Introduction
I
have to start this report mentioning that my main interest is deep sky. I have
spent more than twenty years exploring the sky through a variety of
instruments, from humble binoculars to large dobsonians, but mainly through
reflectors. Up to recently, my interest on refractors was marginal (not
anymore!), since it is well known that aperture rules in deep sky. In spite of
this, I have always admired the purity of refractor images. It is said that a
good refractor does match the practical performance of rather larger reflectors
(i.e., less light gathering, but superior contrast and image quality). The
problem with these instruments is the relatively small lens diameter, which
constrains the applications. In addition, a large achromatic refractor must
have a high F/D ratio to reduce the chromatic aberration (CA) to low levels.
This means a too long and bulky instrument, providing dark images and narrow
fields. Thus, if a small refractor can be F/D 15 to tame CA successfully (i.e.,
1.2 m long for a 80 mm optical tube assembly -OTA-, quite reasonable), larger
achromats must hold higher F/D ratios (F/D 18, 20,...) that are translated in
unpractical OTAs, exceeding 2 meters. Achromatic refractors are, moreover, more
costly than reflectors of similar aperture, and the difference grows
exponentially with the diameter. The alternative (semi)apochromatic refractors
are better color-corrected and can be more compact and luminous, but they are
also much more expensive, especially beyond 100 mm, which was the minimal
reasonable aperture I was looking for. Apochromatic instruments were not the
answer for my second instrument, since aperture was still a main factor for me,
and sincerely, I found them overprized considering the tiny diameters. Nice
images, yes, but too aperture-challenged to enjoy deep sky.
Currently, my
main instrument is a 254 mm Meade LX200. It is very versatile and highly
recommended. Equipped with a focal reducer, an LX200 is a good deep sky
instrument that can be transported to high and dark places, take CCD images and
it is easily governed with a computer. I was looking for a complementary
telescope, smaller but still powerful, more oriented to light polluted skies. I
wanted that it had enough capability of light gathering to give access to deep
sky objects without producing that typical feeling of being quickly constrained
by the limitations of a too small instrument. A telescope that was also valid
for eclipses or casual observing sessions. I am not a purist; I can live with
less perfect images if the overall performance is good, so an achromat was an
appropriate answer for me.
Synta 120 mm F/8.3 as a general purpose
secondary telescope
I
started to look for information and reviews to make my decision. It soon became
evident that Chinese refractors, which once were synonyms of awful quality,
nowadays could be an excellent alternative in the medium F/D range. These
telescopes have been improved with the time to reach quite acceptable standards
at very affordable prices. Nowadays, a huge company (Synta corporation)
manufactures telescopes that are further marketed under different brands (e.g.,
SkyWatcher, Orion, Celestron, Konus, Pentaflex, Meade Bresser, etc). These
intermediate companies can impose their own extra controls and returning policy
to filter better instruments. This is not of minor importance, since the
in-line quality is very varying and luck can equally grant you with a good or with
a mediocre instrument.
An achromatic doublet
only allows the compensation of a narrow wavelength window, normally in the
yellow-green region. The correction level depends, furthermore, on the
telescope diameter and focal ratio. Optically speaking, residual color is
inherent in doublets, and being unavoidable, it is only negligible at long F/D
ratios. Hence, moderate F/D refractors, which were the kind of instrument I was
looking for, can be expected to present some color that can only be compensated
with special and expensive devices. In other words, one has to pay more, or
learn to live with some residual color.
Finally, two months
ago, I bought a Synta 120 mm F/D 8.3 refractor in and EQ5 mount, for which
there was a general agreement of being an outstanding performer. The telescope,
marketed under the brand "Pentaflex", was one of the new 4.7"
achromatic Syntas with a collimable objective cell as a protruding junction,
which generally has a better quality than the old Syntas.
