This is a review of three products.
- The Synta made Skywatcher 120 F8.3 achromatic refractor
- The Aries Chromacor
- RR Achromat 125/F1000 lens
The Skywatcher 120 refractor needs little introduction. Made by Synta Technologies
in China, this scope is a modestly priced medium fast achromat. The Aries Chromacor
is a product that came in to being largely as a result of the mass appeal of
such inexpensive large aperture Synta refractors. The RR Achromat is a replacement
objective lens supplied by Sky Objectives
I purchased the Skywatcher from Scopes Direct in
the UK. I opted to purchase the optical tube assembly only, as I wanted to
use the scope with my existing Vixen GP DX mounting.
Included with the optical tube assembly are mounting rings and a dovetail bracket,
suitable for placing the scope onto the Chinese EQ5 or Vixen GP and GP DX mounts.
In addition the scope came complete with two eyepieces, of 20mm focal length
and 10mm focal length (I believe these eyepieces are typical Plossl designs.)
and a 6X30 finder scope, 2" focuser with 1.25" reducer. The scope
looks smart with its glossy blue paint and adjustable lens cell. In use the
Skywatcher held few surprises.
Optically it is surprisingly good, although I would never claim to be an expert
at star testing, I am experienced enough to determine when a scope star tests
poorly. The Skywatcher displayed none of the trademark optical faults I have
seen in some inexpensive telescopes. Certainly I could detect no signs of astigmatism
or zones. The star test reveals a modest amount of spherical over correction.
Under the night sky, views were pretty sharp. The scope gives the impression
of being very sharp until the view is compared with an optical system that is
very sharp. This occurred when I combined the 120 with the Aries Chromacor.
I attribute this to the moderate amount of spherical over correction.
Not surprisingly for a medium F ratio achromat of this aperture, there is considerable
false colour visible on bright stars, the moon, Jupiter and Venus. Even Saturn
with its lesser surface brightness displays a blue halo of unfocussed light
at higher powers.
Never the less, the views of the gas giants through this scope were impressive,
notwithstanding the false colour. For example the Skywatcher has no problems
resolving Saturn's Cassini division all the way round the rings, it also reveals
Jupiters GRS residing within its hollow together with other low contrast Jovian
Placed side by side with a Pentax 75 SDHF APO and Stellarvue 80mm Achromat,
the Skywatcher easily surpassed both the planetary and deep sky performance
of both the smaller but more expensive scopes . It was useable at higher powers
and revealed Jovian cloud detail that was just not visible in the smaller aperture
Wide field, deep sky views are surprisingly good, although no 5" aperture
scope is ever going to rival the views through much larger instruments, the
views through the Skywatcher are contrasty and its dark sky background works
well in the moderately light polluted conditions of my home.
All in all, the performance of this scope is good in real terms and even better
when you take into consideration the low purchase cost of £198.
There has been much talk of this device on the various news groups, there are
three existing reviews of this accessory on Cloudy Nights, but both these reviews
are in respect of the 150mm Synta/Chromacor combination.
I will restrict my comments here to my experience of using the Chromacor with
the Skywatcher 120.
There are two principal advantages to be gained by using a Chromacor with an
- Much improved colour correction
- Potentially much improved spherical correction.
In my opinion, both of the above advantages are of equal importance.
Spherical aberration is an aberration which occurs when different zones of
the objective lens or mirror have a different focus. When the objective has
an undercorrection of spherical aberration - the outer zones have a shorter
focus than the inner zones. When an objective has overcorrected spherical aberration,
the outer zones have a longer focus than the inner zones. The actual difference
of focus length is related to the amount of spherical aberration
Of course, the best performance happens when all zones of the objective and
all colours have a common focus.
In an average achromat with some amount of spherical aberration, neither the
objective's zones and different wavelenghts of light have a common enough focus
- so the image suffers from both. The Chromacor will fix both these optical
The Chromacor, by using various unspecified and presumably exotic glass elements,
brings various wavelengths of light to a much tighter focus than the standard
achromatic lens can, thereby significantly reducing the amount of false colour
seen in the image and thus improving contrast and definition as well as giving
a more aesthetically appealing image.
According to Aries, the colour correction when used with the Synta 120 F8.3
scope is 1.5 times better than when used with the 150mm F8.
It is worth taking into consideration the necessity to use a 2" Star Diagonal.
The Chromacor screws into the 48mm thread of most 2" diagonals and it is
important to ensure that the distance between the Chromacor thread/flange and
the focal plane is as near to 161mm as possible. Used with an Intes 2"
diagonal and the Synta 2" to 1.25" reducer this distance is almost
It is also worth noting that according to Aries, when used with the 120mm F8.3
Synta, the colour correction can be improved even further by extending this
length to approx 181mm. At this point the theoretical colour correction is 2X
better than achieved with the 150mm F8 Synta/Chromacor.
