Tasco 302911


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Tasco 302911
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After a long period of inactivity (I guess I was more interested in the more mundane pleasures of life) I bought this telescope a number of years ago to revive my dwindling observing career. I had previously owned (and still do ? what do you mean sentimental?) a homebuilt 60 mm (2.4 inch) 1000 mm (F/16.7) refractor, but I had to leave it behind in the Netherlands when I moved to the UK with nothing more but a suitcase. What attracted me to this telescope were its price (? 90.00, i.e. about $ 145.00 second hand) and its compact design. At the time, my fianc饠(who has not got the slightest interest in astronomy) and I were renting a small, single bedroom apartment, and she made it quite clear that she did not want a lightbucket being stored in our bedroom. In addition, I figured that the short focal length would provide me with the large field of view I had been craving after working with a long focal length refractor without viewfinder (ever tried to find anything other than the moon or the brightest planets with a field of view of no more than half a degree?!?).

As far as the field of view was concerned, I was initially not disappointed. I actually managed to find a number of objects I have never previously been able to locate (the double cluster in Perseus for example). But a few major drawbacks immediately became apparent. First of all, there was the viewfinder: Rubbish! Its single element, plastic objective lens made it impossible to see anything but the brightest stars and even these looked rather dim and fuzzy. The inside diameter of the tube is a mere 20 mm, thus vignetting the 24 mm lens. A rolled-up piece of black paper was inserted into the tube in order to eliminate the reflections coming off its smooth plastic surface. This piece of paper was rolled-up so tightly however, that it effectively stopped down the lens to about 12 mm clear aperture. Finally, the construction of the thing made it virtually impossible to reach focus.

Luckily, I managed to find a surplus achromatic lens of the right aperture and focal length to replace the original objective lens. In addition, I made some shims to hold the eyepiece in place so that it stays where it belongs: in focus. Finally, I rearranged the piece of paper so that it no longer vignets the objective, and after some alterations to the outside of the tube (making it possible to align the viewfinder with the main scope), voila, I?ve got myself a serviceable finderscope, which will now allow me to locate stars down to approximately 7th magnitude from 4 - 5 magnitude skies.

Then for the telescope itself: one way to describe it would be to say that it is a cunning piece of design. I got the first impression that there was something amiss when I got one of my old eyepieces (a 40 mm Achromatic Huygens). This eyepiece has a clear aperture in excess of 20 mm and should thus provide a field of view of nearly 2.5 degrees. Not! Looking at the moon, I estimated that the field of view was less than 1.5 degrees. Furthermore, rather than the expected exit pupil of 9 mm, it was less than 6 mm! What is going on? The answer became apparent after I spent a Saturday afternoon investigating the telescope. First of all, I found that the focal length of the mirror was not 500 mm, but 425 mm. That is a SPHERICAL mirror of F/3.8!!! In order to satisfy Airy?s limit, the diameter of a mirror of this focal ratio should not exceed 10 mm!!! ?Luckily?, the Tasco engineers have come to our rescue? The effective minor axis of the secondary mirror measures only 23 mm. On top of that, they have inserted a diaphragm with an aperture of 15 mm inside the focuser tube. In addition to the diaphragm limiting the field of view to less than 1.5 degrees, the effect of all this is, that the primary mirror has effectively been stopped down to an aperture of just 60 mm (!!!), thus reducing the mirror?s focal ratio down to F/7. And lo and behold, this exactly satisfies Airy?s limit! Cunning, huh? You think you buy a 112 mm telescope, and you get 60 mm! The situation is made even worse (yes, that is possible) by the incomprehensibly clumsy way the secondary is supported: the central obstruction is 30 mm, that is a (linear) central obstruction of 50% (yes FIFTY). This will affect the contrast to such an extent that it is hardly any wonder that it was virtually impossible to observe any detail on Jupiter or Saturn. With my 60 mm refractor (which has the same effective aperture and hence resolving power) I regularly managed to observe 4 cloud bands on Jupiter and one on Saturn, but with the Tasco even the two largest cloud bands on Jupiter were a serious challenge.

