Astro Physics 130GT vs the "morris" special... (1 Viewer)

PM01

Member
Top pic is with the AP scope and field flattener. Bottom pic is with the "brand C" lens, stabilized, 800mm class used by a "morris". $14,000 msrp for the lens that took the pic on the bottom half. This was the best sample out of 5 different "brand C" lenses that I've tried.

Both critically focused manually at 10x. Image stabilizer turned off and with mirror lockup.

Always an interesting comparison.

PM01
 

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DanC.Licks

AKA Daniel Bradley
How does it compare without the field flattener? Have you tested a big white 500?
 
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PM01

Member
Without the field flattener the focal length is slightly shorter and field curvature shows on chips as small as the 1.6x 40D. I always leave the flattener on but for bird photography, depending on how much of the frame is filled, I find the flattener to be optional at times.

The 500/4 has similar results to the 800 5.6 shown. Brand C has to fight all the air to glass surfaces. Also remember that brand C is hogged out on a fast production as they aren't really meant for the same usage as the high end telescope optics. Makes quite a difference. The Astro Physics telescope, for 3 elements, 6 sides, may take up to a week of critical polishing and testing before it leaves the factory. This is for a SINGLE SET of lenses. Strehl ratio is at a minimum .98x, most of them being .99x - a strehl of 1.0 is considered perfect.

You'll often hear of another manufacturer, brand "T", claiming "diffraction limited". Diffraction limited is 0.8 strehl and it's quite noticeable comparing a 0.8 strehl vs a .99 strehl of similar focal length.

As for the commercially made camera lenses, they won't give a strehl ratio at all. When I wrote to brand C designers, they said it would take too much time. Cost for producing would jump tremendously.

About the only lens manufacturer that will tout a strehl ratio is Leica. The 280/4 and Telyt modular is edging very close to the .78 to .8 strehl mark. This is commendable for a lens / camera mfgr. But still a far way from the .98+ that AP, Takahashi and TEC routinely do.
 

DanC.Licks

AKA Daniel Bradley
.98 Strehl for a refactor is pretty damn good! My TS 90/800 turns in a solid .94, which is not bad for a "mass produced" scope. Ohara FL53 glass. I don't even own a flattener. I can't see that it would be necessary for nature stuff, and it adds glass.... the more the hairier.;)
 

cango

Well-known member
the difference in quality is quite remarkable... saw that the price was around 45000swedish crowns(6275dollars) compared to 119000 skr for a "brand C" 800mm 5.6 . however the "C" is only 4.5kg and the scope 7ish kilos
 

Paul Corfield

Well-known member
I'm lost, in the last couple of threads we've had something called a "morris" along with brand "n", brand "c", brand "z" and brand "t". 3:)

Welcome and nice to see your rigs PM01. :)

Paul.
 

kurakura

Well-known member
Was thinking the same thing Paul :D

looks quite impressive... Also the scope Collection in the other thread.
 

etudiant

Well-known member
The Astro Physics telescope, for 3 elements, 6 sides, may take up to a week of critical polishing and testing before it leaves the factory. This is for a SINGLE SET of lenses. Strehl ratio is at a minimum .98x, most of them being .99x - a strehl of 1.0 is considered perfect.

You'll often hear of another manufacturer, brand "T", claiming "diffraction limited". Diffraction limited is 0.8 strehl and it's quite noticeable comparing a 0.8 strehl vs a .99 strehl of similar focal length.

About the only lens manufacturer that will tout a strehl ratio is Leica. The 280/4 and Telyt modular is edging very close to the .78 to .8 strehl mark. This is commendable for a lens / camera mfgr. But still a far way from the .98+ that AP, Takahashi and TEC routinely do.

Very interesting perspective on the challenges of manufacturing quality optics. One might wonder whether the new Zeiss Otus and Touit are steps towards narrowing this gap, albeit only at shorter focal lengths.
A more rewarding discussion might be to identify the source of the residual errors. Even if it only narrows the spread between the best and worst acceptable product, it would be well worth doing.
 

PM01

Member
Most of the lens grinders are capable of hogging out parts in 20 minutes, high speed, several thousand rpms. Plenty of room for zonal errors and uneven figures across the lens. The errors will show up on shorter focal lengths but on longer focal lengths, this becomes critical.

The machines that TEC, AP and Takahashi are slow rpm grind and very slow pitch polish, on the order of a few hundred rpms. AP uses a Rochester machine maximizes contact of the polishing tool to the surface of the glass / ED. Makes the surface much smoother. Then they test on a double pass interferometer. To setup the testing instrumentation takes several hours as the room and glass have to stabilize.

Glass is also critical. Not every batch is the same and you can't trust the melt data that the mfgr provides, so they have to test each and every one. Some glass types have a high reject rate and I've seen fluorite blanks that Optron claims as having zero defects to have so many inclusions and fissures that it's unusable for optics.

A thing about lens design. You can either correct for the focal length or the color correction if the batch melt data changes. Can't have both. Most would opt for color correction.

Plenty that goes on into making high end telescope optics, especially refractors. Interestingly enough, the most advanced coatings that I've seen are the ones from AP and TEC. AP only needs 3 layers of broadband coating and the glass literally disappears in many angles. TEC needs 5 layers. If you have too many layers of multicoating, there is a chance that it will delaminate. The multicoatings that TEC and AP have are so advanced that not even Japan or Germany have figured out the recipe. Made in America and it will stay that way. Current military grade high end optics aren't even close to what AP / TEC have.

