TELESCOPE Objective as ULF Lens?

[ruilourosa]

I saw some lenses named telescope objectives with focal lenghts from 600 to 1200 and with maximum f/stops of around f:5.6 to 8, the design is a air spaced achromat (2 elements)

have anyone tryed these?

if so, would it work for ulf?

rui lourosa

[Emmanuel BIGLER]

Hi !

Those lenses are designed to cover only a very narrow angular field, I would say not more than 10° of total field angle.
But being a doublet element, as thin as a single lens element, there is little in the lens itself preventing the user to capture a much broader field than 10°, at the expense of a significant loss of image quality.
It means for 10° of total field angle that the covered image circle would be about 100 mm in diameter, and 200 mm for a 1200 mm of focal length.
Hence it might be worth using, but only if you can make a test and see if the results are satisfactory or not.
And remember than (4/4) air-spaced dialyte-type lenses like the Apo Ronar can cover at least 40° of field, are very compact, and are not very expensive on the used market, at least up to 600 mm.
But may be some of our readers have already experienced this kind of very classical achromatic doublet and can share their feedback here.

To give you an idea of image size behind such a lens, remember that the angular diameter of the full moon is only 0.5°, the diameter of the image of the moon will be 5 mm with a 600 mm lens and 10 mm with a 1200 mm.

An accessory which was often used in addition to the main telescope lens, in order to increase the magnification, is the so-called Barlow lens, I think that you can directly project the image delivered by the telescope lens combined with the Barlow lens to a film.

[William Whitaker]

What Emmanuel said. Off-axis performance is likely to be dismal. If you're intent on trying it, scale it down in size to use a smaller objective on a smaller format such that the angles are about the same. My best guess is that one of Reinhold's Wollaston meniscus lenses will do much better for you. They're available for up to ULF sizes at very reasonable prices.

Cheers

[Maris Rusis]

I've used a variety of telescope achromats on LF cameras and the central image quality is very good BUT off axis things get bad quickly. The most obtrusive aberration in my experience is field curvature with the radius of the field curve about equal to the focal length.

[Drew Bedo]

Ok, it is-what-it-is . . .but . . .how did astronomers get all those great sky survey images on large glass plates?

[William Whitaker]

Probably using a Schmidt camera, which is faster than a conventional achromatic doublet and has a wider field. And pre-digital stitching...

[Steven Tribe]

It is no secret than telescope achromats were the starting point for photographic achromats in the 30's and 40's. I have used late 18th century and slightly later pre-photography era telescope achromats (with their obvious green hue and oil/air spacing). Think long focus landscape meniscii!

[desertrat]

You might be able to get decent results by turning the objective around backwards and putting a stop in front of it to get the effective aperture down to around f15. That's how Chevalier made a camera lens for Louis Daguerre. Chevalier used one of his standard f5 telescope doublets, according to the lore. Some trial and error would be needed to determine the correct spacing of the stop in front of the lens to get a decently flat field.

[Nodda Duma]

Ok, it is-what-it-is . . .but . . .how did astronomers get all those great sky survey images on large glass plates?

Really really really BIG telescopes!

https://www.ucolick.org/images/lh_36in_classic.jpg


http://www.chedbradleyphoto.com/Scie..._JLB1336-L.jpg

[Bernard_L]

The typical survey telescope is a a Schmidt, designed for wide field. Wide field meaning 1-2°. State-of-art LSST (approved project, designed, not built yet) has a field of view of 3°... and a sensor with 3200 megapixels.