In order to remedy this situation, I removed the diaphragm from the focuser tube, so that it can no longer restrict the field of view, and upgraded the secondary mirror to a larger diagonal with a minor axis of 1.25 inch (31.75 mm). This increases the effective aperture to about 90 mm and the focal ratio to F/4.75. While this significantly increases the telescope?s limiting magnitude, it also deteriorates the image quality, as the optical system no longer satisfies Airy?s limit. This becomes very apparent when observing at higher powers: where I could previously resolve some detail on Jupiter, now there is nothing but a blob. And as for Saturn, even its rings are blurred, thus reminding me of some of the early drawings by the likes of Galileo. I suppose I could improve things by using a mask, which stops down the aperture to 60 mm. At low powers however (where this instrument is at its best - in relative terms anyway), the loss of image quality is less noticeable and is far outweighed by the gain in limiting magnitude. The views of some of the Messier objects (M45, M35, M36, M37, M38) and the double cluster in Perseus, for example, are not too bad when using a 25 mm Celestron SMA eyepiece. And although the telescope is now equipped with a larger secondary mirror, the effective central obstruction is actually smaller, thus improving contrast. Although it was at the limit of detection, I managed to catch the Crab Nebula from about magnitude 5 skies.

After all this, it should come as no surprise that the supplied accessories (eyepieces and barlow lens) fall in the same category as the rest of the package: Rubbish. The 4 mm eyepiece is completely useless. Whoever thought it would be a good idea to supply this type of telescope with an eyepiece providing magnifications of 125 X (375 X with the barlow!) deserves to be horsewhipped. This is obviously designed to attract unsuspecting beginners, who do not (yet) realise that magnification is no measure for the quality of a telescope. For the same reason, the barlow is not much use either. In addition to providing useless magnifications, its quality is extremely poor, as it appears to be equipped with one of those plastic, single element lenses the Tasco engineers seem to be so fond of. The 20 mm eyepiece, providing a magnification of 25 X, would be a better option, but unfortunately, its quality is substandard. The field lens is rather small, thus providing only a limited field of view. Also, its image quality and contrast are poor. If anybody wants to buy this telescope (they shouldn?t - there are better options in its price range!), or is unfortunate enough to own one (like me), I suggest that they may want to invest in the aforementioned Celestron eyepiece (or better still, save the money and invest in a decent scope). At a magnification of 17 X, this eyepiece provides a massive field of view (nearly 3 degrees) while maintaining reasonable image quality and contrast.

Finally, the mount does not belie its heritage either; it looks and feels like a cheap version of Orion?s EQ-1 mount or the like. While it is sturdy enough to support the Tasco?s light optical tube at low magnifications, its build quality is abominable. Motions are stiff, screws come loose easily, etc. When mated with another optical tube (a Celestron C5 in this case), its shortcomings become obvious, even annoying.

In summary, this telescope is rather awful, and I could not recommend it to anybody. Ever! I suppose its price level and the choice of outlets (i.e. warehouses or retailers of photographic equipment) pitches the Tasco at the beginning amateur. The poor quality of this telescope and the effect this will have on the pleasure one will ultimately derive from it, may well frustrate budding observers and put them off observing for the rest of their lives. Even after I significantly upgraded this telescopes (and most beginners will not have the knowledge or the inclination to do this themselves) its usefulness is, at best, marginal. I have seen and used telescopes in a similar (or perhaps slightly higher) price bracket (6 inch Dobsonians or the SkyWatcher short tube refractors, for example) that will wipe the floor with the Tasco. The only positive point I can think of, is that purchasing this telescope rekindled my interest in observing the sky. To be frank, I must admit that I had some pleasurable nights out observing with this telescope. But this is a testimony to the beauty of the heavens, rather than the quality of the instrument.



Marc Dubbeldam

c.m.dubbeldam@durham.ac.uk

Overall Rating: 1
Weight: 1 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.106.86)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=338156

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