The telescope arrived well protected and packaged, with good details that made ask myself how it could be so cheap (550 euros). Everything except the focuser wheels, slow motion handles and some covers (RA drive and others), was metal built, resistant, well painted and assembled. The mount was sturdy with steel legs and included two eyepieces (superplossl, 10 and 25 mm), a collimator eyepiece, a 2" diagonal, a small polar finder, several adapters and a nice 50 mm finder, plus all the hardware needed for the setup. The instrument was easy to assemble and no piece was lost. The focuser was very soft and accurate, apparently without the infamous glue-grease that blocks the rack in cold conditions. Since there was no symptom of blocking up to now, I have not found any objective reason to disassemble and remove it.
Once deployed the
telescope, however, I found the legs absurdly short and the tube came too close
to the ground (why?). The mount could be well pointed towards the zenith
without colliding with the legs. I also purchased the motor drives from
Celestron. However, the declination drive protrudes and prevents the OTA to
reach low western areas without making the "OTA tango" (turn and
change the OTA to the east side). So I usually do not mount the declination drive,
but carry it apart to be assembled if really needed. Another reason to not keep
it permanently mounted is that the plug is weak and it is in a risky protruding
position, so it can be damaged if hit involuntarily, or in the transport.
Vibrations are softened quickly and it is easy to operate at high
magnification. The telescope could be transported in three pieces (OTA,
counterweight axis with weights, and remaining mount). The lower triangle
holder can be skipped in some instances; its absence does not cause noticeable
tremors in the mount at the usual magnifications and this makes the
transportation easier and the setup, faster. By the way, there is a certain
risk to leave a 1.25" eyepiece in a 2" hole by accident during the
night.
Images were
exquisitely sharp, reaching a perfection level that I had largely forgotten.
But... what about the false color? Some reports were so negative that I
expected to see something terrible. Using good eyepieces (Naglers and
Panoptics), one has to look for halos deliberately to discover them. The halos
are only perceptible in very bright white-bluish objects exhibiting a very high
surface brightness, such as Jupiter or Venus, or in daytime images. During the
night, I could only find traces in very bright astronomical objects. Bright
stars did not show noticeable halos, except with deep blue filters. In my
opinion, purple residual color is not objectionable, with the exception of some
planets. Two ready solutions: we can filter them (there is plenty of light in a
120 mm objective and we need to enhance the planetary contrasts anyway!), or we
can slightly diaphragm the objective with a mask to increase the telescope
focal ratio (less desirable). Another solution: use a Baader fringe killer
filter, which I also bought with the telescope. The fringe killer filter
removed completely the Jupiter halo, introducing a non-disturbing yellowish
hue. However, for most observations, I found it completely unnecessary. Note
the insistence in good eyepieces: the eyepieces provided with the telescope (25
and 10 mm superplossl), and specially the 2" diagonal, are acceptable but
do not match the quality of the objective and contribute to deteriorate images,
so get good eyepieces and a better diagonal from the same beginning if you can.
The
Moon was absolutely awesome, full of minute craters and precious details
everywhere. The refractor supported very well high magnifications (x300,
exceeding two diameters) without problems, leaving fantastic landscapes. But
the telescope resulted to be not only very good in planetary observations: in
deep sky (DS) it was a real surprise. For double stars it was nearly ideal,
with excellent diffraction patterns. It could resolve very close double stars
at the limiting resolving power (1") and highly uneven pairs such as
Antares. Bright globular clusters were well resolved (M5, M15, M13, etc) with
pinpoint stars, whereas others showed the typical grayish granular texture
close to resolution, and the faint ones with low gradient were unexpectedly easy
(NGC6366, IC1276). Open clusters and Milky Way views were very pleasant. The
smaller available magnifications, faster thermal equilibration, less
weather-demanding quality, and intrinsic perfection of unobstructed optics
yield wide telescopic fields covered with minute stars, perfect points, which
was very aesthetic. Also, some galaxies showed hints of spiral structure (M66,
M51) or inner details (M82), and the contrast of diffuse objects with the
background was comparatively much better than with a SCT. This was quite
shocking. The images with the refractor were darker, but the contrast was much
better, so it allows exploiting better its DS capability. The idea of the
superior performance with regard to reflectors was true, indeed.
