Need help with achromat star test

Hi All,

I need some help with a star test result I am seeing with my 6" F10 IStar achromat.

First off, is it even possible to get any meaningful info out of the test if a narrow band green filter is NOT used (ie. Baader Solar Continuum)? The test was in white light only.

Doing some reading, what it looks like I'm seeing it balanced high-order SA (BHSA). I looked at all the images show on the telescope optics reference here:

Telescope Optics - Abberations

Reading more about it, it looks closer to the bottom sets of images shown here:

http://www.telescope...t/refractor.htm


Interestingly, the description about it mentions that it's more common for an APO refractor, and I couldn't find any mention of it related to a achromat.

One thing that makes it really appear to be this is the in-focus images are different than a traditional pattern where the first ring is brightest, then second is dimmer, and so on. This has many rings seen, all similar in intensity. I really see this in bright stars and at high powers where the image appears "messy".

In previous testing, I now realize I was using way to much de-focus and as I have been reading (and saw) with this aberration, the differences look less obvious in that case.

So, does this seem possible?
What is the cause of it?
How much does it impact the image quality?

I'm also including a (poor) image of a page in my note book of the rough diagram of the pattern I recorded recently while testing.

Thanks!

Testing for HSA and LSA in an achromat can be a bit difficult.

What you have looks like maybe it is a mixture of spherical aberration and higher order spherical aberration.

The chromatic aberration makes it more difficult to know.

My advice would be to test your scope using a 30% obstruction.

Attached is a simulation of .2 waves of higher order spherical aberration with a 30% obstruction in place. This looked about like your drawing, but it is really hard to know.



This will allow you to suppress any higher order spherical aberration and see if there is any LSA.

If the LSA tests passes, then most likely you do have some HSA.

The problem with this is that while very fast refractors an have some HSA, it is usually pretty minor, and by f/10, there should not be that much.

You can also download Aberrator 3.0 and do your own simulations, but to me, it looked like about .2 waves of higher order spherical aberration.

Interesting question, but I agree with Eddgie...use a filter if possible. Anyway, HSA /IS/ spherical aberration, its not a different class of aberration that is like SA. It is SA. It is the amount of actual aberration left over after the initial approximations for the curve of the objective is determined (mathematically.) HSA can and does pop up when another spherical surface or lens is added to the objective changing the correction of the original objective.

So, adding the second lens to an achromat can induce some higher order SA if it affects the shape of the wavefront formed by the objective's initial figure. The amount of that correction needed is driven by and defines HSA. Correcting HSA (by adding some lower order of opposite sign) is what gives HSA it's signature wavefront as displayed in OP top right.

So, does a long focus achromat even have HSA? I am not sure it does, not necessarily. Fast APOs with highly curved and multiple spherical surfaces will have HSA as each lens is added to the objective. The additional surfaces and curves are adjusting (adding LSA) until the residual HSA (and color) is accounted for as LSA, basically. But, in a long focus achromat, it seems adding the second lens to the objective simply corrects for color and does not signifigantly affect spherical correction (not in yellow light, anyway.) Different colors will vary in SA correction, but not in amounts signifigant enough to warrant refiguring the objective (correcting the wafefront for HSA), well in my understanding.

But, it's interesting to know if HSA is a factor in long focus achromats. There may be some, but generally I suspect not enough to warrant changing the shape of the objectives original wavefront. So, if what your seeing looks like correction for HSA, it might not be. If not, then what you are seeing is the shape of the original, uncorrected for HSA objective's wavefront - or simply just the system's SA.

Edit: Initially, it looks like you might have some degree of over correction, or possibly a zone near the center. To me, and initially, anyway. Normally, with SA, the zones coming to focus tend to be brightest as they approach focus. Inside focus, paraxial zones are approaching focus first, so the paraxial (inner) diffraction pattern is brighter. Outside focus (past best focus) marginal rays are finally coming to focus and the edge of the pattern is brighter. That's kind of the over correction pattern I see.

