OPTOPHILE

Δες σε ιστοσελιδες μεταχειρισμενων τον Zeiss Sonnar 200mm f 2.8. Πιθανον να υπαρχει με σπειρωμα Μ42, αλλα με μικρο κοστος ενας προσαρμογεας λυνει το προβλημα. Προκειται για θρυλικη οπτικη κατασκευη. Δειτε κριτικες και γνωμες στο Google. http://www.astrosurf.com/re/halpha.html

Δες το ζητουμενο σχεδιο για τον προσαρμογεα σε πυλωνα

Need Contrast:Baffles + 15% secondary obstruction+Perfect optics (λ/10 wavefront)+electric ventilator. Result MN68 Deluxe

Αυτό εξηγεί τις περίεργες λάμψεις στο βίντεο που έβαλα; Δεν καταλαβαίνω απλα εδωσα την σωστη ερμηνεια της ακτινοβολιας Cherenkov!!!!. Tα υπολοιπα ερμηνευονται κατα βουληση. Τα Laser retroreflectors ( http://www.lpi.usra.edu/lunar/missions/apollo/apollo_14/experiments/lrr/ , http://www-g.oca.eu/cerga/laser/laslune/llr.htm , http://www.apo.nmsu.edu/ , http://www.physics.ucsd.edu/~tmurphy/apollo/apollo.html )που αφεθηκαν στη Σεληνη απο τις αποστολες πριν 40 χρονια, εξακολουθουν μεχρι σημερα να χρησιμοποιουνται για την αποστασιομετρηση του δορυφορου μας. Δειτε τις πολυ εντυπωσιακες φωτογραφιες: http://www.physics.ucsd.edu/~tmurphy/apollo/first_lt.html Επισης μετα τα τοσα γεωλογικα δεδομενα απο τις επομενες αποστολες στη διαθεση μας, μια αναλυση του σεληνιακου δειγματος που πρεπει να υπαρχει στο Ελληνικο υπουργειο Εξωτερικων (δωρο της κυβερνησης των ΗΠΑ στην Ελλαδα), μαλλον θα πεισει και τους πιο δυσπιστους της παρεας.

H ακτινοβολια Cherenkov παραγεται οταν η ταχυτητα φορτισμενων σωματιδιων που διασχιζουν ενα μεσο (οχι το κενο, οπως το νερο ή ο αερας) ειναι μεγαλυτερη απο την ταχυτητα του φωτος στο μεσο αυτο (προσοχη το σχετικιστικο οριο ταχυτητας του φωτο 300.000km/sec ειναι η ταχυτητα στο κενο!!!!!!!). Πχ, ταν το φως διατρεχει το φακο ενος διαθλαστικου τηλεσκοπιου η ταχυτητα του ειναι μικροτερη του c. Το μπλε χρωμα των αντιδραστηρων τυπου "πισινας" ειναι ακτινοβολια Cherenkov. Aτυχώς οι ζωνες Van Allen ονομαστικαν ετσι μετα την αναλυση των αποτελεσματων του δορυφορου Explorer, αλλα ο Φυσικος που προεβλεψε την υπαρξη τους θεωρητικα ήταν ο Nικ. Χριστοφιλου, απο τα εργαστηρια Lawrence-Livermore. O ιδιος ηταν και ο εμπνευστης του πειραματος ARGUS ( http://en.wikipedia.org/wiki/Project_Argus)ισως του πιο σημαντικου στρατιωτικου πειραματος στην νεωτερη ιστορια (το πειραμα ARGUS αποτελει την αφορμη για την ταινια GOLDEN EYE με τον διασημο J. Bond ) . Τέλος, ηταν ο εφευρετης του ΣΥΝΧΡΟΤΡΟΝ επιταχυντη, καθως και της θεωριας εκπομπης της ακτινοβολιας με το ιδιο ονομα, απο τους αστερες νετρονιων, και ενεργους γαλαξιακους πυρηνες.