The instrument resulted
to be superb in terms of performance and price; an excellent purchase. There
were a few problems, leaving apart CA. First, the weight: with more than 20 Kg,
it can hardly be qualified as a grab−and−go telescope! And second,
it resulted to be rather cumbersome to point towards the zenith: the eyepiece
comes too low and specially the finder, in a very uncomfortable position.
Another small problem was the polar finder cap, which is too loose and can be
lost easily. Finally, the dovetail holder left the finder too misaligned, and
it was nearly impossible to center it (i.e., the centering screws had to be
pressed up to the limits).
Globally, I like this
telescope a lot and it does make the second instrument I was looking for,
wonderfully. It is undoubtedly a keeper. But soon I discovered that I
"needed" another telescope: a true grab−and−go one this
time. This eagerness for more telescopes is one the effects of being bitten by
the refractor bug.
And this leads us
to...
Synta 150 mm F/5 as a grab−and−go
deep sky instrument
The
LX200 is a transportable telescope but not a grab−and-go instrument, even
in AZ mode, and the same can be affirmed for the 120 mm Synta in the EQ5 mount.
I still missed a telescope of a reasonable aperture, good for quick deep sky
observing sessions. A telescope that could be stored in the car, set up or
disassembled in a couple of minutes (no collimation), brought to the roof with
accessories and books in a single operation (this was a key point), or used in
those uncertain weather nights that can be interrupted with clouds, or even
storms. I have some other small instruments and binoculars, but none exceeds
100 mm of aperture. But... could a bigger telescope be qualified as a
grab-and-go?
After calibrating the
real impact of color correction in the kind of observation that I like, I was
so satisfied that I began to reckon on a short tube refractor of large aperture
in an azimuth mount, or even only the OTA. This would make a terrific rich
field instrument, filling a remaining gap in my equipment. Up to now, I get the
long focus/rich field complementarity with the 254 mm Meade LX200 and 25x100
Celestron Skymaster binos. These binoculars are very pleasant instruments, but
one could expect that the refractor would allow reaching a new level in the
rich field perspective, similar to a 120-150 mm binocular ...but allowing also
a custom magnification. Imagine you have a matter of 150 mm binoculars with
Nagler eyepieces, and you close one of your eyes. Yes, the image losses, but it
would still be great, isn't it? And you can get more detail magnifying the
images, which is hardly possible in binoculars. As you see, I was focusing the
question in terms of a big binocular.
Naturally, my main
fear was the impact of color in short tube versions, but since the purpose was
rich field observations, F/5 versions could make a great deep sky choice. To be
honest, I had the secret hope of being able to enhance the images diaphragming
the objective to a reasonable value (e.g, 10 cm), when needed. The logical
alternative was a small newtonian reflector, but I must confess that I was
encouraged by the flat images and pinpoint stars I had seen with the 120 mm
Synta: I wanted another refractor. It was irrational, I know, but I was sick: I
had the "refractor fever", undocumented, but surely well-known to
most people in discussion lists.
Since Synta offered
120 and 150 mm short tube models, I went for reviews. I read some reports on
the performance of F/D 5 versions, as full reviews, reports in webpages, or
comments intermingled in discussion forums threads. There were people very
pleased whereas others were horrified. How the conclusions could be so
different? I think that the negative reports had two origins: some people made
their reviews based on an exaggeratedly purist standpoint, whereas others
concentrate their analysis on the planetary performance. In my opinion, both
standpoints are unfair: these are neither APOs nor planetary telescopes. Any
potential owner should have in mind that they are specialized deep sky
instruments, and that CA will be present at certain extent. However, think in
those memorable comments by Walter Scott Houston on his x20 Apogee rich field
refractor (127 mm). I wanted something like that: an instrument that allowed me
a close contact with the sky, offering wide field views with good contrast.
