Refractor Objective Lens Structure

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Pip
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Refractor Objective Lens Structure

Post by Pip »

I have an old brass refractor telescope with a 2.5” objective, made by Broadhurst Clarkson and Co, of Farringdon, London.

The image quality was poor - unsharp and with brown opaque areas round the edges of the image. I found that the objective lens was dirty. It comprises of 2 lenses - one crown element and one flint element. They were stuck together and eventually separated with a bit of effort.

I found the poor image and opaque areas were caused by the material between the 2 lens elements having dried out. Whatever it was, it is now dried and brown near the edges, while more or less clear, but mucky in the centre.

I cleaned the lenses with methanol and they are now clean and shiny – no scratches – I was careful not to rub the faces with fabric etc.

I found in various references that the material between the lens elements can be various oils – such as mineral oil, even olive oil and a few others. But nowhere does it say what it should be – only what has been used. So no idea of the best material. If left dry [that is, simply air], I understand the chromatic properties are not so good. If that is correct, I should like to be guided on what oil or liquid is best to use?

[I actually first thought the material between the objective elements may have been Canada balsam as this is used in microscopy because of its good optical properties. The reason for this initial assumption was because the 2 elements were stuck together. But I discovered that oil over several years can dry out and equally stick the two parts together. As I could find no reference of the use of Canada balsam in telescope lenses, believe that the elements were stuck together by dried oil.]
Pip
David Frydman
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Post by David Frydman »

Dear Pip,
Generally Broadhurst Clarkson objectives are air spaced with nothing in between.
If they were cemented it would br Canada Balsam.
What type of telescope is it. Is it a collapsible drawtube scope. Fixed tube Marine spotter or astro refractor.
What is the focal length,, i.e. disyant object then measure lens to image.
What kind of cell is it in?
What eyepieces are they.
Whatever it is it is a good scope. More information please.

Regards, David
David Frydman
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Post by David Frydman »

Also make sure the lenses are the correct way round and also the whole lens is not back to front.
Are there pencil marks on sides of lenses indicating best position by rotation?

David.
brian livesey
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Post by brian livesey »

It seems unlikely that the objective is an oil-spaced achromat, as there would have been a sealing ring around the objective to prevent the oil from running out.
I'm no expert, but I reckon that it's a flat-back objective and was cemented with Canada Balsam that's decayed. Try it out with three foil spacers and see how it performs.
brian
Barry B
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Refractor objective lens structure

Post by Barry B »

Pip

From your description I would think that your lens is almost certainly a cemented doublet using Canadian balsam as the adesive.
In the 1950's I was an apprentice at the Wray optical company, we humble apprentices were not permitted to cement or mount achromatic obectives as this was considered a skilled and meticulous operation.
With this in mind it may be worth cosidering having the cementing and setting up done by a professional, particuarly if the alignment marks on the side of the lens have been lost.

Regards
Barry B
David Frydman
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Post by David Frydman »

Thanks Barry,
But I have my doubts as to whether Broadhurst Clarkson cemented their doublets, that is why I am trying to find out what focal ratio and what scope type it is.
It is possible a contact doublet was cemented afterwards or this objective is a replacement.
I would not think that if this was a professional camenting job it would be in such a bad way, unless it was somehow distressed.
It is nice to hear you worked at Wray, a very fine firm. Prof. Wynne designed lenses for them. Some of the later aerial lenses are superb.
And interesting if somewhat unreliable cameras.
Regards, David.
Pip
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Post by Pip »

Sorry to delay replying!

The telescope is a brass tube 81cm/32” long from objective lens hood to point of attachment of the eyepieces [the eyepieces are different lengths]. The image from the telescope is the right way up.
It is not a collapsible draw tube – it is a fixed tube, tripod mounted [teak legs] and with rack focusing.
The focal length of the objective is 80cm/31.5”
There are 2 eyepieces:
– one is higher power and appears to be a Huygens eyepiece containing 2 elements, both plano-convex with the flat faces of both towards the eye. The eye element has vey small diameter, and between the 2 elements and nearer the eye element, there is a fixed diaphragm. It is 3cm long, if that helps.

- the other is lower power and is 22cm long [much longer]. I am unsure of the type – could it be a form of compound Huygens? It contains 4 elements in 2 unit pairs. The first unit at the eye end of the eyepiece is 5.5cm long with 2 plano-convex lens elements, both flat faces towards the eye and with a fixed diaphragm between them. This diaphragm is nearer the element away from the eye. The second unit pair are in a similar arrangement, but 7.5cm long and facing the opposite way to the first pair – that is with the flat faces away from the eye and the fixed diaphragm between them nearer the eye.

The objective lens elements – alignment. There are no pencil marks, but both the elements comprising the objective have a short groove engraved on the rim, but it only goes half-way across the flat edge of the rim. Therefore they only line up with the correct faces together as well as the circular orientation correct.

The lenses sit in a small seating and there is a screw ring to hold them in place. There is no seal so I guess they can only be intended to have air or Canada balsam between them.

Hope the above will enable you to get a clear enough picture of the telescope in question – many thanks!!

Regards
Pip
Pip
David Frydman
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Post by David Frydman »

Pip,
On a quick read of your post I think this a coastguard telescope.I think the longer eyepiece is a terrestrial eyepiece giving an upright image. I think this was called a Pancratic eyepiece.
I suspect the two element eyepiece gives an inverted image or it could be some ki nd of Galilean eyepiece, negative giving an upright image.
I have not seen a cemented Broadhurst Clarkson O.G. But I also have not examined their coastguard scopes. I suppose it could be cemented and suffered from a lot of exposure to sea water.
I will read your post more carefully.

Regards, David.

