Re: ach vs. apo


Mar 26, 2003

 


----------------------------

#16329 Mar 26, 2003

Hello

Is there some combination of focal length and f number where an APO really

doesn't provide an advantage over a very well corrected achromatic doublet?

I mean does an APO give any real advantage in a 100mm f/15 scope, or just in

fast systems, or will an APO always visually outperform an achromat just by

definition at all focal lengths and ratios?

Dale



----------------------------

#16331 Mar 26, 2003

Dale,

In all 'practical' focal ratios, a typical doublet (pre ED glass)

will exhibit some secondary spectrum color around bright objects.

I have an old (1876) 6.25" f/16 Byrne refractor that gives beautiful

planetary images but, stopped down to 4" (f/25), some color fringe

still remains.

Comparative observations with an AP 180 f/9 leave no doubt that the

contrast and resolution of a well made APO is significantly better.



Richard



----------------------------

#16332 Mar 26, 2003

I thought the number for a well designed achromat was f/23 to

essentially be color free for visual. I have a GOTO 60mm at f/20

and there is little decernable color.



Stew







--- In ap-ug@yahoogroups.com, "zeeemu" syzygy42@e...> wrote:

> Dale,

> In all 'practical' focal ratios, a typical doublet (pre ED glass)

> will exhibit some secondary spectrum color around bright objects.

> I have an old (1876) 6.25" f/16 Byrne refractor that gives

beautiful

> planetary images but, stopped down to 4" (f/25), some color fringe

> still remains.

> Comparative observations with an AP 180 f/9 leave no doubt that

the

> contrast and resolution of a well made APO is significantly better.

>

> Richard



----------------------------

#16333 Mar 26, 2003

In a message dated 3/26/2003 4:11:20 PM Central Standard Time,

squiress@... writes:



> I have a GOTO 60mm at f/20

> and there is little decernable color.

>



There is little discernible color because the image is too dim to see it. Try

Sirius, or try it during the daytime with the sun shining on a white/black

border. You will see some color. The smaller the aperture, the larger the

Airy disc vs the out-of-focus blue and violet waves. At F20, the Airy disc

size is quite large vs. the blue/violet blur circle. As the aperture grows,

the Airy disc gets smaller with respect to the blue/violet blur circle. At

night, your eyes have limited color sensitivity, so what little brightness

there is gets lost in the dim image of a small aperture.



Roland Christen





[Non-text portions of this message have been removed]



----------------------------

#16335 Mar 26, 2003

Stew,



The f/ratio where the color is "unobjectionable" depends on aperture. I am

somewhat surprised that you can see color in a 60mm f/23. Baker claims "The

focal-length of an achromatic objective with unobjectionable color

aberration is given with sufficient accuracy by the formula f = 5 times D

squared" (where D is the aperture and f is the focal length). So a 3" f/15

is essentially color free, but a 4" really needs to be f/20. I used a 4"

f/15 Clark for a while, and it did show secondary color on bright objects

like Jupiter. I don't recall any in my 60mm f/15 Unitron.



I have also seen the formula written with a multiplier of 3 instead of 5,

but that yields an f/15 in a 5" aperture, and my 5" f/15 Lohmann Brothers

refractor certainly had obvious secondary color.



Clear skies, Alan

----- Original Message -----

From: "samwisethefool" squiress@...>

To: ap-ug@yahoogroups.com>

Sent: Wednesday, March 26, 2003 5:08 PM

Subject: [ap-ug] Re: ach vs. apo





> I thought the number for a well designed achromat was f/23 to

> essentially be color free for visual. I have a GOTO 60mm at f/20

> and there is little decernable color.

>

> Stew

>

>

>

>

> --- In ap-ug@yahoogroups.com, "zeeemu" syzygy42@e...> wrote:

> > Dale,

> > In all 'practical' focal ratios, a typical doublet (pre ED glass)

> > will exhibit some secondary spectrum color around bright objects.

> > I have an old (1876) 6.25" f/16 Byrne refractor that gives

> beautiful

> > planetary images but, stopped down to 4" (f/25), some color fringe

> > still remains.

> > Comparative observations with an AP 180 f/9 leave no doubt that

> the

> > contrast and resolution of a well made APO is significantly better.

