ED Design?

[Regulus]

Here's a thing that puzzle's me and probably says more about my ignorance of optics that anything else.

The price of an ED is considerable higher than an Achro of similar size because essentially the glass is dearer.

So my question is, "Instead of using a large lens in the primary doublet/triplet as the ED element and thus adding to the cost, why can't the designers use a smaller correction lens toward the end of the light path that would use about a 1/3 of the glass a front element would use?"

My own presumption is that the light still needs some distance beyond the lens to bring all frequencies to a common focus, just less than the standard achro configuration. Right, or wrong?

Trevor

[5ash]

I would have thought that the further away you get from the dispersing objective the greater the separation of the the different wavelengths , thus the second correcting element would find it difficult if not impossible to combine them without introducing far worse aberrations /distortions in the final image.
Regards philip

[Satchmo]

Designs like this were published in S and T years ago- certainly a good idea for a large achro refractor. The bottom line is that its easier and cheaper from a manufacxturing and alignment viewpoint to use ED glass. There was a corrector available for a while to sort out the CA in the 6" F8 achros when they first hit the market .

[Merlin66]

The TV Genesis Petzal design incorporates a rear two element lens...
Mark,
It's a pity the CA corrector is no longer available, but I think the pricing would have pushed the total to higher than a good ED.

[5ash]

Quote "So my question is, "Instead of using a large lens in the primary doublet/triplet as the ED element and thus adding to the cost, why can't the designers use a smaller correction lens toward the end of the light path that would use about a 1/3 of the glass a front element would use?""


I read the question as , in the case of an achromat the second element ,(in the doublet) was being reduced to 1/3rd the diameter of the objective and placed further away from the single objective lens, not the achromat had a third corrector lens placed further down the optical path. Hence my comment.
Philip