Posts Made By: Ron Harper

I ran across an idea for a fast achromat the other day that called for grey-tinted glass for the diverging lens. The idea was that in the middle where the glass is thin, there would be little attenuation, but out near the edges where you get into trouble with poorly focussed red and blue, less light would get through, and the result would be less light in the diffraction rings, more in the Airy disc--the apodization idea.

It ocurred to me then that an improved visual achromat could be made by using, not grey tint, but yellow-green. The thick and deeper-colored glass at the edges would attenuate red and blue where they do more harm than good, but the thin glass in the middle would transmit them. This would give a brighter, better-colored image than yellow-green or minus violet filters, which impose their color artifact on all of the light equally.

Batches of vat-dyed glass are used for color filters, but utility glass like that is likely not flint. Also, this glass would need to be lighter-toned than even the light yellow-green Wratten #11, which attenuates 78% over the visible spectrum, in the 1-2mm thickness used in typical filters. You'd want the color to get about as dark as a #11 at the edge. The performance could be helped if a lens could be designed and built with the flint as thin as possible at the center. And of course, you'd HAVE to call it an "apo".

So, light-benders, any merit in this, or should I go adjust my dosage?
Ron

Did I dream this? Once long ago (10-15 yrs.) I saw a big fancy Meade catalog, must have been 3/4" thick, that showed the 4000-series "Super Plossl" design as, well, Super. Erfle is probably a more accurate term. Then, the (also dreamed?) story goes, they went back to a two doublet design, but kept the "Super" in the name, those mega-corporate, lying so-and-sos.

I never see 4000s for sale described as 5-lens. How, after all, could an owner know what's inside? Are there such things, anybody ever knowingly use them, how do they compare to the 4-lens 4000s, and, of course, to the 5-lens Ultimas? And, presuming there were such beasts, which type was reviewed in the now-ancient but still-revered S&T review that found Ultimas and TV Plossls tied for tops ( and, curiously, passed completely over the UO orthos)?
Ron

Lens types,
A perrenial topic around here is the relative performance of an APO, or virtually perfect scope, and a longish, well made Achromat. I don't mean to raise sand gratuitously here, but as a good-achro user, I can't help but wonder what I am missing.

I would like to examine and understand the common claim that an APO will "go to" higher power while still appearing "sharp", without "image breakdown", than a good Achro. My problem is partly that as an Achro guy who doesn't get around much, I have little opportunity to make the comparison myself. Moreover, my Achro appears to me to perform very close to theoretical expectations for its aperture, which suggests the question of how much better an APO could be. There's not much I can do about the first problem, so let me concentrate on the second.

I love double stars, look at lots, and through mindless perserverance have had some success with difficult ones. From firm optical theory, my 5-inch scope produces an Airy disk, or diameter across the first dark ring, of 2.0", but my observations, as well as Rayleigh's and the similar Dawes's limits (which hold for moderately obstructed reflectors, too), indicate that a typical visible "spurious" diffraction disk of a roughly 6th magnitude star is 1.0" across in my scope. That is sure enough how it looks to me.

If this 1.0" is magnified 60x, it looks 1.0' across, still too small for me to make out. 169x makes it 2.8', big enough to see as an extended circular spot, no longer hidden in the spiky glare of aberration produced by imperfect lenses, coatings, and eye, but round and clear. The highest power I commonly use is 217x, giving a spot 3.6' across, which looks comfortably big to my reasonably good eyes.

Vision treatises say that while a normal eye can resolve two narrow black on white lines as such when their separation is only 1.0', fully 2.5-3' separation, or extent, is needed for two points to be made out, or for an extended object to be seen as in fact extended.

So, my achro and eye seem to be performing, on double stars, according to theory and well-established empirical rules for a perfect telescope and normal vision. And, from my experience and reading, including your observing reports and comments presented here, it seems the APO indeed offers no significant advantage, other than aesthetic, on these most contrasty of all astronomical targets.

My planetary views are commensurate with both my double star experience and the optical and vision "rules" as stated above. Planetary limbs, for example, look sharp at around 150x (ie, 30x/inch), but at 217x(ie, 43x/inch) are perceptibly soft. I do use 217x on Mars, just because it is so darned small, and the features, although softened by the magnification, are still easier to see because they are made big enough.

