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Synta 150mm f/5 Refractor


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Synta 150 mm F/5 OTA as a grab-and-go telescope


Introduction

I have to start this report mentioning that my main interest is deep sky. I have spent around 25 years exploring the sky through a variety of instruments, from humble binoculars to large dobsonians. Currently, my main telescope is a 254 mm Meade LX200, very versatile and highly recommended. Equipped with a focal reducer, it is a good deep sky instrument that can be transported to high and dark places, take CCD images and it can be easily governed with a computer. Recently, I had to disassemble my two home-made Newtonian reflectors. To replace them, my idea was purchasing a new secondary telescope, smaller but still powerful, good for light polluted skies but with enough capability of light gathering to give access to deep sky objects without producing the typical feeling of being quickly constrained by the limitations of a too small instrument. A telescope also valid for eclipses or casual observing sessions. Apochromatic instruments were not the answer, since aperture was still a main factor for me, and sincerely, I found them overprized considering the diameters. Nice images, yes, but too aperture-challenged. I am not a purist; I can live with less perfect images if the overall performance is good, so a big achromat was an appropriate choice for me.

Thus, a couple of months ago I purchased a 120 mm F/8.3 mm Synta in an EQ5 mount. That instrument resulted to be excellent in terms of performance and price. Images showed high contrast and were very sharp. Stars showed perfect diffraction patterns. But... what about the false color? Using good eyepieces (Naglers and Panoptics), one had to look for halos deliberately to discover them. The halos were perceptible in very bright white-bluish objects exhibiting high surface brightness, or in daytime images, but they could be easily tamed with a Baader fringe killer filter. During the night, I could only find halos in very bright astronomical objects. Bright stars did not show them, except in deep blue filters. The telescope was not only good for planets and double stars, but also striking in deep sky. Globally, I like that telescope a lot and it does make the second instrument I was looking for. But it was too heavy for casual observations. I "needed" another telescope: a grab-and-go one this time. Neither the SCT nor the 120 mm Synta are grab-and-go instruments. I wanted a telescope of a reasonable aperture, good for quick 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 the key point), or used in those uncertain weather nights that can be interrupted with clouds, or unexpected rain. None of my surviving smaller instruments and binoculars exceeded 100 mm of aperture. But... could a bigger telescope be qualified as a grab-and-go?




A large short tube achromatic refractor as a grab-and-go instrument

After calibrating by myself 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 large aperture short-tube refractor 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 got the long focus/rich field complementarity with the 254 mm Meade LX200 and 25x100 Celestron Skymaster binoculars. The latter are very pleasant instruments, but one could expect that the big 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 3D feeling is lost and the image losses, but it would still be great, isn't it? And you have the chance of getting more detail magnifying the images, which is hardly possible in binoculars. As you see, I was focusing the subject in terms of a big binocular.

Naturally, my main fear was the impact of color in short tube versions, but since the purpose was mainly 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. I was suffering the "refractor fever", an undocumented illness, but surely well-known to most people in the refractors forum.

Synta offered 120 and 150 mm short tube models, so 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. There were people very pleased whereas others were horrified. How could the conclusions be so different? Besides variations in manufacturing quality, 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 an achromat like this is a deep sky instrument, and that inherent CA will be present to a certain extent owing to the short focal ratio. However, think in those memorable comments by Walter Scott Houston on his x20 Apogee rich field refractor (127 mm). I wanted something like that: a handy 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 150 mm F/5 refractors were 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 was 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 frontward 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 OTA, mount, accessory & books 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. The EQ5 mount equipped with deployed aluminum legs was also comfortable: the eyepieces lay at accessible positions (which was not the case with the 120 mm Synta), owing to the short tube length. However, these legs made the EQ5 more prone to tremors.

The focuser was not smooth at all. It stuck when moving inwards, and worse: a careless focus originated comma. Since the focus is very delicate in short tube telescopes, this was an important issue to resolve. In the first session I could learn a way to focus with some accuracy, but without the smoothness of the 120 mm Synta. I worked that way during several nights up to get the tools and solvent I needed. A close inspection revealed that it wasn't a problem coming from the infamous "glue-grease" but related to the regulation of the focuser. There is an internal plate below the locking knob, whose distance is regulated with two Allen screws placed into two side holes. One of those screws was completely released, so the inwards movements of the rack-and-pinion system tilted the focuser tube off axis. That originated the blocking and the 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, despite the regulations, some comma remains, although fortunately not too serious. Apparently there is no way to solve this. The objective cell lacks of collimation screws, and the focuser had very limited possibilities to compensate it.

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 to the naked eye. The Milky Way was spectacular, with lots of faint structures visible in the Scorpio-Ophiuchus-Sagittarius area, etc. 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 this is a grab-and-go instrument, I had the opportunity to bring it in tourist trips, and used it from other good places. The comments below correspond 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, and more important: the portability moves me to observe more.



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 extremely faint Sculptor Dwarf System (finally, 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 especially good for large emission nebulae: North America, Pelican, Omega, Crescent, Eagle, Lagoon..., all of them looked fantastic. The most memorable was the Veil Nebula, absolutely spectacular with UHC or OIII filters. The whole nebula fitted into the 22 mm Panoptic eyepiece. At intermediate magnifications, the eastern arc (NGC 6992-95) 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). 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 2.5º, but incomparably deeper, richer, and more perfect. Sweeping the Milky Way was a pleasure. 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. M33 was seen with a faint spiral structure and, aided with nebular filters and maps, some internal nebular complexes could be glimpsed (beyond NGC 604). Owing to the smaller magnifications and acceptable contrast, large dim objects were easier to see than with the LX200. The brightest globular clusters could be seen well resolved. The image quality was a worse than that of the 120 mm Synta, but the larger aperture compensate this, besides the portability bonus. Lower magnifications (x25) made the seeing effects scarcely perceptible and gave enjoyable views in images otherwise spared by turbulence. Open clusters were always nice to see. Objects such as NGC 6791, a plain and low contrast open cluster, became showpieces (a glowing cloud partially resolved). Sparse and dull open clusters often skipped in large telescopes were 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, owing to the extremely wide FOVs. A object at the very threshold 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 assisted by a very detailed chart. The nebula was steadily seen with OIII or UHC.



Naturally, the colors were there. 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), enhancing the contrast as well. 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. 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 moderately darkened image, the effect of fringes decreases whereas the resolving power is not compromised because we are using the full aperture. 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 surprisingly good system 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, makes the observation more comfortable. Diaphragm plus polarizer gave fairly perfect images.

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 one) 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. The polarizer pair made CA to disappear and left an extraordinary image, where major craters show striations and internal details comparable to the SCT. 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 often hidden by the strong illumination. Dimming gradually the image revealed a considerable number of low contrast structures in the areas far from the terminator. Even the full Moon was surprisingly full of details, and different features make visible in the darkening process.

Finally, double stars. This telescope resolves *lots* of double stars with quite satisfactory images. Usually, I like to diaphragm the telescope so that it yields more pleasant star images at high magnification (specially the brightest doubles). I tried gradually closer pairs, and I was able to resolve 1" pairs. The 120 mm was comparatively better, but the short tube telescope was more than acceptable to see even some of the most demanding double stars. Uneven pairs, and colored stars, are very satisfying. This kind of observation is now becoming a favorite to me for moonlight sessions, otherwise passed by.

Conclusions

I am reasonably pleased. The main drawback is the impossibility of collimation. The refractor was acceptably factory-collimated, but not perfectly, and cannot be enhanced. Leaving this critical problem, this telescope is more than acceptable for casual deep sky observing sessions, light and comfortable. If you are not excessively interested in planets, this is a very suitable grab-and-go telescope, provided a light azimuth mount. 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 some time to cool down, 30-45 minutes in summer. For my purposes, it is perfectly valid and its comfort moves me to observe more, which is what really matters. The telescope presented a small mechanical problem that could be solved easily, and of course residual chromatic aberration. However, the impact of CA was much less bothersome than I 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 if its focal ratio is too short, and if no other serious optical issue exists, the consequences are far less negative than we could a priori think.



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