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Home / A New Refractor Concept: "The Wall Hypochromatic Refractor"
by John Wall 09/01/09 | Email Author

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During research into retrofocally corrected refractor systems, I noted an effect during experiments with an optical bench cum telescope apparatus, that is, when the correcting flint element was removed from the train, the image recovery lenses still formed a back focus image which was smaller but seemed well corrected for colour, in fact it was a better correction than was being had with the flint lens in situ for that particular array. The effect was noted and filed away for future experiment. Later an array was set up having a single element plano convex objective lens , or OG, made from Chance float glass ( which is very good for optical transmission); 150mm dia at F:27. A binocular achromatic objective lens; 50mm dia by 330mm focus was placed at half the focal length. The back focus for the objects being tested at 55m distance was around 268mm. It was noticeable from the start that the colour correction was extremely good when 20 and 15mm Plossl eyepieces were used to observe the multifarious images at hand on the surfaces of houses opposite. Verticals and horizontal window frames being white, showed very little chromatic aberration, ( CA), but lateral CA was evident. This was exciting stuff! Calculations of the dispersal of the Red to Blue back focii were found to be just 2.5mm, whereas the OG had an R to B dispersion of 63mm; this was confirmed by measurement on the bench. This is a reduction of X25 by the reducing lens. A comparison single plano convex lens was to hand having the same effective focal length as the system, which is 774mm at F: 5.2. The R to B dispersion of this lens was measured and found to be 6mm, so the reduction by a reducing lens is not linear, but a much higher order. The ratio of the dispersion of the comparison lens to the back focus dispersion of the reduction lens was 6 to 2.5; this is only a X 2.4 reduction. By introducing a second bino OG of 178mm focus mid way between the first red’ lens and the back focus, the dispersion can be reduced still further, and a displacement of 0.4mm was achieved in one experiment. This is not bad for a refractor without a flint lens! The diagrams below show the actual configuration of the system, and the dispersions in another system compared to a control lens of the same focal length and focal ratio; they depict essentially ,the optical arrangement of a simple refractor .

Key to diagrams:

D= diameter of the OG. EFL=effective focal length.

L= separation distance OG to reduction lens.

BF= back focus R-B= red to blue focus.

The reduction lens suppresses CA to a point where it is acceptable for visual use; the very small distance between the R—B foci means that the images in both colours are practically the same size, and the images are nearly perfectly achromatic. With a 6mm eyepiece the colour starts to show, and the images have a purplish tinge. This is qualitatively very good, but an OSLO analysis shows considerable aberration, however the quality being as it is the system has considerable merit as a visual instrument. The scope makes a very good terrestrial, the actual colour being no worse than that seen in many commercial scopes. Lateral CA was a problem, but this was reduced to an acceptable minimum by having an OG focal ratio of F:40. Objective lenses of shorter F ratio, eg ,F:16 ,will have troublesome lateral colour right up into the centre of the field, although the CA is still very small, however, the shorter system comes into it’s own as a filter refractor, where images need not be obtained in white light, the author is currently investigating a system having a 210mm aperture OG at F:16, where a 70mm reducing lens is placed at one third of the focal length inside the focus. The CA reduction at the BF is about 4mm, but the LCA is too great for the system to be viable as a white light telescope, however, it makes an excellent filter scope,and images with a red filter are very sharp with high resolution, a Helium filter transmits at around 600nm, which is close to the maximum wavelength for the human eye, this was highly successful and yielded images that can only be obtained from a refractor. The author uses terrestrial images to test telescope performance where assessment of the amount of colour in the image can be carried out, also the verticals horizontal diagonals and diffuse grounds such as rough casting or pebble dash yield up for more information about field aberrations in the system, as do the varying contrasts that are present from high to low on any part that one wishes to concentrate on. Another reason is that the research can be done in the daytime, in any weather, and without having to wait for a clear night.

Image of a TV antenna reflector taken through a “ Duplex” ( two reduction lenses), at a distance of 55m, the image is high contrast. The specular reflections of the sun on the horizontal bars show very little colour. Power about X40. The lateral chromatic aberration of the vertical pole is about the amount of colour seen ; this is not ordinary chromatic aberration. An ordinary red acrylic filter will yield a perfect image.

Discussion :

The author had initially named the new system a” Parachromatic “( ref diagrams), refractor on the basis that the system has not been corrected for colour by use of a flint component, but instead the colour has been suppressed to such low limits that the image quality is very acceptable for a general use telescope, however the etymology is incorrect ! and the system should now be designated as an “Hypochromatic Refractor”, (below colour), The purists, no doubt, will dismiss the system as of no use; and they would be right to a certain degree only! however, the brilliancy and sharp definition of the images, having only a very slight colour fringe at the extreme of the field, might induce some to view the new system as an answer to the large aperture refractor problem, namely, that the system is very simple and cheap to make, and one must equate this against the very high cost of even a very moderate aperture eg, 90mm. The system comes into it’s own as an imager’s refractor where say, a large field object such as an extensive gaseous nebula can be imaged with an Oxygen three filter and RGB filter techniques. The author used red filters and a Helium 589nm passband filter and the images were stunningly good.

Two ways to look at the system:

Firstly one can say that an uncorrected OG is being corrected by the suppression effect of a short focus achromat acting as a reduction lens; this has already been stated

Secondly, the system has the effect of increasing the aperture of a small achromat ( the reduction lens) , by placing a weak lens some distance in front of it, if one assumes that the reduction lens is in fact a small refractor. The focal length of the Achromat is also effectively increased from say, 178mm to around 800mm! As the EFL of the system comes into play. The system has an enormous depth of focus ranging from infinity to the front surface of the objective lens, with only a few millimetres shift in the eyepiece, in fact the focus can be brought inside the optical train, in this case it is only the reduction lens that forming the image. The system is a short focus low power scope that will give field diameters around three to five degrees, the focal ratios range from F:2.5 up to F: 6. By using 25 to 10mm focus eyepieces powers can range from X15 to X40. Higher powers can be had by using shorter f/l eyepieces and X200 is feasible, however the CA starts to be noticed. The system works well with a X2 Barlow with no noticeable field aberrations. So far the system is restricted to apertures ranging from 100mm dia to 150mm dia, higher apertures will start to increase the tube length to unacceptable limits. The long focus of the OG is cut in half by the reduction lens, and the remaining separation (L), can be folded three times using two flats. The flats need only be half wave accuracy ; this will make no discernable difference to the image quality, in fact, the author used flats to only two waves correction! The definitive scope made by the author has been folded down to 875mm , that is 34.5 inches. in spite of the fact that the 127mm OG is F:40.

The system is not a dialyte, as there is not a flint element to be separated from a crown element, neither is it a Schupmann deriviative- please note! The system stands alone as an entirely new concept in a class of it’s own. This is a nascent concept, and needs a lot of development, and so the author throws open the idea to other ATM’s to experiment with, you never know, you may be pleasantly surprised.

Provenance for this discovery by the author is established by a published article in the Journal of the British Astronomical Association; 2009 June vol 119 No 3. The BAA has named the system for the author as: “The Wall Refractor.”

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