Πολυ σωστα!!

Tα εργοστασια Carl Zeiss Jena μετα το 1945 βρεθηκαν στη ρωσικη ζωνη κατοχης. Μεγαλο μερος του εξοπλισμου μεταφερθηκε στη Ρωσια οπου και εγκατασταθηκε στη LOMO, στο οπτικο εργοστασιο του Νοβοσιμπιρσκ(βλεπε τηλεσκοπια TAL) , και στο Καζαν (εργοστασιο KOMZ, οπου παραγονται τα καταπληκτικα κυαλια BPC/BPO) Πολλα γερμανικα οπτικα προιοντα συνεχισαν να παραγονται στη Ρωσια και για δεκαετιες τα μηχανικα τους μερη ηταν πληρως εναλλαξιμα με τα γερμανικα (οπως καμερες CONTAX, LEICA, στρατιωτικα οπτικα -παραδειγμα η διπλη διοπτρα του αντιεροπορικου πυροβολου FLAK88,φακοι ZEISS με το ονομα JUPITER). Ταυτοχρονα η αναπτυξη ακαδημαικων τμηματων εδωσε και το απαραιτητο θεωρητικο υλικο πχ Dimitri Maksutov, B. Ioannisiani κ.α.. Η παραγωγη μεγαλων οπτικων συνεχιζεται, πχ τα οπτικα του ΑΡΙΣΤΑΡΧΟΥ ειναι LZOS( http://lzos.ru/en/index.php?option=com_content&task=view&id=131, http://lzos.ru/en/index.php?option=com_content&task=view&id=130 )(οπως και οι αποχρωματικοι φακοι TMB) και για μικρα απο τις TAL, YLENA, INTES MICRO, ASTREYA (super apochromats), Aries (subcontractor στην AstroPhysics για οπτικα μερη Maksutov και SPL προσοφθαλμια) κ.α. http://lzos.ru/en/index.php http://www.lomoplc.com/ http://astreya-optics.narod.ru/eng/index.htm http://www.company7.com/zeiss/history.html http://www.company7.com/astrophy/maks/250f146.html http://www.telescopengineering.com/history/DmitriMaksutov.html Απο τη Ρωσια η οπτικη τεχνολογια απλωθηκε και σε αλλες χωρες οπως η Ρουμανια (IOR), Γιουγκοσλαβια, Ουγγαρια (GPU, http://www.gpuoptical.com/index.htm ) και Κινα (με τα γνωστα αποτελεσματα!).

The link guides you to Lunar and other space misssions conversations: http://www.ehartwell.com/Apollo17/MissionTranscriptCollection.htm

γενικα, προκειται για καλα τηλεσκοπια. Υπαρχουν και εξαιρετικα κομματια απο πλευρας οπτικης ποιοτητας.

O Γερμανος τεχνικος κανει αριστη επισκευη σε κυαλια με προβληματικο παραλληλισμο¨ Αρκει η αξια αγορας τους να δικαιολογει το κοστος επισκευης. http://astro.uni-tuebingen.de/~grzy/8.html

try the UNIVERSITY OPTICS KOENIG 16MM!

TRY THE INTES MICRO MN56, MN55 OR MN66. FLAT FIELD, NO ASTIGMATISM NO COMA!!!! http://www.astrotech.it/it/prodotti/intesmicro/intesmicro.htm. FOR BUDGET SEE THE SKYWATCHER MN190.