> >

> > Richard

>

>

>

> To UNSUBSCRIBE, or for general information on the ap-ug list

> see groups.yahoo.com/group/ap-ug

>

> Your use of Yahoo! Groups is subject to docs.yahoo.com/info/terms/

>

>

>



----------------------------

#16337 Mar 26, 2003

Hello and thanks

So, my "bottom line is.. why doesn't anyone make a or 4 or 5inch f/15 APO

for planetary use? A Barlow is required for good image scale.

Is it because there is no demand since resolution from a 5 inch isn't that

great anyway for planets compared to cheaper larger SCT's?

I was looking for something like an APO 90mm f/15 or 100mm f/12 as a

dedicated solar eclipse photography scope (long, 1200mm, and slow with only

moderate resolution required and minimal number of internal glass surfaces,

light weight) and there just isn't such an animal.

Dale

> -----Original Message-----

> From: chris1011@... [mailto:chris1011@...]

> Sent: Wednesday, March 26, 2003 2:45 PM

> To: ap-ug@yahoogroups.com

> Subject: Re: [ap-ug] Re: ach vs. apo

>

>

> In a message dated 3/26/2003 4:11:20 PM Central Standard Time,

> squiress@... writes:

>

>

> > I have a GOTO 60mm at f/20

> > and there is little decernable color.

> >

>

> There is little discernible color because the image is too dim to

> see it. Try

> Sirius, or try it during the daytime with the sun shining on a

> white/black

> border. You will see some color. The smaller the aperture, the larger the

> Airy disc vs the out-of-focus blue and violet waves. At F20, the

> Airy disc

> size is quite large vs. the blue/violet blur circle. As the

> aperture grows,

> the Airy disc gets smaller with respect to the blue/violet blur

> circle. At

> night, your eyes have limited color sensitivity, so what little

> brightness

> there is gets lost in the dim image of a small aperture.

>

> Roland Christen

>



>







----------------------------

#16338 Mar 26, 2003

I think the market showed that most buyers wanted faster APOs with shorter

tubes.

----- Original Message -----

From: "Dale Ireland" direland@...>

To: ap-ug@yahoogroups.com>

Sent: Wednesday, March 26, 2003 7:10 PM

Subject: RE: [ap-ug] Re: ach vs. apo





> Hello and thanks

> So, my "bottom line is.. why doesn't anyone make a or 4 or 5inch f/15 APO

> for planetary use? [SNIP]



----------------------------

#16340 Mar 26, 2003

In a message dated 3/26/03 7:10:06 PM Eastern Standard Time,

direland@... writes:



< So, my "bottom line is.. why doesn't anyone make a or 4 or 5inch f/15 APO

for planetary use?



As Alan said, the market just isn't there. You might look around for

the rare Astro-Physics 5" f12 Super Planetary (only available in 1986,

according to catalogs of that era) or the 6" f12 Super Planetary which

appeared in 1986 and was still being advertised as late as 1991.



Paul Bock

Hamilton, VA U.S.A.

Visit my homepage at hometown.aol.com/phbjr/myhomepage/profile.html





[Non-text portions of this message have been removed]



----------------------------

#16341 Mar 26, 2003

In a message dated 3/26/03 7:10:06 PM Eastern Standard Time,

direland@... writes:



< So, my "bottom line is.. why doesn't anyone make a or 4 or 5inch f/15 APO

for planetary use?



One other suggestion: Take a look at Eric Jamison's web site (see

below for URL), particularly his planetary drawings. I've looked at hundreds

& hundreds of planetary drawings over the past 15 years, made by skilled,

experienced observers using the best possible telescopes, and I have yet to

see anyone get more out of a comparable aperture refractor of any focal

length then Eric does out of his A-P 5.1" and 7.1" apos - and I think he

still uses altazimuth mounts. I only wish that I could do one-tenth as well.



home.fiam.net/ericj/index.html



Paul Bock

Hamilton, VA U.S.A.

Visit my homepage at hometown.aol.com/phbjr/myhomepage/profile.html





[Non-text portions of this message have been removed]



----------------------------

#16342 Mar 26, 2003

Is there some combination of focal length and f number where an APO

really > doesn't provide an advantage over a very well corrected achromatic

doublet?