Edit, again: You know thinking about this some more. My long focus MCT is corrected (balanced form) for and has some residual HSA visible in the star test. But, that is caused by the strongly curved meniscus that changes everything about the prime focus of the spherical primary mirror. That's HSA induced by the meniscus that needs to be corrected. Adding a second lens to an already long focus obejective probably does not change the objectives spherical correction in any signifigant way, plus long focus corrects SA to some tolerance, already. Along with color, of course.

Edit: Initially, it looks like you might have some degree of undercorrection, or possibly a zone near the edge.

You think some lower order spherical abberation?

Interesting, because that was my guess too.

Seems as if you are putting your book to good use... LOL.

I would obstruct it and use a breakout test to quantify the LSA (if present), but like you, I think we have some mixed abberations present.

But that is why he shold test for LSA with an obstruction first. If there is some, he can then determine if it is the dominante abberations.

Once again, for anyone reading, star testing is a lot more than racking in and out of focus. There are specific methods and a process for testing. It is very easy to do, but if one doesn't know the proper way to do it, one doesn't get a meaningful assessment.

Again though, I suspect that there is more than one error, and to me, it looked like there was an LSA component and maybe even a dominant one.

Ya, I suspect it's primarily over corrected SA...LSA, same thing, really. Ya, it might be good to use the shadow break out to quantify it to some ball park best guess.

It was an interesting question, though, whether or not any HSA existed. Not sure it does, and burned out my cup of Joe thinking about it. LOL

Hi Ed, Norme,

Thanks for the input! I still have questions

Ed, is the obstruction just to localize where the aberration is (the zone) on the lens, or is HSA something that is always localized to the center?

Norme, yeah I understand that this is still SA, I just don't understand how it is different from primary SA and if it impacts performance in any different way. As you were saying, the pattern is very different than primary SA. The inside has a very diffuse/indistinct outer ring followed immediately by a very bright, sharply defined second ring, while the other side of focus just has the outer ring being sharply defined. There is also a difference in appearance to the rings inside the pattern, and then at focus it displays that very distinct series of even brightness rings.

As Ed you are showing in the simulation, the difference becomes less distinct with more waves of de-focus. I think that simulation is more than 7(?) waves? In at only 2-3 waves out, the difference is huge! More like the 0.4 line in the image above.

Some additional info:

-I've been trying out Chromacor's for a bit now. I'm using one now that should be well matched to a neutral corrected (in green) achro of this size

-With the Chromacor in place, the CA is essentially reduced to nothing (very impressive!). However, the pattern remains the same as above without the corrector in place, just very color free

I know that likely isn't a replacement for doing the test just in green light with a narrow band filter, but I thought it was interesting.

Clear skies,

Gord, the HSA in your lens should be insignificant at F10. Also use a green filter and life will be much easier.

Also remember, that as you star test with a real star, if the atmosphere is the least bit turbulent, the extra focal images may well appear a bit soft relative to the intrafocal pattern since you are focusing on the turbulance in the upper atmosphere. This can lead you to believe the lens is undercorrected and is the reason you should conduct star testing over multiple nights with a completely cooled lens (as a lens cools, it will display undercorrection).

Don't sweat central zones unless they are large (span over about 10-20% of the diameter) and/or are pronounced for two reasons. First, the central 10% makes up only 1% of the lens surface. Second, the central portion of the lens does very little "work" on the light. I've an excellent AP 6" F9 oiled lens with a mild zone in the center 10% of the lens. I can easily see it in the star test and in autocollimation. It gives awesome views.

FWIW, my Istar is very slightly undercorrected, with a very smooth polish, and a very, very mild bar shaped zone that bisects the lens, which I believe is the glass. It gives obviously awesome views with the associated cleanliness and sharpness of image charateristic of any fine optic.

Jeff

I just don't understand how it is different from primary SA and if it impacts performance in any different way. As you were saying, the pattern is very different than primary SA.

Hi, Gord, it's an interesting question. Really HSA is simply a mathematical treatment of SA, it's what is left over between the actual focus and mathematical approximation of the objective's curve. So, some surface curve is adjusted if excessive HSA is present so the lower order term more accurately describes the system's SA.

Trick is, at such a long focal length, I am not sure the achromat has any HSA at all - or minimal at best. Therefore, there would be no need to alter any surfaces to correct for it. So, the balanced waveform might not be present at all (waveform in your OP upper right.)

