26 replies to this topic

[Scott in NC]

My next test is that of yet another Astro-Physics refractor, this time a nice sample of an AP130GT f/6.3 from 2013, courtesy of a fellow CN'er.  This is an oil-coupled triplet with a center element made of FPL-53, with 130mm aperture and focal length of 819mm. Despite being over 10 years old it looks as if it could be nearly brand new.

First is the obligatory picture of it set up on the optical bench ready for testing.

 

[Scott in NC]

Green inside/at/outside focus:

 

 

 

Red inside/outside:

 

 

 

Blue inside/outside:

 

 

 

White inside/at/outside:

 

 

 

Edit: I redid all of my photos, this time using the same LED brightness level for my intrafocal and extrafocal images, and tried to get a little better focus this time. I think these images capture what I saw visually much better than my initial images.

Edited by Scott in NC, 24 February 2024 - 05:38 PM.

[CHASLX200]

Looks super good but i think any AP would be good.

[Darren Drake]

Those zones in green seem a bit more than I'd expect.  Or I guess this just emphasizes the extreme sensitivity of the test...

[Jim Waters]

but i think any AP would be good.

+1 

[Scott in NC]

My intrafocal pictures didn't come out as sharp as my extrafocal pictures, so blame that on the photographer and not on the scope or the testing rig. When viewed with the eye the intrafocal images were just as sharp as the extrafocal.  

 

Edit: I redid all the images in post #2 above.

 

Spherical correction looks good in green and red, and a little farther out in blue, as in nearly all of the refractors that I've tested to date.  There appears to be a central zone (at approximately 10% of the radius) and another one at about 60-70% of the radius, and I'm wondering if those zonal variations are a result of hand aspherizing of the optics, as was done in many of the Astro-Physics refractors of this era. These are seen much more easily in green than in white light. Chromatic aberration is very nicely controlled for a fast f/6.3 triplet of this aperture.

 

I had a chance to observe through this scope on a night of average seeing earlier this week. I was treated to some very nice views of double stars and Jupiter, although seeing wasn't good enough for me to push the magnification as high as I would have liked.  The Trapezium E and F stars were clearly visible at all times, with magnification starting as low as 82x using a 10mm Ethos. The bright moon overhead (4 days before full) precluded much in the way of DSO observation. I did catch 2 Plato craterlets (and possibly a 3rd, but my floaters kept getting in the way, so I'll only claim 2).

 

I'm quite pleased with how this scope performs, both visually and under DPAC testing. 

Edited by Scott in NC, 06 March 2024 - 10:05 PM.

[Scott in NC]

I think I'm going to try to redo the images so that the intrafocal and extrafocal are the same level of brightness.  Keep in mind that this difference seen above is just an artifact of the photography process, and doesn't have anything to do with the optics or the test itself. I have a rheostat on my light source and had it set too low for the intrafocal images.

 

Edit: All images in post #2 have been redone, and I think they now reflect much better what I actually saw visually. Better lighting and focus can really make a difference here.

Edited by Scott in NC, 24 February 2024 - 05:45 PM.

[Nerd1]

Pardon my ignorance, but why are the blue a different shape?

[scoale]

Very impressive F/6.3 scope.

 

Scott, I think this is your best testing effort to-date.  The level of detail on the null images (yes, there are zones) and the white images (yes, there is a bit of color) are really superb.

 

I would take this scope in a heartbeat.

[scoale]

Those zones in green seem a bit more than I'd expect.  Or I guess this just emphasizes the extreme sensitivity of the test...

Hi, Darren.  i do feel like the body of DPAC tests indicate that the method is highly sensitive to picking up zones.  I know star testing is able to pickup zones as shallow as 1/50.  All of my refractors have zones.  The Televue is very shallow and dead center.  The Stellarvue is bigger, deeper, and in the central part of the lens.  The TAK and TEC have outer zones (worst place to have them). 

[Scott in NC]

Pardon my ignorance, but why are the blue a different shape?

Good question. That's the effect of spherochromatism (changes in level of spherical aberration which vary with changes in wavelength, i.e., color). It's impossible to produce refractive optics which focus all colors at the same point and which correct for spherical aberration (SA) equally well in all colors. The optical designer will typically strive to figure the lens so that it has best correction in green (or somewhere close to that), as that's where our eyes are most sensitive. Most well-corrected apochromatic scopes that I've seen have very good correction in red as well, and blue tends to fall a little farther off.  If the lens designer tried to perfect the correction in the blue wavelengths of light, red and green would likely look much worse, and I suspect that the scope's visual and photographic images would suffer quite a bit as a result.

[Scott in NC]

Very impressive F/6.3 scope.

 

Scott, I think this is your best testing effort to-date.  The level of detail on the null images (yes, there are zones) and the white images (yes, there is a bit of color) are really superb.

