Achromat - which way round

[Stub Mandrel]

I bought a cheap 50mm 180mm f/l achromat lens to make a finderscope... which way round should I use it? One side bulges more than the other, and this is the side with the thinner element.

[Chris]

Hi, the outer element is usually the crown element  (the bi convex one), and the inner element is usually the flint (this is concave on one side so it can mate with the convex crown element). 

With this is mind, I reckon the bulge should be pointing outward out the finder (and this element should in theory look thinner on the edge because it's bi convex). 

[DaveS]

Bulge forward I should think.

If you're looking at the edge, then the thin bit is the thin edge of the converging element, probably biconvex, while the thick bit is the edge of the concave diverging element.

This assumes, of course, that it's a bog-standard achro doublet .

[Chris]

Here we go:

[Ajohn]

It will be the most curved side to the sky Neil. These lenses sometimes have a longer radius on the back side which goes towards the focus rather than being flat. The 50mm one is close to flat on that side. Mine is still in it's packet but I can feel that one side is close to flat.

John

-

[Stub Mandrel]

Thanks, it's about 3mm bulge on one side and only about 1mm on the 'back'.

Not coated, but will be fun to play with, I will try the cross-hair eyepiece from a cheap 30mm RA finder that I don't get on with.

[Ajohn]

They are coated Neil. It appears to be BBAR - broad band anti reflection. It's more or less colourless but if you look at it under a fluorescent strip light at various angles you will see some colours in the reflections. The colours are caused by the depth of coating. The reflections will be pretty weak compared with polished glass anyway.

The usual thing with lenses is curved side toward "infinity" and flat side towards the focus - can't make you mind up same curve on each side. The weak curve on the back towards focus can be applied to plano convex lenses as it improves all sorts of things and is often used in better  quality magnifying glasses even with a single piece of glass.

John 

-

[Mr Spock]

Curved side out