The 6 inch f/8 Celestron
Guido E. Santacana
San Juan, Puerto Rico, USA
In my quest for
unobstructed optics of good but manageable size and cost I decided
upon the C6-R about a year ago. The order to Astronomics was only for
a tube assembly since I already owned an Orion Sky View Pro
equatorial mount that can handle this telescope quite well.
As many of you already
know the C6-R is manufactured in China by Synta Optical Co. The
refractor is sold under various other brand names. It is an
achromatic refractor of the Fraunhofer design with a focal length
of 1,200mm. The short focal ratio (f/8) and possibility of
significant chromatic aberration was not a concern due to my review
of the existing literature about this telescope.
The C6-R arrived in a
big long box two days after ordering. It was very well packed and
survived the long trip across the pond without a scratch. After
unpacking the double boxed tube assembly I found a good quality 20mm
Plossl eyepiece, a very good 1.25” mirror diagonal and a
Cheshire collimating eyepiece all well packed in a small box. The
tube assembly had the mounting rings and dovetail attachment for the
Sky View Pro mount. An excellent 9 X 50mm finder scope was also
included. (see figures 1 and 2).
The only assembly
required was that of the focuser and finder. The alignment of the
focuser to attach it to the optical tube is quite simple. The holes
for the screws that attach the focuser to the tube align in only one
position. The finder mount is pushed into a slot built right in the
focuser. The finder is then placed in the mount and later aligned
using only two adjustment screws. After 44 years in amateur astronomy
this is one of the best alignment mechanisms for a finder telescope
that I have used. The next step was to mount the optical tube in the
mount and I was ready to check the collimation with the Cheshire
eyepiece provided for this purpose. The Cheshire test indicated that
the telescope objective was a bit out of collimation. A simple
adjustment of the objective cell push-pull collimating screws solved
the problem and now the telescope was ready for first-light. Figure 3
shows the objective cell with one pair of the three push-pull screws.
C6-R in the Orion Skyview Pro mount.
Detail of the C6-R focuser with diagonal and Finder
Details of the C6-R objective lens cell and push-pull crews.
Before taking the
telescope out for observing I noticed that objects in the zenith
would require some contortions of my physique that included lying
very low in the ground. This was evident even with the tripod legs
fully extended. I realized then that the 16 inch mount extension sold
by Orion for the Sky View Pro mount would be obligatory but in the
meantime I had to do with what I had at hand. Another additional
accessory to get is an extra counterweight for the mount to be able
to balance the telescope in the SkyView Pro equatorial mount.
As usual the “new
telescope syndrome” attacked and I had a few days of clouds
that did not allow even a peek at the sky. Finally in the fourth day
clear skies were available and I went out for the obligatory star and
Ronchi tests. After allowing about an hour for thermal equilibrium of
the optics I chose my test star. Arcturus was was chosen for testing.
The seeing was not at its best but adequate for the test at hand. The
test star was first focused at medium power (120x). At this level of
magnification it showed very little if any false color and I could
focus it to a very tight point. I increased the power to 300x and
defocused the star 3mm on either side of focus. The results are shown
in figures 4 and 5 where you can see the extra and intrafocal images
of the star obtained as a still image with a Phillips Webcam. The
outer ring in the intrafocal image is a bit tighter and brighter than
that of the extrafocal image indicating some degree of spherical
aberration. Nonetheless the images do not show any signs of
astigmatism or zones. These images were obtained without a green
filter. Further testing with my Ronchi grating and four parallel
lines in the field also indicated well controlled spherical
aberration probably ¼ wave or less. The focused image of the
star showed a bright concentric first ring around the Airy disc and
another dimmer secondary ring with the hint of a third one. My
evaluation of the optics was that they were very good with good color
correction and acceptable spherical correction. Further observations
corroborated my early findings.
Figure 4. Extrafocal image of Arcturus with the C6-R.
Taken as a single image.
Figura 5. Intrafocal image of Arcturus. Taken as
a single image.
apparition came just in time to test the telescope. Visual
observations of the planet indicated that the C6-R performed very
well in discerning detail of the cloud belts and zonesr. Low power
images of the planet showed a diffuse purple halo around the
planetary disc that did not interfere much with the observation of
details. The use of the Orion minus violet filter improved contrast
and made a lot of the detail more visible. In fact, the visible
detail was so much that it precluded any drawing of the planet. Using
my Phillips 900 PC Webcam coupled to an infrared filter and the minus
violet filter plus a 2x Barlow I was able to obtain the image in
figure 6. Even though this image is the result of the processing of
multiple frames, you can see that the C6-R is capable of capturing a
significant amount of detail in Jupiter and the round shape of one of
it’s moons. Figure 7 shows a recent image of Saturn taken
recently under not so good seeing conditions. A recent image taken
with the Naval Observatory 8” Clark refractor and posted in the
Web doesn’t show more detail than this one. I am not, by a long
shot, trying to imply that this refractor is as good as the Clark but
the fact is that it is capable of very good performance on the
Lunar observations show
very dark crater shadows with no hint of purple haze. There is some
purple rim around the moon but no more that that which I find when I
use my 76mm f/16 Japanese refractor from the 60s. The 76mm has superb
optics. Details on the moon with the C6-R are sharp with high
Another area where I
have found the C6-R to excel is in double stars. Under good to
excellent observing conditions this telescope is capable of splitting
close doubles of equal magnitude to the limit established for its
aperture. On doubles with unequal magnitudes, where a refractor
should excel, the C6-R does an excellent job. Such was the case when
I first observed Antares. The greenish companion came out well
resolved. I have been trying to resolve Sirius companion, the Pup but
no success yet.
One item that has
produced some discussions about this telescope is the focuser and the
fact that it is usually not well aligned with the optical axis and
there’s that heavy grease in the mechanism etc. I found the
focuser quite acceptable and was able to align it using the two Allen
wrench screws that avoid slop in the focusing mechanism. The focuser
runs smoothly and in my climate this grease is not heavy at all
especially in August with temperatures peaking during the day at 95
with very high humidity. The average temperature here during the year
is 80 degrees.
More recently I ordered
the 16 inch extension for the telescope mount. This is definitely
necessary to keep the telescope at a more comfortable altitude when
with the C6-R and minus violet filter.
Notice the detail captured in
the cloud belts and the Great Red Spot (GRS).
7. Saturn imagen recently with the C6-R. The seeing was about
The greenish color is due to the minus violet filter.
The Optical evaluation
of the C6-R resulted in excellent chromatic correction for a fast
telescope. There was also some spherical aberration but in my opinion
it fell well at or under the established limits for good optics.
Observation of extended objects like the planets resulted in good
resolution of detail and good contrast for such a short focal length
refractor of this aperture. The use of a minus violet filter improves
contrast and removes any residual color except in very bright objects
In my opinion this
specimen of the C6-R passes the test as a very capable medium
aperture refractor telescope. The short focal length permits the
telescope to be utilized in the observation of deep sky without
compromising planetary and lunar observations, a field where the
refractor is still king. Mechanically the telescope is well
constructed. I found the focuser quite adequate and in my case well
collimated with the optical axis. At this level of cost this is a
well made end effective telescope for refractor lovers. I would say
it is a “keeper” but my wife will never believe that.