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Testing a refractor objective cell


Panagiotis Papadakos
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Hello everybody.

I bought this lens from ebay http://www.ebay.co.uk/itm/370661390588?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

and I am planning to build a refractor, assuming that everything is Ok with the lens and that it is a telescope lens! :)

The lens had a bit of fungus. So I cleaned it by taking apart the cell and letting the glasses inside some water, with vinegar and hyperoxide.

Except from some marks on the coating, I believe the lens is now cleaned.

The real diameter of the lens is around 19cm, but only 17.2cm are used, since the rest are under the top ring of the cell.

It seems like an achromat, but unfortunately the is no information on the lens. I measured its focal length, which is around

2700mm. I placed the lens on a chair and used an eyepiece 20mm (no tube around) and the image was a bit hazy and somehow

stretced, with very distinct color.

So, since I am a newbie, but really eager to learn, I would like some guidelines about how to test the lens, how to be sure

that the cell is assembled correctly, etc.

I have no optical tools available.

Thank you

Regards

Papadakos Panagiotis.

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Hello Papadakos.

The focal length suggests that this is not a photographic condenser lens (often the case with large eBay lenses). When you dismantled the cell to clean the lenses were the two glass components closely separated or were they spaced apart each end of the cell. Check the orientation of each component, ususually the front lens is biconvex with the shallower curve outwards followed by the second lens usually planoconcave with the plano towards the rear. It can be quite tricky examining objectives in daylight with no tube as the stray light washes out the image. There should not be much false colour in a lens of this focal length which is why I mention the orientation. Hope this helps. :smiley:

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Hello Papadakos.

The focal length suggests that this is not a photographic condenser lens (often the case with large eBay lenses). When you dismantled the cell to clean the lenses were the two glass components closely separated or were they spaced apart each end of the cell. Check the orientation of each component, ususually the front lens is biconvex with the shallower curve outwards followed by the second lens usually planoconcave with the plano towards the rear. It can be quite tricky examining objectives in daylight with no tube as the stray light washes out the image. There should not be much false colour in a lens of this focal length which is why I mention the orientation. Hope this helps. :smiley:

Thanks for the reply Peter. The two elements of the cell, were separated by 3 small pieces of boxing tape. Probably the lens had be opened again, since there was a big arrow at the sides of the elements to align them. The front lens is biconvex, with the the same curve I believe, and the back is like what you described. So do you believe that the low image quality, the stretched image, etc. can be a result of not using a tube?

Regards

Panagiotis

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It looks DIY acromatic objective, and if its any good, for f/15-16 it should not show almost any false colour in normal use. Distortion and false colour may be effect if you keep eyepiece out of optical axis, i think it can happen quite easily with chair testing method.

It is difficult to say anything of quality, i think you have to find tube for that and try again, or make testbench of wood. With 2,7m focal length it will be real beast to handle, mine DIY achro is only 145/2250 :grin:.

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Hello again Panagiotis (apologies if I had your name in the wrong order).

From your description it would appear that it is at least a proper telescope objective. As Ransu says, it is very difficult to assess a lens without a tube or some other temporary construction to hold the optics in line. Keep us informed of your progress.

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