 

I would take this scope in a heartbeat.

Thanks, Stephen. Despite the visible zones, this scope puts up very good images under the skies (or at least it did the one night that I used it under suboptimal seeing).

 

I do think that I'm getting better at capturing the images photographically. That's of course a good thing, but the better that I'm able to get lighting and focus under control, the more visible subtle findings will appear. You may have noticed that the images that I've been posting are rather large, and of course the larger the image is, the more easy it is to see any aberrations.  I could have easily taken less well-focused images, inadvertently added a little blur from not holding the camera perfectly steady, and then posted the images at a smaller size, and any surface deviations from normal would have either been invisible or barely visible.  But of course I always want to get the best images possible, otherwise what's the point in doing the test?  

 

Anyway, it's been a work in progress, and a fun journey that I started a year ago in February 2023.

[Spikey131]

Whew, so glad that it passed the test.

 

I guess I’ll keep my GT after all, and go on out and look at Jupiter…

[Nerd1]

Good question. That's the effect of spherochromatism (changes in level of spherical aberration which vary with changes in wavelength, i.e., color). It's impossible to produce refractive optics which focus all colors at the same point and which correct for spherical aberration (SA) equally well in all colors. The optical designer will typically strive to figure the lens so that it has best correction in green (or somewhere close to that), as that's where our eyes are most sensitive. Most well-corrected apochromatic scopes that I've seen have very good correction in red as well, and blue tends to fall a little farther off. If the lens designer tried to perfect the correction in the blue wavelengths of light, red and green would likely look much worse, and I suspect that the scope's visual and photographic images would suffer quite a bit as a result.

Thanks for the explanation, I suspected that was the case but wasn't sure. Very interesting tests now that I'm starting to understand what I'm looking at.Thanks again for your time and effort.

Sent from my moto g power (2022) using Tapatalk

[Scott in NC]

Speaking about zones, this interesting article "Understanding Aspheric Lenses" has some nice figures (especially Fig. 2) showing how aspherical lenses differ from spherical lenses.

 

I'm far from an expert on this, but my understanding is that figuring a lens with an aspherical correction is going to produce a wavefront that will look like a zone on DPAC testing. I guess technically that area of aspherization really is a zone as it's an area with slightly different curvature from the rest of the lens. If the lens were left completely spherical it would look much smoother without visible zones on DPAC testing. But even though the "at focus" image would look better, the intrafocal and extrafocal Ronchigrams would likely show significantly more spherical aberration.

 

https://onlinelibrar.../opph.201800033

 

Now can someone who actually knows what they're talking about please review what I just said and make any needed corrections?

[Scott in NC]

Here's a newer AP130GTX that Jeff B. tested in 2022. Notice how the "in focus" image looks similar to the AP130GT that I just tested.

 

https://www.cloudyni...746/?p=12301682

[Eddgie]

Good question. That's the effect of spherochromatism (changes in level of spherical aberration which vary with changes in wavelength, i.e., color). It's impossible to produce refractive optics which focus all colors at the same point and which correct for spherical aberration (SA) equally well in all colors. The optical designer will typically strive to figure the lens so that it has best correction in green (or somewhere close to that), as that's where our eyes are most sensitive. Most well-corrected apochromatic scopes that I've seen have very good correction in red as well, and blue tends to fall a little farther off.  If the lens designer tried to perfect the correction in the blue wavelengths of light, red and green would likely look much worse, and I suspect that the scope's visual and photographic images would suffer quite a bit as a result.

A very good answer, but I think the designers today lean towards a 620nm figure for faster scopes that are most likely to be used for imaging. This is because many  astro-camera sensors are regular sensors that don't have the IR cut filter on the sensor.  

 

 

 

For example, here are some Takahshi 106ED scopes. 

 

The first one is tested 532nm and the Strehl is .91. This was probably a bit unfair because while it is what it is in green light, if the system is optimized for 620nm, this test is under-rating the Strehl, though to be fair, this scope isn't the best that Tak has made. You can clearly see that there is some astigmatism and coma. Now the optical quality is still good, but not what one would expect from a brand that is renowned for its quality. I think the major issue though is they tested a scope that was likely optimized for 620nm at 532nm. 

 

http://fidgor.ru/Obs...t/test_241.html

 

This second one has been tested at 620nm and the scope does much better with a Strehl of .963

 

http://fidgor.ru/Obs...t/test_186.html

 

Another one tested at 620nm, and here we have a really excellent instrument, with a Strhel of .971.

