Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Announcements and News >> Discussion of CN Articles, Reviews, and Reports

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | (show all)
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: BillP]
      #4074784 - 09/26/10 04:36 PM

Quote:

Quote:

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...




As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine.




When it comes to image quality, talking peak HP isn't a good analogy. If folks want maximum visual "horse power", they should get a huge Newtonian.

But, that's veering ever further off topic.

Reviewing my comments about Neil's article I think they're rather blunt and/or unkind.
My apologies for that.

Neil has done a great job producing his report.
He's a masterful writer.
It obviously involved a lot of research on his part, with the assistance of Vlad.
Lots of interesting questions were raised and explored.

It just doesn't jibe with my experiences, which is why I have a problem with some of the findings.
Mathematical modeling, graphs and spot diagrams sometimes don't accurately describe what's going on. Or, they do but context is misinterpreted.

Case in point--
Some enthusiasts of a small, very well made Maksutov-Cass telescope are known to extoll it's virtues with great exuberance. That instrument's central obstruction has been argued (along with proof derived from optical modeling) to increase the scope's resolving power, compared to an unobstructed system. However, in real-world usage, it's obstructed optic doesn't perceptibly improve resolution and in the vast majority of situations, image quality is degraded. But that never seems to inhibit the diehard enthusiasts from revisiting the "central obstruction is good" viewpoint.

So, around and around we go.

Best wishes to Neil.
He is a great ambassador of the long focus achromat.


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4074788 - 09/26/10 04:37 PM

Quote:

If somebody has a vested interest in championing the alleged superiority of a particular design, then it's not surprising that they will continue to promote it ad nauseum. Present lots of convincing looking charts, graphs and statistics, which apparently prove their point.




Another conspiracy theory, eh? The thing is, all the optics related graphs are reproducible with raytrace. Oh, well - didn't think of it before - OSLO had to be rigged too, just to support long focus achromat. Or were it those who created diffraction calculation itself? Oh, man...


Quote:

Can somebody point me to the 4" f/6.3 doublet apo mentioned in this article? I'd like to *actually* compare it to my 4" f/15 achromat.




Here are the specs:

100mm f/6.3 FPL53/KZFSN2
SRF RADIUS THICKNESS APERTURE RADIUS GLASS SPE NOTE
OBJ -- 1.3160e+20 1.1485e+18 AIR

AST 350.056000 13.160000 50.008000 AS O_S-FPL53 C
2 -184.230000 0.130000 49.513451 S AIR

3 -188.714400 6.580000 49.491049 S KZFSN2 C
4 -605.360000 618.462178 49.384613 S AIR

IMS -250.040000 -0.046529 5.478249 S

It is a top apo doublet, comparable to TMB 130*BLEEP* in color correction (when scaled up to 130mm and f/7). It does not
quite satisfy apo requierement, but it's close. The achromat is standard Fraunhofer.

Quote:

Then again, I have observed with a 4.3" f/5.95 ED doublet stopped down to 4" F/6.3 and can report that it outperforms the 4" f/15 achro in all respects, except for sensitivity to seeing.




Which proves that that particular apo was better than that particular achromat. The article considers very specific case of an apo with moderate amount of e-line correction error (0.050 wave RMS, equivalent of 1/6 wave p-v wavefront error of lower order spherical), and an achromat with an effective 1/8 wave p-v e-line error. That seems very realistic. You may have gotten luckier with your apo, and/or unlucky with the achromat...

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4074836 - 09/26/10 04:59 PM

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars.
This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo.
Only trouble was, the modeling didn't match what I observed.
So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors.
There was no conspiracy at work with the software he was using.
There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4074886 - 09/26/10 05:27 PM

Quote:

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars.
This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo.
Only trouble was, the modeling didn't match what I observed.
So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors.
There was no conspiracy at work with the software he was using.
There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.




First off, switching the subject usually does not help
resolve an issue. And so doesn't not understanding (or misrepresenting?) optics and raytrace. The thing is, we do not look at the spot diagrams, and they have nothing to do with diffraction calculation.

