Αχρωματικό διοπτρικό με Chromacor.

[DBAROUNIS]

Νομιζω οτι δεν εχει νοημα η συγκριση. Προσωπικα ειμαι πολυ ευχαριστημενος, και δεν διαθετω αλλα χρηματα για τηλεσκοπιο (C8, ETX, και 2 διοπτρικα Vixen ειναι σε αποθυκευση), ενω καποιος αλλος θα δυσκολευτει αρκετα πλεον να κστασκευασει κατι αντιστοιχο.

Ελπιζω να καταλαβατε οτι δεν μιλω με αυταρεσκεια απλα ρεαλιστικα. Επιμενω οτι η ουσια στο χομπυ βρισκεται στην ευχαριστηση που αντλει καποιος απο αυτο, σε συνδυασμο βεβαια με την γνωση που αποκτα σταδιακα προχωρωντας. Οπως ξαναειπα μας ενδιαφερει ο ουρανος μονο.

 

Σημ. Το τρεχον project μου αφορα την επαναφορα σε χρηση ενος αποχρωματικου τριπλου αστρογραφου Zeiss-Goerz 360 mm f/3.5 κατασκευης 1916.

[DBAROUNIS]

Για μια εξαιρετικη απλη και ακριβη μεθοδο μετρησης σφαιρικης και χρωματικης εκτροπης διαβαστε στο SKY AND TELESCOPE - APRIL 2004-E. PHANNENSCHIMDT.

[ngc4565]

Νομιζω οτι δεν εχει νοημα η συγκριση

 

Εντούτοις για τους φίλους που μπορούν να ενδιαφερθούν για το Chromacor oι Side by Side συγκρίσεις είναι πολύ πρακτικές και ένας αποτελεσματικός τρόπος να συγκριθούν visually της εικόνες ενός τηλεσκοπίου (Seeing is believing)

.

Όπως δήλωσα προηγουμένως υπάρχουν μερικοί που είναι satisfied με το Chromacor - είναι εμφανές ότι είστε σε αυτήν την ομάδα.

 

Προσωπικά πιστεύω ότι (για Διοπτρικά) δεν υπάρχει καμία αντικατάσταση για τις APO εικόνες που μας δίνουν τα Astro Physics, τα Takahashi και τα Borg.

 

Επιχειρήσεις που παράγουν "substitute" προϊόντα συχνά έχουν (μερικούς) φίλους που έπαινουν ιδιαίτερα τα προϊόντα τους - στην πραγματικότητα δυστυχώς δεν υπάρχει κανένα "magical" solution or substitution.

 

To be cost effective, για τον ερασιτέχνη ένα Nευτώνειο 8-10in (ή μεγαλύτερο) είναι η καλύτερη επιλογή και είναι εντελώς color free - ενα πραγματικό APO

[DBAROUNIS]

substitute δεν ειναι. Δειτε το κειμενο του T. Back Defining apochromatism.... Ρωτω και εγω αν πραγματικα η ευρεια κοινοτητα γνωριζει τι ειναι αποχρωματικο. Μηπως ενα διοπτρικο με λευκα fuzzy ειδωλα. Με τον κατασκευαστη δεν εχω καμμια εμπορικη σχεση ουτε προσπαθω να πουλησω το προιον. Για το κατοπτρικο συμφωνω. Επισης διαβαστε στο βιβλιο ΤΗΕ ΕPIC MOON για τον παρατηρητη FAUTH και το θρυλικο διοπτρικο του 15 ιντσων.

[OPTOPHILE]

I WILL BUY THIS EXOTIC

 

 

http://www.zerochromat.com/

[OPTOPHILE]

I WILL BUY THIS EXOTIC MONSTER OF EIGHT INCH

APERTURE

 

http://www.zerochromat.com/

[OPTOPHILE]

gia tou talaiporous toy chromacor vrika auto:

 

ARIES Chromacor Installation/Collimation

INTRODUCTION

 

Installing the Chromacor I or Chromacor II is relatively easy by following these instructions, step by step with some patience and accuracy.

