Телескоп исключительно планетник - по дешевле и по меньше.

[vilisvir]

Я не ставлю для себя задачи, что бы сравнить этот телескоп, который покажет столько же или больше чем 125-Аполар, для меня покупка SW15012 скорее приобщение к классике. 

Тогда уместнее длинный рефрактор с малой светосилой. Есть тут у кого-то на форуме немецкий рефрактор 102 мм F/11. Но лучше F/15. И это, кстати, по теме данного топика - исключительно планетник.

[Шмидт Кассегрен]

А зачем мне 100 мм, когда и 125 мм апохромата откровенно  не хватало, вот 150 мм рефрактор для меня представляет интерес. 

[Андрей Смирнов]

По детализации изображения планет, качественный 100мм АПО, способен превзойти только качественный Ньютон от 200мм, а может и от 250 мм  (если брать Ньютон с относительным отверстием 1:5), да и то не всегда

Эти всё сравнения с личного опыта ?

[Шмидт Кассегрен]

Есть тут у кого-то на форуме немецкий рефрактор 102 мм F/11. Но лучше F/15. И это, кстати, по теме данного топика - исключительно планетник.

По поводу длинных рефракторов:

In the late evening, I set up two nice refractors; my faithful companion Tonya; a Russian made 5” f/9 achromatic refractor and a newly acquired 3.1 inch (80mm) f/15 achromatic refractor, which I have christened, ‘Bessy’. Coincidentally, both telescopes have very comparable focal lengths (1125mm and 1200mm, respectively).

The larger Tal, as one might come to expect from its specifications, throws up modest levels of secondary spectrum round bright objects. In contrast, the 3.1 inch is, to all intents and purposes, completely colour free. The 5” Tal has 66 per cent  greater resolution and has about 260 percent more light gathering power than the smaller 3.1” instrument.

Tonya displays modest spherical overcorrection which brings it just marginally above the diffraction limit (arbitrarily set at ¼ wave PV) and, as a result displays a very shallow depth of focus even for an f/9 instrument. Bessy, on the other hand, has much better spherical correction (at least 1/6-1/8 wave PV) and at f/15 renders a much more generous depth of focus (scaling as F^2).

Beginning at about 11.20 pm local time, ambient air temperatures were at or slightly below freezing. A thin ground frost had enveloped the landscape and the grass was crisp under foot. There was little or no wind and sky conditions were clear to start with, turning hazier as the minutes passed, and by vigil’s end (about an hour later), a thin blanket of cloud had all but extinguished the light from the planet.

Both telescopes were well acclimated and I began by comparing the views in a low power (18mm) eyepiece,  delivering enlargements of  x 63 in the 5” and x 66 in the 3.1” instrument. Right off the bat, the little 3.1” was offering a razor sharp, high contrast image of the Giant Planet with one of the Galilean satellites just emerging from its western limb. The image was easy to focus and indeed, I could move the position of focus quite a bit inward and outward before noticing a perceptible drop in image sharpness. Placing the same eyepiece in the larger Tal, the image came to a sharp focus but it was definitely more difficult to find the optimal focus spot. The image of the Jovian globe was considerably brighter with a halo of unfocused blue-violet light  observed round the planet’s limb. Still, I was able to see finer details in the larger instrument even at this sub-optimal power.

Switching to a 7mm orthoscopic, yielding 163x in the 5” refractor, the image was more difficult to focus still. The planetary image had impressive levels of details but it wasn’t easy to decide where the ‘optimal focus’ might be. It was at this moment that I reached for the Baader fringe killer, which removed the majority of the secondary spectrum and most definitely helped to acquire the best focus position. Some people might baulk at using such a filter but I have changed my mind about its efficacy. It definitely makes finding best focus easier in achromatic refractors without attenuating the brightness of the image too much. Indeed, it even seems to improve the image on much more expensive ED instruments, especially in apertures of 5” and over. It’s a tool to aid observation, plain and simple.

When I placed the 7mm in the smaller 3.1” instrument, I found that the resulting enlargement (x 171) to be too much for the little refractor. The fine, high contrast details one could discern at lower powers were simply washed out at this uber high magnification (in excess of 50x per inch). The image was much better in the 3.1” when I backed down to 120x (by employing a 10mm Plossl). Returning to the images rendered by the telescopes, I came to the conclusion that both instruments had meritorious attributes, but for different reasons.

The larger refractor served up a more dynamic and information-rich image, with much finer details in the vast planetary belts being more readily discerned, so much so that sketching it would have been (for me at least) an exercise in futility. The little refractor however produced a ‘prettier’ more ‘porcelain’ image with the main details appearing more sharply defined and thus easier to sketch. Again finding best focus in the smaller telescope was much easier.

As sky transparency fell off with the onset of haze, I found myself turning more and more to the larger instrument to pick out fine detail. Haze is a wonderful natural filter for secondary spectrum, as I’d learned from many previous outings with Tonya, and, as a result, I removed the fringe killer from the 7mm ocular. Details in the Jovian atmosphere were crisp and of high contrast, very similar to my recollections of the planet with a 5” ED refractor I had previously owned. The smaller instrument, with its much lower light gathering power was, by now, producing too dim an image and I found myself backing down to ~ 100x (using a 12.5mm ortho) as the haze began to really assert itself. And as it gave way to thicker cloud, the 5-inch instrument was the only viable option.

So, which is the better telescope? I think that depends on what you want from an image. If you’re the kind of observer that wishes to study the ‘porcelain’ or ‘salient’ nature of Jupiter, than the smaller instrument is quite sufficient, but if you want to study finer details, the 5-incher is unquestionably the better choice.

What kind of image do you prefer?

 Want to explore more? Follow me on my Twittface  Space.

[vilisvir]

    Есть тут у кого-то на форуме немецкий рефрактор 102 мм F/11. Но лучше F/15. И это, кстати, по теме данного топика - исключительно планетник.

По поводу длинных рефракторов:

Если бы у автора был длинный и короткий 5", то я думаю, что перевес был бы на стороне длинного, при прочих равных. А так тут речь шла больше об апертуре, почти в 2 раза меньшей.

[Радист]

Вообще то вот мои слова:

В качественный Ньютон (1/F=1:5) апертурой от 250 мм можно, при определенных условиях, увидеть на планетах больше деталей, чем в 100 мм АПО рефрактор. Особенно если вывести оба телескопа за пределы атмосферы.

Эти всё сравнения с личного опыта

Да. В Черни в твой 250 мм Ньютон БКП было почему я не увидел. Зато его было отчетливо видно, с подробностями, в рядом стоящий 110/770мм АПО.

[Андрей Смирнов]

Эти всё сравнения с личного опыта

Да. В Черни в твой 250 мм Ньютон БКП было почему я не увидел. Зато его было отчетливо видно, с подробностями, в рядом стоящий 110/770мм АПО.

Странно, а я отлично видел, и Антон ПНО тоже вроде как, если я не ошибаюсь. Значит так хотелось увидет, но не буду спорить.

[Dornier]

  БКП в 250 не увидел?? При том, что рядом стоял рефрактор в который было видно и другие говорят, что видели в этот 250.  Может глаза не открывал?