G Scheffer
Introduction
Today the
amateur astronomer has a large choice of telescope accessories to choose from.
The most popular suppliers are the wellknown brand names such as Celestron,
Meade and Orion. The telescopes as well as their accessories are marketed and
distributed from the USA. A fact that is not that well known is that many of
these products are manufactured in Japan and more recently in Russia and China.
The
products are then exported to the USA and from there they are distributed
worldwide using a brand label.
Product Description
The ‘One
Star’ kit (distributed by Eridanus Optics CC) shown in photo 1 consists of the
following:
·
A 15mm
focal length Plössl with a 1.25” barrel
·
A 2X
Barlow lens
·
A moon
filter
·
Black
carrying case
Photo 1
The three optical components in the kit have no identifying marks to indicate their source of manufacture.
Properties
Eyepiece mechanical
The
eyepiece mechanical quality is very high, having the same quality as expected
of a Japanese optical product. The barrel of the eyepiece is chrome-plated and
has an indentation to prevent the eyepiece falling out should the focuser
thumbscrew not be fully tightened. The comparison eyepiece, a 12.5mm
orthoscopic from University Optics and of Japanese origin does not have this
indentation on it’s barrel and as a result the user must be extremely careful
to avoid damage to the eyepiece as a result of the eyepiece falling out of the
focuser. The eyepiece also has a rubber ring with a knurled surface giving the
eyepiece a nice feel when handled. The eyepiece rubber eyeguard can easily be
removed and be replaced if desired.
Eyepiece optical
The Plössl
eyepiece is the most popular eyepiece used by amateur astronomers today having
taken over from the orthoscopic eyepiece. The Plössl uses 2 doublet lenses instead of a triplet. The Plössl is easier
to manufacture and has very similar optical properties to the orthoscopic.
·
The
focal length is 15mm
·
The
eyepiece is fully multicoated with antireflection coatings on each air-glass
surface.
·
The
apparent field of view (FOV) = 52°
Barlow Mechanical
The Barlow
lens is of the same quality as the eyepiece but does not have an indentation on
the barrel, as a result extreme caution must be observed when handling the
Barlow with eyepiece in the dark. When the Barlow falls out the eyepiece comes
out with it. It would have been nice if the chromed barrel of the Barlow lens
also had an indentation.
Barlow Optical
The Barlow
lens is useful to have with the popular f4 to f5 focal ratio Newtonians
doubling the focal length of the telescope. With the Barlow lens the user
effectively has 2 different eyepieces with each eyepiece purchased. With the
15mm Plössl and Barlow lens the user also has a 15/2=7.5mm eyepiece. Future
eyepiece focal lengths must therefore be chosen not to be multiples of 2. Normally 3 to 4 eyepieces are enough for
observing and the user should initially decide on 3 eyepieces or a Barlow with
2 eyepieces.
The Moon Filter
The moon
filter has a yellow-green color when viewing a white surface. The percentage
transmission is 18%. Viewing the moon without a filter is an unpleasant
experience especially with a large aperture telescope at low magnifications and
a filter is highly recommended. Viewing the moon with the moon filter also
gives a more pleasant viewing color than is obtained when viewing with a
polarizing filter.
Results During Observation
The kit was
used on an Orion Atlas 10” equatorial telescope with a f4.8 focal ratio. The
telescope is of Chinese origin, being manufactured by Skywatcher(Synta)
and being resold by Orion in USA.
Two
eyepieces were used to compare:
A 10mm
Orion Sirius Plössl which comes standard with the telescope and a 12.5mm
orthoscopic of Japanese origin from University Optics in USA.
A
side-by-side comparison reveals no difference in the quality of the products.
The Orion
Sirius is marked fully coated implying that a single antireflection coating is
used on each air-glass surface as opposed to the multi-coatings of the Eridanus
Optics eyepiece. The orthoscopic
eyepiece is also multi-coated.
Three
objects were used for viewing:
1:Alpha
Centaurii
2:Jupiter
3:Moon
In the case
of α-Centaurii the double star was viewed in the center of the field and
then slowly moved to the edge of the field while being on the lookout for image
deterioration. In all 3 eyepieces the image deterioration was dominated by the
coma(off-axis image distortion) caused by the main mirror and not by the
eyepieces.
It was
however noticed that the eyerelief of the Eridanus Optics eyepiece was a bit
less than that of the orthoscopic and Orion eyepieces.
Looking at
Jupiter the aim was to determine if one of the eyepieces revealed more contrast
or detail on the planets surface. Again what the one eyepiece displayed all the
others displayed too indicating that the eyepieces are all similar and of high
quality.
The moon
was used to detect internal eyepiece reflections due to bright sources.
Again the
eyepieces performed as if it was the same eyepiece.
Next the
Barlow lens was used with the 15mm eyepiece from Eridanus Optics giving an
effective magnification of X163 and equivalent eyepiece fl=7.5mm.
The surface
of Jupiter was viewed for detail and image quality.
The Barlow
was then used with the 12.5mm ortho and then the 10mm Orion Sirius.
It was
found that increased magnification improved detail with the 12.5mm ortho but
not with the 10mm Orion. The conclusion reached was that 6mm eyepiece focal
length is the shortest that can effectively be used with this telescope.
Next the
moon filter was tested using another 25mm Orion Sirius eyepiece. The 10”
aperture with the X48 magnification and 18% transmission was found to be too
bright for comfort when viewing the moon after 1st quarter. Using
the higher magnifications of the 15mm, 12.5mm and 10mm without the Barlow
solved the glare problem. Viewing the moon through the filter with it’s
yellow-green color is nicer than with a variable polarizing filter.
Conclusions
My 1st
conclusion was that the quality of the optical products from China is becoming
as good as those from Japan and at a lower cost. The complete Eridanus Optics
Kit costs the same as a single Japanese eyepiece of similar quality. If you
need a 15mm eyepiece the Eridanus Optics Kit gives you a 15mm eyepiece an
equivalent of a 7.5mm and a moon filter
for the cost of a single eyepiece.