As good as expected

[Ben the Ignorant]

Teleskop Spezialisten is an outlet that tests and optimizes telescopes before shipment at no extra charge, how about that? They also publish their optical tests, compiling a very educational 220 results to examine and learn from. The one I want to show you, the very latest apparently, is about the new and coveted Sky-Watcher 72mm doublet apo.

https://www.teleskop-spezialisten.de/shop/Astro-Blog-Erfahrungsberichte-Testaufnahmen/Testaufnahmen/Ein-Testbericht-mit-Sterntest-und-Ronchi-Aunahme-fuer-einen-SkyWatcher-EVOSTAR-72-ED-DS-PRO-OTA-ED-APO::3724.html

Here is the enlarged image:

How do I diagnose that, you ask? First, the Ronchi lines are sharp, straight on both sides of focus, and show no violet and yellow, or green and magenta fringing. Also, within the black spaces are secondary (very pale, turn your screen brightness to the max) very fine lines.

Sharp lines mean smooth polishing of the lenses.

Straight lines mean no spherical aberration and no astigmatism, no tilt, no decentering. The cell and factory collimation are as good as the lenses' manufacture.

No color fringing means no chromatic aberration to speak of.

The fine secondary lines are an extra indication of very low level or residual defects. All optics have them but only the very best have a negligible amount, which is the case here.

 

On the second row of images, both large defocus disks show a continuous set of rings from center to edge, with the critical outer and brighter ring being of the same brightness and sharpness on both sides, another proof of the absence of spherical defect. A ghost of a zone creates a tiny discontinuity two thirds from center, but that is only a cosmetic defect in the hypersensitive star test, not something that degrades the image in a meaningful way. All the rings have the same luminosity and sharpness in the extrafocal and intrafocal images, in a very rare near-perfection of symmetry.

The overall color in each disk is nearly white, and uniform. The intra disk is maybe extremely pale green with a pale indigo line around the outer ring while the extrafocal figure is a very diluted blue with a discrete green line around it. Stunning and better than many large affordable triplets. The in-focus artificial stars are tiny, neatly round with no hairiness and no colored halo. Splendid and a bit hard to believe when you convert the scope's price at First Light Optics to euros, around 300! 

The third row tells the same tale except higher magnification makes the residual imperfections more visible. A second zone that was very hard to notice in the previous image now displays itself, it occupies only one third of the radius, and is quite negligible. That kind of thing is called a cosmetic defect of the star test because it has no discernible effect on the lenses' performance. It might account for a couple percent reduction in Strehl ratio at most.

The same near-whiteness (no false color spreading) and evenness of the rings is obvious in this blown-up star test.

 

To put this in perspective for those with little experience in star testing, here is a set of images which show exactly what I see when I star test my Astro-Professional 80mm f/7 doublet, an excellent wide-field and high-resolution performer with only a trace of visual color fringing at high power:

Save for the colors that are more saturated than what I see with my eyes - this depends on the camera Teleskop Spezialisten use, and they might have changed over time - this is the result for a very good semi-apo. My 9mm 100° Myriad makes what little false color there is disappear, although a hint of it can be glimpsed through other quality eyepieces. Very close to focus some haloing happens around the bright laboratory star, that almost vanishes at exact focus, precisely what to expect from a nice semi-apo of small diameter.

 

So, despite being only an f/6 doublet, this 72 is a true apo, not a semi-apo or quasi-apo. Is this sample typical? It hope it is because a large and experienced maker like Sky-Watcher has no excuse for failing to execute small lenses perfectly each time. The positive feedback from users seems to confirm this (altough I'm such a die-hard skeptic I still want to check that myself). I know lots of these 72's are used for imaging only but it would be a shame to not enjoy such image purity in high resolution. It should do 200x with no issues.

I personally don't need one because I already own three 80's but if I was looking for a 72 I wouldn't bother with a triplet. This doublet has everything.

[ollypenrice]

1 hour ago, Ben the Ignorant said:

 

I personally don't need one because I already own three 80's but if I was looking for a 72 I wouldn't bother with a triplet. This doublet has everything.

You've seen the tests, you've read the words: now take the pictures.

Sorry, I'm a pragmatist.

Olly

[johninderby]

Interesting to compare the results of the AP ED80 test you show with one I had which had been tuned up by them.

[Ben the Ignorant]

Yes, your former Astro-Pro 80  had a little bit more curvature in the Ronchi lines, but a whiter star test. Seems the colors in the other 80 sheet were too dense, those in my own Astro-Pro are in-between. The centering had been adjusted perfectly, by the way. Why didn't you keep it?

Maybe it has to do with the camera or settings, but I can't help thinking the 72's sheet is extraordinarily clean.

[johninderby]

Replaced with a Zenithstar 80FD. 

Makes me wonder just how consistent their test results are. They seem to be fairly accurate but not 100% every time. 

[smr]

So does this mean optically the 72ED is great for imaging? Unfortunately it doesn't sound as though the focuser is brilliant.

[Ben the Ignorant]

21 minutes ago, smr said:

So does this mean optically the 72ED is great for imaging? Unfortunately it doesn't sound as though the focuser is brilliant.

I don't know about the focuser in imaging duty, it seems to be the standard 1:10 two-inch gear. Two of my visual scopes are fitted with this kind of focuser and work well. Optically the doublet is excellent, really a great level of cleanliness.

[smr]

3 minutes ago, Ben the Ignorant said:

I don't know about the focuser in imaging duty, it seems to be the standard 1:10 two-inch gear. Two of my visual scopes are fitted with this kind of focuser and work well. Optically the doublet is excellent, really a great level of cleanliness.

I'd buy one then if it's that good for imaging, would save me a lot of money compared to the Z73 for instance... but if you see here: 

 

There are problems with back focusing, spacing etc. stuff I don't really understand because I've only ever used camera lenses.

 

[Ben the Ignorant]

Spacing is a recurring question in imaging but I'm only visual. You'll find your answers, plenty of imagers here.