I would like to see a comparison of Strehl ratios between two similar

size and F ratio apo & achro. For example a comparison between AP6"

f/12 super planetary and a 6" f/12 and f/15 D&G achromats would be

very interesting, if their Strehl's represent all three colors and

not just the single wavelength of light that the lens is corrected

for.



Thanks,

Vahe



----------------------------

#16344 Mar 26, 2003

Have to jump in here. The APOMAX is not any kind of achro. It is a

Super APO bringing four colors to focus. It is a long focus

refractor, and very similar to the AP Super Planetary's that Paul

Bock has mentioned. And, sadly, it was not fully appreciated till it

was over and done with. Still have mine and will have it for a long

time yet. (Mechanically it is a work of art!) I think D&G has done

alright with their achro designs. I have a bunch of f/15-16 achro

refractors from 40-100mm from the late 50's. They are fun to set up

and use, but the big problem with them remains poor mounting

stability. One of my favorite scopes is a little Mayflower 60mm that

with a Tak 1.25 diagonal and Pentax orthos is absolutely wonderful.



Stew

--- In ap-ug@yahoogroups.com, "Enyo" Enyo@w...> wrote:

> The APOMax was just such a beast. Not many folks wanted one. When

I was

> making my decision I went with the AP130. I had the choice of

either the F6

> or F8.3 and chose the F6. Now I am just waiting for a nice MC for

planets

> with either active cooling (TEC) or fast cool (AP) but if my name

comes up

> on the 155 list I will have some choices to make.



----------------------------

#16345 Mar 26, 2003

As I recall the 4th crossing was in the far red/near IR as the intended use

was with CCD cameras. The thought was it would be a compliment to a large

Dob. Both APOMAX and AP would setup at Astrofest and it was easy to compare

the two. I agree the APOMAX was first class in all respects.



----------------------------

#16346 Mar 26, 2003

In a message dated 3/26/03 8:47:12 PM Eastern Standard Time,

squiress@... writes:



< I think D&G has done alright with their achro designs.



As the owner of a 5" f/10 D&G for 9 years, I'll second that. If I

wanted to own another 5" or larger achro I'd call Barry Greiner (the "G" in D&

G) first. He won't make anything smaller than 5", BTW..



< It also says that my 8" f/12 achromat should show a lot of color and needs

to be f/40 to be unobjectionable.. Certainly some color there, but not as bad

as this sounds.



Funny you should mention that, Stew. When I owned my D&G 5" f/10 I

once commented on some forum or other how suprisingly unobtrusive the color

was (less instrusive than my Jaegers 4.125" f/15 as far as I could tell). I

was immediately taken to task by someone quoting theory and telling me how

completely useless a 5" achro would be at f/10. I'm not belittling the

theory, but all I know is what I see so maybe my eyes are a lot worse than I

thought....... You're not color-blind by chance, are you? wink>



But to get back on topic, as a "convertee" to the apo design I really

can't see any worthwhile points for debate other than the obvious ones of

field flatness, criticality of focusing and magnification for a given

eyepiece between, say, an f/6 apo and an f/8.5 or so. These issues aside, I

think that all of the top end apos are so stunningly good that one would be

hard-pressed to feel "unhappy" with any of them.



Paul Bock

Hamilton, VA U.S.A.

Visit my homepage at hometown.aol.com/phbjr/myhomepage/profile.html





[Non-text portions of this message have been removed]







----------------------------

#16347 Mar 26, 2003

--- In ap-ug@yahoogroups.com, phbjr@a... wrote: > You're not color-blind by chance, are you? wink>



No, I see color just fine. Although I remember observing with a

very enthusiastic individual once and looking at nebula in his

modified 10" MakCass. He was seeing all sorts of color in these

objects and describing it as you might see in a film or CCD shots. I

didn't see any during this session, only shades of gray. :-)

> field flatness, criticality of focusing and magnification for a

given > eyepiece between, say, an f/6 apo and an f/8.5 or so.



I think these are pretty important issue for the visual observer.

