Doublet APO? How does that work?

[John]

"... there's not a lot of difference in visual observing between a good Achro doublet and an Apo triplet ...."

Not sure that is the conclusion but perhaps today is not a good time to prolong this thread !

Happy holidays to you too

 

[FZ1]

Oops, sorry John - just my reading of the discussion so far with no intent to cut it short. Let’s just call it a mid-discussion summary from just one point of view! 😄

Everyone Please crack-on with adding any additional value ! 👍

Cheers

Jon

Edited December 26, 2019 by FZ1

[mikeDnight]

On 25/12/2019 at 23:08, FZ1 said:

Thanks for all the info folks.  The tech info is really what I was after, and I got that in spades, but discussion about different scopes is always interesting (Like I said in the OP, I'm not looking to buy, I'm just interested in the science, terminology and theory)

In summary [1] for anyone browsing this thread in future:

In terms of use, it seems that there's not a lot of difference in visual observing between a a good Achro doublet and an Apo triplet., so the definitions of each are less relevant or meaningful.  However,  for  astro photography, long exposures with CCD or CMOS sensors are  more sensitive than our eyes, so GENERALLY a triplet Apo will be better colour-corrected and therefor help give better photos.

[1] - This is a generalisation of all input above.  Everyone knows someone who paid 30-bob for doublet that takes better photos than the Hubble or knows of a £5,000 Quad that is rubbish .... but this is a summary! 

Cheers and Happy Christmas

Jon

I suppose it boils down to how you define a "good achro". Today with a high number of Chinese chromats in circulation among amateurs, its highly likely that most believe they have a "good achro" in their possession, and if all they use them for is low power sweeping of dso's, then they probably could be classed as "good". But how well do they do when compared alongside a doublet ED? In 1999 i bought a Helios 150mm F8 achromat and it was a wonderful scope for star fields, comets and deep sky objects. The moon and planets looked amazing and the colour fringing that everyone seems to complain about, was to my mind, very well controlled. Then on one Saturday night in January 2003, someone donated a Vixen FL102 doublet apo to my local astro club. When I arrived at the club there were three refractors lined up outside the main observatory building, a 150mm Helios F8 achromat, a 100mm Tal achromat and the Vixen fluorite. It was still twilight and all three scopes were aimed at Saturn low in the east. All three showed a very similar level of detail, but I was confident that as Saturn got higher in the sky the 150mm would wipe the floor with the other two smaller scopes, after all aperture is King!? Well it isn't!

The 102mm doublet fluorite all but destroyed the 150 F8 achro and the Tal, so much so Infact that I can't remember ever wanting to look through the 150 again after that night. When a 102mm scope reveals voyager like views while a 150mm shows a much more subdued view, there's a serious rethink needed. The 150mm was a nice scope, and its optical figure was good too showing perfectly straight ronchi lines and perfect diffraction rings, but it was no match for the Vixen. It was akin to Les Kellet vs Bruce Lee.

Edited December 27, 2019 by mikeDnight