First remove the focuser and housing. The draw tube will be loose and adjusting the allen screws only adjust the tube down and alignment could be worse. Get that caliper again and measure the hole to the scope tube. Is it round? Is the draw tube centered? If not there are two ramps cast in the holder with pads on them. You can build up the pads or what I did was to slightly bend the casting blocks where the pads are glued. A screw driver with a little pressure in the hollow opening will make them a few thousands taller then use the top Allen screw to adjust the remaining play. Add a touch of lithium or molly grease to the pads and upper tension block where the Allen screws are put back together. Now the tricky part. Remove the lens cell and measure the end of tube to see if it is square. Mark the high spot with felt pen. Got to love those felt pens LOL. Now mark the outer lens with a dot and the cell. Remove the ring and remove the outer glass lens. Mark the second lens and cell. Measure the seat where the lens sets is it square. Without going in detail use tiny slivers of black carbon paper or wax paper to get everything square and adjust according to where you put your marks on tube and cell. This might take a few tries to get it dead on but the results are worth it because a lens assembly that is off just a tad will produce poor performance at high powers where at low powers you won"t even notice it. Now another problem with Synta refractors (China made) is some are suffering from astigmatism. Not much can be done because it is in the figure of the glass. My unit had a problem with astigmatism at high powers on stars, however I noticed most planets were great. Here is how to check this problem. Using high power within the scopes ability, say 200x 4" 250x 4.5" 300x 5" etc. focus on a bright star. Not too bright but bright and look for a elongation of image just before or after focus. If it stretches then jumps to a pinpoint you have astigmatism. My scope had it on one side of focus only. Did I fix it? Well did I HaHa. Yes I got rid of 90% of it. How you say. Take your outer lens ring off and remove the lens leave the second negative lens in place and mark it. Turn the outer lens 180-degrees and put it back. Start star testing. Is it worse or better? Move lens 2-4 degrees with suction cup only. Loosen ring so you can turn and leave out rubber ring or gasket. Now lightly snug ring each time to hold glass firm. Star test better or worse? Keep doing this small rotation on the outer lens and if you are lucky you will find the flaw is a combination of both lenses as in my case. When you got the best you can get hold the lens down while you tighten the ring with the gasket or rubber ring. In my case down. You are done This might not fix the problem depending on the nature of the aberration Astigmatism For me it helped allot not perfect but much better.. This can be done in house during the day with a bright sun. Get a small shiny ball bearing and mount it on a stick with super glue. Set it up about 60-100 feet outside in the sun from in the house with an open window or door. You can do some very accurate star testing. Actually I prefer to use this over a real star test because the seeing is perfect on a nice day at 100 feet. Also it helps to use a dark green filter to make all adjustments as the filter kills the flare and hollows which make precise dialing in a task. My 120 skywatcher is pretty darn good for a achromat and a light yellow filter say #8 or #12 depending on how much color you have works fine and if you got some bucks get a chromacor or mv1 or other corrector. I heard they work very good but kinda pricey. If after the collimation with a cheshire and laser you have the same image, then the "coma" picture is sign of objective de-centering. So you have to send the scope to the factory for repair. NEVER TOUCH THE OBJECTIVE ELEMENTS ALONE THE ELEMENTS HAVE SPECIFIC DISTANCE BETWEEN THEM AND SPECIFIC ROTATION!! !!!!!!!!!!!! !!![

COLLIMATION

OUT OF COLLIMATION......