A squishy part of my attempt to connect star and planetary images is the fact that dim stars' disks look smaller, and bright ones larger, than assumed by Rayleigh and Dawes. One might suspect that low surface brightness planets should be easier to resolve, ignoring the effect that dim things are hard to see. Saturn is currently 0.0 magnitude, and 23" diameter. Consideriing the area of the rings, this works out to a bit dimmer than 6th magnitude/square arcsecond, but still close to the Dawes limit standard star. Jupiter and Mars have considerably greater surface brightness, and might be expected to appear softer-limbed than Saturn by this sort of thinking (which I am not sure is valid, but consider as an objection to my argument that could be raised).

So now I come to the APO high-magnification claims. I can believe that due to better color control, low-contrast planetary features appear more easily visible in the APO, and also colors would appear true, instead of yellowed, as in my achro. But, how can it be that APOs look "sharp" up to the 70x or so per inch frequently claimed? At that magnification the disc of a roughly 6th mag star is a whopping 5.9' in diameter. If an observer can't distinguish such a bloated image from "sharp", something must be wrong with his eyes.

It is my guess that some APO fiends are guilty in such cases, not of lying, nor blindness, but of inarticulateness. They see something in the APO that beats the achro, and inaccurately call it "sharper". Am I correct, APO heads, or is there something that I just don't get?

Ron

Spot diagrams are commonly used to show color error in refractors. They are very useful, at least for their main purpose of selling apos! But I have a problem with them. They are comparing apples and oranges.

The little circle at the center represents the size of the Airy disk, usually in green, and as such recognizes the diffraction behavior of light. The green ring pattern is not shown on the usual diagram, but we are supposed to know it's there. It is my understanding that the color spots, on the other hand, result from geometric ray-trace alone, and show how the light would be arranged in the absence of diffraction.

I'll pose a question that is relevant to us achromat victims. How does a red or blue color spot that would be, say, 4x the size of the Airy disc in the absence of diffraction, really look when it arrives at the focal plane? Is there smeared light between the diffraction rings of the well-focused green image? Or, as I suspect, does the poorly focused red and blue actually arrange itself in rings nearly coincident with the yellow-green rings, making those rings brighter than in a perfect telescope, but still well defined?

This has important implications for double star observing, where one is often trying to spot a dim companion between the central, visible part of the Airy disc, and the first ring. In my achro, that region looks dark to me. But is it, really?

Ron

I posted this little issue I have a while back and got not one response. I could say it got swallowed up in the Herb thing, but it actually languished unappreciated for a couple of days before all that hit. I just can't accept that nobody cares, sniff. So, I'm trying again. I have a weak mind and actually wonder about such picayune matters!

Optics buffs,
Color spot diagrams are commonly used to show color error in refractors. But I have a problem with them. They are comparing apples and oranges.

The little circle at the center represents the size of the Airy disk, usually in green, and as such recognizes the diffraction behavior of light. The green ring pattern is not shown on the usual diagram, but we are supposed to know it's there. The color spots, on the other hand, result from geometric ray-trace alone, and show how the light would be arranged in the absence of diffraction.

I'll pose a question that is relevant to us achromat victims. How does a red or blue color spot that would be, say, 4x the size of the Airy disc in the absence of diffraction, really look when it arrives at the focal plane? Is there smeared light between the diffraction rings of the well-focused green image? Or, as I suspect, does the poorly focused red and blue actually arrange itself in rings nearly coincident with the yellow-green rings, making those rings brighter than in a perfect telescope, but still well defined?

This has important implications for double star observing, where one is often trying to spot a dim companion between the central, visible part of the Airy disc, and the first ring. In my 5" achro, that region looks dark to me. ( I saw the companion to Propus tonight, plain as could be, right inside the first ring.) But is it, "really"?

So, I suspect that this may be part of the answer to "as terrible as the spot diagrams look, how come you can see anything at all through an achromat?" Namely, there's a bright, neat Airy disc, in any color. The converse is also unfortunately true. Now, did I say something bad about APOs?
Ron

Some of my modern-generation friends regard me as a deep sky walking go-to, but I am in fact pitiful. I am weak on the southern M's because I don't have good southern sky in my yard, where I do practically all my observing. I've seen all the M's more than once, but the globulars in Oph, Sgt and Sco are mostly a blur in my mind. The Virgo cluster, with almost no bright stars to guide the eye, is like seriouly deep space. And if you say M-some number, I might go into a panic for a minute before I can think what and where it is, even if it is a "favorite". I know pretty cold the Ms I can see from my yard, but the first time they pop up in the east for the year can sometimes take a bit of thought.