First of all, it is necessary to state that the Chinese achromatic refractors that have become available have mechanics with loose manufacturing tolerances. Therefore, it is not unusual for these telescopes to possess miscollimation or decentering of one or more components in their optical paths.

 

These must be collimated, to ensure that the Chromacor will work as designed. The Chromacor is basically an optical element which reduces or removes an inherent and undesirable trait of all achromatic refractor designs, known as "false color" or chromatic aberration. If one has selected the O or U versions, the Chromacor also participates in reducing spherical aberration on the wave front formation.

 

Among more than 300 samples of Chromacors tested for collimateability (prior to their shipment), using 120mm F/8.3 and 150mm F/8 telescopes, only a few tens worked well without additional collimation and tweaking. . Even these samples didn’t work identically in these telescopes and needed some adjustments to work as designed. The reason is that from telescope to telescope, there were differences in the collimation of objectives, focusers, diagonal mirrors, diagonal threads, etc. Taking into account this variance in Chinese refractors, it was impossible to simply make the Chromacor’s cell assembly super-precise and know that its installation would work without any collimation procedures done by the end user. The controlling factor would still be the telescope's need for collimation with Chromacor.

 

With this in mind, to make Chromacor work as designed, it is necessary to first properly collimate your refractor. This is not any more difficult than collimating Newtonian or Cassegrain telescopes. It is even easier if you take the following steps in order.

 

Some amount of collimation procedure is necessary each time after the Chromacor and/or diagonal is removed, if best results are desired, even if it was fully collimated previously. The following will give you a sense of how to most easily make its installation and (re)collimation as accurate as possible. During numerous testing of all Chromacors, I found the way to make a Chromacor a true "plug & play" device for a given telescope and better yet, even for two or more scopes. For this one will need two metal rings from 48mm color filters with the glass removed, 2" star-diagonal from Intes Co., 2" laser collimator, and slow-drying glue for metals.

(Astrobuffet notes from John: You don't need to trade your MaxBright, EverBrite, William Optics, or other fine 2" diagonal for an Intes. As Valery mentions later, substitution of other 2" diagonals is allowable, provided the adjustment for any difference in optical path length is made. Use of a 1.25" laser collimator in the same 2-1.25" adapter your eyepiece will be used in, is also acceptable. Although it may be less accurate for the straight-through scope collimation, it can be equally or more accurate for use with your diagonal, if care is taken to repeatably set it up identically for the 1.25" eyepiece and the 1.25" laser collimator. Nobody makes a more accurate laser collimator than Howie Glatter: I personally use his 1.25" version. Perhaps the combination 2"/1.25" version would be ideal, with machined surfaces of both diameters. The Hoya 48mm filter rings which I sell are well suited as the empty 48mm filters, and each adds 5.6mm to the optical path.)

 

It is also recommended that the 2" stock visual back found on the end of the focuser's drawtube should be modified with one more locking screw. This means that there should be a total of three screws 120 degrees from each other. We will use them not only for locking a star diagonal in its position, but for collimation as well. For owners of the Chromacor I, a centering ring may be added for best results. It can be made easily in any machine shop, and will be described later.

 

Proper Distance: One can use different star-diagonals and adapters. The only important thing is that the distance between Chromacor's threaded rear flange, (where it’s 48mm thread begins) and eyepiece field stop must be 161mm +/-2mm. So, the entire assembly, (star-diagonal, 2"/1.25" adapter, 48mm extension rings), should measure at this 161mm +/-2mm distance between Chromacor flange and eyepiece's field stop at the focal plane. Note: the field stop location depends upon the eyepiece, but should not be confused with the eyepiece barrel flange location, nor the top of the 2-1.25 adapter, which are two ways of describing "another, more easily described location"! You need to figure out where the focal plane is for your particular eyepieces of interest: namely how far below the "more easily described location".

