Apochromatic lenses

Apo-lenses for amateur telescopes
made on base of OK-4 glass type

The main advantage of lenses systems in comparison with mirror or lenses-mirror one one is absence of central shielding, which negatively affects the image quality characteristics of the optical system. Influence of central shielding in mirror telescope is expressed in image contrast reducing in sufficient wide range of spatial frequencies in comparison with non-shielding system, what has bad influence during visual observing of most objects and also during taking pictures.

However, if the properties of mirror surface reflection are equal for the rays of any colour, then the refractive materials have dispersion - the refractive index dependence of emanation wave length what results to the beams of different colours refraction at the “air-glass” border at different angles. In optical systems this effect results to the chromatic aberrations, which appear to be as different colored halos during for instance star observation.

Selecting combinations of usual optical glasses is succeeded to bring the images generated by beams of any two colours (usually it is made for red and blue beams situated near the borders of visual spectral range) to one point. At that the image generated by the beams of third green colour (the middle of the visual range) doesn’t coincide with this point. This lack of coincidence is called the secondary spectrum and objectives with such type of correction of aberration are called achromats. The secondary spectrum from usual glasses in achromats is sufficiently great and image colour reduces the image quality and limits the possibility of objective aperture ratio developing especially long-focus. Objectives with the corrected secondary focus (for which the star image looks monochromatic as for mirror systems) are called apochromat.

Using the usual optical glasses to correct the secondary spectrum is impossible. For this purpose optical materials of special trace of dispersion curve are used – crystals and “special” glasses. Traditionally the optical crystal of calcium fluoride is considered to be the most suitable material which allows to make the apochromatic combinations including usual glasses. Fluoride is a unique and expensive material which in addition has some disadvantages which are given below.

Russian scientists designed and developed special types of optical glass - fluorophosphate crown glasses, which has optical specifications similar to fluorite. LZOS experts developed and melted a glass from the fluorophosphate crown glasses group - special crown of OK4 type. Having near fluorite optical constants OK4 glass at the same time has the some advantages in comparison with fluorite: first of all it is a higher optical homogeneity, absence of cleavage usual for fluoride optical crystals. Apart from that OK4 glass has small thermo-optical constant Wt (3 times less in absolute value than fluorite has) that allows to avoid of thermowave aberrations, i.e. aberrations which appeared under environment temperature changes.
Schemes of LZOS astronomical lenses objectives-apochromats are designed with the use of OK4 glass. These are three-lenses or triplet systems which have ideal colour correction for visual range of spectrum and excellent correction of monochromatic aberrations in the point at axis.

picture taken via telescope with an apo-objective

Concerning optical quality of produced systems our company as most of the world manufacturers of astronomic optics confirms its for the each objective by report of system wave aberration testing by interferometric method. Traditionally interferometric measurements are carried out with the use of helium-neon laser the wavelength of which is 633m. This wavelength is in the red area of visual spectrum range (i.e. essentially at its long-wave cutoff) that is why its use can’t always give the adequate evaluation of visual system image quality. In fact our company became the first manufacturer of astronomical objectives which provides its certification according to the results of interferometric testing with the use of green laser (wavelength is 0.532m). This wavelength is in the middle between maximums of eye sensitivity for twilight vision and diurnal vision and that is why gives evaluation coincides with the experts evaluation of system quality according to the results of point object image under observation.

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