Canon 500mm F4 vs AP155 F7

[Peter Ward]

Did a small review on these

the link is here (http://www.atscope.com.au/BRO/tutorials/televsphoto.html)

[leon]

Very interesting review Peter.

Leon

[dannat]

One of the members on ausphotography is looking to purchase the 500/4 now for birding, it is a big lens though. Nice comparison

[DavidTrap]

Interesting comparison - but to quote Sean Connery in The Untouchables, "You don't take a knife to a gun fight!"

DT

[bratislav]

Peter,

"This now "unobtanium glass" was used in one of the three elements of the AP155 oil spaced refractor...that even Christien himself has acknowledged has better I/R correction than their latest 160mm F 7.5 flagship refractor."

the "NASA glass" (KzFS-1) is actually inferior to both Fluorite and FPL53 as far as "abnormality" of dispersion. I'd be surprised if a modern EDT (with two best matching Crowns) couldn't surpass what the old 155 could do.
See copy of Roger Ceragioli's analysis of APO refractor lenses (now only kept at Rohr's site AFAIK)

KzFS1 150mm f/10 Oil-Spaced Triplet maximum chromatic focal shift : 794 microns
FPL53/ZKN7 150mm f/9 (!) Oil-Spaced Triplet Apochromat maximum chromatic focal shift : 59 microns

http://rohr.aiax.de/chapter%204b.htm

Yes, it looks like in far IR FPL53/ZKN7 may get worse, but where it counts (400-800nm) it is far superior.

Bratislav

[Peter Ward]

The dispersion was not what made that glass special.

Foundries like Schott have a homogeniety index, which gives a lamba error for a given thickness of glass. Typically from H1 to H3. H3 is often unavailable. (higher = less distortion)

I understand the glass Roland used in the 155's was H4. ;)

While I don't have the test data on my 155, I was lucky enough to get it for my AP 130mm. It has a strehl of 0.99.

The star-testing both scopes, they are very hard to split, hence I can only assume the 155 is a similar spec.

[Peter Ward]

I found this 2004 reference from Tom Back, who penned some lines on the nature of the AP155 glass:

"Roland was very fortunate to find a large supply of an abnormal dispersion flint, similar to Schott KzFS-1, but even better in its color correcting properties. In fact, it was ordered by NASA, but was never used. When Roland found that the supply was available for sale, he gladly bought it up. By-the-way, this "NASA" flint glass was the ultimate in short KZ flint glass. No manufacture supplies a flint glass that can match its abnormal dispersion properties today."

As to whether this is still true, I can't say, but given things are driven by cost these days, rather than performance, it probably is.

[bratislav]

It is largely semantic argument, and it is dubious to call those glasses anything in 'ordinary' terms, but FPL is in fact more 'crown' than 'flint', so in that view late Thomas was right.
But both Fluorite and FPL53 are much farther from "normal" glass line than KzFS-1 (hence better color correction/spherochromatism possible).

BTW, Fluorite has higher homogenity than ANY produced glass. If there was H10, it would be called that. The Russian OK4 would be very similar.

[Peter Ward]

And I must apologise!

I was wrong about the AP155 EDF .... your comments jogged a distant memory, and I should have dug a little deeper prior to making any comment or review. :ashamed:

I had corresponded some years ago with Roland on the EDF and frankly had forgotten the details.

His comments are now in the review, which has been revised accordingly! :)

[Moon]

Peter,
You should perform a similar comparison on a typical daytime subject somehow - perhaps a landscape or a portrait shot.
James

[Peter Ward]

It did cross my mind... but I don't currently have a portable tripod/mount that will cope with the AP155.

That said, I could do a similar write-up looking at the Canon 500mm and say FSQ106.......

[jase]

Nice comparison Peter. 'Horses for courses' comes to mind.

As you highlight image circle size is a let down for the Canon 500mm lens, especially in today's big chip world.

[Phil Hart]

thanks for the review Peter :thumbsup:

from my experience, camera lenses are a great astro option in terms of price/performance up to 300mm focal length. much above that the equation (both numerator and denominator!) turns rapidly in favour of scopes built for the purpose.

now if only i could distill the collective wisdom of both you and bratislav then i'd really know something about refractors!

phil

[Peter Ward]

A pleasure.... frankly it is a little short on detail, but I figured pictures often tell 1000's of words.

My (refractor telescope) advice?

If you ever have to opportunity to purchase an AP telescope (1993 to present).

Just buy it!


P.S.
My AP155 is not for sale :)

[Waxing_Gibbous]

I tried using my (now Leon's) 500/4 L as a telescope with mixed results.
The image quality was very, very good, but the eight-blade diaphram gave objects multiple spikes and it was incredibly difficult to balance.
As Jase said: "Horses for courses".