Highest feasible terrestrial magnification? (1 Viewer)

Hi Lars,
There was a spectroheliograph that a well known British astronomer had that stretched the length of his house in the basement.
The Sun was projected by front surface mirrors into the device, one of two in Britain.

(Commander Henry Hatfield, who wrote the Admiralty Manual of Hydrographic Surveying. See The Telegraph obituary. He mistakenly fired a 15 inch shell from 13 miles that landed in Genoa Cathedral and is still there to this day. It didn't explode because it was designed for thick armour and the Cathedral was soft material).

A friend also uses front surface mirrors in his own made mirror mount for use with binoculars. I used one for a while, but it showed the terrible quality of well liked binoculars as the view drifted towards the edge of the field. I had not realised how awful many binoculars are at the edge of the field.

I swapped a wonderful Zeiss 120cm f/7 Aero triplet, probably hand aspherised, from 1920, for Harold Ridley's large optical flat that originally was at the National Physics laboratory. Such a mirror could be used to see Anholt.
The owner of the Zeiss Aero triplet would not take money, only another optic. It took a year before I could persuade him to part with the lens. This lens was used to take many wonderful comet photos on 5x4 inch film. The enlarger lens was, If I remember correctly, a Ross wide angle 5 inch f/4 six element aero lens.
Zeiss later, I think, split one of the Aero triplet lenses into two elements, as the triplets were so difficult to make by top Zeiss technicians.
Harold Ridley also liked trains and had lovely large photos taken with a large Voigtlaender folding camera.
I drove 130 miles in the early hours to his house in Somerset to hand deliver the lens, had tea and drove back another 130 miles with the large optical flat for my friend at about 3 a.m.
Harold found me sitting outside his house a half an hour after we parted, as I rarely saw such a dark sky and was just skywatching.
Another friend bought his house after Harold passed away and put my donated 14.5 inch f/5 Newtonian in his observatory dome that had housed a large refracting astrograph.

Could you get a Skywatcher Maksutov locally, so you could check that it is a good one or would you buy one from Britain?
Can you get a discount locally because of your trade?
Are there any secondhand or new Russian ones available?

Elof Hansson of Gothenburg was one of my good customers in the 1960s and 1970s.

Regards,
B.

Hi Lars,
Is the southish facing window on the right of the plan, in post 20, about 40cm wide?

If a concrete pier was used, could there be an air gap of 5cm between the window on the right of the plan and the back facing telescope tube. This should prevent temperature gradient problems and any boundary layer problems near the right window glass.

Is the balcony glass double or single glazing?

My 14.5 inch f/5 Newtonian was bought secondhand and there were some problems with the optics.
Jim Hysom identified the problem as a too thin diagonal flat that was distorting or bending. He made a top quality thicker flat and also refigured the primary to a high standard.

If any flat front surface mirror is used it should be of good quality, but I don't know how good, and also the correct thickness.
Large flats are expensive. My friend got them ex government. Optical labs may have old surplus ones.
New ones are listed as say lamda/10, lamda/4 etc. at different prices, but all big ones are expensive new.

If a front surface mirror is used it could be motorised to turn slowly and accurately.

I think that the spectroheliograph had motorised movement, maybe half the Sun movement speed? Maybe several mirrors projected the beam into the basement of the house.

Horace Dall's camera obscura had a very high quality mirror mounted right at the top of his house that rotated and moved up and down.
This directed light to the camera obscura optics and eventually onto the large white viewing table, which had a movable magnifier for detailed views at a mile or so. I remember seeing blades of grass at great distance moving in the breeze.

If a mirror is used on the balcony it would alter the orientation, but so would a star diagonal. I am not sure what the final image orientation would be, But it wouldn't matter much if you saw the lighthouse at 54 kms.

Additionally, could the mirror be mounted externally to the balcony to provide a larger than 40 degree sweep towards the south.
This presents its own problems, with a housing or cowling, and exposure to the outside air which could damage the mirror surface. And also temperature effects.

It may be that careful positioning of a mirror in the balcony would slightly increase the sweep.

Regards,
B.

Binastro said:

I swapped a wonderful Zeiss 120cm f/7 Aero triplet, probably hand aspherised, from 1920, for Harold Ridley's large optical flat that originally was at the National Physics laboratory. Such a mirror could be used to see Anholt.

Click to expand...

Hi,

interesting idea to use an optical flat - a newton secondary comes to mind - should not bee too expensive at 4"...

But what about the Zeiss triplet you mentioned, was it a telescope objective (air spaced triplet) or some kind of camera lens? And what about the data 120cm f7 - focal length of 1200mm and a focal ratio of 7?

Joachim

The Zeiss Aero triplet was a Cooke triplet camera lens of very high quality.
1200mm focal length and f/7. Aperture 172mm.
Harold Ridley had to tap it about 1,000 times to unscrew it and clean it internally. It had probably never been opened since new.

Harold Ridley specifically did not want a telephoto lens.
I had a very fine Ross 50 inch f/8 telephoto, which I would have given him, but he wanted a non telephoto lens.
These and 60 inch versions were used on cricket cameras and for photographing Royal weddings at great distances.

The Zeiss Telikon 4 element 75cm f/6.3 for 30cm square images on the very large and heavy Zeiss survey cameras was exceedingly high quality, but again it was a telephoto. But a very strange telephoto, as the back elements are considerably larger than the front. It has an internal Venetian blind shutter. I think 1930s.
Again Harold did not want one of those.

In fact, I have never seen another 120cm f/7 Zeiss Aero triplet.
It is still being used after being passed on to another astro photographer who still uses large size film.

For a 150mm Maksutov, a minimum width for the diagonal is 6 inches.
For a 180 scope, over 7 inches.
Good optical flats can be found secondhand if one has access to ex government or ex lab sources.
I would think that an optician may have contacts who could find a good one at a fair price.

Hi Binastro,

thanks for the info about the Zeiss made Cooke triplet - sounds like a great lens but if you got a good and large flat for it, that's also nothing to scoff at.

About the newton secondary idea - at 6" and beyond they get a bit rarer and more expensive... I was thinking about 95mm aperture for some reason... I guess I posted in the Swaro BTX thread before...

Joachim

I notice on Astromart a two mirror vertical beam Heliostat/Coelostat for sale in New Jersey for $300.
Mirrors 1/8 wave and 1/3 wave. I think 6 inch mirrors. If an offer was made probably cheaper.

For a 150mm Maksutov to receive a remote beam, one probably needs a 160mm times 230mm optical flat mirror, so considerably larger.
But a secondhand Coelostat or mirror/s from one might be a reasonable price.
Such mirrors are also used in physics labs and similar.

Looks they "Coelostat" are all sold on Astromart. Not that I was interested.