It could still I suppose be possible to mount the whole O.G. back to front, This would give soft images.
David Frydman
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Post by David Frydman »

Thinking about it further it might be one of their starboy astro refractors with a terrestrial and an astro eyeoiece.
Usually the tripods were mahogany.
It could be a much earlier telescope where they did cement the O/Gs.

recall the coastguard scopes may have been finished in black or leather but I am not sure.

Are the eyepieces screw thread about 1.25inch outer diameter on eyepiece screw?

The tripods sometimes had a brass pillar on top about 10inches long with a mounting curved plate.
Or is it a different type of head.

David.
Pip
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Post by Pip »

Dear David,

The telescope is brass throughout – no leather and no black parts or other colours.
As you thought, the eyepieces have a diameter of 1.25 inches across the screw threads.
The telescope is mounted on a brass pillar as you suggest. The actual pillar is 8 inches tall, but with the swivel joints [pan and tilt] and the mounting bracket, the pillar measures 10 inches overall. There is a short cradle attached to the top of the swivel joints and the telescope attaches to this by means of 2 milled screws.
My earlier message regarding the images was not quite correct – sorry for the confusion! The image formed with the low power eyepiece [with lens 4 elements] is the right way round, while the higher powered eyepiece [the Huygens??] gives an image that is inverted.

I should add that the telescope has previously been used for astronomical work and it has a dense dark filter for the Huygens eyepiece for viewing bright objects [although certainly NOT for viewing the Sun!!!].

Yes, I guess the legs are mahogany rather than teak – thanks!

Regarding the crown and flint pair of objective lenses - If it helps, after thoroughly cleaning the crown and flint elements I have tried both with simply an air junction and with an oil . I think the oil film altered the focal length of the objective lens, the focusing range was insufficient to focus the final image. After cleaning the objectives of all trace of the oil, and reassembling the objective lenses, the image resulting was clear an in focus – so air separation seems correct. So I wonder if Canada balsam might behave similarly to the oil and adversely alter the focal length – do you have a view???? Otherwise, it seems air separation in the objective elements gives best results. No colour aberrations were readily visible, although I have only tried it in daylight looking a distant trees etc.

Hope this helps resolve your thoughts on the telescope and whether the objectives should be mounted only with air between them? Also would it seem likely if the telescope is designed to be used both for astronomical as well as terrestrial use – hence the lower powered upright image lens - ???

Many thanks again for your comments!
Regards,
Pip
Pip
David Frydman
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Post by David Frydman »

Dear Pip,
Is the separation between the two monting bolts 10 inches?
I think the telescope is a standard 2.5 inch astrp refractor. The terrestrial eyepiece is an extra for day use. There were probably extra astro eyepieces as few people stuck to one. I think the standard supplied one was half inch, but there were quartre inch, thre eights, five eights, three quarter, one inch and a comet eyepice fatter as well as orthos, kelners, monocentrics etc.
Star diagonals, finders and so on. Stellar micrometers, spectroscopes, a whole range of extras. Fitted case.
I think the O.G. was originally airspaced. If scope was affected by salt some of it would have gone green.
It is possible somebody tried cement or oil, or the original O.G. was broken and this a replacement from somewhere else.
There were Moon filters and Sun filters on back of eyepiece.
Unfortunately it is possible the filter you have was for solar use. If so it should be discarded so future accidents don't occur. They shatter and you risk shards of glass or serious eye damage.

Is the brass top vertical pillar hollow?

I will enquire about whether canada balsam affects optics. I think not, but I am not sure.

I think the Moon filters may have ben blue and the solar filter green.

Regards, David.
Lady Isabella
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Post by Lady Isabella »

The scope does sound like the smaller Starboy scope similar
to this one from the 1920s.

http://www.philharrington.net/broadhurst1-200.JPG
David Frydman
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Post by David Frydman »

Yes it does. Unfortunately it talks about a solar filter which I think Pip has.
Also it talks of the three inch having a 40x terrestrial eyepiece, but I think I measured mine at 53x or 50x. I measured it a long time ago.
Also the eyepiece focal lengths were not very accurately stated. They were marked in pencil on the eyepieces.
The ordinary Starboys say 3inch were 3 inch diameter but only 73mm aperture.
Their better scopes up to 6 inch I think had genuine apertures rather than diameters.
In addition to the extras I also had the erecting prism for thye astro eyepieces.

As to canada balsam affecting the focal length. It could depending on the shape of the airspace. I think the contact doublets were not quite contact and just touched at one part.
I cannot recall cemented BC O.Gs but perhaps someone has met them.

David.
Lady Isabella
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Post by Lady Isabella »

I must admit that I've never seen a cemented lens in a BC astronomical scope before.

Out of interest, are a couple of links to Patrick Moore's BC scope.
http://www.telescopehouse.com/acatalog/Servicing.html

http://www.telescopehouse.com/acatalog/ ... copes.html
David Frydman
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Post by David Frydman »

Various tree balsams were tried but Canada balsam was preferred. It was probably available commercially for glues etc.
The cemented doublets had the same curves at the glued surfaces, the layer was thin and the focal length would not change.
If oil spaced paraffin oil is reasonable but castor oil is preferred and there would be a spacer. From the cell one could see if there is room for a spacer.

The Pancratic or terrestrial eyepiece makes a very good magnifier of high quality.

I cannot remember if it was BC but someone kept pet spiders to provide the fine wires for the stellar micrometer. The problem I think was to stop spiders eating each other.

My 3inch Starboy projected wonderful images of the Sun on my bedroom wall with the curtains closed around the scop. Highly detailed views with a large solar diameter I think by eyepiece projection.

I probably had Sun and Moon filters but I hope I did not use the Sun filter.

Regards, David.
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