I think what you're seeing is under correction, or simply LSA and some uncorrected HSA (if, indeed, any is present at all at that focal ratio.) Only if it IS balanced will that waveform (in OP) be present resulting from correcting the objective's surfaces. This is necessary in fast APOs, but probably not in log focus objectives. Adding a lens to an achromat corrects for color, and very likely not correcting any residual SA /not/ defined in the lower order term - i.e., HSA.

...is HSA something that is always localized to the center?

Not really, as I understand it, balancing for HSA (putting the SA back into the lower order term, basically) entails adding a little LSA of opposite sign. This is done by lengthening the primary objective's focal length - turning back the edge.

The shadow break out helps visualize how each zone comes to focus. Observing and measuring this, one can estimate the approximate amount of SA correction and whether or not it is over or undercorrected. Observing HSA (the waveform in the OP at upper right) needs a star test, just as you're attempting to observe the effects on ring brightness. The balanced form will be better corrected, though, and should give a better shadow break out - I think - since the resulting waveform more closely resembles a sphere (turning back the outter zones retard the marginal ray so it curls back and hugs the perfect reference sphere centered on best focus.)

Man, this stuff is hard to keep simple...LOL Fun though.

Edit: Jeff is correct.

I just don't understand how it is different from primary SA and if it impacts performance in any different way

While both are forms of spherical abberation, the differnce between LSA and HSA is to be found in the distribution of the light that does not go into the Airy Disk.

With lower order spherical abberation, the light that does not go into the Airy Disk goes mostly into the first diffraction ring, which affects medium frequencey detail (detail on your target that is a couple of Airy Disk diameters in size.. For example, if your Airy Disk is 1.5 arc seconds across, pure SA would do the most damage detail that was about 3 arc seconds or so in size.

Pure HSA though has the quality of putting the light further out into the extended diffraction rings.

This means that the light is spread out more, so less contrast is lost in general, but for larger details.

This is what makes HSA a concern (when it is present). While LSA might lower contrast on a very area of the target, HSA won't lower the contrast as much, but it will lower it over a wider area.

Neither is good, but HSA is rarely found in refractors until they get rather fast and have a lot of elements, and even then it is rarely severe, though AP I think may use some level of aspherizing the optics in one or more elements to further improve their scopes performance. But these scopes are quite fast and have to be perfect for them to be worthy of the brand.

The Obstrucion minimizes the presence of HSA duruing ths star test. This allows you to see if there is any lower order SA present.

If you do this test and the scope passes, then you may have HSA or zones (easy to test for zones too). If it does not pass 100% for LSA, then of course you may be seeing some lower order spherical abberation, but the question would then be if it is the dominant error.

If the scope fails a 3/1 breakout test, then SA may be your principle error.

If on the other hand, it passes with maybe a 2/1 ratio, the LSA is not at all bad enough to be an issue, but it becomes harder to know if you are seeing a balance of HSA and LSA.

This is when it becomes difficult. If the aberrations are mixed, you may need to first estimate the SA, then try some models with HSA (using Aberrator 3.0) to find a fit.

I want to be very clear on this.... Just because a telescope does not present a perfect star test doesn't mean that the telescope is defective.

If you think there is a more serious issue, I personally believe that the burden is on you to substantiate your belief.

HSA has to be pretty severe before it will make a scope fall below the bar.

But HSA lowers contrast for larger areas of the target than LSA does, and that was your question.
Most manufacturers only claim that their scopes are diffraction liminted, and you can have some HSA and LSA and still be easily inside that rather low (and ambigious) threshold.

Here is a simulation that shows how LSA differs from HSA.

HSA has to be pretty bad to do serious damage.

These plots show what I was talking about in my last post.

In both cases, there is .25 wave of error, but the left one is .25 waves of Higher order spercial abberation, and the right is .25 waves of lower order SA.

Note that for the HSA, the damage is done on the left side of the chart which represents large detail. Light is in effect scattered over a very large area. It doesn not lower the contrast of any one feature very much (the line does not sag much) but it lowers contrast on a very large scale.

