Old Old telescope

[Tycho Brahe]

Kids woke me up at 2:30 this morning... Ended up being thankful, cause its really clear, so got my scope out. But have been wondering how good the victorian reflectors were? There seem to be more old refractord than reflectors, is that cause of the problems they had silvering them?

[essex sky]

hi tycho , you must be knocking on a bit now:D

i think it was to do with the the fact that the early mirrors were

rather poor at reflecting any light that they collected.

it was not until the mid 1800's that mirrors were really begining to be made out of glass and

be silvered, the early one were bare metal and just polished.

usually a mix of copper and tin.

plus the fact that back the a smallish lens was far easier to manufacture than a large mirror .

clear skies

lance

[Capricorn]

Guess it was a case of accuracy. They couldn't make them as well as we can now.

It will have been a spherical mirror also for the same reasons. Still as long as it allows you to see things then no worries.

[AndyH]

Vintage refractors do seem to have lasted longer.

As a side note......

No need for Parabolic mirrors if the newt is F8 or slower. eg: Tal's 'slow' spherical mirrors are superb and highly regarded for their views. Even so called optical experts have raved about the views through em.

Faster mirrors are a different story though !!

Cheers,

Andy.

[essex sky]

yes it was not until , the silvering process was perfected

that mirrors could be made of glass, it then became a lot easier to

cast a good one.

plus the victorians just loved refractors;)

lance

[acey]

A nice insight is given by TW Webb's "Celestial Objects For Common Telescopes", first published 1859 and available in a modern Dover edition. He reports on comparisons between reflector and refractor performance.

"Maskelyne estimated the apertures of metallic reflectors and achromatics of equal brightness as 8 to 5. Dawes gives this value for Gregorians, but like J. Herschel rates Newtonians as 7 to 5... with very large apertures the advantage of the achromatic disappears. The silver-on-glass specula, invented by Foucault and Steinheil, but perfected in England, take their place between the metal Newtonian and the achromatic, approaching more nearly to the latter, especially when the plane mirror [i.e. secondary] is replaced by a prism... Buffham assigns equal light to silvered Newtonians of 9, 6.5 and 4.5, and achromatics of 8, 5.75 and 4 inches respectively."

Webb's scope was a 3.7 inch achromat of 5.5 feet focal length (i.e f/17.8) "of fair defining quality". He advocates testing scopes on a star, checking equality of intra/extra-focal views, much as we would do today. He also recommends that reflectors have "a tube perforated with large and numerous openings" to deal with tube currents. In those days without light pollution, an aperture of 2 to 4 inches was enough to give good views of hundreds of DSOs, and the best scope of that aperture was an achromat.

Herschel, over half a century earlier, used mirrors made from the alloy speculum (2 parts copper, 1 part tin), cast and then ground and polished to the correct figure, and tested on stars during the process. Because of its limited reflectivity (about 68%), and perhaps also for convenience, in his larger scopes he removed the secondary altogether and looked down the front end of the tube, the primary mirror being tilted appropriately. On the picture of his 48-inch (below) you can see the platform hanging off the front end where Herschel stood, looking straight down at the tilted primary.

[ollypenrice]

Yes, it was a sound idea of Hershel's to dispense with the secondary since good levels of refectivity were impossible to acheive. One (or two?) of Lord Rosse's mirrors were on show in the science musem last time I went.

A good book covering the transition from refractor to relector is Victorian Telescope Makers. I can't remember more than that and my copy is in the UK.

Olly

[AndyH]

How very interesting. :icon_salut:Cheers, guys. Hadn't heard of that before.

:)Andy.

[Mark at Beaufort]

If anyone wants to undertake more research into Victorian astronomy and equipment here is a link to Webb-Share - Welcome to the Spacewatch Website

The Lottery Fund allocated finance for the following project -

Webb-SHARE: Celebrating our Victorian Astronomy Heritage is a 3-year project led by The SHARE Initiative and funded by the Heritage Lottery. It celebrates the lives of three Victorian astronomers – Rev Thomas William Webb, Rev Henry Cooper Key and George Henry With – who all lived in Herefordshire in the mid-nineteenth century. Webb was the Vicar of Hardwicke and a great populariser of astronomy (the ‘Patrick Moore’ of his day). Key was Rector of Stretton Sugwas and an early telescope pioneer and inventor. With was Head Teacher at the Blue Coat School and made over 200 mirrors for amateur telescopes. The project includes training workshops for volunteer researchers, museum loan boxes for schools, constructing replica telescopes, talking tactile diagrams for visually-impaired learners, star parties, family learning events, a touring exhibition and a DVD.

Paul Haley is leading the research and has unearthed a huge amount of info about Webb's telescopes and other pioneering Victorian astronomers. I have seen a With mirror which Webb-Share has obtained.

