Astronomical Society of Edinburgh
Journal 42

Binoculars with 153mm Diameter Mirrors

A precis of an article written by Laurent Souchu of the ASE and published in the French periodical Astronomie Magazine no.14 June 2000.
Translation by Ron Livesey

Introduction

Binoculars are not necessarily a lightweight instrument slung around one's neck. This is a 153mm instrument, ideal for observing comets, the moon and the Milky Way. With a mount there are some advantages : simple to aim, wide field, and comfort while observing. It is a good DIY project.

It's a question of personal choice. I avoid the technicalities of CCDs and automatically-aimed instruments controlled from the comfort of a warm room. I like the intimacy of the fresh night air, and find my way around using my "natural detectors". I prefer wide-field viewing of open spaces which excite the imagination. Having practised with one eye with its frustrating discomfort for a long time I decided in 1994 to construct binoculars with mirrors. Little information is available on binocular construction and this conservatism, which I can't explain, deprives us of an incomparable observing pleasure. The instrument presented here will, I think, give a little boost to Hervé Burillier who, in the October 1999 issue of Astronomie Magazine was worried about the lack of interest in observing, with more being devoted to "galloping technologies".

Difficulties in construction

The principles of binocular construction are not fundamentally different to those of other types of instrument but there are particular difficulties :

Mirrors

The maximum difference in focal lengths must be limited to 1%. Above this value the unequal lengths carry a constraint incompatible with binocular vision. It is not a question of using here the method of mould templates or thickness wedges sufficient for making a single telescope mirror, but a spherometer, as described in Jean Texereau's book. I obtained a micrometer screw having a 26mm travel and adapted the three-point support used by Jean-Claude Dubsay. I prefer a mirror tool using rough to fine stages, working the mirror above and below so that the values given by the spherometer are identical to within 1/200mm. The Foucault test then gave a divergence less than 1mm which is only 1/7 of the 1% limit.

The mirror supports

Due to the conic section produced by the method of grinding, the mirrors lie in their supports with a lateral clearance of 0.5mm. Without this precaution, uncontrolled displacements of the mirrors would render the optical tuning very difficult. The necessity of obtaining a perfect fusion of the two ocular fields required me to think up and construct a specific system. With 4 knurled adjusting screws each ocular can be independently moved in the longitudinal and transverse (x, y) axes, thereby providing optimal alignment. A vernier facilitates the setting in x for the distance between the eyes.

The prisms

It is perfectly possible to utilise four flat mirrors to bring in the 160mm separation of the optical axes to the average separation of the eye pupils, which for me personally is 64mm. I prefer two prisms with total reflection which give a useful 39mm field sufficient to adapt to variations in anatomy. They are in BK 7 and I thank Michel Dodard for making them.

The Eyepieces

Before satisfying my taste for wide fields my choice was for the following 3 combinations:

  1. 33mm focal length, magnification x23, 6.7mm pupil diameter, field 2° 12'.
  2. Eudiascopic 25mm focal length, x27, 5.6mm pupil, field 1° 50'.
  3. Panoptic 19mm focal length, x36, 4.3mm pupil, field 1° 54'.

Note that it is not always possible to use wide-field oculars with long focal lengths which because of their large diameters renders it difficult to adjust the separation of the eyes (x) or their installation on the mounting plate.

Adjustment

Besides the optical adjustments familiar to owners of Newtonian telescopes the fusion of the two ocular fields presents no particular difficulty, it is carried out in two stages. Azimuth - point the instrument at any region of the sky and put a star at the edge of the field of one of the oculars by moving the tube left or right. Then verify the same position in the other ocular using the horizontal transverse (x) screw. The Altitude is corrected in the same manner using the vertical (y) screw. This operation takes less than a minute and is necessary after disconnecting and transporting the instrument.

A stable and functional mounting

The mounting described in Lunettes et télescopes by Danjon & Couder answers perfectly. The weight of the tube and its small dimension gives it a remarkable insensitivity to wind. The large field and absence of movement on the altitude and azimuth axes give it a simplicity of operation. The only improvement, a spirit level, permits the horizontal setting of the assembly, making corrections in azimuth unnecessary. The instrument is easily transportable, it comes apart as feet, fork and optical tube and can be assembled in 5 minutes. It goes into a trunk which can fit into a small car.

Observations

The 2° field makes a finder telescope unnecessary. The instrument is easily pointed at objects using the upper edges of the tube. It takes 7 minutes for a star to traverse the field, so there is no need to constantly re-centre when a group is observing. A motor drive is not needed. I am now able to spend nights of up to 5 hours continuous observing without undue fatigue or the bleary vision of monocular use. There is no appreciable chromatic aberration and it doesn't mist up. The first impression is of observing in a different dimension, not like looking through a keyhole. The instrument is particularly good for comets, close conjunctions, lunar occultations, star fields and eclipses. At x23 the crescent moon and its Earthshine offer an incomparable spectacle, a globe "floating" in a field about 4½ times its diameter. Jupiter and its 4 satellites offer the feeling of isolation as they float among pinpoints of stars. The Orion Nebula, the Pleiades, the Andromeda Galaxy, the M27 planetary, the lacework of the Cygnus loop or the Rosette Nebula are unforgettable, with a depth and relief which make one reluctant to return to monocular viewing. I have an indelible memory of observing Comet Hale-Bopp at the Viaileix Observatory and of the solar eclipse of 11 August, where these binoculars without doubt gave me the greatest benefit.

At gatherings such as in Saint-Aubin de Courteraie, at the home of Pierre and Agnès Bourge, or the third European Amateur Astronomical Meeting organised by SAN, where these binoculars were voted the premier prize in the category of an instrument made by a craftsman, I could appreciate more objectively the value of this instrument and take a real pleasure in seeing the enthusiasm it has generated

Optical characteristics

Primary mirrors : clear diameter 153mm, focal length 679mm, focal ratio f.4.44. surface precision λ/13 to 15.
Secondary mirrors : minor axis 45mm, precision λ/10.
Prisms : total internal reflection.
Stellar limiting magnitude 13.2

Down-to-Earth considerations

Here are details of the costs of materials. (approx 10F to the £)

Primary mirrors (blanks, abrasives, aluminising) 2000 F
Secondary mirrors 1300 F
Prisms 3000 F
Various materials (wood, aluminium, rubber) 1500 F
Oculars 1900 F
Total 9700 F

While the cost seems high it was spread over the period of construction. The following table gives the comparative cost of other instruments :

Diameter mm Collecting surface cm2 Limiting mag. Price (F) (fixed ocular)
153 183.9 13.2 45,000
125 122.7 12.7 27,000
100 78.5 12.3 8,600
80 50.2 11.8 4,500

It doesn't stop there. I am completing the construction of binoculars with 306mm diameter mirrors at f.7 on a motorised mounting, specially designed for high-resolution lunar and planetary observations and suitable for photography.

Bibliography

André Danjon & André Couder, Lunettes et Télescopes.
Jean Texereau, La construction du télescope d'amateur.
Pierre Bourge & Jean-Marc Becker, Mon télescope, mon observatoire pourquoi pas.
Karine & Jean-Marc Lecleire, Réalisez votre télescope.

Diagram of layout of current binoculars

Design of 153m binocular using prisms