Due to its nature (i.e. less exponential spread of energy toward farther off non-optimized wavelengths), sperochromatism does apear less colorful than secondary spectrum *at identical level of chromatic error*. Roland Christen goes as far to say that sperochrimatism is "white" (I would assume it refers to well balanced spherochromatism).

The point is, one cannot accurately judge the level of chromatic error based on the magnitude of color visible in an apo vs. achromat.

Vla

Edited by wh48gs (09/26/10 05:31 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4074965 - 09/26/10 06:08 PM

Quote:

You may have gotten luckier with your apo, and/or unlucky with the achromat...

Vla




I usually try to optimize the system, when it comes to ED doublets and triplets.
In the case of my apos, employing a prism diagonal (either BK7 or BAK4, depending on the objective lens characteristics) has often resulted in improved chromatic and spherical correction.
Perhaps that accounts for what I've experienced.

Question for Vla--
When formulating the 4" f/6.3 apo doublet, did you optimize correction for the yellow/green or blue/green?


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4075242 - 09/26/10 08:52 PM

Quote:

I usually try to optimize the system, when it comes to ED doublets and triplets.
In the case of my apos, employing a prism diagonal (either BK7 or BAK4, depending on the objective lens characteristics) has often resulted in improved chromatic and spherical correction.




If apo correction is imperfect, a prism can be beneficial, or make it worse. With 1" prism (BK7) this particular 100mm f/6.3 apo went from 0.92 to 0.95 polychromatic Strehl. The prism improved both, red and blue, but worsened violet and deep red (the latter nearly don't matter for visual, so the overall visual Strehl went up; if it was photosensitivity, the two probably would mainly offset each other). That indicated that the color curves in the apo were slightly more separated than the optimum; increasing front radius by 3mm and increasing lens spacing by 0.06mm, the Strehl went to 0.943. The 2-3 ratio points differential is bellow the level of perception (comparable to 0.97-0.98 and 1 Strehl), but it is always better to be up than down.

If the apo went onto the graph with 0.943 Strehl, it would be down to 0.85, not 0.83 after adding 0.050 wave RMS e-line error. Not quite as the achromat at 0.86 Strehl, but practically identical.

However, adding 2" prism lowered the Strehl to 0.906; the curves got too close, and best foci of other colors were past the e-line focus plane.

So with one same apo the two different prism sizes had the opposite effect.

The other part of the equation is the achromat. While it is very hard to produce sufficient deviations in the radii or spacing to result in a significant error (frankly , it would require very sloppy fabrication/assembly), some other issues, sort of below the radar, can affect its quality: glass homogeneity, deviations of actual glass from design indici, surface figure integrity and smoothness, wedge control. As little as 0.01 degree wedge (little over 1/60 mm) in the 100mm f/15 achromat lowers its poly-Strehl by 3 ratio points.

Assuming that you've got this particular f/6.3 apo, and used it with 1" prism, you would have a 0.95 Strehl instrument. You could consider such scenario very lucky indeed. And being less lucky with the achromat could have it down to 0.80 Strehl or so. That is a possibility. In the article, an attempt was made to present what would be a reasonably likely average scenario with both instruments with high quality optics. Somewhat greater e-line error in the apo is simply to expect considering *much* tighter tolerances for the radii values and lens separation (in addition to other quality factors being as tight, or more so than in the achromat).

Quote:

When formulating the 4" f/6.3 apo doublet, did you optimize correction for the yellow/green or blue/green?




They are both (apo and achromat) optimized for minimum error in the e-line, with the red and blue ends roughly balanced. That makes them comparable.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4075514 - 09/26/10 11:13 PM

Thanks for the information, Vla.
Much appreciated.


Post Extras: Print Post   Remind Me!   Notify Moderator  
roscoe
curmudgeon
*****

Reged: 02/04/09

Loc: NW Mass, inches from VT
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4075879 - 09/27/10 08:13 AM

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............

