. On the other hand, if you are doing tricolor or other individual (e.g. emission line colorizations), with refocus for each... that will still suffer the spherochromatism, but be substantially-compensated for the traditional chromatics (longitudinal and delta mag ~lateral~).    Tom



#8 Eddgie

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Posted 20 February 2019 - 10:04 PM

But remember as mentioned polychromatic Strehl is variable depending on circumstances. 

 Well, polychromatic Strehl never varies for a given design. It is a characteristic of the design.  Now you can use techniques to minimize the effect of CA (not spherochromatism though) but while one can take three images with a badly corrected scope, the user of a reflector or high Polychromatic Strehl triplet or a reflector can do one image and get a great result. 

 

And if you don't care that the scope has poor Polychromatic Strehl, then that is absolutely wonderful for you.   Irrelevant to the OPs question though.  He wanted to know if chromatic aberration lowers the scopes Strehl, and to answer, it is important to differentiate between the quality to which the scope is made, and the actual design characteristics of these scope.

 

A 6" f/8 Achromat could bench test with a Strehl of .98, but it is only putting about 65% of the energy in the Airy Disk even if it is perfectly made.  That is what the OP I think was trying to find out, and hopefully the references I gave him will be useful to his pursuit of the realities of optical design. 


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#9 SeattleScott

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Posted 20 February 2019 - 10:09 PM

Absolutely agree, and to clarify, I don’t care that it has low polychromatic Strehl for low-mid power DSO viewing. Having an Apo instead would definitely be advantageous for planets or higher power viewing. I do have a 4” Apo for planets. So I do appreciate Apos and reflectors and recognize the limitations of the big achro. I certainly wouldn’t have it as an only scope.

Scott

#10 AtmosFearIC

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Posted 21 February 2019 - 03:51 AM

Correct me if I am wrong but you should theoretically be able to have an achromat with high strehl value for red, green and blue but a shocking polychromatic strehl. A smooth figure should be able to give a high monochromatic strehl at RGB but they'll be at different focal lengths giving a poor polychromatic strehl.



#11 Jon Isaacs

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Posted 21 February 2019 - 06:32 AM

But remember as mentioned polychromatic Strehl is variable depending on circumstances. CA is not an issue for H alpha solar observing, narrow band nebulae observing, etc. And minus violet filters remove much of the CA increasing the contrast, at the expense of a dimmer image and a yellow cast.

The type of viewing makes a big difference. I don’t care what the polychromatic Strehl of my AR6 is, it still bests my 8” Newt on Double Cluster or Pleiades. And they have the same FOV so it isn’t an issue of just framing them better. Stars just look nicer at low to medium power in the big achro. But the Newt gets its revenge on high power lunar viewing.

Scott

Scott:

 

For viewing the Pleiades and viewing the double cluster, not only is the polychromatic Strehl not so important, neither is the Strehl..  

 

As far as the 8 inch Newt, it's F/4, what eyepieces, which coma corrector are you using?  

 

The Strehl is a measure of optical quality, not of design quality.  As I understand it, it is the ratio between the actual energy in the central disk versus the theoretical energy in central disk for the given design.  

 

Jon



#12 starman876

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Posted 21 February 2019 - 08:32 AM

These are always wonderful educational discussions. 

 

Thanks



#13 Asbytec

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Posted 21 February 2019 - 09:06 AM

As I understand it, it is the ratio between the actual energy in the central disk versus the theoretical energy in central disk for the given design.  

 

Jon

My understanding is similar, except that the theoretical energy in the central disc is for a perfect aperture given the unavoidable diffraction that comes standard with every aperture. In other words, due to the diffraction as you know already, only 83.8% of the light is theoretically possible in the central Airy disc - as opposed to the visible spurious disc. That 84% figure after normal diffraction loss is normalized to Strehl = 1, then aberrated performance is compared to it as a ratio with a value less than 1. It was mentioned above, and I agree, it's the final peak intensity of the central disc that is what matters. I am not telling you anything you don't already know, it's just worth repeating because that is where the rubber meets the sky. 

 

I am still not sure if Strehl accounts for the diffraction of an obstruction, or not...I think it's only for clear aperture. Some argue otherwise. But, if so, when comparing a obstructed to unobstructed, Strehl looses it's beauty as a measure of optical quality only...post diffraction (so goes the argument for the obstruction). To my way of thinking, we can add obstruction effects later as merely being obstructed has no (sic) affect on optical quality and only on the final peak intensity. Otherwise, as Eddgie points out, all obstructed SCTs will have a Strehl of 1 with only 0.8 * 0.84 ~ 0.67 light in it's central peak rather than the standard 0.84...which is the maximum possible at Strehl 1. The "optical quality" of a premium obstructed scope is at a disadvantage right out of the gate which is probably not the case...if we want to compare it with a 6" APO, that is. 

