Wat is actually an APO ?

[skysurfer]

I have the Televue Genesis and for me it is a good (travel) scope. Many discussions on CloudyNights are about : "Does modern (semi-)apos perform better than the 30 year old Genesis ? " Or "is the Genesis an APO ?".

I would say: Yes and No.

Yes: it corrects colors better than achromats because of the rear-end Petzval elements and even at high power, only slight hints to violet halos are visible @ 200x and high-contrast targets.

No: According to Wikipedia an Apo is a *three* element objective putting three wavelengths in one point. An objective which corrects for only two wavelengts is an achromat as the Genesis does.

So some WO FLT triplets are apos, but even the NP-101 which is an improvement over the Genesis (better glasses used) and probably the best 10cm refractor in the market beside the Tak FSQ-105 but has still the same Petzval design and is just an achro but a VERY GOOD one.

What are your ideas ?

[Merlin66 (Ken)]

The general definition of a Apochromatic optical system is one which brings three wavelengths to a common focus (we hope!)
In traditional lens designs this would require a minimum of three different lens/ glass specs.
I'd say any optical system, whether it has three (or four in the Petzal design) which produces an APO image is a Apochromatic instrument.
( I still use my Genesis, s/n 1007)

[Wavytone]

Wikipedia is often useful but it is not a definitive, authoritative source. From my optics texts, to be called "apochromatic" a lens group must be corrected for spherical aberration at two wavelengths or colors and for chromatic aberration at three wavelengths.

Comments:

Reflecting telescopes are inherently apochromatic.

A two-element lens can meet the condition using either fluorite or ED glass, as can many designs with 3 or more elements.

A "lens group" means any number of elements in close proximity ie cemented or airspaced. While the Petzval design can be effectively as good, technically it doesn't meet the definition as it has two groups - a positive objective, and a second group midway to the focal plane. It doesn't take Einstein to add more groups, that's an easy way to find a solution but at the expense of net light loss, scattering and internal reflections, not to mention increased manufacturing cost.

[Kal (Andrew)]

Thomas Back wrote an essay titled 'defining apochromatism' that you might find interesting.

In it he states "The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma. "

That essay specifically mentions Al Naglers petzvel telescopes.

[asimov (John)]

'Reflecting telescopes are inherently apochromatic' is incorrect then, according to Thomas Back.

[gregbradley]

To the original poster. Yes I think you have that right.

The reason why reflecting telescopes are considered APO (well the term isn't really applied to them) is because a mirror reflects without splitting white light into its colours unlike an objective lens.

Hence FPL53 etc which splits the light into its various colours far less than regular glasses.

3 elements in an objective is inapplicable for a definition. Roland Christen has made the comment that he could make an APO doublet. Tak FS series were close to APO but had some false colour.

Greg.