Η ΠΡΑΣΙΝΗ ΓΡΑΜΜΗ ΜΑΛΛΟΝ ΔΗΛΩΝΕΙ LATERAL CHROMATIC ABBERATION, KAI ΠΡΟΕΡΧΕΤΑΙ ΠΙΘΑΝΑ ΑΠΟ ΤΟ ΠΡΟΣΟΦΘΑΛΜΙΟ. ΤΟ ΣΦΑΛΜΑ ΑΥΤΟ ΕΙΝΑΙ ΟΡΑΤΟ ΑΚΟΜΑ ΚΑΙ ΣΕ ΚΑΤΟΠΤΡΙΚΑ ΚΑΙ ΣΕ ΚΑΤΑΔΙΟΠΤΡΙΚΑ. OYΣΙΑΣΤΙΚΑ, ΠΡΟΚΕΙΤΑΙ ΓΙΑ ΔΙΑΦΟΡΕΤΙΚΗ "ΜΕΓΕΘΥΝΣΗ" ΠΟΥ ΔΙΝΕΙ ΤΟ ΠΡΟΣΟΦΘΑΛΜΙΟ ΑΝΑ ΜΗΚΟΣ ΚΥΜΑΤΟΣ ΦΩΤΟΣ ΠΟΥ ΕΣΤΙΑΖΕΤΑΙ ΑΠΟ ΤΟ ΚΥΡΙΩΣ ΟΠΤΙΚΟ ΣΥΣΤΗΜΑ 'Η ΣΕ ΦΩΣ ΠΟΥ ΕΣΤΙΖΕΤΑΙ ΕΚΤΟΣ ΚΥΡΙΟΥ ΟΠΤΙΚΟΥ ΑΞΟΝΑ ΤΟΥ ΣΥΣΤΗΜΑΤΟΣ. ΓΙΑ ΤΗΝ ΙΣΤΟΡΙΑ ΤΟ ΠΡΟΣΟΦΘΑΛΜΙΟ ΠΟΥ ΔΕΝ ΕΧΕΙ ΤΟ ΣΦΑΛΜΑ ΑΥΤΟ ΕΙΝΑΙ Ο ΤΥΠΟΣ HUYGENS 2 ΣΤΟΙΧΕΙΩΝ (ΣΧΕΔΙΑΣΗ-ΚΑΤΑΣΚΕΥΗ HUYGENS 1670 ΠΕΡΙΠΟΥ). ΠΟΛΥ ΣΠΑΝΙΟΣ ΣΤΙΣ ΜΕΡΕΣ ΜΑΣ!!!.

EΞΑΙΡΕΤΙΚΟΣ ΟΠΤΙΚΟΣ, ΜΕ ΠΑΝΕΠΙΣΤΗΜΙΑΚΟ ΕΡΓΑΣΤΗΡΙΟ, ΑΛΛΑ ΚΑΙ ΠΡΟΣΩΠΙΚΟ ΟΠΤΙΚΟ - ΜΗΧΑΝΟΥΡΓΙΚΟ ΕΡΓΑΣΤΗΡΙΟ, ΑΞΑΙΡΕΤΙΚΗ ΠΟΙΟΤΗΤΑ ΕΡΓΑΣΙΑΣ, ΚΑΙ ΚΑΛΟ ΧΑΡΑΚΤΗΡΑ.

http://www.aktistar.gr/CR150HDTest.htm

http://astro.uni-tuebingen.de/~grzy/index.html

Για επισκευη με εξαιρετικη ποιοτητα εργασιας http://astro.uni-tuebingen.de/~grzy/index.html