With Mak Cass at f/11-f/20, MSC at f/10-f11, all these scopes

optimized for imaging at f/6 or less, I still think of a nice all

around scope as f/7-9. The long focus scopes don't provide wide

fields; the short fast scopes require barlows to get sufficient image

scale.



I was talking to a guy that has a Tak FS. He just doesn't notice

color anymore as it's the only scope he has. I am fortunate enough

to use a lot of different scopes and when you see a bright star image

in an AP like the f/8.35 130 against an FS series Tak, it does have a

tendency to jump out at you. That was the most obvious thing about

the Tak TOA. What you tend to notice is that there is no color, not

that there is a hint of it or that there is less than some other

scope. I think that if you're observing with an achromat at f/15,

you will quickly get to the point that color is a non-issue.



Stew



----------------------------

#16348 Mar 26, 2003

Hi



I had an eye on a D & G 8" f12 refractor for use with an ap900 mount (-: wh=

en

St. Christen sends it to me :-) It maybe too long for this mount but it wou=

ld be a

good value for planetary viewing

--- In ap-ug@yahoogroups.com, phbjr@a... wrote:

> In a message dated 3/26/03 8:47:12 PM Eastern Standard Time,

> squiress@t... writes:

>

> < I think D&G has done alright with their achro designs.

>

> As the owner of a 5" f/10 D&G for 9 years, I'll second that. If I=



> wanted to own another 5" or larger achro I'd call Barry Greiner (the "G" =

in D&

> G) first. He won't make anything smaller than 5", BTW..

>

> < It also says that my 8" f/12 achromat should show a lot of color and

needs

> to be f/40 to be unobjectionable.. Certainly some color there, but not as=

bad

> as this sounds.

>

> Funny you should mention that, Stew. When I owned my D&G 5" f/10 =

I

> once commented on some forum or other how suprisingly unobtrusive the

color

> was (less instrusive than my Jaegers 4.125" f/15 as far as I could tell).=

I

> was immediately taken to task by someone quoting theory and telling me

how

> completely useless a 5" achro would be at f/10. I'm not belittling the

> theory, but all I know is what I see so maybe my eyes are a lot worse tha=

n I

> thought....... You're not color-blind by chance, are you? wink>

>

> But to get back on topic, as a "convertee" to the apo design I rea=

lly

> can't see any worthwhile points for debate other than the obvious ones of=



> field flatness, criticality of focusing and magnification for a given

> eyepiece between, say, an f/6 apo and an f/8.5 or so. These issues aside=

, I

> think that all of the top end apos are so stunningly good that one would =

be

> hard-pressed to feel "unhappy" with any of them.

>

> Paul Bock

> Hamilton, VA U.S.A.

> Visit my homepage at hometown.aol.com/phbjr/myhomepage/

profile.html

>

>

> [Non-text portions of this message have been removed]



----------------------------

#16349 Mar 26, 2003

--- In ap-ug@yahoogroups.com, "Alan French" adfrench@n...> wrote: > I think the market showed that most buyers wanted faster APOs with

shorter > tubes.



Indeed, I read somewhere that the refractors almost went the way

of the dinosaurs until Roland Christen and Al Nagler first made

a breakthrough with shorter focal ratio APO refractors, especially

with the larger aperture AstroPhysics APO. IMnewbieO, the shorter

faster APO is much more flexible, being able to give such wide FOV

but still able to attain such high magnifications for planets, not

to mention being easier to mount, much lighter, and faster for

astrophography. The only disadvantage I see is the $$$$($) :-(.



Ron B[ee]



----------------------------

#16360 Mar 27, 2003

In a message dated 3/26/2003 7:35:21 PM Central Standard Time, vahe@...

writes:



> For example a comparison between AP6"

> f/12 super planetary and a 6" f/12 and f/15 D&G achromats would be

> very interesting, if their Strehl's represent all three colors and

> not just the single wavelength of light that the lens is corrected

> for.

>



Strehl ratio for an achromat would be very low for all colors except the

principal one that is in focus. For instance, if the green was in focus, then

red, blue and violet would have very low Strehl at that focus point. Long

focus Apos can have very high Stehl for more than one color at the best

focus. The defocus in achromats is so high that Strehl ratios will be below

.5 for colors outside of best focus.



