True APO ??

I've seen this *term ( True APO ) used quite frequently here on the Forum, and it's *meaning is self explanatory to the connoisseur, however the term has been abused when discribing lower end refractors. As a consensus of opinion take the ad currently posted here on the Mart in "Refractors" # 636748 a Celestron ED 80 (APO?) where would this variety of scope fall? Achromatic? APO? or True APO ? as you can see there is this nebulas area.. thank you for your indulgence.. JM

Practically speaking, based on what I have read here, "true apo" should be reserved for instuments with color correction necessary for astrophotography and would probably require a well designed triplet configuration. ED doublets, however, and even long focal length achromats might in some cases have good enough color correction for visual use. I will leave others to explain the technical information. Currently I am very happy with my 100mm f/9 ED refractor for visual use.

John Meiners said:

I've seen this *term ( True APO ) used quite frequently here on the Forum, and it's *meaning is self explanatory to the connoisseur, however the term has been abused when discribing lower end refractors. As a consensus of opinion take the ad currently posted here on the Mart in "Refractors" # 636748 a Celestron ED 80 (APO?) where would this variety of scope fall? Achromatic? APO? or True APO ? as you can see there is this nebulas area.. thank you for your indulgence.. JM

This is worth a read:

http://geogdata.csun.edu/~voltaire/roland/musing.html

And the images are worth a look as are the remaining essays at the bottom of the page.

http://geogdata.csun.edu/~voltaire/roland/

As far as the Celestron ED-80 being a "true APO." Visually these scopes have very good color correction, even on a tough target like Venus or Rigel, I could not see any violet.

Jon

Well here is a topic that has never been discussed LOL

That particular model is not a Apo at least not in the true definition of apochromatic......."Apo" has become a marketing "buzzword" of late being used to describe visual performance and from what I read the 80ED is performance wise visually very close. It is a very well corrected doublet that gets about as close as any doublet (at its focal length)can but it still is not a true apochomat. My opinion of course

John Meiners said:

I've seen this *term ( True APO ) used quite frequently here on the Forum, and it's *meaning is self explanatory to the connoisseur, however the term has been abused when discribing lower end refractors. As a consensus of opinion take the ad currently posted here on the Mart in "Refractors" # 636748 a Celestron ED 80 (APO?) where would this variety of scope fall? Achromatic? APO? or True APO ? as you can see there is this nebulas area.. thank you for your indulgence.. JM

It uses the same lens as the Orion ED80. A couple
years ago I measured the color correction -- the
focus change between red / green / violet -- for
the Orion version. The ED80 color correction results
were superior to AP Traveler EDT (ED triplet) and
TeleVue 85 (ED doublet). The ED80 had a red-violet
focus change of 0.12mm, the AP was 0.20mm and the
TV85 was 0.22mm. Call it whatever you want, but it
is very fine lens.

Focus shift: Red (#25) / Green (#56+#58) / Violet (#47)
Orion ED-80 (600mm): +0.12mm / 0mm / +0.03mm
TV85 (595mm): +0.08mm / 0mm / +0.22mm
AP105 EDT (620mm): +0.09mm / 0mm / -0.11mm

I know it is a lost cause, but APO stands for Army Post Office or Adirondack Public Observatory. If apochromat requires an abbreviation, it would be apo or Apo.

Perhaps the usage of the bizarre "APO" is a reflection of the confusion over its precise definition. My take is that a lens that reveals no out of focus color when in focus and shows sharp, contrasty, high power views of the planets is good enough to be called an apochromat (perhaps it would be more prudent to simply say such a lens keeps me very happy). Imagers might be fussier since they have to deal with a wider portion of the spectrum than our eyes perceive.

It would be helpful if the manufacture or seller provided some details on the lens - the chromatic focal shift at the very least.

The "true" definition that gets quoted was actually written by Abbe with microscope objectives in mind.

Clear skies, Alan

david goldman said:the one scope that i know that has the best color correction ever seen by me is the Zeiss APQ.there is absolutely no bleeding over of false color[purple haze]on vega,and the design is also very well corrected in the deep red.the late Thomas Back sent me a Zemax print out showing spot diagrams at various wave lenghts inticating that fact.i also have other diagrams from him from the same program [which i do not understand]on the APQ and ap superplanetary as a comparison.

