A Summary of January’s main meeting

Bob Marriott (BAA Instruments’ Section) talked about the ‘Silver on glass revolution’.

The 1850s witnessed a massive revolution in science and astronomy.It was long known that reflector telescopes could give good results and that experimentation found that a copper and tin combination (two thirds copper, one third tin) gave reasonably good reflectivity, especially when a little arsenic was added. Unfortunately the arsenic made the surface tarnish very easily, so the hunt was on to see if it was possible to deposit a metal film on glass. (When John Herschel was working in Cape Town in the 1830s he used three 18” mirrors in rotation in his telescope. They had to be repolished and refigured every two to three weeks.)

Both Justus Liebig, an industrial chemist (also of Oxo and Marmite fame), and Michael Faraday experimented with using alkalides to deposit metals on glass. Faraday was only about 18 at that point.
John Stenhouse (who invented the carbon filter which became the basis for the gas mask) did lots of experimental work, adding various oils into the mix.

By the 1850s a silver nitrate preparation with a glucose reducing agent (look it up!) was able to give a good result, but unfortunately the glass itself was still not optically good enough for use in reflectors, even the best Venetian glass.

The 1850s were very busy as regards events: the Hyde Park Great Exhibition (1851), Cholera epidemic (1854), and Crimean war (end 1850s onwards). James Clerk Maxwell was playing with photography and by 1861 had managed to take the first colour photograph. In the meantime Warren de la Rue wrote on the progress of silvering and Webb’s ‘Celestial objects for Common Telescopes’ came out in 1859, the same year as the massive Carrington event (solar eruption).

The art of silvering glass was finally perfected by Leon Foucault in 1857, but speculum metal mirrors continued to be made.

What pervaded Ian’s high-speed talk was the enthusiasm that the Victorian astronomers and scientists had. Many of the names are familiar to us; do look them up: William Lascelles, James Nasmyth, William Huggins, Henry With, George Calver. And they were almost all amateurs.

Gwyneth Hueter.

A summary of December’s main meeting

Chris Hooker of Newbury AS has graced us before, and this time his offering was ‘Solar System imaging with a webcam’.

Ironically he immediately set about explaining that astro images are no longer made with basic webcams as a whole.  They were designed for surveillance originally.   Nowadays you can get proper ones designed specifically for planets.  (exx. Celestron, Meade, Orion Optics) The top end ones can cost up to £800.

They may not necessarily have more pixels but will be less noisy. Chris gave us plenty of advice on getting the best out of a webcam. The important thing is to be able to stack images and use processing software like Registax.  This is quite old but still does most things and is free.  The more images you can stack the better.  It also means you can make observations even if seeing is not great.  In the Antoniadi scale of seeing where 1 is excellent and 5 is rubbish even 4 can give you glimpses of good seeing, so you can select the good images.

For planetary imaging it is always best to use a long focal length.  He uses an 8” telescope with a Barlow at x2 or x3, and set at f/30 he can take 1/80th of second shots at 60 frames a second.Don’t get the polar alignment too spot on because if the camera’s CCD has a few duff pixels they won’t be in the same place on each image. He also tells us not to worry too much about getting the focus spot on: scan through the range where it comes into focus and goes out again then put it somewhere in the middle of that acceptable range. The focus is likely to change a bit anyway, as the scope’s tube shrinks as it gets colder during the session.

He does not recommend colour cameras as they are a black and white matrix with a colour matrix overlay, so the resolution will be far better with a black and white camera and superimposing images taken through red, green, and blue filters. Chris finished off with some of his specials.

He likes the Aristarchus area on the Moon because there are so many different features in a small area.  The Sun is best in monochrome and Venus’ clouds are best through a UV filter.  He has even managed some good shots of the International Space Station.

If you want more advice on using webcams, we have quite a few experts in the society and the Committee can point them out to you.

Gwyneth Hueter.