Unfortunately,
just some weeks after purchasing the 120 mm telescope, finding a large short
tube achromat in Europe (United Kingdom, Germany and Spain) became nearly
impossible. No 120 mm short tube in AZ3 mount seemed to be available, whereas
the 150 mm F/5 refractor was only sold in equatorial mounts that duplicate my
EQ5; moreover, this was against my idea of portability. I contacted with
several distributors with no luck. Perhaps this is a commercial strategy to
launch new lines of ED instruments, time will tell. Finally, after nearly
having lost all hopes, I could get a 150 mm OTA in a "minidobson"
mount, which was a hybrid mount made with the aluminum base of an AZ3 mount
finished with wooden dobson axes. After enhancing it by increasing the friction
in the height motion to decrease balance problems, the OTA could be used in
three different mounts: (1) the "minidobson" mount, (2) the EQ5 from
the 120 mm Synta, and (3) an azimuth mount with slow motions from an old Mizar
refractor (a small Japanese 68 mm F/15 telescope), which was my final election
for grab-and-go.
The
first mount was too bulky. Mechanically, the best was the 2nd mount, but in
portability terms, the most compact and lightest was the 3rd one. If the OTA is
placed slightly unbalanced towards the observer, it can never tilt by its own
weight and can be equally raised or lowered without balancing problems. I made
this mount lighter by replacing the original wooden legs with aluminum legs,
getting thus a very light telescope. I discovered with pleasure that I could
transport the OTA, mount, accessory & books aluminum case, and two aluminum
chairs to the roof, in a single operation. One of the chairs acts as support
for the case, which was thus converted in a small table. It was perfect: I had
got the grab-and-go condition successfully. However, for long observing
sessions, the EQ5 mount was the prevalent choice, owing to its higher accuracy.
The focuser was not
smooth at all. It stuck when moving inwards. Since the focus is very delicate
in short tube telescopes, this was an important issue to resolve. A careless
focus originated a matter of comma. In the first session I could learn a way to
focus with some accuracy, but without the smoothness of the 120 mm Synta.
Obviously, it was a provisional solution. I worked that way during several
nights. Finally, a close inspection revealed that it wasn't a problem coming
from the grease but related to the design. There is an internal plate below the
focuser locking knob, whose distance is regulated with two Allen screws placed
into two side holes. One of those screws was completely released, so the
focuser tube was not well fitted except when pressed with the locking knob. It
tended to tilt. That forced the inwards blocking. When we focused, the plate
made the tube to tilt and the eyepiece came outside the optical axis, producing
comma. I removed the grease and regulate the pressure carefully, and the accuracy
was restored at the same level as in the 120 mm Synta. However, the lenses were
still slightly miscollimated, and apparently there is no way to solve this.
The first light was
in a mountain place in eastern Spain called the Javalambre summit, 2000 meters
height above the sea level and far from any city. The sky was excellent, dark,
clean, and dry, with a limiting magnitude (LM) close to 7.0 at the naked eye.
The Milky Way was spectacular, with lots of low light features plainly visible.
I spent there three nights. Later, I tried my two usual observing places, the
first 1000 m height with LM=6.5, and the second 225 meters, within a village
and surrounded by parasite lights (LM=5.0-5.5). Also, since it is a grab-and-go
instrument, I used it from other good places. The comments below correspond
mainly to all these places. Naturally, an excellent observing place adds
points, but the performance from normal places was much more than rewarding. I
am discovering that most of time I do not miss any other instrument to enjoy
the night sky.