ΤΙ ΕΙΝΑΙ ΑΠΟΧΡΩΜΑΤΙΚΟ??? (ΤΗΟΜΑS BACK OF TMB OPTICS) With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma. Here is a more detailed analysis for those that are interested. The term "Apochromat" is loosely used by many manufacturers and amateurs astronomers. Let's look at the history of the definition, and maybe a more modern one. Ernst Abbe, in 1875, met and worked for Carl Zeiss, a small microscope, magnifier and optical accessory company. They realized that they needed to find improved glass types, if they were going to make progress with the optical microscope. In 1879, Abbe met Otto Schott. Together they introduce the first abnormal dispersion glasses under the name of Schott and Sons. Abbe discovered that by using optically clear, polished natural fluorite, in a microscope objective, that apochromatism could be achieved. These first true apochromatic microscope objectives were so superior to the competition, that Zeiss gained nearly the entire high end market. So secret was the use of fluorite, that Abbe marked an "X" on the data sheet for the fluorite element, so as to keep it secret from the other optical companies. Abbe's definition of apochromatism was the following. Apochromat: an objective corrected parfocally for three widely spaced wavelengths and corrected for spherical aberration and coma for two widely separated wavelengths. This definition is not as simple as it sounds. I have designed thousands of lenses: simple achromats, complex achromats, semi-apos, apochromats, super-achromats, hyper-achromats, and Baker super-apochromats. Abbe's definition, to put it in clearer terms (I hope) is that a true apochromat is an objective that has three color crossings that are spaced far apart in the visual spectrum (~4000A, deep violet to ~7000A, deep red). However, just because a lens has three color crossings, doesn't mean that it is well corrected. Let's say that a 4" lens has three color crossings at the F, e and C wavelengths (4861A, 5461A and 6563A). Fine, this objective is now considered an apochromat by most amateurs and even optical designers because it has three color crossings in the blue, green and red -- the common definition of an apochromat. But what about the levels of spherical aberration at each of these wavelengths? If the lens is 2 waves overcorrected at 4861A, and 1.5 waves undercorrected at 6563A, is it still an apochromat? No. It is no better than an achromat, as the OPD wavefront error is worse than a 4" f/15 achromat. Abbe, in his definition of apochromat, states that spherical aberration must be corrected for two widely spaced wavelengths. Now I will tell you what happens when you correct spherical for two widely spaced wavelengths: you correct for all the wavelengths between them too. This is called correcting for spherochromatism (the variation of spherical aberration with a change in wavelength). Only with extremely long focal lengths, advanced Petzval designs, aspherics, large air spaces, or a combination of these designs/factors, can you correct for this aberration. It is the designer that must come up with a good compromise of color correction, lack of spherical aberration (3rd order and zonal) and controlling spherochromatism, so as not to degrade the image contrast.

ΚΟΙΝΟΣ ΜΕΓΕΝΘΥΝΤΙΚΟΣ ΦΑΚΟΣ ΓΙΑ ΤΗΛΕΣΚΟΠΙΟ???? ΑΦΘΟΝΗ ΧΡΩΜΑΤΙΚΗ ΚΑΙ ΣΦΑΙΡΙΚΗ ΕΚΤΡΟΠΗ. ΠΑΡ'ΟΛΑ ΑΥΤΑ ΘΑ ΔΙΝΕΙ ΕΞΑΙΡΕΤΙΚΕΣ ΨΥΧΕΔΕΛΙΚΕΣ ΕΙΚΟΝΕΣ.

ΤΑ ΚΛΑΣΣΙΚΑ uo ORTHO ΕΙΝΑΙ ΟΤΙ ΚΑΛΥΤΕΡΟ (EKTOS MARKETING) ΚΥΚΛΟΦΟΡΕΙ ΣΤΗΝ ΑΓΟΡΑ. ΣΕ APO 150 F/8 ΣΕ ΣΥΝΔΥΑΣΜΟ ΜΕ UO KLEE 2.8X (ΠΡΟΕΙΝΕΤΑΙ ΓΙΑ ΑΓΟΡΑ) ΦΤΑΝΕΙ 120/ΙΝΤΣΑ ΜΕ ΤΕΛΕΙΟ ΕΙΔΩΛΟ. ΑΝ MΠΟΡΕΙΣ ΠΑΡΕ ΚΑΙ ΤΟ 25,18, 9.

