Impressions on two achromatic refractors

       Synta 120 mm F/8.3 and Synta 150 mm F/5


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José R. Torres

 

 

 

   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.