How
did the telescope perform? Surprisingly well in deep sky. In fact so well that
it is becoming my most used deep sky instrument (...and the best telescope is
the one you use more, isn't it?). With it, I have seen objects that I could
never have seen clearly before, such as the Sculptor Dwarf System (finally,
extremely faint, but it was there!!!) or the California Nebula (with Lumicon Hb
filter), and lots of dark nebulae. The best point is the appearance of stars,
as points of light, which makes observing clusters delicious: small points of
light pop out everywhere with averted vision. Not bad for an OTA costing only
590 euros. No bothersome optical imperfections were perceptible in deep sky
views, for which this telescope is designed. The view of the Double Cluster was
absolutely breathtaking, with swarms of minute stars glittering in the cores.
This telescope is great for large emission nebulae: North America, Pelican,
Omega, Crescent, Eagle, Lagoon..., all of them were fantastic. The Veil Nebula
was also breathtaking (UHC and OIII filter), not so intricate as with the 254
mm SCT, but nevertheless showing composite of filaments of light. The whole
nebula fits into the 22 mm Panoptic eyepiece. At intermediate magnifications, the
full eastern arc (NGC 6992 and 6995) fitted the eyepiece field, the same as the
Pleiades and its accompanying reflection nebulae (not only southeast Merope!).
A 31 mm Nagler, if it
could be tried, would provide 4.1º at x24. I am planning to add it to my
collection. The widest field I could get with the available eyepieces was 2.8º
with a 35 mm Plossl (7 mm exit pupil), and the magnifications were in the
x21-x310 range. For panoramic views I preferred the 22 mm Panoptic (2.3º field,
4.4 mm exit pupil), with a similar apparent field but more contrast. It was
equivalent to the view through the large Skymaster 25x100 binoculars, whose FOV
is also around 2.3º, but incomparably deeper, richer, and more perfect.
Sweeping the Milky Way was shocking. Dark nebulae in Sagittarius area showed
extraordinary structures because we can see much fainter stars than with large
binoculars of similar exit pupil, such as 25x100. I have never tried a rich
field instrument like this before and I enjoyed the experience as in my first
years. For general observing, I liked particularly the performance of the 13
and 9 mm Nagler (1.7º and 1.2º respectively; exit pupils 2.6 and 1.8 mm). The
first one is able to show 13.9 magnitude stars (naked limiting magnitude of
6.5) at a comfortable field of view of around 2º and near x60, enough to start
to resolve the brightest globular clusters, showing at the same time an
extremely wide view. That combination rapidly engaged me.
M31 was seen with two
dark lanes within a bluish outer area surrounding the golden core, an
extraordinary image to remember, as well as M33, with a faint spiral structure
and knots. Large dim objects were easier than with the LX200 owing to the
smaller magnification and good contrast. The brightest globular clusters could
be resolved, although the limited aperture made the large SCT better. The image
quality is a bit worse than that of the 120 mm Synta, but the larger aperture
compensate this (besides the portability bonus). Diffuse objects such as
galaxies were surprisingly close to SCT images, though. Because of the lower
magnifications, the seeing effects are less visible (only at low
magnifications!), and gives very enjoyable images, especially for all kind of
clusters. Some plain low contrast open clusters usually unresolved (NGC 6791)
became showpieces, even without reaching resolution. Wide and dull open
clusters often bypassed in large telescopes (e.g., Cygnus area), are beautiful
here. The low surface brightness galaxy NGC 6822 (the Barnard galaxy) showed
some irregular structures, not seen before. I also had no problem seeing NGC
6749 (the reddened globular in Aquila), and the planetary nebulae in the same
constellation (NGC 6790, 6778, 6772, etc) were very simple to find. In fact,
starhopping was easier than in any other telescope I have ever tried. A
limiting object was the planetary nebulae NGC 7094 in Pegasus (13.5 mag,
99" diam), close to M15. It could be glimpsed without nebular filters at
x60, and was was steadily seen with OIII or UHC.