The Petzval Telescope & Sub-Aperture Color Correctors. Two other ways to reach apochromatic correction are either a) by means of a "Petzval" telescope; or b) by means of a sub-aperture color corrector. The former consists of two widely separated doublet lenses which are constructed so as to compensate one another's aberrations and produce a better focus than any single doublet can. The latter consists of a small group of lenses which are inserted into the light-train of a pre-existing finished achromat in order to correct the achromat's secondary spectrum. Versions of the sub-aperture corrector have been marketed commercially, but I do not propose to speculate about these. Rather, I will show several corrective systems of my own devising, the first of which is indebted to an article published by Roland Christen in 1985. The Petzval telescope and sub-aperture color correctors belong to a class of systems called "dialytic" or simply "dialytes," meaning the they consist of widely "separated" lens elements. Proposals for dialytic refractors go back 175 years to Alexander Rogers in 1828 [A. Rogers, "On the Construction of large Achromatic Telescopes," Memoirs of the Astronomical Society of London 3.2 (1829), pp. 229-233; cf. also H. King, History of the Telescope (Dover reprint, 1979), pp 191; R. Riekher, Fernrohre und ihre Meister, 2nd ed. (Verlag Technik, 1990), pp. 231-232; and importantly, A. Danjon & A. Couder, Lunettes et Télescopes (Paris, 1935), pp. 254-255]. The problem Rogers was trying to address was not the removal of secondary spectrum, but something more fundamental. Until the middle of the 19th century, it was difficult to obtain large homogeneous disks of flint glass [cf. Riekher, pp. 144ff. & p. 231; and King, pp. 176ff.]. Whereas crowns were far more easily made. Rogers' proposal, therefore, was to form the objective lens from a large singlet of crown glass, and then to insert a much smaller doublet of both flint and crown, positioned about halfway down the light-train in order to correct the primary spectrum and spherical aberration of the singlet objective. The reason why Rogers proposed a doublet corrector, instead of just a small singlet of flint [as. J.J. von Littrow did in the same year: "Ein Beitrag zur Verbesserung achromatischer Objektive," in Baugartners und Ettinghausens Zeitschrift für Physik und Mathematik 4 (1828), pp. 257-276], was that he wished to keep the optical power of the objective unchanged, and simply to correct the objective's primary spectrum by means of the flint. A singlet corrector of flint would necessarily change the telescope's optical power as it developed enough dispersion to correct the primary spectrum.