The right image shows how LSA affects the image. Notice that the sag is much deeper. This is because the same amount of energy is encircled very close to an area the size of the Airy Disk and first diffraction ring. This means that detail the size of about 3 Airy Disk diameters will have much lower contrast than an instrument without SA.

For the HSA scope, this smaller detail gets affected much less, and since the same amount of light (1/4th of the energy more or less) is thown out into the 4th through 6th diffraction rings, the energy being scattered over each unit of area on the target is lower, but more units of area are affected. Contrast is lowered out maybe 5 or 6 Airy Disk diameters.

Now that does not sound like much, but if your scope produces an Airy Disk that is 1.2 Arc Seconds across, this would equate to an area bigger than Mars... A very small target like a faint Planetary would not stand out as well from the background as in a perfect apeture.

But until HSA gets to be about .4 waves, it simply is not usually serious enough to worry about.

Commercial MCTs often have in excess of .2 waves of HSA and most people will rightfully report that the image seems fine.

Hi guys,

Thanks for all the input. Wow, there's a lot to digest! But what you guys are confirming is that HSA shouldn't be as huge a detriment as compared to LSA alone. It sounds like there is more I need to do on testing still.

On a few points...

Jeff, I think I'm good in terms of conditions and other factors. This is something I'm seeing consistently (over many outings) and I don't have much cooling issues are everything is kept close to ambient. Also, you mention under-correction which brings up an interesting point. I believe I'm seeing over-correction since the brighter, more distinct ring is seen on the outside of focus.

1. Is this correct?
2. Could this be a mis-identification of the correction if there are multiple aberrations at play?

Perhaps a little more to this, I have tested two Chromacor's now. One was a Chromacor 1 O1, and the other a Chromacor 2 U1. Both correct the color very well, with the 2 version being best. I also found the images of Jupiter to be noticeably sharper with the Chromacor 2 and consulting with Valery he said this would be the case as it is naturally a better fit due to this being an F10, and that if the lens was over-corrected to being with, the O1 would make it worse still. That seems to match with what I am seeing as even though the color was better with the O1, I didn't find the detail to be any improved. The detail (on Jupiter) is noticeably better with the 2 version.

This is all leading me to believe that it is definitely over-correction I'm seeing.

The other thing is the in-focus pattern and all the even intensity rings. Ed, your description of the HSA throwing the light into the outer rings vs. LSA throwing it into the first ring would seem to match this description, no?

The other reason I'm wondering about the HSA is that it's my (perhaps incorrect...) understanding that LSA may be correctable by adjusting the spacing of the elements to bring things closer to neutral. I have the impression that this isn't something that could be done for HSA.

Finally, I don't think this is a defective lens, but could be a lens with some defect. Defective to me would be un-usable, and this lens is useable. Terrific at low powers and even at medium and high on deep sky and fainter stars. But it doesn't show bright stars at higher powers nicely and it doesn't keep up well to my other scopes on planetary (C8's, the newt, etc.). It shows a nice image and details in the view itself, just not relative to the others. With the Chromacor 2 in place, I'm finally seeing details that I think it will be able to start comparing against the C8. Haven't got them side-by-side yet, but a well corrected lens with the corrector should be ahead of the C8 from what I understand of the experiences people have shared.

And Norme, you are right, this stuff really give the old noodle a workout!

Clear skies,

While both are forms of spherical abberation, the differnce between LSA and HSA...

The Obstrucion minimizes the presence of HSA duruing ths star test.

But HSA lowers contrast for larger areas of the target than LSA does, and that was your question.

Here is a simulation that shows how LSA differs from HSA.

Eddgie, correct me if I am wrong, but LSA and HSA are not different forms of the same abberation, they ARE spherical aberration expressed in terms of lower and higher orders mathematically. There are not two forms of SA - just one expressed differently.

"Higher-order spherical aberration is merely a consequence of using (close) approximation of the conic surface, instead of the actual surface, in calculating the aberration.