 

Anywhoo...tonight was a wonderful example of how the atmosphere rules all who observe under it. Seeing went form good to terrible while star hopping to my next double star attempt. Tiny fuzz balls everywhere. And I think my scope has a pretty good Strehl. All for naught, apparently. smile.gif


Edited by Asbytec, 21 February 2019 - 09:08 AM.


#14 SeattleScott

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Posted 21 February 2019 - 09:17 AM

Scott:

For viewing the Pleiades and viewing the double cluster, not only is the polychromatic Strehl not so important, neither is the Strehl..

As far as the 8 inch Newt, it's F/4, what eyepieces, which coma corrector are you using?

The Strehl is a measure of optical quality, not of design quality. As I understand it, it is the ratio between the actual energy in the central disk versus the theoretical energy in central disk for the given design.

Jon

I am using the coma corrector designed specifically for the R200. It never leaves the focuser. Works nicely. No complaints with the R200. It gets more use than the big achro due to being more portable.

I am just making the argument that achros can have a role and do it well despite CA. The clear aperture gives more pinpoint stars. So yes CA is a limiting factor for certain types of viewing. My 4” Apo is more versatile as it can do open clusters, planets or EAA. But it doesn’t go as deep visually as the AR6, and my Rev isn’t great on open clusters, so the AR6 is kind of my weapon of choice for the showpiece winter open clusters. Sure, a 6” Apo would be better, and I may just do that in 18 years after the kids are through college. But for now I can use the AR6 for open clusters and my other scopes for planets.

Scott
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#15 Stephen Kennedy

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Posted 21 February 2019 - 03:32 PM

CA essentially causes the same type of aberration that SA does.  A spherical mirror can not bring rays of light from an object infinitely far away (all astronomical bodies) to a single focal point.  Rays striking different parts of the spherical mirror are reflected to different focal points causing the image to look blurry.  This was the problem with the HST when it was launched.  The astronomers who first tried to use it found that they could not get it to a good focus.  Newtonians, classical cassagrains and gregorians solve this problem by shaping the surface of the mirror into a paraboloid.  It can be demonstrated mathematically that a paraboloid can focus all the light that enters the telescope to a single point. 

 

In refractors, because the speed of light passing through glass is different for different wavelengths, the light can not be brought to a single point of focus.  You notice the false color it causes fairly readily but need to be aware that the image is degraded in the same way as SA since different wavelengths of light do not all focus at the same distance from the lens.  There really is no way to completely eliminate CA and the SA type effects it causes because the number of wavelengths is essentially infinite.  Makers of refractors try to overcome CA by using types of glass that allow different wavelengths of light to traverse it at less disparate speeds and add additional lens elements that attempt to neutralize the abberations caused by the other lenses.  


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#16 Steve Allison

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Posted 21 February 2019 - 04:40 PM

Thanks for all the great information!

 

I have some beautifully corrected, long focal length achros that provide wonderfully sharp images, with at times a hint of color that I hardly notice. I was just wondering if an apo that produced less sharp images would have a higher Strehl solely because it displayed less color.

 

Regardless of optical theory, I find chromatic aberration (unless excessive) to be less deleterious to image sharpness that spherical aberration. I have seen better images in my Royal Astro 90mm F/15.6 refractor than in a couple of inexpensive apos of around the same aperture, at least at the magnifications my usual seeing conditions permit.

 

Thanks again, people.

 

Steve


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#17 Eddgie

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Posted 23 February 2019 - 12:01 AM

The test reports do not account for diffraction.  They only look at the wavefront of the telescope.  They typically measure hundreds of distinct points on the lens (or mirror) and derive a Sthrel figure from the deviation of perfection. 

 

Now let's look at the wisdom of that.   If you really want to know what the quality of manufacture is, the size of the aperture and the presence of a secondary obstruction don't matter.  You are only concerned about how the wavefront converges on the focal plane.  It either propagates perfectly or it does not.

 

As I mentioned earlier, the polychromatic Strehl is not really influenced so much by the manufactured quality.   In a refractive optic, it is set by the types of glass used, curves in the glass chosen by the designer, and the spacing of those curves.  This is what sets the polychromatic Strehl.   No amount of precision in manufacter can raise this number.  It is not usually included in bench tests though because again, the interferometer test is really just looking at how well the curves were generated.  It does not really know where the different focal points of the light are.  Now if you tested in several colors you could derive the polychromatic Sthrehl, but why would you do that if you were really just looking to see how well the scope was made?

 

To the OP though, your question was excellent.  