 

REFRACTOR COLLIMATION

 

Collimation of the telescope itself. Remove the star-diagonal. Insert a 2" laser collimator in 2" visual back. Rack out the focuser to maximum distance and see if the laser collimator beam dot is exactly in the center of the objective. If not, use two Allen wrench screws at the upper side of the focuser to bring the dot in the center along its vertical axis. Then rack the focuser fully in and if the dot moves. Using these screws and if necessary three screws where the focuser attaches to the tube, one should achieve focuser collimation so that the dot is always in the center of the objective, for all focuser positions. Of course, the laser dot will always slightly change its position if it is rotated while inserted inside the visual back. But these changes should be as small as possible. When the drawtube is fully racked-in, the dot must be in the center of the objective. After focuser collimation is reached, the screws where the focuser attaches to the tube must be tightened to maximum and never touched again.

 

The next step is to collimate the objective if it has the newer push-pull lens cell. Using push and pull screws, one needs to collimate the objective with a Cheshire eyepiece (Astrobuffet note: I'll add a section on Cheshire eyepieces!) and then check the collimation looking on a star at highest magnification. No coma should be seen, and all diffraction rings should be circular on both sides of focus.

 

 

Collimation of the star-diagonal: The next possible source of telescope miscollimation is the star-diagonal. Insert your 2" star-diagonal in a scope. Insert a 2" laser in the star-diagonal. See where is the laser dot. If the star-diagonal is well made, then the dot must be in the center of the objective. Using screws in the star-diagonal cell and thin pieces of paper as shims, one should reach such collimation of the star-diagonal, that the laser dot will be in the center of the objective. This is especially important when a drawtube is fully racked-in. This will mimic the position of the Chromacor when in use. It is a good idea to remove the diagonal and re-check straight-through collimation, before investing too much time in shimming your diagonal to perfection.

 

 

 

PRELIMINARY DRAW TUBE MODIFICATIONS

 

Here are some preliminary drawtube modifications that are necessary in order to install the Chromacor properly. Please refer to Diagram 1.

 

 

1. Centering Ring: This is very important for proper Chromacor collimation. In Diagram 1, it is at position # 5. Using thin narrow pieces of tape, # 6, one need to make that the centering ring will fit tightly around the Chromacor's nose. At the same time it should fit the inside diameter of the focuser draw tube with a little friction. So, when Chromacor is installed inside the drawtube, the Chromacor's nose will be more or less centered – enough to go further. On new Chromacor-I and Chromacor-II models this centering ring is "built-in" as part of the Chromacor cell, as seen in Diagram 2.

 

 

2. Remove the stock Chinese 2" visual back from a telescope and drill a hole (of proper diameter to match a tap) in its wall right 120 degrees from both locking screws already there. Using a tap, thread the hole and add a third screw. Now these screws can lock the star-diagonal in it as well as can be used as collimation screws. The stock Chinese 2" visual back has an inside diameter that is usually larger than star-diagonal entrance tube. The gap is enough for collimation. If this gap is not enough for collimation, we need to increase the visual back's inside diameter, by perhaps 1mm, since the star diagonal's outside diameter can vary from manufacturer to manufacturer.

 

Remove the small screw which holds the focusing rack teeth to the drawtube. Make the screw as short as necessary that it will not enter the inside of the draw tube, but still holds the rack teeth firmly. Wire cutting pliers/nippers are a good tool to use to shorten this screw's length. (It would have been nice if they hadn't made the screw protrude inside a 2" inside diameter, but they did. The same is true on the Meade AR series achromats.)

 

Remove the last stop baffle in the focuser drawtube to prevent its interference with Chromacor’s centering ring. This can be done by pushing a long enough screwdriver against one side of the baffle in order to tilt it sideways. Once this is done, either long needle nose pliers or something that can hook on to the baffle that can be used to pull this baffle out of the drawtube.