"...does have some significance in certain types of amateur telescopes with typically more strongly curved optical surfaces. In particular, in most apochromatic refractors, as well as Maksutov-Cassegrain (MCT) and Schmidt-Cassegrain (SCT) telescopes."
"When significant, it needs to be minimized by correcting the surfaces in their lower-order approximation so that they better approximate the higher-aberration term surface profile."

http://telescope-opt..._aberration.htm

So, while you can discuss HSA separately resulting from corrections to the caustic focus due to adding another highly curved surface, both terms are simply an expression of pure, classical spherical aberration we all know and love. Nothing more, nothing less. HSA does not exist as a separate, weird form of SA. It is SA not accounted for in the lower order approximation of the objective's surface and is induced by changing that approximation by adding another highly curved lens.

The obstruction minimizes the classic "hill" in the balanced wavefront. It obscures that central wavefront deviation thereby eliminating it's contribution to the overall P-V and rms error. You can see that "hill" in the OP image at upper right. By the way, this "hill" is formed by a corrector or meniscus altering the waveform so that the paraxial rays strike the central primary before the 70% zone does. Further correction, or balancing, is achieved by retarding the marginal rays so their reflection lags the 70% zone, the way I visualize it. This is what both the SCT corrector and MCT meniscus attempt to do, and the altered wavefront can be seen in the diffraction pattern (shown in the OP.)

Again, this is probably not required in a long focus achromat, the "lower order approximation" is likely left untouched. So HSA might be present but minimal. You probably wont see the balanced form as shown in the OP.

In my Mak, it was highly likely the secondary baffle vingnetted the deviation from shperical at the marginal wavefront or edge. You can see that in the OP upper right as the wavefront curling back at its edge. By the way, most of this wavefront formation is done at the Schmidt corrector as opposed to the maniscus/primary combo in an MCT. This is what they mean by the corrector has the higher order term ground into it. It's taken care of before the wavefront hits the SCT primary, not so in a MCT OR APO. Both of the latter have some residual.

When HSA is present, I believe you are correct about the contrast loss. Also, aberrator HSA is the balanced form. When you add HSA to the diffraction image, you are also adding LSA at the same time. You're manipulating BHSA, not HSA alone.

I believe I'm seeing over-correction...

...understanding that LSA may be correctable by adjusting the spacing of the elements to bring things closer to neutral. I have the impression that this isn't something that could be done for HSA.

I think over correction, too, without further inspection.

Correcting for LSA in this way might work, balancing for HSA would require a surface refigure. But both are mute points, really, because I would not mess with the separation. Doing so might add color and do nothing for SA....LSA, that is. HSA is likely minimal, already, at that focal length and with that set of achromatic lenses. A long focal ratio achromat with more shallow spherical surfaces will likely not change it's caustic focus much with the introduction of a lens designed to control color - which is probably why it's hard to find a reference for it. Even the image you posted was titled "APO" on Vlad's site.

I could be wrong, though, ask Istar if they balance for HSA. I doubt it, though, because it's prevelent in faster lenses, such as a very fast MCT primary and it's steeply curved meniscus or fast APO objective lenses, probably not so in already long focus achromats. Heck, I'd be interested to know, too, if they do "touch up" their lenses in such a way.

In sum, I guess what I am asserting based on what I understand is, you cannot test for either LSA or HSA...both do not exist except in mathematical expressions. What you test for is purely classical SA. Now, if the scope is balanced for HSA because it needed to be, then it's star test will show it. The surfaces have been touched up to better approximate the system's total SA in the lower order term. That can be seen, but is neither LSA nor HSA...it's the result of both and is simply SA.

Hi Norme,

That's a detailed explanation, and I think I even followed it!

We got into a discussion of HSA vs. LSA, but as you got into in your last posts, there is the balance of the two. That was the link/pictures that I originally referenced (talking about BHSA).

I realize it was talking specifically about APO's, but all I did was look a the list of aberrations (first link in my OP) and find the one that most closely matches what I'm seeing. Could it matter any that this is a Steinheil (which has slightly steeper curves as I understand it)?

The thing that makes it distinct over primary (LSA) is that bright ring just inside the in-distinct outer ring on the inside of focus. Without that, it would look more like just LSA.

Looking again at Ed's aberrator simulation he showed earlier, that looks to me like the pattern, but the number of waves of defocus, it looks more like the 0.4 p-v example (at least to me).

But aside from re-figuring, this isn't fixable?