 

And here is the hypocrisy of the refractor forum.  I have seen this many times over the last 20 years.. Someone in the refractor forum will complain that Strehl testing of reflectors does not really tell the full story, because well, you know, if you included the obstruction, the Stehl of a C8 would never be better than .8 so they say that all C8s have a Strehl of .8 at best.  (And of course this is malarky because the Strehl value is as I have repeated several times, a mesurement of optical quality and optical quality alone). 

 

These same people though simply choose to ignore that when CA and Spherochromatism, they have the exact same result as a secondary obstruction, which is that they lower contrast.   As the tables I referenced above show, a 100mm f/10 achromat has about the same contrast loss as would be experiences with a 100mm reflector with a 30% obstruction.

 

People can demonize the contrast loss of a secondary obstruction but give a total head in the sand pass on contrast loss caused by CA and spherochromatism, which can both be quite bad in large, fast achomats. By comparison, the contrast loss of a secondary in a marvelous scope like the Intes Micro MN66 is almost nothing, being less than some of the fast ED doublets out there. 

 

Anyone that is a refractor lover and that thinks that a secondary obstruction is the end of the world should beg for a peek though an MN76 if they ever get to look though one at a star party.  Spectacular telescopes.   Most people on CN will never own a refractor that will beat an MN76 on contrast.  Heck most will never own a refractor than can beat an MN 66 on contrast.   Buy comparisn to CA in many scopes, a 17% obstruction is nothing. 


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#18 SeattleScott

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Posted 23 February 2019 - 01:29 AM

Valid points Eddgie. But I do know from personal experience that my 6” F8 achro provides sharper/clearer low-medium power views of open clusters than my 8” newt. Granted the newt has over 30% CO. I’m not saying my achro will best an Intes Mak Newt. But even with a coma corrector in the Newt and premium eyepieces the views are more aesthetically pleasing in the refractor. For low-medium power DSO viewing, especially open clusters. Now is that a result of clear aperture? Is it the result of looking through a lens rather than bouncing light off mirrors? Is my Newt mirror dirty? I did clean it last year. I don’t know exactly what it is, but there is something about the presentation of stars in a refractor. For 15 years I didn’t buy a big refractor because on paper they don’t make sense. Too long, too heavy, too much CA. A 10lb 27” long 6” F5 Newt will deliver better views for a fraction of the cost because it is free from CA. I finally found a used AR6 at a price too good to pass up, so it was time to find out for myself if a big achro can really have that refractor magic. Well I did a shootout with a buddy with a very sharp 6” F5 Newt and we compared the view of Andromeda (not even an open cluster). He said it was like night and day. The refractor was brighter and clearer than the Newt. He also said it was like seeing the double cluster for the first time, and the view of the Moon at 150x was the best he has had, splash of purple and all (he also has a mediocre C8). So the refractor has bested 6” and 8” newts with very good star tests. Granted planetary is not its strong suit. It is more of a DSO up to 150x specialist, although if the planets are out they are certainly worth a look (I just may be inclined to get a different scope out if the planets are out). So while I have been a long time skeptic I am now a believer in the aesthetic presentation of stars in a refractor. Maybe an optimized newt could provide similar presentation with better planetary contrast? I would like to think so but not many optimized newts running around (although a buddy just got an 8” Cave so that could be a good benchmark). I don’t deny the statistics about CA. The numbers are what they are. CA is certainly visible on bright objects. Everything you and others have said is probably absolutely correct. And yet those beautiful pinpoint stars call to me...I cannot explain it with statistics. I can’t prove my point on paper. But I know what I see. As long as you stick with low-medium power DSO viewing a 6” achro can be an impressive instrument.

Edited by SeattleScott, 23 February 2019 - 01:31 AM.

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#19 Steve Allison

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Posted 23 February 2019 - 02:26 AM

Some great posts to consider!

 

For most viewing, I personally prefer a refractor, either an apo or an optically perfect, long-focus achro. But I must concede that the best image of Jupiter I have seen (so far) was through an Edmunds 6 inch F/8 reflector with a refigured, near-perfect mirror.

 

Saturn has looked the best through my previously owned 6 inch refractors, a D&G F/15 and a Celestron F/8. I have not yet had a chance to look at Jupiter or Saturn with my Takahashi FS-102NSW or my just acquired Brandon F/15 achro.

 

I have to say, though, that a view I had of Saturn through my Sears (Royal Astro) 90mm F/15.6 achro one night last summer, when the dew was dripping off in rivulets, was little short of stunning! I was either at 175 or 200 power, I don't remember which, and the sharpness and contrast of the image was a something to behold.

 

What all this has taught me is that a perfect optic, whether it be an apo, achro or mirror is the secret to beautiful images!

 

Steve

 

P.S.- My little Meade 4 inch SCT has given me very sharp images of Saturn and other high-contrast targets, once again reinforcing my belief that optical quality is a major factor in what you can see!


