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I'm as sure as I can be that this scope employs the same optical tube
assembly as that marketed by Celestron as the "Celestron
Tabletop 76mm"
and under the Tasco name as
the "
Tasco - 76mm 200x Catadioptric Telescope".
The Celestron Tabletop,
which is finished in black rather than blue, comes with a 4mm eyepiece in
place of the 6mm and includes a 2x Barlow, otherwise it has an identical
mount and accessories. The Tasco version also includes a 2x Barlow and
appears to be finished in black, but has a modified mount allowing
the scope to be used equatorially - personally I doubt many people will
use that feature.
It should
be noted that I have only
tested the one Danubia
Venus 30 that I personally own. There is no guarantee that all such
scopes, including those marketed under the Tasco and Celestron names that
I presume are the same OTA (optical tube assembly) will perform
identically. |
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The design - In the
much the same way as the manufacturers it seems, I have struggled to come
up with a name for this optical design. It is a Catadioptric telescope,
but that is far too general a term - covering both Schmidt and Maksutov
designs among others and this is neither.
In the page title I have
referred to it as a "Newtonian hybrid", as seen elsewhere and
that describes it well.
The
piece of glass at the front of the scope (seen in the photograph to the
right) posing as a Schmidt corrector is simply an optical window that
seals the tube and serves as support for the secondary mirror.
The basic design is that of a Newtonian.
Most budget 'short tube' scopes employ a spherical primary mirror with a
focal length of around half the final value. In ours the focal length was
found to be approximately 275mm*. An internal two element Barlow, built into the focusing tube,
increases the effective focal length to it's specified 600mm. In most
scopes of this type the internal Barlow also acts as a 'sub aperture'
corrector for spherical aberration introduced by having a
spherical primary mirror (where this is used).
I have to admit
to disliking this optical arrangement, and to be honest I was a little
disappointed that it's performance proved far harder to fault than
expected...
A brief
review -
As you can see from the pictures this small scope is, I think, rather
attractive and is well finished. There are some plastic parts used, but
these are well made and don't detract from the overall appearance . As you would expect for a budget scope there are a few
negatives - firstly the finder is rubbish, although in my opinion
still preferable to the 'gun sights' found on some budget scopes. The
eyepieces supplied aren't good either, the big minus is that they are 0.96"
format and
that the focus tube doesn't lend itself to attaching the more common 1.25" eyepieces;
however there are adaptors available to overcome this.
The main optics are surprisingly good and actually work quite well with
the eyepieces provided - mine came with a 20mm (giving 30X magnification) and a 6mm
(giving 100X) - These are the only 0.96"
eyepieces I have. However using a makeshift adaptor I managed to test the
scope with my 1.25" plóssls.
At up to 80x this scope really holds it's own, and surprisingly so,
particularly at the edge of the FOV. At 100x you start to detect the first
real signs of CA (chromatic aberration) from the internal Barlow, although
this is by no means obtrusive and is only obvious with intra and extra
focal images. CA is considerably less noticeable than in a lot of budget
refractors. In focus, which is quite sensitive, the images are still good
at this power and certainly usable but there is noticeable 'softening'. I
would place a maximum useful magnification on this scope of 120x.
 The
mount for me was the biggest surprise, because at first glance I simply
thought "rubbish", but being an alt-az it is very easy to use and it works
amazingly well. At 30x to 50x it is really quite usable. Yes it does
vibrate a bit as you focus – but the important thing is that it also
settles very fast. For casually browsing the night sky at low power I just
can't fault it. It has a huge advantage here over hand held binoculars and
of course you can easily show other people without the usual "I can't find
it".
One 'feature' that
caused some amusement is the inclusion of a 24mm 'mini aperture' in the
lens cap which is normally plugged by a small plastic cap. This 'mini
aperture' enables the scope to be used as a 24mm clear-aperture reflector
- I just think the manufacturer showed a good sense of humour!
In summary, for a beginner on a tight budget
that wants a grab and go telescope either as a first or second scope, I
think this scope would make a pretty good choice! Especially if you bear
in mind you can now pick up 0.96" eyepieces for as little as £1 second
hand. However to get the best out of this scope you really need to buy a
0.96" to 1.25" adaptor and use better quality eyepieces.
This is a 3" inch scope and a budget one at that so in my opinion
it would be nonsense to compare it to larger and/or more expensive
instruments , therefore I have avoided doing so. Among telescopes of
around the same price, especially if you are looking for a compact
instrument, this one will compare quite favourably.
*Edited
3rd April 2005
When this article was originally written it was assumed that the scope
employed a spherical primary mirror of approx 300mm focal length.
Subsequent tests suggest that it actually employs a primary
mirror with a focal length of 275mm.
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