5 inch ISTAR objective lens (price: £822 + delivery)

5-inch (127 mm) aperture achromatic doublet objective lens by I R Poyser
Our 5-inch (127 mm) aperture achromatic doublet objective lens

These lenses were designed and made at our request by ISTAR Optical, of Arizona, because we had not found it possible to obtain sufficiently good objective lenses on the open market.

These superb lenses are intended for use in our range of large brass refractors, but we are also going to offer some for sale to those amateur telescope makers who have reached the stage of building their own large refractor.

Why did we want a lens of this design?

We wanted the maximum aperture and longest focal ratio that was consistent with the telescope being supported on a tripod. A 127 mm clear aperture lens gives excellent light grasp (over 1½ times the light-gathering capacity of a 100 mm aperture lens) and a focal ratio of f/12 gives excellent image resolution. The finished telescope is neither too heavy nor too bulky to require a permanent mount, such as a concrete pier; it can be supported on a sturdy tripod.

Over the years many astronomers have asked us if we supply a 6-inch diameter objective lens; usually they want it to have a focal ratio of f/15 because they have read that a long focal ratio will give superior image quality (which is true) and a large aperture will give access to fainter celestial objects (which is also true). The problem with such lenses is not the building of the telescope but the obtaining or building of a mount that will hold the telescope with sufficient rigidity that it will give satisfying views.

The difficulty is weight; a 6” objective lens in its cell is a heavy item and an f/12 or f/15 focal ratio will mean that the telescope is very long (between 6feet and 7½ feet long). The weight of the lens cell at one end, counterbalanced by the finder telescope, the eyepiece and the star diagonal at the other end means that there is a large turning force on the mount and the worst problem is the telescope wagging from side to side once it has been jolted, perhaps by adjusting the focus or interchanging the eyepieces. Such long, heavy telescopes do require a permanent support, such as a concrete pier or a large metal tube sunk deep into the ground with a massive mount fixed to the top. Such arrangements are usually out of the compass of the amateur astronomer, at least in his early years.

For these reasons, then, we have chosen this prescription of objective lens, based on our own experience and which we commend to you.

Here are the technical details:

Design: ISTAR Optical call this prescription their ‘Anastigmat’. The R30 lens is an air-spaced achromatic doublet. Istar claim that its optical performance is equivalent to an apochromat because the optical performance is superior to a conventional achromatic doublet, due to their choice of glasses and their design of the curves.

Finish: All four surfaces are multi-coated

Diameter: The clear aperture is 127 mm (5 inches)

Focal length: 1500 mm

Weight: 2.72 kg

Cell: Each lens is supplied in an alloy cell, black anodized. The cell is of the ‘push-pull’ design to aid squaring-on when the lens is fitted to the optical tube assembly. Usually, however, squaring-on will not be necessary.

Outside diameter of cell flange: 168 mm
Holes in cell flange: pairs of M5 threaded and 5 mm plain holes
Pitch circle diameter of holes in flange: 158 mm

Performance: Each lens is hand finished and is individually star-tested before it is shipped to us.

Availability: These lenses are made for us by ISTAR. Only we hold them in stock.

Price: £822.00 plus delivery

If you are going to build a large astronomical telescope and you are seeking an objective lens – choose this one.