Edited by Steve Allison, 23 February 2019 - 02:42 AM.


#20 SandyHouTex

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Posted 23 February 2019 - 10:48 AM

Dumb question time.

 

Does the presence of chromatic aberration in a lens lower its Strehl rating.

 

Say you have achromat that has absolutely perfect spherical correction and the best color correction possible for its crown and flint design. Would it have a lower Strehl than an apo with the same level of spherical correction?

 

I read a post recently to the effect that an early Clark lens was so perfectly corrected for spherical aberration that it was beyond the 1/40th wave detection limit of the testing apparatus. No Strehl rating was given, however.

 

Anyone know?

 

Thanks.

 

Steve

From what I’ve read, Strehl is the ratio of how well an optic is made, compared to the ideal telescope of that design.  So a triplet APO could have a Strehl of 1.0 if perfectly made.  The problem with refractors is that the first lens is a prism, and all of the lenses after that try to put “Humpty Dumpty” back together again.  Unfortunately, they can’t.


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#21 Asbytec

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Posted 23 February 2019 - 11:49 AM

It's been a while since I've looked through a nice refractor, and no doubt there are reasons to appreciate thier star images. I suspect the reason is the first diffraction ring is diminished compared to an obstucted design and probably regardless of polychromatic Strehl which peaks at some frequency.

I get nice color corrected stars in my 8" f/6 when it's cooled and well collimated. However, on even middling magnitude stars, I can still see the first diffraction ring. I think they look cool. Surely that ring is all but gone in a refractor on the same star making them look like true pin points.

I was chatting with a refractor owner talking about a color halo he sees on the moons limb. I mentioned seeing a similar diffraction artifact along the limb in my MCT. We both agreed neither effect bothered us and the moon was cool, anyway.

As I understand it, refractors have very good throughput, too. So, they seem to have some favorable qualities we can see, even if not perfectly corrected for color with a lower poly Strehl. I don't mind diffraction rings except when they make certain tight doubles more difficult. My best views of Jove were through my obstructed 6" MCT on the zenith in excellent tropical seeing.

Edited by Asbytec, 23 February 2019 - 11:50 AM.


#22 Stephen Kennedy

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Posted 23 February 2019 - 03:05 PM

When we talk about the diffraction, we have to remember that a CO is only a minor component of total limitations imposed by diffraction when viewing through a circular aperture. It is the size of the circular aperture itself that by far contributes the most image degrading diffraction. This diffraction is a direct inverse function of aperture. The larger the aperture, the less diffraction. Reflectors and refractors of the same aperture would both suffer the same about of circular aperture diffraction.

The thing is that when we are comparing refractors to reflectors, it is rare for them to have equal apertures. Normally, reflectors used by amateurs have considerably larger apertures than amateur refractors. A medium size refractor will have an aperture of about 100 mm while a moderate sized reflector will have an aperture of about 200 mm. The result is that circular aperture diffraction will be twice as much in the typical refractor than in the typical reflector.

#23 SeattleScott

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Posted 23 February 2019 - 03:33 PM

Well, I understood about zilch of that, but I do prefer the views through an 8” obstructed scope over a 4” refractor, Apo or achro. Refractors are nice but that is too much of an aperture advantage to overcome.

Scott

Edited by SeattleScott, 23 February 2019 - 03:34 PM.

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#24 Asbytec

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Posted 23 February 2019 - 08:41 PM

This diffraction is a direct inverse function of aperture. The larger the aperture, the less diffraction. Reflectors and refractors of the same aperture would both suffer the same about of circular aperture diffraction.
 

If I get what you're saying, you are talking about resolution. Less diffraction meaning a smaller diffraction artifact, not about the intensity distribution itself which sets the limit on Strehl at 0.838 real (presumably full spectrum) light in the central peak due to diffraction in all apertures which is further modified by the added diffraction of any obstruction. 

 

Seattle Scott mentioned something I do not know the answer to, but seems reasonable. Chromatic aberration disperses light but does not seem to brighten the diffraction rings in the same way an obstruction does. Ideally, it also peaks in terms of Strehl at some frequency. So, even though light is dispersed from the central peak (by color vs all colors by diffraction), the peak (Strehl) at that frequency can be quite high and thus pleasing to look at. Couple that with a fainter first ring and you have pleasing star images even though the full spectrum Strehl is not as good as a single frequency Strehl. Maybe?  



#25 SeattleScott

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Posted 23 February 2019 - 09:36 PM

Sounds right to me, seems like an achro is better than obstructed scopes for low power but worse for high power, given similar apertures. I don’t know all the science behind it but that has been my experience. Of course an ultra premium obstructed scope could change the equation, but if you can afford that, why would you have an achro?

Scott


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