Roland Christen





[Non-text portions of this message have been removed]







----------------------------

#16367 Mar 27, 2003

Can you educate us a little more,



assume that green is getting .95, then if I understood

you correctly, red, blue and violet would be likely

about .475 ?



that would make me think a camera would see twice the

energy in green as the other 3, and pics of say, the

moon, should come out distinctly green, not just on

the

edges, but across the entire field. (or blue etc,

depending on which color was most in focus). Is

my logic wrong? I must be wrong, since my cheap achr

has purple fringe on the moon, but the color of the

moon seems much closer to white that this would

predict.



thanks



Kurt



--- chris1011@... wrote: > In a message dated 3/26/2003 7:35:21 PM Central

> Standard Time, vahe@...

> writes:

>

>

> > For example a comparison between AP6"

> > f/12 super planetary and a 6" f/12 and f/15 D&G

> achromats would be

> > very interesting, if their Strehl's represent all

> three colors and

> > not just the single wavelength of light that the

> lens is corrected

> > for.

> >

>

> Strehl ratio for an achromat would be very low for

> all colors except the

> principal one that is in focus. For instance, if the

> green was in focus, then

> red, blue and violet would have very low Strehl at

> that focus point. Long

> focus Apos can have very high Stehl for more than

> one color at the best

> focus. The defocus in achromats is so high that

> Strehl ratios will be below

> .5 for colors outside of best focus.

>

> Roland Christen

>

>

> [Non-text portions of this message have been

> removed]

>

>

>

> To UNSUBSCRIBE, or for general information on the

> ap-ug list

> see groups.yahoo.com/group/ap-ug

>

> Your use of Yahoo! Groups is subject to

> docs.yahoo.com/info/terms/

>

>





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----------------------------

#16369 Mar 27, 2003

I've owned an excellent Pentax 60mm fl=700mm achromatic refractor. It

displayed the standard purple fringing on telephone wires at over

100x.



Roland's original design was quite a bit better, even the f/6 units.

Planetary observing and imaging requires high strehl at all visible

wavelengths and the achromats just don't do it.



Better off with a good long focus 6" or 8" Newtonian for starters.



Don



----------------------------

#16373 Mar 27, 2003

In a message dated 3/27/2003 11:06:14 AM Central Standard Time,

kurt_friedrich@... writes:



> that would make me think a camera would see twice the

> energy in green as the other 3, and pics of say, the

> moon, should come out distinctly green, not just on

> the

> edges, but across the entire field.



No, the energy is still there, it is simply not sharp at red and blue. You

can easily do a simulation. Take a black&white image of a lunar crater, using

Photoshop, place the image in an RGB mode. Then choose Channels, blur the

blue channel perhaps 5 pixels, and red channels perhaps 2 pixels, and then

look at the combined color image again. You will see the dark areas will look

distinctly blue instead of black, but the overall lunar landscape is not

green. This is almost exactly how the Moon will appear in an achromat. You

will not loose any resolution, even though you have blurred two of the three

channels. You will see a tiny bit of contrast loss.



Roland Christen



Roland Christen





[Non-text portions of this message have been removed]



----------------------------

#16376 Mar 27, 2003

--- In ap-ug@yahoogroups.com, chris1011@a... wrote: >

> No, the energy is still there, it is simply not sharp at red and

blue. You > can easily do a simulation. Take a black&white image of a lunar

crater, using > Photoshop, place the image in an RGB mode. Then choose Channels,

blur the > blue channel perhaps 5 pixels, and red channels perhaps 2 pixels,

and then > look at the combined color image again. You will see the dark areas

will look > distinctly blue instead of black, but the overall lunar landscape

is not > green. This is almost exactly how the Moon will appear in an

achromat. You > will not loose any resolution, even though you have blurred two of

the three > channels. You will see a tiny bit of contrast loss.

>

> Roland Christen



I saw purple (crater) shadow instead of black in my old C102-HD

achromat (f/9.8). It's interesting though for me to read some

observer's report of black shadow from fast f/6.9 achromats. Do you

think it's the eye sensitivity?



Thanks,

Ron B[ee]


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