So, my question to you Messier marathoners is, do some of you have perfect mastery? By this I mean the ability to think of, point at, and see without surprise, all the Messier objects, without written or high-tech aid? I think that would be so cool. I hope to live in the country in retirement, and to observe enough to be able to do that. The number of Messiers seems tractable, but the sky is so large!

Holy gurus, step forward, and show me the way.
Ron

The local public library was recently given a beautiful set of full-sky photographic star charts the like of which I have not seen. This is the "Photographischer Stern-Atlas" by Dr. Hans Vehrenberg, publisherd by Treugesell-Verlag in 1963. It consists of two boxes of very nice reproductions of positive (ie, black stars on white background) photos on heavy 8.5x11.5-inch matte paper, The photo covers 7x7 inches on the page, and depicts a 1 degee square of sky. Each box contains 303 such charts, 0-12h in one box, 12h back to 0 in the other, covering +90 to -20 degrees. No Idea how deep it goes, hoo, what a lot of stars, but faint fuzzies don't show up very well for some reason. There is no labeling on the charts, this is not for sissies. It must have cost a lot, but pales in utility next to Uranometria. It is in unused condition.

My mission is to determine its worth and report back to the used bookstore. Help! This thing is staggering. Maybe if one of you wants it I could let you have it cheap and give the money to the library. I have a feeling it won't sell down there and end up in the junk heap with old Clark telescopes, etc. You could cut out the pics, stick them together, and paper an entire room with the sky, blasphemous idea that it is!
Thanks, Ron

Would some of you lens designing and making types (Roland, especially, but also anybody else who can help) please straighten me out a bit concerning the mystery of "eco-glass"? Less toxic chemicals spread around is surely a good thing, but is the glass quality as good, and are as good achro and apo combinations of glasses available, as with old-fashioned lead, arsenic, cadmium, etc. compounds? How prevalent is the use of eco-glass in modern consumer optics? Should the "conniseur" demand toxic substances?
Thanks,
Ron

Through-Lookers,
It is often said that, in most eyepieces, off-axis astigmatism is the dominant aberration in visual observing. Naglers and a few other high-toned many-lensed designs are the exceptions. My experience agrees with this.

It is also well known that astigmatism in the eye will cause stars, especially at eye-filling exit pupils, to look wonky.

I have enough astigmatism in my favorite eye to notice when the exit pupil gets large. But, tonight I discovered something cool, that other people may have noticed but I was unaware of. I was looking at Alberio through my 10", f/6 Newt, using a 40mm UO 70-degree Konig, a dated design, with only fair edge correction. By moving Alberio to a spot about 1/3 way to the edge, towards the lower right, I was able to eliminate the wonkyness that I see in the center, and got the best view of my life at this power--neat, tight, round spots. Whee!

This was not inspired by, but reminds me of, passages on the ATM-ing of achromatic lenses that describe how astigmatic crown and flint elements can be rotated to reduce or almost eliminate net astigmatism, by playing one lens's aberration against the other's.

Those of you with mild astigmatism who don't like wearing glasse to observe, try this trick using your lowest power eyepiece. Of course the direction and distance from center where you get cancellation will depend on your own eye, so you'll have to experiment. I'd like to hear if anybody else can make it work.
Ron

I have often looked at the faint cluster ngc2158 near M35. It interests me because of the perspective it gives. I have read that it is physically very similar to M35, only more distant. I love views like that, where you get a real 3-D feeling of space. In fact I had a view of it in a 20" scope on a really steady night that looked just about like M35 in a small scope, really driving home the point.

But just a bit further from M35, in about the same direction, lies open cluster IC2157, which I had never seen before tonight.

I could see it in a 16x70 binocular. It appeared similar to the nearby NGC2158, a hazy patch.

In a 5-inch scope, the cluster was seen easily at 29x, as a fairly well defined patch of stars. Under higher power, the cluster appeared to be about 6 arcmin in diameter, with 10 stars easily visible. With averted vision, at least that many more, and some unresolved glow, was seen. It is quite different from the neighboring NGC2158, which is a more tightly packed mass of dimmer stars.

I am happy to learn of this second companion to M35. It is not difficult, and I recommend it to all!
Ron