On a related note (and as if I don't have enough projects on the go...), I now have two additional 6" F10 lens sets. They are Jaeger's. I need to do some work to get them mounted, but I'll be interested to see how they compare.

Clear skies,

Hi Gord, ya you mentioned the balanced form. I'd be interested to know of Istar goes through the trouble of actually balancing it. I guess the point being, they probably don't need to at that focal length.

I kind of got confused in the discussion as it sounded like one could actually test for either LSA (primary) or HSA. Thinking about that, I am not sure one can. All we see is classic SA defined in the lower order term with possibly some residual - resulting is just some classic over correction. But, if the curves are figured to the balanced form then those can be seen as you describe...if it was done.

Anyway, I think you're correct. That is what you're seeing: simple over correction. Well, at least for now, anyway. The shadow break out Eddgie describes should give some indication of how much is there.

Great news on aquiring the additional 6" lenses. Nice to have them ready to go.

Anyway, thanks for the interesting discussion. I find star testing and optics fascinating. Maybe in my next life...either rock and roll star or master optician. LOL

Hi All,

I need some help with a star test result I am seeing with my 6" F10 IStar achromat.

First off, is it even possible to get any meaningful info out of the test if a narrow band green filter is NOT used (ie. Baader Solar Continuum)? The test was in white light only.

The star test is notoriously hard to interpret accurately because its so sensitive and doubly so in achromats without a green filter. I had an A-P Superplanetary triplet whose star test was hard to assess without a green interference filter but it gave superb planetary views nonetheless.

If you have a planetary imaging webcam, you can easily test your achromat using the Roddier test but you will need a green filter (I've heard a green + yellow filter stacked works well as well).

Tanveer.

The star test is notoriously hard to interpret accurately

There are many components to the star test, and the only one that is more difficult to interpret because of chromatic abbaration is the test for Spherical Abberation.

The problem is that the mixing of light can give the same result as a turned edge, where the internal rings are softer on one side of focus, and more defined on the other side.

The use of a green filter can clean up some of this to make it easier to see that the internal rings are a uniform width.

The central obstruction will allow a much better look at lower order spheical abberation.

The reason that the star test has limitations is usually more to do with the person doing the star test than with the test itself in that most people have never even read the book on how to do it correctly.

Using the full range of tests (and there are several) combined with Suiters images, and complimented with Aberrator 3.0, one can get a pretty good estimation of optical quality regardless of the kind of scope being tested.

Everything is in the book. It is so clearly laid out and so logical that with a little practice, anyone can identify a lemon, and after star testing 20 or 30 scopes, can render a pretty good assesment with only a little effort.

I would still encourage the OP to do the breakout test. If the test shows lower order spherical abberation, then that is ususally the more severe error, and he will know it. If it passes, then what he sees may be higher order SA, and if so, we can attempt to model in in Aberrator 3.0.

If there is no lower order SA and the HSA is below .4, then the scope is bascially diffraction limited, and the effect is to slightly lower contrast in the field, but only slightly.

And since it is an achromat, it could be that the light most affected is far away from green, so that the degradation in visual performance is to small to be concerned with. This is why violet is allowed to be uncorrected. Even though it can scatter across many many Airy Disk diameters, we simply don't see the effects of lowered contrast that much.

This is why it is so important to eliminate lower order spherical abberation frist.

The star test is notoriously hard to interpret accurately

Using the full range of tests (and there are several) combined with Suiters images, and complimented with Aberrator 3.0, one can get a pretty good estimation of optical quality regardless of the kind of scope being tested.

I would still encourage the OP to do the breakout test. If the test shows lower order spherical abberation, then that is ususally the more severe error, and he will know it.

If there is no lower order SA and the HSA is below .4, then the scope is bascially diffraction limited.

Agreed. One can get a good ballpark figure, though an interferometer takes no prisoners. Star tests can be close.

Yes, a little study and time and effort in the field using those tools works.

Yes, strange that 0.4 wave HSA wont hurt RMS as bad as 0.4 P-V LSA. I believe because HSA is really present (and corrected) at the edge of the wavefront. It does not emcompass the entire wave as does classic, pure spherical aberration (generally described in the lower order term.)

