Comparo between APM 152 semi-apo doublet and Takahashi 150 full-apo triplet

[Ben the Ignorant]

Doesn't matter if you don't read italian, just look at the two Jupiter pics in the article:

http://www.dark-star.it/astronomia-articoli-e-test/test-strumentali/apm-152-1200-ed-apo/

The new APM 152 has been debated a lot, I understand that; I love apos and semi-apos, I own one of each kind. Everyone wonders how good a planetary scope it is while deep-sky performance is pretty much a given, even my 80mm f/7.5 achro gives stunning views of globulars from my orange-skied city. Cute stellar Airy disks.

But planetary constrast together with the required fine resolution is another question, you're not guaranteed to have them just because you're using a large refractor. So here is a comparo. The APM pic, as expected given the price difference and the extra lens in the japanese scope, has somewhat less contrast, less detail and less color fidelity, but colors are still bold, and the finer features would surely show better in great seeing. (Sorry, I don't post the pictures here, I assume they are copyrighted)

The Takahashi pic is beyond reproach, no need to comment it. There you have it, if you consider buying the APM you now know a little more to guide your choice, and if like me, you were just curious about it, you understand a little better where semi-apo technology currently is. Would be natural to add the soon-to-be-released Sky-Watcher 150 doublet to the comparo, let's wait.

[johnturley]

The colours in the photo taken with the APM are not natural, but I don't know whether thats due to the lens or the processing.

 

John

 

[John]

5 minutes ago, johnturley said:

The colours in the photo taken with the APM are not natural, but I don't know whether thats due to the lens or the processing.

 

John

 

I'm not sure what those images of Jupiter actually tell us about the scopes optics really ?

 

[Louis D]

I just wonder why, on a $3000 scope, they didn't use FPL-53 instead of FPL-51.  Were they trying to hit a particular price point?

I can see some color at 125x in my 72ED and it uses FPL-51.  The newer 72EDII uses FPL-53 instead for better color correction for less than a 20% increase in price over the older version.  I'm tempted to try one sometime.

[John]

13 minutes ago, Louis D said:

...  Were they trying to hit a particular price point?

 

Thats very likely. The latest APM 152 ED doublets state ".... equivalent to FPL-51" which leaves a couple of other options.

Damien Peach liked the scope though:

152ED_AN-2.pdf

[johnturley]

10 minutes ago, John said:

Thats very likely. The latest APM 152 ED doublets state ".... equivalent to FPL-51" which leaves a couple of other options.

Damien Peach liked the scope though:

152ED_AN-2.pdf

APM also do a 140mm doublet which uses FPL-53 for a slightly lower price (around £2,799), I wonder which would be better for viewing planets, I assume the larger 152mm would be better on DSO's.

John

[John]

The 140mm FPL-53 is an interesting scope I agree. You see plenty written about the 152 but relatively little about the 140.

[Ben the Ignorant]

I forgot to mention the APM pic was taken through a Baader SemiApo filter; without it colors would be a bit less natural, and some contrast would be lost to the blur of unfocused wavelengths. The quality of seeing was similar for both pics. Limited info, I agree, but we take what we find.

Knowing at length the improvement from my 80mm f/7.5 achro to my 80mm f/7 FPL-51 quasi-apo, I can state the intensity and width of the purple halo are reduced about four times. With that special 9mm/100° Myriad eyepiece it vanishes so much the scope becomes really visually apochromatic in focus while some color seperation is still obvious outside focus, no magic here. The lowly achro becomes almost a semi-apo thanks to that peculiar 9mm (moderate power).

This allows me to guesstimate the change from the club's Sky-Watcher six-inch f/8 achro to APM's 152 FPL-51 equivalent doublet. Chromatism in the Sky-Watcher is unacceptable to me, even at the lowest powers I find it shocking. So a four-fold improvement would not put the APM in the semi-apo class, but make it a very much improved achromat.

My 80mm FPL-51 has only a dim thin fringe, and surprisingly, in several evenings of Venus observation, chromatism was so weak it was overhelmed by the residual sunset's dark blue sky color. That's why I call it a quasi-apo.

A scope with almost twice the diameter (chromatism is exponential with diameter) but a slightly more relaxed f/ratio won't be a semi-apo in my opinion (no need to fill pages about opinions being subjective, everybody knows). So I'd say, given its narrow diffraction-limited band in the chromatism chart, the APM is an improved achromat.

I'm not going to buy any of these, but if I was I would gor for the 140 f/7 with FPL-53 glass. The diffraction-limited section of spectrum is broader, and the reduction in light grasp is only 18%, probably what has to be removed with the SemiApo filter.

To Louis D : I think the price of larger blanks of FPL-53 is much higher. Personally I would always chose the smaller but better corrected scope.

[Tim]

In my humble opinion, the comparison is interesting but essentially an unproductive exercise. It says similar seeing conditions but gives  no quantification of them. Would have to be photos taken simultaneously from the same location on the same mount, with the same camera to be useful for comparitive evaluation.

 

 

[garryblueboy]

I had one of the APM for a while nice views still preferred the view from my Old vixen 150ED which Damien happens to own now ?