Super Apo's

Hi All,

Lately, well over the past few months, I have come across the term 'Super Apo' in the description of Refractors. I have only seen this description used with Russian refractors, APM and in the description of the HT Precision Optics APOMAX 130. Whilst Triplets are alleged to be corrected for three colours Super Apo's claim to be corrected for four (this maybe a very simplistic and inaccurate description of the differences). Question: Is the term Super Apo a marketing ploy to put them above the many triplets available or are these the next generation of Apo refractors.

I think it is both some accurate description and some
marketing hype. One probably needs to actually test the
scopes side by side to know what is going on, as there are
many variables.

I know of only one current production Super APO with correction
at four colors, and it is the lenses from this company Astreya:
http://astreya-optics.narod.ru/eng/prod/lens152.htm
There are four differnet wavelengths with the same focal shift.
The lenses have four elements. The actual focal shift on the
plot is about 95 microns between 4678 and 6670 Anstroms (corresponding
to visual use), or 0.009%.

However, the focal shift is not hugely better than some
other triplet APOs. For example the AP 160 -- there is a
plot on the AP website, and for the same wavelengths the
shift is 105 microns, or 0.009% (same as Astreya).
So while one is technically "Super APO" and the other is
a APO (triplet) the percentage focal shift appears to be
the same.

APM has described their TMB 80mm lenses as Super APO.
There are some focal shift curves given at
http://www.apm-telescopes.de/info_bilder.php?action=show&id=3027&artikel_id=72637
and for the same wavelength range as above, the shift is
61 microns or 0.013%. So I do not see that 0.013% shift is somehow
extraordinary APO performance. I will say from experience
the color correction is superb -- whatever you call them, they are
very fine lenses. Also, if you used a wider wavelength range from say
4000 to 10000 Angstroms -- corresponding to imaging use -- then
yes, the focal shift does seem unusually small. So the term Super APO here
is somewhat hype and somewhat descriptive. I think they are an
example of excellent APO design, but not some high-tech breakthrough
with magical glass.

I have not commented on the APOMAX, since I have not seen
focal shift plots for it. They are supposedly diffraction limited
over a very wide wavelength range, but of course they are also F12.
Here I would say Super APO is used in a descriptive fashion, but
there is no miracle high-tech glass. They are just well designed
F12 triplets.

Anyway, thats my personal take on it.

Sometimes you will get 4 color crossings if the index data used in your calculations is only to 5 decimal places, however when you use 6 place data, there will be only 3 crossings. The number of crossings is not important, rather what you want to see is overall color error or the percentage defocus over a large wavelength spectrum. You could theoretically have a 2 color crossing lens that has better performance than a 3 or 4 color crossing lens.

Rolando