Refractor objectives

Refractor Objectives

I am presently seriously considering making oiled, two-element objectives in sizes from 6" to 9" at f/15 available as a regular item. They are to be BaK1, F2 coma-free designs. These objectives work stunningly well and represent a link with our optical past. If this is of interest to you, e-mail me and let me know.

These are all airspaced or oiled objectives with identical R2 and R3 curves and are made from BaK1 and F2 glass. They are completely coma free over 1/2 degree. Coating all surfaces results in an excellent wavefront, minimal scattering, no ghost images. These objectives are carefully hand worked and figured until a complete null in autocollimation is achieved.

The BaK1 and F2 oiled design is a favorite of mine. This combination, which uses a barium crown glass (BaK1) as opposed to a borosilicate crown glass (BK7) reduces the effects of the secondary spectrum by nearly 15 percent, resulting in a noticeable reduction in secondary color. The barium crown glass is only very slightly more expensive than BK7 and does not increase the cost of the objective over BK7. Spot diagrams are shown below for comparison purposes and are of 6" f/15 objectives with the D line expanded to fill the Airy disk and the C line (red) and F line (blue) allowed to run out. While the difference is not dramatic, every little bit helps.

BK7 - F2

BaK1 - F2

Aperture	Price Lenses Only	Price Cell	Total Price Lenses and Cell
6"	$1,990	TBD
7"	$2,700	TBD
8"	$2,990	TBD

We are capable of producing refractor doublets from 6" to 12" and of any reasonable focal ratio from f/5 upwards. These objectives are conventional glass doublet designs either air-spaced or oiled depending upon the requirements of the purchaser and individually optimized during manufacture. These are not mass produced objectives but are carefully hand-crafted products. They are typically made of either Schott BK7 and F4 or BaK1 and F2, but any glass combination is possible depending upon availability and customer requirements.

We do not recommend the construction of refractor objectives for serious planetary or lunar astronomical work in ratios less than f/15. This has been considered the nominal standard for acceptable performance since the mid 1800s when the Clarks began making virtually all of their refractors in focal length to aperture ratios of 15 to 1. However, the use of the BaK1 glass does allow for f/12 focal ratios in 6" sizes if customers insist - though f/15 is still preferred. I am occasionally asked to make refractors of 6" and 8" at f/10. I avoid the production of these refractors inasmuch as they will invariably produce a rather strong secondary color spectrum. The ability to recognize secondary color is interesting in that while some people do not see it at first, progressive use of the instrument and an increasing skill and awareness will eventually result in dissatisfaction. The existence of secondary color is nothing more than a defocused blur in the red and blue and a loss of contrast and information.

All prices are quoted individually and purchasers of specialty refractor doublets should be aware of the large up-front setup and tooling costs in the form of fixtures, test plates and other paraphernalia associated with the production of unique multi-element refracting objectives.