As for the Zeiss Otus, remember that those are small glass parts in comparison with the larger telephoto lenses. They are relying on lots of correction in the optical formula, but the strehl ratio would be on the low side as well as the overall light transmission of the entire optical system. Plus most lenses are air to glass (aka air spacing) which makes the design a bit more tolerant to changing glass melt data than the oil spaced ones which AP/TEC uses. There are pros and cons to both.

Huge factor with camera lens mfgrs is the time and labor standpoint. They do a good job for what they put into the lens but for top shelf optical quality, it's going to be the high end refractor. The camera mfgr guys won't measure strehl ratio, nor give scientific color correction charts for each lens. The numbers would be a bit on the low side and that's understandable considering how many they have to hog out for the working pro that needs autofocus and DOF control.
 

Kevin Conville

yardbirder
A modern telephoto lens like C or N also focuses much closer that a refractor telescope, and undoubtedly demands more elements.

They also are auto focus thus not allowing very close tolerances as the focus elements have to move easily.

They also have adjustable apertures demanding a more complex optical design.

Astronomical telescopes are also much heavier, are not water resistant, and have rack and pinion focus mechanisms that act as bellows drawing dust into the scope which is less than ideal for birding in the field.

As for "getting the shot" there really is no comparison. You'll be onto your next bird with the telephoto while your buddy is still focussing with the refractor, if the bird is patient enough to wait around. Good luck ever getting a warbler or a tit or a bird in flight!

For pure optical excellence, of course, a good refractor can't be bettered. But it is a bit unfair to compare the two designs for general photography.
 
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FernandoBatista

Well-known member
Sorry, but will disagree with some of what you wrote.

A modern telephoto lens like C or N also focuses much closer that a refractor telescope, and undoubtedly demands more elements.
A super telephoto lens doesn't focus much closer, a 500 F4 has a MFD of 3,7 meters, witch is roughly the same as my 480mm telescope at 3,9 meters.
But the telescope has zero breathing so it gets a higher magnification factor in the end, witch defeats the purpose of needing to get closer.

They also have adjustable apertures demanding a more complex optical design.
My scope has an adjustable aperture from F/6 to F/16 and it has the same simple optical design.

Astronomical telescopes are also much heavier, are not water resistant, and have rack and pinion focus mechanisms that act as bellows drawing dust into the scope which is less than ideal for birding in the field.
Depends on the scope and the lens, my 480mm weights roughly 2,5Kg, that's slighly more than a bigma. And a bigma isn't a lens I would compare to this telescope.
As for water resistance, true their not. But I feel much safer using my telescope under heavy rain or dust than a lens. I can easily clean my scope inside and out. And a lens, even a weather resistant one will eventually get dust in, even if it may take years, good luck cleaning that yourself.

As for "getting the shot" there really is no comparison. You'll be onto your next bird with the telephoto while your buddy is still focussing with the refractor, if the bird is patient enough to wait around. Good luck ever getting a warbler or a tit or a bird in flight!

Again, depends on the lens. I would take my 480mm over many AF lenses for action shots, obviuosly I'm not refering to Nikon/Canon supertelephotos here, I would prefer using a 500mm F/4 than a scope for the AF alone.

Still a lot of flight/action images can be done with the scope:
http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1812981667&k=xZrsg64&lb=1&s=A

http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1181707895&k=WJf9bhn&lb=1&s=A

http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1281150147&k=tGNxfBb&lb=1&s=A

http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1281150290&k=t7rfV4R&lb=1&s=A

http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1281150257&k=qtr8XgX&lb=1&s=A

http://www.smugmug.com/gallery/21552659_dMpZVq#!i=1281150065&k=Qhf95TG&lb=1&s=A
 

Paul Corfield

Well-known member
A modern telephoto lens like C or N also focuses much closer that a refractor telescope, and undoubtedly demands more elements.

As for "getting the shot" there really is no comparison. You'll be onto your next bird with the telephoto while your buddy is still focussing with the refractor, if the bird is patient enough to wait around. Good luck ever getting a warbler or a tit or a bird in flight!

I can focus down to around 2m with my 600mm. There really is no limit to how close you can go, you just add extra extension tubes between the camera and the scope.

When you have used a refractor for a while you can focus in a split second while setting shutter speed, aiming the scope etc. It just becomes second nature. If I'm carrying my scope over my shoulder on the tripod as I usually do then I can have it set on the ground and focused with correct camera settings in less than 5 seconds and that's for static subjects or birds in flight.

Scroll down this gallery for my tits in flight photos. http://www.flickr.com/photos/[email protected]/sets/72157625290080046/

Also some other birds in flight on my signature tag at the bottom of my posts.

Birds in flight are quite easy as long as you understand what is required to capture them with the equipment you have.

Paul.
 

Paul Corfield

Well-known member
n=nothing
c=crap
z=zilch
t=trash
morris.... as in mini
or do I have something wrong?

:smoke:

In the UK a morris is Cockney rhyming slang for something quite vulgar. I guess that's not the context being used here though.

I was thinking brand c was Canon and brand n was Nikon, hence the confusion. Why not just name names if a photo has been taken with a particular lens as in the first post?

Paul.
 

PM01

Member
Paul,

It's a habit of being "pc". Believe me, I wanted to put the names in the article for Nature Photographer but they wanted me to concentrate only on the telescopes and not put comparison names / pics in the article. They were afraid that it would hurt sale advertisements from the vendors.

PM01

In the UK a morris is Cockney rhyming slang for something quite vulgar. I guess that's not the context being used here though.

I was thinking brand c was Canon and brand n was Nikon, hence the confusion. Why not just name names if a photo has been taken with a particular lens as in the first post?

Paul.
 

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