Naturally, the colors
were there, as expected. In contrast with the 120 mm Synta, here bright blue
stars present faint halos. This is not a telescope for planetary observers, but
it still can be used. In this case, the most ready strategy is to diaphragm the
objective. The cover can be disassembled yielding a working aperture of around
11.5 cm, still large and allowing an F/D ratio much better for planets than the
original F/D 5 (F/D increases to 6.5 with the mask). The fringe killer filter
also contributes to decrease the halos. In spite of the residual color, the
details are there, and playing with filters is good to simultaneously decrease
the CA and increase the planetary details. Naturally, blue filters contribute
to intensify halos; these are the ones that work worse, pity. I could only try
Jupiter and it was only 25º height at the nautical twilight, at mid August
2006. The CA does not affect deep sky targets at all. Think in the enormous
difference in surface brightness for Venus, the Moon or Jupiter (0.8, 3.6 and
5.4 mag·arcsec-2, respectively), with regard to the highest surface
brightness deep sky objects such as NGC 6572 or M 57 (14.4 and 17.9 mag·arcsec-2,
respectively).
I
would recommend to any user to try a polarizer/analyzer filter pair to decrease
the weight of CA. It is particularly effective for the
Moon and planets: in a darkened image, the effect of fringes dramatically
decreases whereas the resolving power is not compromised this way. The halos
were bothersome for brighter objects, wasn't it? Hence, let's reduce the light
and the halos will diminish. This is a simple and very effective strategy
for bright objects. The image of Jupiter darkened with a polarizer pair is not
too different to the image with the fringe killer filter, but the color remains
unaltered. For visual purposes, decreasing the brightness, less than
introducing problems, is a quite convenient strategy for long observations.
For the Moon, a green
filter, a 10% transmittance grey filter or a polarizer/analyzer pair, mitigates
fairly well the color effects. The gray filter, or the polarizer pair, are
optimal (particularly the last) to reveal subtle differences in the lunar
surface. A green filter (#56 or #58A) is also very good, leaving an image that
rivals the best pictures you have ever seen, and at a more comfortable light
level. I was really surprised with the good lunar performance of this rich
field telescope at x260. The polarizer pair made CA to disappear and left an
extraordinary image, where major craters show striations and internal details
only seen before in the SCT perfectly collimated. To give an idea, the details
were more complex than those shown in the Virtual Moon Atlas at the highest
magnification. The polarizer pair also was very useful to reveal low contrast
details out of the terminator area, or in the full moon, which are often hidden
by the strong illumination. Dimming gradually the image revealed a considerable
number of low contrast structures.
Finally, for double
stars, the achromat was, in just a word, wonderful. It resolves lots of double
stars with very satisfactory images. Usually, I like to diaphragm the
telescope, so that it yields even more pleasant star images at high
magnification (specially the brightest doubles), but this is not required. I
have tried gradually closer pairs, and I was able to resolve 1" without
problems. Uneven pairs, and colored stars, are very satisfying. This kind of
observation is now becoming a favorite to me for moonlight sessions.
As a conclusion, I am
reasonably pleased. The main drawback is the impossibility of collimate the
objective. At least, I have found no way to correct the slight miscollimation
it presents. Leaving this important problem, this telescope is excellent for
deep sky observing, light and comfortable. It is a very suitable grab-and-go
telescope. Planetary observers, however, should think in longer F/D ratios. The
contrast is worse that in the 120 mm Synta but the greater aperture compensates
this fairly well, and we always have the possibility of diaphragm the telescope
to 11.5 cm. It requires time to cool down (30-45 minutes in summer). For my
purposes, it is perfectly adequate. The telescope presented a small mechanical
problem that could be solved easily, and of course the presence of residual
chromatic aberration. However, the impact of CA was much less bothersome than
expected, and it could be partially overcome. I think that our sub conscience
makes us to equal an instrument with CA with an instrument with deficiently
figured and polished glasses. It is likely the influence of junk telescopes,
where CA is one of the symptoms of a deficient quality. However, any refractive
instrument can present inherent CA by its focal ratio, and if no other serious
optical issue exists, the consequences of CA are far less negative than we
could a priori think.