Thursday, January 27, 2005

Home comparison: C90 vs. Ranger

Tonight, I've done a comparison for the Celestron C90 and the Tele Vue Ranger at home. As an ordinary Hong Kong Citizens, I don't have a backyard or even a roof top. I don't have a balcony either. I just open my Window and point my scopes to the night sky. The field of view is around 5 degree by 10 degree, so you know how bad it is. And of course, I'm in the "town center", and thus the light pollution is servere.

The effective aperture of the C90 is 5867 sq. mm in area while the effective aperture of the Ranger is just 3848 sq. mm. The difference is thus more than 1.5X, and according to the "aperture always win" rule, there should be a difference and the C90 should be better.

Due to the limited window size, I cannot choose or even identify the objects for the test, so I randomly search for whatever targets. Besides the light pollution, the atmosphere is not very clear and I can see only a single star within the viewing window with a 10x50 bino.

In order to have a fair comparison of the two different instruments with two different focal lengths. The C90 is a 1000mm instrument while the Ranger is a 480mm instrument, I've inserted a 2x barlow (Orion Shorty) in the Ranger for the comparison.

I setup the C90 and look into the viewing window with a 32mm Sirius Plossl, after an exhaustive search, I can only find two stars and they fit into the 31X field of view nicely. I then switch to a 20mm Tele Vue Plossl, the sky is blackened and with averted vision, I noticed that there're three stars in the view (50X).

Then, I setup the Ranger and look into the viewing window with the 32mm Sirius Plossl on top of the barlow, after an exhaustive search, I can find more than 5 stars and I try to fit the two stars, which I found with the C90, into the 30X field of view. With averted vision,

I can notice that there're three stars in the view. Then I swapped the 32mm Sirius Plossl with the 20mm Tele Vue Plossl, I can clearly see that there're three stars in the view (48X).

Obviously, the Ranger performs better than the C90. I can find more stars with the Ranger than that with the C90, even the C90 has more aperture.

I make a conclusion that it is due to the quality of the coating and the focusing mechanisms. When looking at the objective of the C90, I can see some white light is reflected and it is a sign of poor or even no coating. However for the Ranger, I can see a greenish purple coloring, and it is a sign of good coating. During the comparison, I found that it's a difficult task to make a star a pin-point in the C90 while for the Ranger, it's a joy to do the focusing and the resulted image looks like a real points of light. I can't have similar star images in the C90 at all, but it might as well, be the inherient difference of a refractor and a central-obstructed Mak.

The result is obvious, the smaller Tele Vue Ranger is the winner. But when portability is concerned, the C90 is still the (slightly) better candidate due to the compactness of the Makotov-Cassegrain design. For the weight, the two scopes are essentially the same.

Optical quality wins, even aperture wins.

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