A simple test to see the differences of some small refractors

Date: 21/01/2012

There are 3 basic categories of refractors in the market: the achromatic, semi-apo, and apochromatic refractors. Achromats correct for 2 colours, semi-apos correct for 3 colours only for low magnifications, and full apos correct for 3 colours over most of its operating range. Well, at least that's the theory. I decided to put some refractors to the test to see if I could capture the differences photographically. The following refractors were all coupled with a Canon 550D at prime focus, and the raw results as well as a cropped and zoomed photos (to aid comparisons) were made.

Hope you enjoy seeing the results as much as I did getting them !!

Joo Beng

 

Telescope	Raw photo	Cropped	Zoomed	Notes
Galileoscope (50mm) Achro				Relatively tougher to take a photo through this setup but it's possible. Photos are soft as it's difficult to focus, and also because of the small size of the lens. Slight yellow fringing visible at the zoom level only.
Bosma Beta RE 3.5" Achro				Noticeable violet fringing on all sides and generally very soft image. Non-rotatable focuser was a nuisance.
Orion Shorttube 80 Achro				Violet fringing is clearly visible but the image is relatively crisp. Non-rotatable focuser was a nuisance.
Orion ED80 Semi-apo				No fringing observed at all. Amazing for a semi-apo! And the image is very well defined with good detail resolvable.
Orion ED102 Semi-apo				Very slight violet fringing observed. Detail resolvable is noticeably better than the 80ED due to the extra 20mm diameter.
William Optics Zenithstar 66 Apo				Incredible contrast with colours popping out. Absolutely no fringing at all. Details are good, but noticeably softer than the 80mm's as this is the smallest aperture of the lot.
Borg 77EDII Apo				Slight violet fringing observed but colours are good and detail is relatively fine.

And the winner is...

For this test, I would say the William Optics Zenithstar 66 Apo performed the best in terms of colour/contrast, lack of fringing, and level of detail given its diminutive aperture. The runner up would the the Orion ED 80mm, which performed exceptionally well given its price. At the zoom levels concerned, it exhibited unnoticeable fringing, which were readily apparent in lesser scopes.