Could a 10 lines/mm (254 lpi) Ronchi grating eyepiece be used to differentiate and/or confirm these suspected aberrations?

Does the use of full spectrum starlight for Ronchi testing invalidate the test in a similar fashion that full spectrum light invalidates the star test?

------
C

Well, that is due to the way the light is distributed.

With lower order SA, it is more concentrated in the first and second ring, and mostly the first ring.

With HSA, it is thown further out, and because it is spread over a much larger area, no one area gets a lot of intensity. The strength is "diluted".

What this means though is that for small targets like faint planetary nebula, their contrast against the background sky is very slightly lowered, of for planets the contrast of the larger features is very slightly lowered.

But this lowering is soooo slight that until the HSA gets very severe (.4 wave) it simply doesn't loose enogh contrast to matter. It will still stand out against the background sky.

Lower order SA damages the smaller details. For example, in a small aperture, a 2 or 3 arc second faint oval on Jupiter would be damaged more by LSA than HSA. The damage is kept close to the border separating these two features.

With HSA though, it is scattered out further so it does not fall with the same intensity into the small oval, but rather is spread out past it, and into the surrounding details.

This is the value of MTF. Almost all defects (and obstructions) take light out of the Airy Disk.

The question now becomes "Where does it go?" A turned edge or HSA both lower contrast in the background. That is why Suiter kind of lumps them together in Chapter 11. Circular zones, turned edges, and HSA all can have the effect of damaging the sky background.

People talk like a central obstruction lowers sky background, but it does not really do that. Most of the damage is done very near the edge of the target area, and little light goes out into the sky around the target.

But with HSA, turned edges, and zones, the light is scattered much further out into the field. This is what makes it harder to pick out a very faint, very small object like a planetary nebula.

MTF describes where the light is going, and if it is falling in the .2 area on the right side of the MTF chart, it means that the damage extends very far away from the Airy disk.

Edggie, I dont have time to digest your comment, but I think you mean the balanced form has such a slight degradation on the image. In the unbalanced form, the marginal rays are uncorrected and come to focus as normal for SA. They provide some light in the area around the central disc in the form of transverse aberration. That can be severe in terms of blur size.

In the balanced form, the marginal wavefront is retarded so the marginal wave more closely hugs the perfect reference sphere centered on best focus. This delaying the marginal wavefront reduces the PV error (improves RMS) while lengthening the marginal focal length. This reduces both transverse error and longitudinal error, so the 'stray' light brightens the rings slightly, I believe, toward the center.

The result is a smaller, slightly brigher blur, generally, and less blur at best focus (using defocus from gausian.) There is an excellent illustration on Vlad's site (he seems to update it often.)

http://www.telescope..._aberration.htm

You may be right, though. Okay, gotta run, let me mull that over...over dinner.

EDIT: Back from dinner. So, yea, when one grinds a spherical mirror, the lower order term approximates the SA depending on how close the surface is to the mathematical approximation. The rest of that classic, pure SA is handled in the higher order terms if for some reason the actual surface is not ground closely to spec.

Same thing happens when another highly curved surface is introduced, such as a meniscus or another lens in a refractor obsjective. A highly curved surface will throw the focal point way off so that the lower order term no longer describes the 'system' SA. For example, tossing a meniscus in front of a f/2 primary changed the focal ratio and the level of correction. That difference is, again, handled in the higher order terms. It's just SA being handled mathematically. HSA does not have a unique surface that can be seen in a star test, it's simply residual SA not described in the lower order term.

Now, the balanced form of SA, balanced HSA, does have a unique waveform that can be seen in a star test. It differs from LSA (the original primary SA approximation) in that the paraxial and marginal zones are modified to more closely resemble a sphere...improved P-V and RMS. This shifts the best focus location (70% zone) out toward paraxial focus and extends marginal focus in the same direction. Best focus and smallest blur swap locations along the caustic between paraxial and marginal focus.

Adding lower order of opposite sign means lenghtening the focal length or the primary turning back the marginal and 70% zones a tiny bit. In conjunction with the meniscus or lens, the waveform takes on it's balanced form. The Schmidt corrector takes care of this wafeform variation at the corrector.