My understanding of APQ performance compared to other top apo’s is that APQ is optimized for best visual performance as opposed to some of the other top brands which offer extended coverage to better perform with CCD’s extended spectrum sensitivity.

Vahe

To muddy the waters further, here's what Al Nagler had to say on the subject some years ago on the now defunct Ask Al section of the Tele Vue site:

Phil Barker asks about descriptions or standards regarding color
correction in refractors.

This is a complex issue, Phil. The residual color can be altered by
design, by manufacture and even by the individual "melt" production in
glass. Spherical aberration also varies with color (spherochromatism).
Then there's the matter of how much residual color is visible and the
nature of the hue. There are "classic" definitions for standard
achromats, but considering fluorite and other exotic doublets, triplets,
and 4-element systems, there's lots of wiggle room on APO definition. I
consider a system an APO when there's little or no color fringing on a
bright star at 200x, and semi-apo when performance is significantly
better than an achromat for a given focal ratio. Earned reputations are
indeed your best guideline at this point.

My previous numbers for the color correction in the
Orion ED-80 led to some interesting comments and
controversy. I went back and repeated them using a CCD
camera. Rather than measure focus shifts, I decide to
simply present the star images in the three different
colors -- violet, green, and red. The scopes were all
focused for the green filter, and then I took the violet
and red images without changing the focus, so you get some
feel for the chromatic and other aberrations.

There had been a comment that maybe while the ED-80 had
a small focal shift, it might have bad chromo-spherical
which was not really addressed by the focus shift number.
I thought posting the images would help resolve this issue.
As you can see, there is no significant spherical in
the violet; maybe theres is a little bit in the red.

The attached image shows the results for the Orion 80-ED,
TMB 80/600, TV85, Orion 80/500, 6" F5 Newtonian, and the
AP Traveler. Color runs left-to-right violet, green, red.

Some notes on the individual scopes:

Orion 80-ED: Green and Violet are both well focused.
Red is a little soft -- some extra light in the Airy ring
-- a lot like the TV85 in the red. Good Airy disks
in all colors. Very close to the TMB. No evidence for
significant spherical error in violet. This is an ED doublet.

TMB 80/600: Nearly text-book perfect images in all
three colors. This is labeled "Super APO" on the lens,
whatever that means. It looks pretty good. This is
an air-spaced ED triplet lens.

TV85: Green is OK. Red definitely soft. Violet is
a fuzz ball; lots of de-focus, etc. ED doublet lens.

Orion 80/500: This has sometimes been called semi-APO, etc.
It is more or less identical to the old University Optics
80 mm scope. Green gives a good image. Red is soft
- some light in the first Airy ring. Violet is
far out of focus -- I had to shrink the image by 4x to fit it in
(some hot pixels in the CCD also show up in the violet image
due to long exposures). Doublet lens; glasses rumored to be
FK5 and SF2.

Newtonian 6" F5: Good focus at all three filters.
A little coma and other aberrations in the violet.
I included the Newt just to check there were no errors
in my setup or method.

AP Traveler 105 F6 EDT: Reasonably good image in the
green. Red is a little soft. Violet has about 1 wave
defocus and some combination low and high-order spherical.
I repeated this one three times to make sure. This is
of course an F6 scope, vs. the others around F7 to F7.5,
so a little more color/aberration is expected. This is
an oiled ED triplet lens.

How images were made: These are all lab measurements
on an optical bench. I used a 16" collimator which effectively
projects an artificial star at infinity.
Setup is collimator - scope - TV 5x PowerMate - filter -
Imaging Source DMK CCD camera. Images are all at the same
pixel scale (except Orion 80/500 in violet) -- you will notice
the Airy disk is smaller in violet, and smaller for faster
F-ratios, as expected. The scope was focused using
the green filter. Then images were taken in green, violet, red.
Then focus was checked again in the green filter. I exposed
for about 1 minute in each filter at about 8 fps, and then
stacked in K3CDD, and stretched the contrast to put the peak
around 230/255 counts, but no other processing.

I think the bottom line is still unchanged from previous
discussion. Whatever you call the ED-80, it is a very fine
scope. The optics compare very well to other refractors
costing 3-5 times more $$.

I should also add, of course, these are tests on one particular
randomly selected sample of each model. The one you get might
be better or worse, etc.

Attached Image: