I just finished the China made 60mm refractor (C60) testing, bulk order from a small China factory. As the absolute measurement is not possible, I compared it side by side with another 60mm Takahashi FS60C. The price difference is 9 times, one would expect there would be big difference. So this report roughly tells you what it means by big difference.
At a first glance, the finish of C60 is bad, however the design is good. It has an extension tube and the eyepiece holder is even equipped with compression ring! One just needs to screw the extension tube into the end of the focuser and it works for webcam.
The handcraft is by no mean good! So what about the optics? I used Nagler 2-4mm and 3-6mm zoom in both C60 and FS60C. The big problem of the C60 is that chromatic aberration in any magnification is pronounced, especially when viewing high contrast objects. In fact I tried to use Baader Planetarium 8-24mm zoom to test the low magnification performance of it, the problem persisted. Another bad design of the C60 is that the inner tube of the focuser is very long and the inner diameter is smaller than 60mm. I guess this makes the effective aperture of C60 smaller! Look at the attached photo. On the other hand, the chromatic aberration in FS60C is very small. One can detects it when the magnification is very high, say larger then 355/2.5=142X. Of course this is a rough figure. The result may vary from different viewing conditions. My subjective opinion about the maximum acceptable magnification of both scope are:
C60: 220/2=110X
FS60C: 355/3x2.5=296X
Beyond the above magnifications, the contrast would be too low.
I used to use 5-point scale to rank telescope. If I keep on use this scale, I must give the C60 negative score! However I found that I was spoiled by those ED/APOs! I have several refractors. Some are achromatic and some are EDs or so-called APOs. After I got the ED/APOs, all my achromatic refractors are sitting in the store room and gather no more photons. I do most of the visual observations and imaging with ED/APO refractors. I never seriously look back the achromatic refractors again. Until this test, my instant feeling was that I bought garbage! Then I dug out my Meade ETX70. Although the finish is good compared with C60, the chromatic aberration is very bad. I do remember I was once satisfied with these achromatic refractors. Now I say they are garbage! I was really spoiled by those ED/APOs! In most of the time when I do optical testing of those ED/APOs, I deliberately push the magnification to limit, point to bright and high contrast objects and do rigorous star tests etc. Radically fault picking the scopes. Sometimes the differences can be only be identified by experienced telescope users. In fact if one is not doing a side by side test, I doubt the results are reliable. I asked myself is it the way to promote astronomy. I can say virtually all the telescopes we used nowadays are much better than the one used by Galileo 400 years ago. His achievement in astronomy was tremendous! I reckon that always talk about prestigious telescopes may not be the way to promote astronomy because the price of those prestigious scopes are too high in some sense. We should introduce more good value scopes, for example Sky-watcher Mak 127, Celestron Nexstar 4SE or even binoculars, to the public. These good value scopes once lead me to the world of astronomy. Anyway, in terms of optics, test result is follow (10-point scale):
C60: 4 (Moderate blue and red aberration, but more contrast than ETX70)
Mead ETX70:5 (Severe violet aberration, but sharper than C60)
Takahashi FS60C: 8 (Sharp, high contrast, by all mean winner in this test)
At a first glance, the finish of C60 is bad, however the design is good. It has an extension tube and the eyepiece holder is even equipped with compression ring! One just needs to screw the extension tube into the end of the focuser and it works for webcam.
The handcraft is by no mean good! So what about the optics? I used Nagler 2-4mm and 3-6mm zoom in both C60 and FS60C. The big problem of the C60 is that chromatic aberration in any magnification is pronounced, especially when viewing high contrast objects. In fact I tried to use Baader Planetarium 8-24mm zoom to test the low magnification performance of it, the problem persisted. Another bad design of the C60 is that the inner tube of the focuser is very long and the inner diameter is smaller than 60mm. I guess this makes the effective aperture of C60 smaller! Look at the attached photo. On the other hand, the chromatic aberration in FS60C is very small. One can detects it when the magnification is very high, say larger then 355/2.5=142X. Of course this is a rough figure. The result may vary from different viewing conditions. My subjective opinion about the maximum acceptable magnification of both scope are:
C60: 220/2=110X
FS60C: 355/3x2.5=296X
Beyond the above magnifications, the contrast would be too low.
I used to use 5-point scale to rank telescope. If I keep on use this scale, I must give the C60 negative score! However I found that I was spoiled by those ED/APOs! I have several refractors. Some are achromatic and some are EDs or so-called APOs. After I got the ED/APOs, all my achromatic refractors are sitting in the store room and gather no more photons. I do most of the visual observations and imaging with ED/APO refractors. I never seriously look back the achromatic refractors again. Until this test, my instant feeling was that I bought garbage! Then I dug out my Meade ETX70. Although the finish is good compared with C60, the chromatic aberration is very bad. I do remember I was once satisfied with these achromatic refractors. Now I say they are garbage! I was really spoiled by those ED/APOs! In most of the time when I do optical testing of those ED/APOs, I deliberately push the magnification to limit, point to bright and high contrast objects and do rigorous star tests etc. Radically fault picking the scopes. Sometimes the differences can be only be identified by experienced telescope users. In fact if one is not doing a side by side test, I doubt the results are reliable. I asked myself is it the way to promote astronomy. I can say virtually all the telescopes we used nowadays are much better than the one used by Galileo 400 years ago. His achievement in astronomy was tremendous! I reckon that always talk about prestigious telescopes may not be the way to promote astronomy because the price of those prestigious scopes are too high in some sense. We should introduce more good value scopes, for example Sky-watcher Mak 127, Celestron Nexstar 4SE or even binoculars, to the public. These good value scopes once lead me to the world of astronomy. Anyway, in terms of optics, test result is follow (10-point scale):
C60: 4 (Moderate blue and red aberration, but more contrast than ETX70)
Mead ETX70:5 (Severe violet aberration, but sharper than C60)
Takahashi FS60C: 8 (Sharp, high contrast, by all mean winner in this test)
FS60C Vs C60
Look! The large inner diameter of the focuser tube!
C60 Vs ETX70
Testing the limit of FS60C (355/3x2.5=296X)
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