Ed, Norme, great discussion going on! I need to do some more testing.

Ed, you mention using a central obstruction. This would eliminate anything being contributed by the center zone of the lens, but since this is the easiest part to get right, it's more important to look at the condition of the outer parts. Would that be a correct way to paraphrase it?

Jeff also mentions using an aperture mask to reduce the outer portion of the lens as well. This got me thinking... would there be benefit to using a series of rings (aperture masks) and disks (central obstructions) and testing the lens at various different zones? If so, how narrow of a range would be best focused upon? Would 20% increments make sense?

Seems this would allow you to say how the performance of any area of the lens was, although it's possible a few could have general issues (ex. the outer 50%). I'm assuming what you would be looking for was a perfect test on each of the zones.

Now if only the weather would co-operate! We were outside and it was nice. Then a few minutes later not. Then clear again when I looked out. Then 10 minutes later covered. Yikes! Longest run of ^%$#^% weather I've seen in ages!

Clear skies (enough already!),

Gord, certainly masking the center would remove any aberration from that zone and improve RMS somewhat, but the point of the shadow break out is not to mask anything. It's to use the shadow to determine how far inside nd outside it can be clearly seen. That distance from best focus tells you something about the steepness of the zones coming to focus.

For example, the central zone comes to focus a bit beyond best focus (under corrected.) This light will tend to obscure the shadow until it is far enough beyond the caustic where the marginal rays diverge sufficiently to allow the shadow to 'break out.' The longer the central focal length, the further outside focus the shadow stays hidden.

Inside of focus, same thing only with the marginal rays coming to focus. This difference in break out distances gives you some idea about the level of correction.

So, now the ratio of inside and outside break out will give you some idea of the correction - the distance (roughly) between marginal and paraxial focus. For example, if they break out at exactly the same distance on both sides, that's perfect correction. A ratio of 2 to 1, if memory serves, is about 1/8th wave SA.

You can mask the outter zones and the inner zones, but then you'd hae a 3" refractor. LOL Dont worry about the outer (marginal zones), you need them to determine the break out ratio. Anyway, over corrected will change things a bit, but it's the ratio you are after.

Eddgie is the go-to (pun intended) guy for this, so he can correct me if I am wrong.

Hi All,

Just a little update from this evening. I'm now thinking there is some sort of edge defect at play. I tried playing with two things tonight:

1. a 30% obstruction
2. aperture mask of 5"

I have some notes on using the obstruction and see differences, but don't think I understand the test enough to say what I'm seeing yet. I'll need to do some more reading and playing with aberrator.

The real surprise though is with the 5" mask. Jeff suggested trying that test in combination with the Chromacor-II. Wow, what a difference that made in the pattern. *Much* closer to matching between inside and out.

And, the in focus diffraction pattern has improved a lot to and is looking more close to what I expect (less rings, first one brightest, etc.).

I swung over to Saturn as it had finally cleared the houses, but the seeing is pretty blah here tonight. A poor view. For giggles, I decided to throw the aperture mask on and see what it looks like. To my surprise, the image got sharper. More details were popping out. I played with taking it off and putting it on, and there was a consistent difference. Still a poor evening seeing wise, but definitely sharper with more details being seen. This was all with the Chromacor in place BTW. Color wasn't seen, other than some atmospheric dispersion.

Could a turned edge lead to that fuzzy outer ring appearing on the inside of focus? It went away as soon as the mask was introduced.

Clear skies,

Its interesting your image got sharper with the 5" mask. Need to mull that over. Certainly you are masking the marginal zone and probably improving PV error, but you're loosing resolution. Perfect inside and out, while textbook perfect patterns, are not necessarily the best patterns in a Spherical system.

If you're going to add the CO and perform the shadow break out test, you might need to leave the mask off because you will need to see the behavoir of the marginal rays to determine the amount of error. The central zone necessarily masks the paraxial rays because the axis is where those zones come to focus and we want to see how the shadow masking that zone pops into view. What you need is the ratio of breakout from inside and out to estimate the distance between marginal and paraxial focus along the caustic. For that, you need the marginal rays.

Eddgie, is this correct?

Anyway, following your progress with interest.