 

CHROMACOR INSTALLATION

 

Now we have a fully collimated telescope. With the draw tube modified to accept the Chromacor, all that remains is to install the Chromacor and collimate it properly, thus making it a "plug & play" device in the future for a given telescope. (You'll notice that this first time, it is more like "play before you plug in", if you want it done right.)

 

Please refer to the diagrams for your appropriate Chromacor model to use with the instructions that follow.

 

1. Insert a star-diagonal entrance tube, # 9, in the stock 2" visual back, # 3. Screw in the first 48mm extension ring, # 8, into the star diagonal's entrance tube, # 9. Make sure it is in tight. Screw in your Chromacor, # 1, into a second 48mm extension ring, # 7, and make sure that the connection is also tight. Screw the Chromacor assembly's ring # 7, (already tightly connected to the Chromacor) into the diagonal assembly's ring #8, (already tightly connected to the star-diagonal). The two rings should be screwed in fully, but not as tightly as each of the rings is screwed into its respective part. The idea is to be able to join and separate the two rings, without the rings coming off the diagonal or Chromacor.

 

Insert the whole Chromacor/diagonal/visual-back assembly inside the focuser drawtube and move it further inside slowly and carefully. Try to hold it fully parallel to the axis of the drawtube so it doesn't get jammed. If it starts to jam, move it backward until sliding again. Move it in even further and thread the stock 2" visual back, # 3, in its place (on the thread at the end of the drawtube). Tighten the 2" adapter, #3, in its place.

 

Fully insert the star-diagonal, extension rings and Chromacor assembly to its limit with the diagonal body against the 2" visual back, # 3. Lock all three screws, # 4, in the 2" visual back, # 3.

 

Star test: After full cooldown of the scope (including Chromacor), choose a star about 2nd magnitude at least 45 degrees above horizon and point the telescope at it. Use a high magnification of about 30-35 per inch of aperture, about a 6mm eyepiece. Do not to use any Barlow lens at this time, nor a self-Barlowing eyepiece like a Nagler. Place a star in the center of the field and focus the scope. See if the star is fully symmetrical and has no sign of coma. If the star is fully symmetrical without trace of any coma flare, then your scope is about 90% collimated. The remaining 10% can be accomplished by adjusting three centering screws, # 4.

 

If the star image has comatic or flared tail or asymmetrical diffraction rings, and then we need to remove the Chromacor-diagonal assembly from the drawtube to the degree where 48mm extension rings # 7 & # 8, will be outside and accessible. The 2" visual back, # 3 must remain in place. Turn extension ring # 7 relative to extension ring # 8 slightly and place the Chromacor-star-diagonal assembly (CSA) back and lightly lock the screws, # 4. See if a coma flare or asymmetry in diffraction rings still remains. If it does, repeat the procedure by slightly unscrewing between extension rings # 7 and # 8 - this will rotate Chromacor step by step.

 

You will see that the coma flare or asymmetrical rings will change in orientation and size. By repeating this procedure further several times we need to find the ring # 7 orientation to ring # 8 where no coma flare or asymmetry will be seen and the star image will be fully symmetrical and show the Airy disk with a concentric and circular first diffraction ring.

 

 

When the coma flare is removed and a star shows a fully symmetrical diffraction rings, we need to mark this relative position of extension rings # 7 and # 8. Making a scratch across the two rings is a time-honored method. You can also scratch a mark on ring # 7 at the upper point for orientation purposes as shown in Diagram 3.

 

Now remove the Chromacor/ star diagonal assembly from the focuser drawtube only far enough so that both extension rings # 7 and # 8 would be accessible. Unscrew the extension rings # 7 and # 8 and apply a couple of small drops of slow drying glue for metals on the thread of the ring # 8. Distribute the glue more or less uniformly all over the circle.

 

Remove any remaining excess glue with a cotton cloth. Screw-in both rings together to the limit and then unscrew them back to the degree, when a marking scratch on the ring # 7 will be exactly at the upper point. Carefully insert the Chromacor/star diagonal assembly inside the drawtube trying to not change the orientation of the rings relative to each other.