Mark

[Tycho Brahe]

Wow, thats interesting stuff, cheers lads... have to digest that and have a think... I love the pic of Hershels...

What do you mean by a slow mirror, is that one with a focal ratio of less or more than 8? And whats the difference between a parabolic mirror and a normal concave glass mirror? I know its the shape, but hows the shape different?

Cheers,

Zane

[AndyH]

Ah sorry. Folks seem to class as 'slow' any scope who's focal ratio is >F8. So I guess my 60mm F16.7 must be uber slow then ;-)

A parabola and sphere. It's geometry I think. Something to do with bringing light to a point of focus? I'm sure someone will pipe up soon enough and explain it. I'm a bit rubbish when it gets that technical. Takes me back to school, sitting at the back of the maths class and staring at the blackboard thinking 'what the hell is he talking about':icon_scratch: I still get flashbacks.

Andy.

[Tycho Brahe]

Ah, thanks... I'm really interested in this cause I want to grind a mirror for a dob, and have also found an old, possibly victorian mirror which the silvering has all come off... I'm just thinking what an ace thought it would be to bring such an old mirror back to life in a modern dob! Imagine who may have looked at the stars with a mirror that old!!! So that will prob be a slow, so it will prob have a long focal length so should make a good planetary dob???

[Peter Drew]

Hello Tycho. Let's assume that a "normal" concave mirror is spherical in surface shape. Now, as every secondary schoolboy should know, the angle of incidence equals the angle of reflection. On that basis, draw a spherical curve and note where lines drawn perpendicular to the surface would be reflected to a centre axis line. You will find that those at the extreme edge of your drawing cross the centre line much nearer to the surface than those drawn nearer the centre. If this was a telescope mirror then the focus of the outer parts of the sphere would be different from the central parts and as the eyepiece could not focus on both positions at the same time the image would be useless. Looking at the drawing again, it should be obvious that if the outer parts of the curve were flattened out then the light from any part of the curve should reach the same point. The curve that satifies this condition is the parabola, hence its importance for a telescope mirror. With long focus mirrors the difference between the parabola and the sphere is so small the spherical shape will perform well. I know what I am trying to say here, I hope it comes over clearly enough!

[Astro_Baby]

I was pondering the same thing about more recent 'classics' in the classis group of late. You see a lot of 1960s circa refractors now being classed as classics but releativelt few newtonians.

Part of its down to the fact that newtonians from even 30 years (in the main) lack the performance of modern scopes whereas long focus refractors will still give good images compared to modern equivalients. Theres also the aspect that newtonians have their optical surfaces exposed to the atmosphere so their coatings can be destroyed more easily but by far the biggest reason I suspect is that reflectors were often home made affors with mirrors polished by the owner.

The simplicity of the newtonians design lends itself to home made telescopes wgereas it much harder to grind lenses for a refractor. This mean refractors almost always had to be made by a manufacturer and could therefore acquire a pedigree whereas refelectors could be made by almost anyone and as a result could never acquire any sort of pedigree. Obvioulst that very generalised and there are some beautiful reflectors made by quality companies but there are a lot fewer of them and quite often they are the result of small production runs or one offs.

Thats my guess anyway.

[Tycho Brahe]

If I get this victorian mirror, I was thinking it could be quite funny to clean off the old silvering, and deposit a TiO2 dielectric mirror in its place... That would be a cool step for such an old mirror, from one of the earliest mirror techniques to one of the most advanced! Do astronomers ever use dielectric mirrors?

[FraserClarke]

Do astronomers ever use dielectric mirrors?

Yes, but it is (as I'm sure you know) hard to get a very wide band pass from dielectric coatings. So if your instrument has to cover a wide wavelength range (as most professional instruments do), it is hard to make a suitable coating. You can cover the entire visible band though, so for a purely visual telescope, it might be feasible.

If you put on a lot of layers (to get a broader coating), you may get to the point where the deform the optical surface (stress it) by several waves. That might not be a problem for primaries (it will just shift the focus (to first order)), but you certainly don't want to do that on any flat fold mirror, as you'll introduce astigmatism.

[Tycho Brahe]

Ahhh, right... It is to be a purely visual telescope... I dont know much about it cause the coatings I work with a single layers that simply absorb the light, but the machines I get to use are capable of making complex multilayer films... so I'm just starting to read about it... I didnt think about that but its obvious now you said it! 'cause the effect on the light is caused by the film spacing's relation to the lights wavelength it will be difficult to cover a wide bandwidth... Maybe just stick with aluminium... after all I know I can do that!

[skye at night]

If I get this victorian mirror, I was thinking it could be quite funny to clean off the old silvering, and deposit a TiO2 dielectric mirror in its place... That would be a cool step for such an old mirror, from one of the earliest mirror techniques to one of the most advanced! Do astronomers ever use dielectric mirrors?

but how well prepared and free from error is the surface?

what about a photo here of your old scope?

steve