I decided to put this theory into practice. I had an extended diagonal barrel
machined, this increased the distance between the Chromacor and the focal plane
to approx 180mm. Indeed the colour correction of the system was improved further
still, although not by a large margin. It was also a little more sensitive to
collimation in this guise.
In addition to its colour correcting abilities, the Chromacor can be supplied
with a pre set amount of spherical under or over correction. The purpose of
this is to nullify the spherical aberration of the telescopes Objective lens,
thus providing sharper images and better performance at higher power.
That's the theory! How does it work under the night sky?
I initially used a Chromacor O-1, which in view of the slight over correction
on the Synta lens was the wrong choice as the O-1's built in over correction
will simply add to the scope's spherical aberration rather than reduce it.
The Chromacor O-1 had been purchased for use with my under corrected Celestron
CR150 refractor and I initially used it with the Skywatcher 120 in the full
knowledge that it was the wrong choice in view of the spherical aberration issues,
but I was keen to see how much improved the colour correction would be.
Dealing with the colour correcting properties first, I found that provided
I took care to ensure the 2" focuser was correctly squared on to the objective,
the colour correcting performance of the Chromacor was very effective.
It is important to ensure correct collimation of the focuser, to assist this
I used a Cheshire eyepiece.
Using my favourite BW Optik Bino viewers at magnifications between 150X and
240X I can detect no false colour halo around the limb of Jupiter or Saturn.
It is worth remembering, that when observing Jupiter in particular, without
the Chromacor the scope throws up a large halo of unfocussed light that extends
several Jovian diameters around the planet. Bright stars likewise show a large
Rigel for example normally displays a large halo of unfocused light which appears
ever more obvious the higher the power used. With the Chromacor, this halo is
totally invisible at low to medium powers and only a very faint hint of violet
unfocused light can be seen at higher powers.
Using the O-1 and notwithstanding the doubled up spherical aberration ,the
views were still an improvement over the standard scope.
I decided to obtain a Chromacor U-1 from APM Telescopes (APM-Telescopes.De),
this model has a built in under correction. In theory this should counter the
over correction and result in a virtually null corrected optic.
The difference was immediately obvious.
The star test using a green filter showed the improved spherical correction.
Before, the outer fresnel ring inside of focus was thicker and had a slightly
"woolly" appearance. With the U-1 installed I could not determine
any difference in the outer rings on either side of focus.
Although I shy away from providing actual estimates of spherical correction,
to give an illustration of the Chromacor's spherical correcting properties,
if one assumes the scope has over correction of say 1/5 wave, dividing this
by the Chromacor's 1/6 wave under correction (1/5/1/6) the net result is a scope
with 1/30 wave over correction.
This is simply outstanding and explains why this combination delivers such
sharp views. The Chromacor U-1 therefore has a two pronged approach to image
improvement. It corrects for much of the light left unfocused by the simple
achromatic object lens, thus improving image sharpness, definition and the overall
aesthetic appeal of images delivered. It also corrects for the spherical aberration
of the objective lens, delivering sharper images and allowing the effective
use of higher powers.
But the most important test for me came when I turned the scope onto Jupiter.
Again the difference was immediately obvious. The scope snapped straight into
focus in a way that demonstrated perfectly the improved optical performance.
There is a definite improvement in image sharpness when comparing the views
with the U-1 to the O-1 or the telescope without either Chromacor. Using the
slightly coarse Synta focuser, there was never any doubt when the scope was
in perfect focus.
Jupiter was beautiful because of its sharpness, the limb was hard edged against
the dark sky background, as the GRS crept into view, the darker outline around
one side of the hollow was visible with a clarity the scope had not been able
to achieve before. One night whilst I was conducting this test, the Chromacor
U-1 and Synta 120 combination delivered the best views I have had of two faint
festoons extending from Jupiters NEB into the equatorial region The scope retains
its sharpness at high powers, the seeing seems to be the limiting factor at
my observing location.
RR Achromat 125mm/F1000 Objective Lens
I obtained this lens from Hans Winkelaar of Sky Objectives in The Netherlands.
The lens was received before the U-1 Chromacor and I was interested to assess
its optical quality and to determine whether it would be a better match for
my O-1 Chromacor.
The lens was easy to assemble, it is an air spaced achromat that fits perfectly
into the Synta adjustable lens cell and gives a clear aperture of 120mm. There
are clear alignment marks on the two lens components and I experienced no collimation
Star testing the lens showed it was indeed superior to the stock Synta lens,
it took me a long time testing before I eventually concluded this optic has
a very small amount of spherical under correction. I could see no evidence of
any other optical defects.
I tested the lens without the Chromacor. It threw up an identical amount of
false colour to the Synta lens and was very sharp and operated well at higher
Hans Winkelaar was very helpful and supportive answering all my questions.