Test of six small refractors (1 Viewer)

[John Russell]

The German magazine, "Interstellarum" recently published a test of six small astronomical refractors and I thought this might be of interest to BF readers.

Astronomical telescopes have several disadvantages for birding. None is waterproof, they are often heavier than comparable spotting scopes and mirror or prism diagonals result in a reversed left to right image. Amici prisms available as accessories give a correct image but most are allegedly not very good at higher magnifications. However, the tested scopes ranged in price from only €289 to €759 and may in some cases be optically superior to spotting scopes of similar size.

There were four contenders from China, the Omegon 66ED, the Astro-Professional 66ED Carbon (otherwise identical but 85 g lighter due to the carbon fibre tube), the IN ED70 from Telescope Service in Munich and the William Optics Megrez 72. The two from Japan were the Borg Mini ED60 and the Takahashi FS-60CB. All have focal ratios around f/6. and Crayford focussers with the exception of the Borg, which has a helical focusser.

Although some of these scopes are described as apochromatic, none of them (all doublets, the Takahshi with a calcium flouride element) met this criterion.
The arithmetic mean of the longitudinal chromatic error of red and blue was set in relation to the depth of focus of green light within the diffraction-limited Airy disc. For an apo this factor should be <1, for a semi-apo 1-2 and for an achromatic scope >2. The values measured were 2,6 for the Omegon, 2,5 for the Astro-Professional and Telescope Service, 2,9 for the William, 1,7 for the Borg and 2,4 for the Takahashi. Both the William and the Takahashi profited from an additional 50 mm glass path (erecting prism intead of mirror) and the values improved to 1,6 and 1,7 respectively.

The Strehl ratios for green light approached perfection with values between 0,94 and 0,99.

For terrestrial use, most scopes gave excellent results with magnifications of 50x and for astronomical use, above 100x. CA was apparent in the Telescope Service at 40x due to a 116 µm longitudinal error in the blue. The William with the largest aperture (and highest weight of 2350 g) was the best visually followed by the Takahashi and Borg, the latter being the lightest at 632 g.

Hope my interpretation of this test makes some sense. Being "comprehension-limited" I'm not sure that I understand it all myself!

John

 

[DRodrigues]

Thanks for the info.
I would curious to see a review of this type to the new 80 models with triplet FP53 glass.
Which image erectors were used by the WO and the Taka?

 

[John Russell]

Which image erectors were used by the WO and the Taka?

David,

No mention was made in the text, which prism diagonals were used, perhaps the Takahashi, which is an accessory for the FS-60.
The scopes were illustrated with the same 90° (mirror?) diagonal and the Astro-Professional was shown with a 60° (!?) Amici correct image prism.
I believe Henry Link mentioned that the only really good Amici prism was from Baader Planetarium. On their website it states that it is manufactured to Zeiss standards with a roof angle accuracy of 2" and can be used on astronomical telescopes up to 400x.

John

 

[dannat]

i find the 2.4 fo the little tak a little surprising, having seen many astro photos taken with it the colour is very good, i am very surprised the borg beat it -tough they are a quality scope

 

[John Russell]

i find the 2.4 fo the little tak a little surprising, having seen many astro photos taken with it the colour is very good, i am very surprised the borg beat it -tough they are a quality scope

Well it did improve to 1,7 with an erecting prism and the initial value was only "spoiled" by one measurement in the red. Perhaps field flatteners for astro photography effect a similar improvement.
Here are the values for the Tak:

Wavelength, Longitudinal error, With 50 mm glass path

486 nm, -10 µm, +48 µm
510 nm, -27 µm, +14 µm
546 nm, -, 0 µm
587 nm, +57 µm, +14 µm
656 nm, +173 µm, +84 µm

John

Sorry, my attempt to show this as a table failed. I hope everyone can make sense of the above.

 

[henry link]

Thanks for posting the test results, John.

I see the same change in color correction when my Tak SKY90 (f/5.6 Fluorite doublet) is combined with a prism diagonal. The CA fringes appear red without the prism and change to blue/purple with the prism. The prism also adds spherical correction which improves the SKY90's undercorrected spherical aberration.

As for the 45 degree "Amici" diagonals, every one I know of is actually a Schmidt roof prism like the ones used in spotting scopes. I've yet to hear of one that is phase coated and the few I've seen have been so poor that they have ruined the scope's performance. I much prefer reversed images to poor quality of these things.

Henry

 

[woodhornbirder]

William Optics 1.25" 45 degree Erecting Prism Diagonal

Have you tried this one?

 

[mooreorless]

Here is a review of the 2" William Optics erecting prism.

http://www.cloudynights.com/item.php?item_id=1428

Here is some on the Baader Amici Prism

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3747228/Main/3745969

I know the first link is for 2" but it isn't going to be any better for the 1.25"

Thanks John for posting this.

 

[woodhornbirder]

@As a daytime spotting scope, at lower magnification, the 45 degree erecting prism performs quite well. It provides a sharp and bright image when used at powers from 35x to 60x.@

SOunds ok for 30 quid then.

 

[henry link]

The Cloudy Nights review of the William Optics Schmidt diagonal is odd in that it initially describes such poor optical quality that a star can't be brought to good focus and then claims that doesn't matter for terrestrial use. In terrestrial tests, diagonals of this design I've tried compromised the image at 20X and completely ruined it at 60x. Judging from the terrible star-test image in the review I think the reviewer allowed his standards and expectations to fall too low in the terrestrial tests.

The Baader diagonal is a different animal, an Amici instead of a Schmidt. I don't think there are any 45* Amici's available for use on telescopes. My brief experience with the 90* Baader suggested that even that well made and very expensive Amici comprises the image a little compared to a good star diagonal.

 

[Kevin Purcell]

The Cloudy Nights review of the William Optics Schmidt diagonal is odd in that it initially describes such poor optical quality that a star can't be brought to good focus and then claims that doesn't matter for terrestrial use.

That would be the astro guy bias showing. Terrestrial stuff is just trivial ...

 

[rmel66]

Here is a site showing how prisms work. Pay attention to the single reflective, isosceles, roof and three reflective, isosceles, roof diagrams. These probably represent >95% of all image erectors in use today.

http://www.tecplusplus.de/ManualLu/prisms.htm