 

This is sadly not a very good Takahashi, but it I am including it here simply because there is still relevant information.  The scope was tested at 632nm, but the amount of coma that is present gives the scope a rather low Strehl for such an expensive brand.  The top test chart shows the Strehl to be .888 with the coma, but in the second report, the tester has turned off the coma, which is by far and away the most serious error. The important thing though is that this lets the tester see how good the scope would be if the error was removed and as can be seen, if you look at the correction at 632nm with the coma removed, it is now .958, which would make it in the excellent category.

 

Coma removed, .958 Strehl

 

http://fidgor.ru/Obs...st/test_93.html

 

This last test is probably the most important:  

Here, we see that the test was run twice. The first test was done at 53nm and the second test was done at 632nm. The first test is made without deductions for things like coma and astigmatism, and the Strehl at 632nm is .963.

 

The second test is with the coma removed and the Strehl without this error would be .98. The last test show though is the comparison without deductions at 532nm and note that the Strehl is lower than it was at 632nm, coming int a .938.

 

I would pay attention to the lower order spherical aberration figure, which is .011 at 632nm and .337 at 532nm. It would appear that this scope is indeed optimized for 632, and I do believe that this has become much more common for scopes where the designer thinks the primary use will be for imaging. 

 

http://fidgor.ru/Obs...t/test_186.html

 

 

Scopes that are likely more aimed at visual observers, such as the 125EDL or the SkyWatcher 120ED, are probably more optimized for 532nm. 

 

The reason of course is that while our eye cannot see red fringing as easily as blue, the camera with no IR block filter will see the blue and red as equally bright, but the camera will also see 655nm (H-a) very bright as well. 

[bobhen]

A very good answer, but I think the designers today lean towards a 620nm figure for faster scopes that are most likely to be used for imaging. This is because many  astro-camera sensors are regular sensors that don't have the IR cut filter on the sensor.  

 

 

HERE is a link to the new Astro-Physics 110mm F6 refractor. The 110 is a fast, imaging refractor. The scope is nulled in green at 540nm with a Strehl at .99 in green.

 

Bob

[davidc135]

Speaking about zones, this interesting article "Understanding Aspheric Lenses" has some nice figures (especially Fig. 2) showing how aspherical lenses differ from spherical lenses.

 

I'm far from an expert on this, but my understanding is that figuring a lens with an aspherical correction is going to produce a wavefront that will look like a zone on DPAC testing. I guess technically that area of aspherization really is a zone as it's an area with slightly different curvature from the rest of the lens. If the lens were left completely spherical it would look much smoother without visible zones on DPAC testing. But even though the "at focus" image would look better, the intrafocal and extrafocal Ronchigrams would likely show significantly more spherical aberration.

 

https://onlinelibrar.../opph.201800033

 

Now can someone who actually knows what they're talking about please review what I just said and make any needed corrections?

I'm no expert either but my take on these faint zones is that they are simply figuring errors that are acceptably small and not worth the producer's extra time making them even smaller.

I don't think that they are necessarily related to aspherising but, where that's needed, it gives greater opportunities for their occurence.

 

The testing of refractors (in one wavelength) is comparable to the testing of a paraboloid. Any zones showing in the star test or on the bench are unwanted. Just like the well made Newt mirror in DPAC, the properly aspherised objective should show straight Ronchi bars, at a chosen wavelength.

 

David

Edited by davidc135, 25 February 2024 - 02:59 PM.

[Maciek_Cz]

Hi,
In this case, based on the shape of the ronchigram bands, I do not see that the zones shown in the at focus image are related to aspherization.
In particular, it was possible to simulate ronchigrams using a precisely defined conic constant for the entire lens surface.
Here are my results (strictly from OPD):
red ~0.97
green ~0.99
blue ~0.86

[Scott in NC]

Thanks, Maciek! Sorry that I forgot to measure the drawtube position yet again. 

[Maciek_Cz]

This telescope is simply great. The analysis is pure pleasure and the focuser position is not required because the shape of the ronchigram bands is easy to simulate.

[Spikey131]

Mine tested well last night in a very subjective assessment.  There was there were very nice views of the GRS on a very distant Jupiter, with the Galilean moons, sharp little worlds all in a row.  Very nice splits of 52 Ori, Epsilon Arietis, Sirius and  Tegmine.  The last was a neat triangle of airy discs and diffraction rings.  And of course the near full moon all blacks, greys and whites (I am one of those Lunatics who likes the full moon…).

 

Yeah, I think that I will keep it.

[scoale]

Hi,
In this case, based on the shape of the ronchigram bands, I do not see that the zones shown in the at focus image are related to aspherization.
In particular, it was possible to simulate ronchigrams using a precisely defined conic constant for the entire lens surface.
Here are my results (strictly from OPD):
red ~0.97
green ~0.99
blue ~0.86

Hi, Maciek.  Does your program account for astigmatism and/or coma?

[Maciek_Cz]

Hi,

Unfortunately, AOS does not allow for the analysis of astigmatism and coma (for now).