Possiedo il 60 cm da circa un pio di mesi e ho avuto modo di usarlo circa una mezza dozzina di volte in alta risoluzione. In un paio di queste occasioni ho anche fatto dei report osservativi: il 3 marzo ( link ) e il giorno April 8 (link ).
I enclose a photograph of 15 February (the cloth hood was short but I stretched). The comparison with the GSO 8 "speaks for itself.
After about 6 observations in high resolution I think it's time to take stock. The nights are consistent with a typical situation of seeing and not seeing a particularly good (it would be was Jan. 23, when I looked at 480x in GSO). The best was probably seeing the April 8, but unfortunately (as explained in the report), the battery of the fan was half full. By this I mean that it is therefore performance "average" or typical, not exceptional cases, taking into account the average seeing conditions and also imperfect use.
The best night, despite the fan discharge, was perhaps the 8 April. Do not repeat the description of cpose observed. I just want to make a budget and some considerations on the conditions that when met allow us to use a large opening in the highest resolution.
Last night, April 14, I put the side back to the GSO, to evaluate a bit 'better advantage of the large opening. The GSO could be used to about 240x on Saturn (compared with 480x of 23 January to get an idea of \u200b\u200bthe quality of seeing conditions than the better). The 60 cm had no problem, now, to support the order of 400x magnification. With the passing of time the boundary layer narrowed on the mirror (the mirror as they went in temperature) and even improved vision. The most obvious difference was that the Saturn in the 8 "at 240x seemed small and dark, while in 60 cm was large and clear. The size and clarity brought with them the ability to distinguish colors and an endless variation of colors in the cups. Without colors and small-scale (240x) the planet's ice caps were slightly stretched, but the colors and the cover twice the size can be resolved in a series of colored bands that ranged from yellow to blue with an orange stripe and the other ( that the band was more evident in 8 ").
Obviously this description, made up of only adjectives can not make a quantitative sense in the reader. Some might say that he sees "very colorful" or "dry, clean and marble" and there would be no way of knowing "how" dry and marble.
To give then a more objective idea of \u200b\u200bthe quality of images obtained with a large opening and then describe the vision of Titan on April 8. The satellite, when the mirror was enough acclimatized, appeared as a disk of appreciable size (and a nice orange color). The other satellites, neighbors, were significantly more point. To assess the feelings of this observation scaled disc Ariya an opening of 15 cm and the diameter of Titan.
Titan has an apparent diameter of 0.8 ". Even a telescope of 15 cm has a" resolution "according to the Rayleigh criterion of 0.8. However it should be noted that the "resolution" in Rayleigh is equal to the radius from the center of the Airy disk to the first minimum in the ring black (between the disc and the first diffraction ring). If a point source is located at a distance of 0.8 "arc, ends up at the dark ring and then" resolved, but this obviously refers to the ability to separate two point sources and is something else than the possibility to actually see the diameter of a disk. The overall diameter of the Airy disk, in fact, is equal to 1.6 "(at least the black light). As the figure shows Titan's all well contained in the Airy disk. It follows that with an aperture of 15 cm to see Titan's diameter is necessary to enlarge the valuable point that even the stars have a diameter even larger. This means that you can not see Titan as a floppy disk and at the same time pinpoint stars. To be honest, neither is it true that you can actually see Titan as a floppy disk, because what you see is just an enlargement of the Airy disk. Instead
the Titan was a 60 cm disk several times larger than the other satellites. And we're talking about a situation of seeing typical (six nights without any particular selection choices over two months).
This observation, that anyone would try to play, obviously definitively debunks the idea that more than 15-20 cm aperture can not be exploited. No Titan 15-20 cm may show as a disk and at the same time the smallest stars.
Ma .. there is a but. What do I need in order to achieve this?
1) First thing is a matter of good workmanship. In particular it must be countered board and should not be wrinkled. Both spread the light very well reducing the contrast. These defects can be produced without working with tight deadlines. Optics very smooth, I have seen only in the best handicrafts, work very slowly and extending the polishing stage for a long time (I suggest you read the Yahoo group of Zambia). For the board replied the issue is different in the sense that some handicrafts to other smooth-edge can happen replied. Note that these are two flaws that are not easily seen with an interferometer.
2) Temperature control. The temperature control is essential. Should always check (in intrafocale blurring) the state of convective cells ( link). For the extraction of the boundary layer using a system like this (link , another link). I enclose a few pictures of the system that is currently installed at 60 cm.
3) eyepieces. This in a sense is a surprise. Looking back I had already sensed something, but only 60 cm allowed me realizing it. Last night (April 14) I used different types of eye and I noticed that the Nagler type 6 have sbordature of light and color effects of off-axis that are not visible in smaller telescopes. To put it more accurately, a 420x, 1.5 mm exit pupil of the 7 mm Nagler type 6 shows the effects of light similar to those of turbulence (sharp edges and the traced edges of light). In smaller telescopes to 1.5 mm exit pupil light of this neighborhood I had ever seen, but I always had wrongly attributed the effect al fatto che l'immagine fosse "troppo luminosa e troppo piccola". Di tutti gli oculari il Nagler è quello che mostrava questi effetti in maniera più evidente, ma anche gli altri ne erano soggetti: l'ETHOS 13 e i Pentax mostravano effetti simili ma in misura meno apprezzabile. Curiosamente il Burgess da 5 mm da questo punto di vista andava meglio (o forse era favorito dal fatto che a 5 mm la pupilla di uscita di 1 mm cominciava a nascondere i bordi di luce). Fuori asse solo l'ETHOS e i pentax non introducono coma. La cosa sorprendente, però, è stato il risultato ottenuto con un pentax 21 con barlow 2,8x (400x come il Nagler 7): nessun bordo di luce. Immagine molto "secca" a 400x. La mia interpretazione è che gli oculari siano stati disegnati per telescopi di diametro inferiore e meno luminosi, e che la maggiore raccolta di luce metta in evidenza "difetti" che non possono essere nemmeno considerati tali, visto che un oculare è un perfetto bilanciamento di diversi requisiti e che sarebbe poco sensato ottimizzare gli oculari per i telescopi meno frequenti. La barlow, ovviamente, stringe il fascio ottico e rilassa lo stress per l'oculare. Nel confrontare strumenti più o meno aperti e più o meno grandi occorre quindi tenere conto che anche ottimi oculari potrebbero essere finiti in un campo di uso per il quale non sono ottimizzati (prima di passare alle conclusioni)