I've always been a long skinny tube guy, (although lately my 'main' scope is a short f/8), so I'm delighted to read Neil and Vladimir's work, and I'm sure
I'll read it a couple more times to really understand it all.
I feel a bit bad for all you folks with a gazillion dollars tied up in finely crafted APO's, beautiful 2-speed focusers, and gorgeous hand-grenade EP's, when it turns out that a couple pieces of glass and a simple focuser does indeed get the job done........... but then again, those APO's actually fit in an auto, and a decent mount for a 12-foot-long yard cannon might just cost as much as a decent used car.........
I am a bit dissapointed to find out, though, that my beloved skinny-tubes don't win out in the faint fuzzy department..........ah well.......
Thanks, Neil and Vlad, for your impressive research!
Russ


Post Extras: Print Post   Remind Me!   Notify Moderator  
Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: roscoe]
      #4075910 - 09/27/10 08:37 AM

Quote:

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............
(snip)

Russ




Russ,

I think having an objective well above the ground and well away from the observer is a significant advantage. It may actually be the single biggest advantage.

Clear skies, Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4075913 - 09/27/10 08:40 AM

What puzzles me somewhat is the polychromatic Strehl value of 0.9 for a 4" f/15 achromat.
That seems remarkably high.
IIRC, a value closer to 0.81 has been mentioned in the past for an achro of that size and f/ratio.
In a posting earlier this year, Vla refers to a 4" f/30 achro as having a polychromatic Strehl at just a tick over 0.9, so getting that from a 4" f/15 is intriguing.

Vla's website elaborates about it:
http://www.telescope-optics.net/polychromatic_psf.htm

"The numbers for achromats may look somewhat optimistic, but that is what the raytracing results imply."

Indeed.

P.s.,
more rereading and trying to understand the "Sacek Effect"...
it's mentioned that this results in a "slightly enlarged central maxima". Does this mean that the Airy disk of an achromat is larger than what's normally calculated for a scope of a given aperture?

Edited by Clive Gibbons (09/27/10 10:52 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Doug76
Long Achro Junkie
*****

Reged: 12/05/07

Loc: Refractor Heaven
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4076306 - 09/27/10 12:45 PM

I think all the emphasis on no CA in an apo vs some CA in a long achro ignores a very important point.
CA isn't the only noticeable aberration, and some are worse than CA, and most of those other aberrations are much more prevalent in a short scope.
I've said it before, I'll say it again.
For me, a long achro, always, for viewing. Apo's are best used for imaging.
Just my opinion, but it's the opinion I trust most.


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4076471 - 09/27/10 02:12 PM

Quote:

What puzzles me somewhat is the polychromatic Strehl value of 0.9 for a 4" f/15 achromat.
That seems remarkably high.
IIRC, a value closer to 0.81 has been mentioned in the past for an achro of that size and f/ratio.
In a posting earlier this year, Vla refers to a 4" f/30 achro as having a polychromatic Strehl at just a tick over 0.9, so getting that from a 4" f/15 is intriguing.




The 0.91 polychromatic Strehl is for the e-line focus, for a e-line optimized 100mm f/30 achromat.

E-line Strehl for a 100mm f/15 achromat is 0.82. On the graph in the article, this is the Strehl value of the solid red line at the zero defocus line.

What I found out when looking into achromats defocus is that its poly-Strehl improves going from e-line toward red/blue focus. This is actually very logical, and almost obvious consequence of all the wavelengths focusing farther than e-line, including those to which the eye is highly sensitive. The highest polychromatic Strehl for the achromat, 0.86, is shifted 0.13mm toward the red/blue focus (which is 0.84mm from the e-line focus). That is the top value on the solid red line.


The effect of slightly enlarged central maxima - another phenomenon directly related to the chromatic defocus - is that peak diffraction intensity does not as closely match (relative) encircled energy within the central maxima as it does with other common monochromatic aberrations of similar magnitude. This is something I knew before, and was described on my site for a couple of years. With spherical aberration, for instance, the encircled energy is nearly identical in value to the peak intensity: 1/4 wave p-v will result on 0.80 Strehl, and the central maxima is encircling 0.80 (or 80%) of the energy that is there with zero aberrations.