 

Look at the star again. If the star image remains fully symmetrical, then lock the screws, # 4, in the 2" visual back, # 3, making sure that the star image remains fully symmetrical and leave the Chromacor inside the tube till the glue will set completely. Both rings # 7 and # 8 will be impossible to divide.

 

If after "gluing procedure" and restoring rings orientation according to the marking scratch the star will show small coma again, (very low probability, but it is a good idea to check for it, before the glue dries!), then repeat the procedure of step by step rotation of Chromacor & ring#7 in ring #8. When you find the position with no trace of coma, lock the screws, # 4 in the visual back # 3 making sure that a star image is remains fully symmetrical. Now leave the Chromacor inside the tube to the time when the glue will dry up completely and both rings # 7 and # 8 will be impossible to divide.

 

After the time of slow drying glue is completely dry we can continue. First of all, we need to again check if a star's image remains fully symmetrical with no trace of coma. If yes, we can continue to adjust a Chromacor.

 

Point the scope at Jupiter and focus at high magnification (30-35x per inch). See if the planet disk is free of any asymmetrical false color. If yes, then defocus the scope on either side of focus and see if a residual false color halo is symmetrical around the planet disk. The halo should be greenish when the focuser is racked outside of focus and deep violet when racked inside of focus. If both halos are fully uniformly symmetrical around the planet disk, then your telescope is fully collimated and will work as designed. Tighten the locking screws, # 4, in 2" visual back, # 3, firmly and start to use your scope with near Apochromatic performance with the Chromacor-I, or with complete Apochromatic performance with the Chromacor-II.

 

8. If the circumference of Jupiter shows asymmetrical color glow in focus, (one side is deep violet and the opposite side is greenish), this means that the Chromacor-star-diagonal assembly optical axis is not perfectly collimated with the telescope's optical axis, (# 10 and # 11 in Diagrams 1 or 2).

 

Defocus your scope to outside of focus and you will see an asymmetrical greenish halo (much fainter in a Chromacor-II). Using screws, # 4, you can tilt the Chromacor/star diagonal assembly and make its axis very close to parallel. What you want to see is a fully symmetrical color halo on both sides of focus.

 

When you will reach such collimation (fully symmetrical color halos outside of focus and no colored halo in focus) on Jupiter, the collimation procedure is done. Then start to observe.

 

How to reach even better color correction.

The original Chromacor design parameters recommended that the space between the Chromacor's threaded flange and eyepiece field stop to be at 161mm. After collimating your scope and Chromacor as previously described, you can improve the color correction even more. Just move an eyepiece (or 2"/1.25" adapter with eyepiece) farther out to increase a distance between Chromacor and eyepiece. Color correction becomes better.

 

With the Chromacor-II an increased distance of about 15mm will delivers better color correction. Inside of focus images of any object will not show any false colors. Outside of focus images will also be color-free indicating that your scope is perfectly color corrected now. This is the case even for a 6" F/8 telescope.

 

However, I need to again remind you that this level of improved color correction is possible only if you already reached perfect collimation of your scope and Chromacor.

 

 

Please Note: Anytime you remove the Chromacor from the telescope, (in the case that a focuser will not be re-adjusted and screws, which hold a focuser on a tube, will not be touched), and install it back in the telescope's draw tube, the Chromacor will keep its collimation. The only procedure you will need to perform again is step 7 (??? Uh-oh, need to check step numbering from DOC-TXT conversion) in the Chromacor Installation section. After two or three times, you will complete it within 1 min.

 

Congratulations! You now can enjoy sharper images and all benefits that better color correction can offer. The best of luck in future observations.

 

 

Valery Deryuzhin (author)

Sol Robbins (editing, proofreading)

John Hopper (editing, proofreading, notes, wisecracks)

[OPTOPHILE]

http://www.geekjoy.com/sketchgallery/sol_robbins_jupiter_sketches.html

[OPTOPHILE]

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.

[OPTOPHILE]

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.