An Introduction To Chromatic Aberration In Refractors The popularity of high-quality refractors draws attention to color correction in such instruments. There are several point of confusion and misconceptions. I am not an optical designer - just a used physicist who occasionally masquerades as an amateur telescope maker or amateur astronomer - but perhaps I can clear things up a bit. LONGITUDINAL CHROMATIC ABERRATION: Nobody makes refractors with single-lens objectives because of longitudinal chromatic aberration, briefly called "longitudinal color". A simple lens has different focal lengths at different wavelengths. A well made lens will give a sharp image in any color you like, but that image will be blurred by the out-of-focus images of all other colors, combined. If you plot the focal length of a simple lens as a function of wavelength, across the visible spectrum, you will find that the difference between the minimum focal length (for blue) and the maximum (red) is about one and a half percent of the average focal length. Thus, a simple lens of nominal focal length 1000 mm might have a focal length in the red of 1007.5 mm and in the blue of 992.5 mm. Such a lens is said to have longitudinal chromatic aberration, across the visual, of one and a half percent, or 15 mm for a 1000 mm focal length. Suppose you make such a lens, that is 100 mm diameter, and use it as an f/10 objective in a telescope. If you adjust an eyepiece to be in focus half way between red and blue, at the 1000 mm position, images of bright stars will be surrounded by an overlapping violet glow - the out-of-focus red and blue images mixed - that is 7.5 mm divided by 10, or 750 microns in diameter. The diameter of the Airy disc of a diffraction-limited f/10 objective is only about 13 microns, over 50 times smaller, so our simple-lens refractor won't work very well. ACHROMATS: Using two kinds of glass helps a lot, if the glass types are well chosen. With a positive lens of crown glass, and a negative one less strong, of flint glass, one can design a lens with longitudinal color reduced to about 0.05 percent of focal length. A lens that is a combination of two simple lenses is called a doublet. Such lenses are usually designed to focus blue and red light to the same position, with all other colors focusing at either slightly longer or slightly shorter focal lengths. A lens that brings two colors to a single focal point is called an achromat. That 0.05 percent is the longitudinal distance between where red and blue focus, and where green focuses. For such an achromat, the longitudinal chromatic aberration on our 100 mm f/10 objective is only 500 microns. If we focus for best sharpness in the green, star images will be surrounded by a purple haze that is only 50 microns in diameter - four times the diameter of the Airy disc. The eye is most sensitive to green light, and some common glass combinations do better than I have described, hence a modern four-inch f/10 achromat made from conventional glasses is pretty good with respect to chromatic aberration. I have one - a Vixen - and though it does show a tad of purple haze, it is a fine instrument. Whizzier optical materials make whizzier achromats. Use of first-generation "ED" glasses can reduce the longitudinal color by a factor of four, to 0.0125 percent of the focal length, and later ED glasses or fluorite can get another factor of two. A well-made four-inch f/10 doublet achromat using ED glass can have the purple blur the same size as the Airy disc, and a fluorite doublet of the same size can have the blur smaller than the Airy disc. That's why people like ED and fluorite doublets. APOCHROMATS: Still more pieces of glass can help more. A triplet objective - using three pieces of glass - can be designed to bring three colors of light to the same focal point: Remember that a garden variety achromatic doublet does so for just two. A lens system which brings three colors to the same focal point is called an apochromat. Three instead of two is not necessarily better, however - it is a win only if the total length of the longitudinal blur, where all the other colors come to focus, is reduced. There are even some wild glass types, with odd optical properties, whose use permits the design of a two-lens apochromat - just two pieces of glass can bring three colors to the same focal point. Last I heard, however, these glasses were expensive, difficult to work, and not very durable. But perhaps we will see more of them in the future. APOCHROMATS AND ACHROMATS COMPARED: Some early triplet apochromats had as much longitudinal color as achromats of their day. For ease of photography, they were designed simultaneously to be in focus for visual use and for imaging with early, blue-sensitive plates. Photography is still a consideration. Fluorite doublets have excellent color correction for visual work, but that correction often fails rapidly beyond the blue end of the spectrum, at wavelengths to which many photographic plates respond: If we could see just a little farther into the blue than we actually do, fluorite doublets would make lousy visual telescopes. But we don't, so they are superb: I've got one - a Vixen 90 mm f/9 - and I love it. In the visual wavelengths, the comparison is a little different. Most modern triplet apochromats have less longitudinal color than modern doublets that use conventional glasses, or early ED glasses, but good fluorite doublets generally equal the best of triplets, at least, across the visual. Triplets have other advantages, though, particularly for getting excellent images across a wide field, as useful for photography. SCALING LONGITUDINAL COLOR: How about for other sizes of telescopes? Well, things scale as follows: The linear diameter of the Airy disc depends only on the f number of the telescope, and on the wavelength. The actual formula is: Airy disc diameter = 2.44 * lambda * f, where lambda is the wavelength and f is the f-number. If you are puzzled because you thought bigger telescopes gave better resolution - smaller