With the same Strehl level resulting from defocus (1/3.89 wave p-v), the central maxima encircles nearly 0.87 of the energy that is there with aberrations absent. This means less energy in the rings, and better contrast transfer in the left side of an MTF graph. Since this includes all detail sizes from Airy disc up, it covers most of details observed. The price it pays for it in larger central maxima is negligible: it is 3.6% larger, linearly, corresponding to the central maxima size of 96.4mm aperture. Theoretically, it would result in slightly inferior resolution of near-equal doubles, but it hardly has any practical significance. As for the contrast/resolution of details smaller than the Airy disc size, it affects only bright contrasty objects, since this detail size range is beyond the limit of resolution for low contrast objects.

In short, at identical central diffraction intensity (Strehl), defocus error will result in more energy in the central maxima, and less in the rings. For the majority of objects, it will act as if it has higher effective Strehl. This effective Strehl for the achromat is plotted as the top red dashed line on the graph. Its highest spot is at 0.91 polychromatic Strehl. It is obtained by applying empirical approximation that fits the raytrace data to the best polychromatic Strehl (0.86)(F+3)/(F+2), where F is the achromat's focal ratio.

The 0.91 Strehl for the 100mm f/30 is its e-line Strehl. I didn't check its best Strehl value, but would expect it to be around 0.95. And its effective Strehl, due to higher encircled energy, is still somewhat higher.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: wh48gs]
      #4076501 - 09/27/10 02:26 PM

One thing I've often wondered about - and this might have some bearing here - is what the eye perceives as "best focus." In relation to the chromatic focal variation, where do we set the focus? Does it vary among people? I would guess it would be somewhat toward the blue/red focus and inside the green focus, but it would be interesting to see some experiments along this line.

Clear skies, Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4076512 - 09/27/10 02:30 PM

Thanks for the detailed explanation, Vlad.



Post Extras: Print Post   Remind Me!   Notify Moderator  
jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: astroneil]
      #4076926 - 09/27/10 05:54 PM

Well, now that you mention it Neil, I do have a bone to pick with your article.

Specifically, I note that it has not been peer reviewed by your mum and a cadre of garden gnomes.



- Jim


Post Extras: Print Post   Remind Me!   Notify Moderator  
Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: jrbarnett]
      #4076951 - 09/27/10 06:08 PM

Yeah, but those garden gnomes move so slowly you can't tell they're doing anything.

Clear skies, Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
astroneil
Pooh-Bah
*****

Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: jrbarnett]
      #4076985 - 09/27/10 06:27 PM

Quote:

Well, now that you mention it Neil, I do have a bone to pick with your article.

Specifically, I note that it has not been peer reviewed by your mum and a cadre of garden gnomes.



- Jim




Unfortunately,
The gnomes were away at a conference and my mum rejected it outright, as there wasn't enough math in it!

Regards,

Neil.

Edited by astroneil (09/27/10 06:31 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Stranger than fiction! new [Re: roscoe]
      #4077176 - 09/27/10 08:22 PM Attachment (56 downloads)

Quote:

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............





The Federal Aviation Administration has requested that I put strobe lights on my dewcap. And of course, I can continue my observing programme (in honor of Neil) during rainy weather.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Stranger than fiction! new [Re: Alan French]
      #4077180 - 09/27/10 08:25 PM

Quote:

One thing I've often wondered about - and this might have some bearing here - is what the eye perceives as "best focus." In relation to the chromatic focal variation, where do we set the focus? Does it vary among people? I would guess it would be somewhat toward the blue/red focus and inside the green focus, but it would be interesting to see some experiments along this line.

Clear skies, Alan




It could also be that focus varies with age and eye accommodation. Eyepieces that are "parfocal" don't seem to be that way for me anymore. Maybe I am picky about focus.

Yes, I prefer that explanation.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Stranger than fiction! new [Re: Preston Smith]
      #4077247 - 09/27/10 08:55 PM

Quote:

The answer is simple folks.

Buy one of each.




Overcome. Adapt. Well done Preston!


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | (show all)


Extra information
1 registered and 3 anonymous users are browsing this forum.

Moderator:  iceblaze 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 26328

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics