Dear Ladies and Gentlemen!
I was reading this thread for a long time and now I would like to do a contribution.
First, let me say that I´m not able to write in your mother language Russian, I’m apologize for this fact. I hope it´s not too discourteous for you to publish such a long article in English. I´m sorry and I hope you can understand some of my sentences.
I´m a user of a TAL-250K in Germany we here in Germany try to detect the advantages and disadvantages of the TAL-250K telescope very clearly. You are comparing a TAL-250K with a 10” Newton telescope from Skywatcher (f/4.7). So let me tell you my opinion on the basis of 20 compared aspects.
Dimensions:
1) The TAL-250K has a very short overall-length. It´s transportable and not sensitive to wind. It could be carried with a (N)EQ6 mount quite good - the BKP2501 not (picture #1). I bought the TAL-250K just because of this reason.
System components:
2) The TAL-250K is an open system, just like the Newton. With its curved spiders you will have no spikes around stars, like Newtonian telescopes do.
3) The curved spiders of the TAL-250K are rather thick (4mm). The Newtonian spiders are much thinner – better for image quality.
Thermal behavior:
4) With a long tube like the BKP2501 has, it is more difficult to reach a thermal stabile situation without thermal cords in the tube. You will need fans at the end of the tube. Modifying a Klevtsov system in such a way (with fans) is more difficult, but possible (Anton Drokin did such a modification).
Optical system and construction:
5) The central obstruction of a TAL-250K is bigger (90mm = 36% linear diameter - picture #2). The BKP2501 has an obstruction of 58mm = 24%. It is to expect that the BKP2501 will show (under the same atmospheric and thermal conditions) higher contrast images.
6) Because in a Newtonian system only mirrors are used you will not have any chromatic aberration –a TAL-250K will have this error always (in principal). Our measurements show that a diffraction limited TAL-250K has a chromatic aberration like an average apochromatic refractor (but it could be much worse)
7) You are losing less light in a Newtonian system because you have only two (reflecting) optical surfaces. Skywatcher publishing a reflectivity of 94%, NPZ is publishing a 96%-92% reflectivity of the mirrors (at visual wavelengths), but additional the light has to pass 2 lenses twice. When we compared a TAL-250K with a 10” Schmidt-Cassegrain the TAL-250K showed always the weaker image (little bit, but remarkable). Clearly seen is the difference at faint nebula structures (M42) and galaxies (M101).
A Newtonian system (like the BKP2501) will show coma at the edge of the field and you will need some kind of coma corrector and may be a flattener too. But there are many reliable correctors available. A TAL-250K is coma free, but on the other side the photographic field is not completely flat. It´s a little bit curved like you can see at the pictures. However the seeing influences are leveling this effect rather completely.
9) With a TAL-250K the biggest field of view is approximately 1.56 degrees (for example with a Vixen LVW 42mm eyepiece).
With the BKP2501 you will reach a maximum field of view of more than 2 degrees (for example with the 31mm Nagler T5 eyepiece).
10) With an aperture ratio of f/8.5 or f/6 (Reducer) you can use a big variety of eyepieces with the TAL-250K. The BKP2501 has an aperture ratio of f/4.8. At this ratio some eyepieces won’t perform well any longer.
11) On the other side – for astrophotography again the “faster” aperture ratio is better. You will need only 60% of time a TAL-250K needs to collect the same amount of light (80% if the NPZ-reducer is used).
12) The BKP2501 is much lighter than the TAL-250K (3kg – 13kg) but also much longer.
Focuser
13) The TAL-Crayford focuser is rather good, fine and smooth. But heavier equipment (more than 600 gr.) making problems (sliding). The Crayford has no reduction (1:10 or 1:
too, and no adaption for a motorized focus. Meanwhile it´s common procedure in Western Europe to exchange the TAL crayford with a Baader Steeltrack, JMI-EV1, or Lacerta Crayford which perform better (pictures #3 and #4).
14) The adapter plate of my TAL crayford is fixed to the focuser itself. Therefore it´s impossible to align a tilted focuser, but this is an important feature for astrophotography. But it looks like that the newer TAL crayford focusers are adjustable. I had to construct a new adapter plate for adapting the JMI-EV1 focuser (picture #5)
Finder
15) The 8x50 TAL finderscope has an outstanding quality. I have the newer version which has an illuminated reticle. The older version with the big eyepiece is a little bit more comfortable. But it´s unthinkable that you will find a considerable finder.
Dovetail
16) The dovetail of the TAL-250K is used to connect the tube to the MTS mounts. This dovetail doesn´t fit to other mounts. The other disadvantage is that this dovetail is rather heavy (1.2 kg). I constructed another “double faced” dovetail connective the tube to the mount. In addition this dovetail weights only 840gr and I could save some weight too (pictures #6 and #7)
Quality
17) A 10” Newtonian system needs a parabolized mirror and a flat secondary. It looks like GSO Ltd. (Skywatcher) did some fabrication improvements or increased quality checks. Really bad main mirrors we can´t find anymore, despite more difficult parabolizing. The secondary mirrors doesn´t perform as well, in some cases you have to exchange the secondary with a higher quality mirror. But summing up: Most of the Skywatcher Newtonian telescopes here in Western Europe are diffraction limited. Of course it´s mass production and neither optical peak value telescopes you won’t find.
18) The TAL-250K has only spherical surfaces that are much easier to manufacture. But the secondary seems to have narrow tolerances. Together with the very ambitioned curve radiuses (f/1.9 main mirror) and quality problems (may be with the glass) we found here in Western Europe much TAL-250K having problems. They don´t work diffraction limited. The biggest problem is a severe spherical aberration. We hold the secondary unit responsible for this. 30-50% of the TAL-250K I had interferometry data perform badly. But keep in mind that of course especially telescopes supposing problems are tested. So the absolute percentage may be lower, but low quality TAL-250K are definitely existing in a substantial amount (Picture#8)
Price
19) The TAL-250K is competing with other mass production Schmidt-Cassegrain telescopes in Western Europe. Here is a short overview of the German prices for the tubes:
TAL-250K – 2800€
10” Meade ACF - 2400€
9,25” Celestron - 1600€
11” Celestron - 2100€
(Both Celestrons are not the Edge HD versions)
10” Skywatcher BKP2501 – 500€
It´s not a surprise that the BKP2501 is the cheapest telescope in this list.
But look at the Schmidt-Cassegrain systems. Comparing the TAL-250K with other Schmidt-Cassegrain shows, that the TAL is the most expensive telescope. This is due the massive advance in price NPZ did during the last years (I bought my TAL-250K 3 years ago for 1900€). With this price the TAL-250K will have severe problems in Western Europe. Additional it´s difficult to submit a warranty claim because the shipping costs to Russia and the waiting time.
Alignment/Collimation
20) A Newtonian system has a simple procedure of aligning it´s optics. Even if everything is misaligned you´ll be able to align the optics rather quickly. You can change the backfocus by moving the position of the main mirror towards secondary. You can decide to use a smaller/bigger secondary to reduce obstruction/increase CCD size. But normally “perfect” alignment is lost after transporting the telescope. So if you are a perfectionist you have to do a short alignment of the telescope before each observation.
The TAL-250K and its optics are very rigid. Collimation may last over a years and even transporting the telescope (in a normal way) won´t change a thing. But if the secondary lost his direction (tilted) you have do a very precise re-alignment with semiprofessional equipment (Helium-Neon laser, cross table, perfect fitting bushings …)
Conclusion:
Will there be a winner in general? I think it´s the wrong question. It depends from want you want, what you need, what priorities you have. For example: If you´re looking primarily for a compact telescope the TAL-250K will have many advantages. If you´re looking for a big aperture paying less money, the BKP2501 is better. So, take a look at all the aspects and rate each one of them for you.
Regards, Andreas
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