diffraction patterns - you are right, but so am I. This formula is for linear diameter, not angular diameter. An 8-inch f/10 telescope has twice the scale at the image plane, as a four-inch f/10, so that same-linear-size Airy disc takes up only half as much angle on the celestial sphere. If you hold the focal length constant, and change the aperture of the lens, the diameter of the purple blur circle changes in proportion to aperture. But if you hold the aperture constant and change the focal length, the diameter of the purple blur circle remains constant. So if you go from a four-inch f/10 to an eight-inch f/10, by simply scaling up the lens design by a factor of two, you get twice as big a blur (twice as much aperture), but you still have the same size Airy disc (still f/10), so the ratio of purple blur circle diameter to Airy disc diameter is doubled. If you want it back the way it was, you have to make your eight-inch telescope f/20 instead of f/10. That doesn't change the blur circle size, but it makes the diffraction disc twice as large. Now you see why large refractors often have long focal ratios. Alternatively, if you speed up your four-inch f/10 to f/5, you haven't changed the blur circle size, but you have cut the diameter of the Airy disc in half, so all of a sudden that blur circle looks a lot bigger in proportion. Now you understand about all those colored star images in little rich-field refractors. Unfortunately, there is more to color correction of refractors than longitudinal color. There is a major aberration we haven't considered. SPHEROCHROMATISM: Spherochromatism, or change in spherical aberration with wavelength, is a little confusing. To see what is going on, imagine we have set up a telescope for testing its figure - by star-testing, by null-testing with a knife-edge, by a Ronchi screen, or whatever. Suppose further, that we have arranged to perform the test in various wavelengths of light - perhaps by using a bunch of narrow-bandpass filters. The question is, is the figure equally good (or equally bad) at all wavelengths? The answer is, not necessarily, and a lens which has different figures at different wavelengths of light is said to have spherochromatism. Now clearly, correction for spherical aberration is a big deal in telescope objectives - after all, that is the only thing that people who make paraboloids worry about. So it follows, that spherochromatism in a refractor matters a lot. Our imaginary test setup provides a good way to understand the difference between longitudinal color and spherochromatism. A lens with no spherochromatism would give a perfect test result at every wavelength, but if it had longitudinal color, we would have to refocus the test apparatus, or change the knife-edge position, every time we changed filters. A lens with no longitudinal color would not require refocusing on wavelength change, but if it had spherochromatism, it would give different test results at different wavelengths. What the designer of a simple visual objective will probably do is make the spherical aberration small at the wavelength to which the human eye is most sensitive - green - and trade off spherical aberration at other wavelengths till the correction is about the same in magnitude but opposite in sign in blue and red. It is hard to separate the effects of longitudinal color and spherochromatism under test, and indeed, what counts is their combination - that's why designers publish spot diagrams in many colors. But for understanding, it is useful to distinguish them. If a lens had no longitudinal color, then spherochromatism would be visible in the star test, as color in the out-of-focus images of a star. Perhaps the edge of the out of focus diffraction pattern would look purplish, and the center greenish, or the other way around. I sometimes think I see this effect in my 98 mm f/6.7 Brandon refractor, whose objective is an one of Roland Christen's early triplet apochromats. SCALING SPHEROCHROMATISM: I don't have any generalizations about how spherochromatism varies with type of lens design. But its significance scales with the design, if you change both aperture and focal length in the same proportion. That is, if a four-inch f/10 focuses red light from the 70 percent zone half a wave longer than red rays from the center, then when you double all lens dimensions (making an 8-inch f/10), that half wave will double, too, and increase to a whole wave. Once again, large objectives are a difficult proposition. LATERAL COLOR: One chromatic aberration of eyepieces is worth mentioning, simply because it is common: In systems with decent objectives, it is often the most obvious chromatic aberration of the whole telescope. That is chromatic difference of magnification, or lateral color. It has nothing to do with color correction of the objective - for the time being, we can assume that the objective is textbook perfect. The problem occurs when eyepiece focal length varies with wavelength. When that happens, since magnification is inversely proportional to eyepiece focal length, the magnification will be different at different wavelengths. Suppose you are looking at the Moon. The image you see, perhaps, will be a small red Moon, a medium-sized green Moon, and a large blue Moon, all superimposed. (Red and blue might be reversed - it depends on the eyepiece design.) The edge of the Moon will appear bluish - that's the large blue image, sticking out from the superposition, and the boundaries of shadows and of materials of different brightnesses will have red and blue colored fringes. Lots of binoculars have lateral color. You can see it in daylight as colored fringes at boundaries between light and dark areas, that are bigger at the edges of the field of view. REFERENCES: Most of the material I have described here is basic optics. It will be covered in most books on optics, and will be touched upon lightly in introductory college physics texts. A good source for more detail is Harrie Rutten and Martin van Venrooij, 1988. _Telescope_Optics_, Willmann-Bell.