by Raffaello Braga
Some time ago I’ve got an achromatic refractor of 150mm aperture at f/5, the well known Skywatcher Startravel 150 with black tube, with the intention to use it for solar imaging in H-alpha light. It is in fact a relatively inexpensive tube that is in available in the used marked for only 300-350 euro, a modest price in relation to its diameter: since the filtering parts (etalon, blocking filter, front ERF) instead are much more expensive, saving money on purchasing at least the optics would be a good starting point. Although Fraunhofer refractors are not corrected in H-alpha, the relatively large diameter of this instrument largely compensates this small handicap. My sample has very good correction in green light, delivering surprisingly regular and textbook-perfect diffraction patterns, even more than several Chinese apochromats I’ve used so far.
Before to modify the instrument for my purpose I pointed it to the Sun, Moon, planets and double stars, just curious to check its potential in high resolution observing in spite of its very unfavorable focal ratio. Here I am reporting the results of my test.
A BAD REPUTATION
If one asks around what people think of the 150 mm f/5 as a telescope for high magnification observations, the answer would invariably be that it is the worst possible instrument for this purpose due to its intolerable chromatic aberration. In fact it is usually labeled as a “brush” for Milky-way observing and therefore mainly used to observe open clusters and diffused nebulae, often with the aid of a binocular viewer, after – horribile dictu! – mutilation of the tube for reaching focus without OCS, or in pair with an identical tube to form a binoscope.
This reputation is not entirely unjustified – some old specimens of this tube I looked through were in fact less than satisfactory – but it is only part of the potentials of the 150mm: one shouldn’t forget that it’s still a 15cm refractor, an instrument that a few decades ago would have made the joy of the observers of the planets, now so squeamish about every little hint of chromatic aberration (CA, in the following). About the latter I have already had occasion to write that its effects are not so harmful as it is generally believed (the MTF graphs that are used to show the effects of the CA on the contrast are in fact pessimistic because they do not take into account the actual eye sensitivity to a restricted ranges of colors). As I am showing herebelow, with the aid of some filtering also a refractor with a chromatic aberration index so low (0.85 when the recommended minimum would be 3) can be used profitably in high resolution within the limits of its optics and without asking it for apo-like performances.
It should also be considered that professional refractors, those that were built until the first half of the XX century, all have very low CA index and very obvious CA, but this did not prevent them to be used to get remarkable results in observational astronomy. Putting side by side, for example, the 150mm achromat and my Takahashi Sky-90 – a true fluorite apochromat with its focal extender that almost eliminates any residual color and spherochromatism – there’s no history: the achromat shows more details on both the Sun and the Moon and on Mars and Saturn also, not counting the fact that with the achromat I can easily push magnification up to 300x on certain subjects as Moon, Saturn and double stars, while this magnification is decidedly excessive for the small albeit perfect apo. Only in observing Jupiter I found most pleasing the image provided by the apochromat although the 150 performed well considering the forced aperture ratio.
But let’s see in more detail some of the observations I did.
Inserting into the eyepiece a Solar Continuum filter, which has a very narrow bandwidth around 540 nm, the secondary spectrum becomes entirely suppressed and the images of the solar photosphere returned by 150 become indistinguishable from those of a true apochromatic or of a reflector, at any magnification. A full aperture filter in front of the objective or a Herschel wedge are of course necessary to reduce sunlight to a tolerable and safe level. Personally I prefer the Sun in its natural color, that is white (because the Sun is white, not yellow or orange as someone believes) and for this purpose I use a 130 mm modified Newtonian with a non-aluminized main mirror. However once provided with the above filters the achromat shows exactly the same details one can see in the newtonian, indeed something more having a diameter slightly larger. I must add that using lighter filters such as the excellent Lumicon W11 or the Baader green 500 nm, the secondary spectrum peers at the edge of the solar disk but the details on the surface remains the same as with the Continuum filter. Without filters, however, spots appears surrounded by a slight blue halo although no details are lost. For imaging in the Calcium-H line I have to stop down the aperture to 75 mm as at these wavelength the objective at full aperture doesn’t form a sharp image, while in H-alpha line the aperture is only slightly reduced in order to reach a focal ratio of f/25 – 30 by means of a focal extender.
Being practically monochromatic (yes, I know, this is not entirely true but visually Moon colors are very soft and can be better seen in photographs) the lunar landscape can be filtered without losing any of the details, even increasing the contrast between the zones of light and shadow, and this is true even using any telescope completely free from CA. A yellow filter works fine as well as the above mentioned W11 or the Baader Contrast Booster, which, however, with the 150 f/5 is not really effective being the objective very fast for this filter. However I have been able to observe along the terminator without filters up to 300x with a still pleasant image with only a bit of spurious color at the boundaries between light and dark zones.
Below I have collected some lunar images taken with the instrument:
Without filters, in full daylight, the planet looks vaguely yellowish instead of white, because that’s the color for which the objective is most corrected and focalizes: it is a characteristic of all achromatic refractors, fast or slow. However Venus is never observed without filters otherwise nothing would be seen, and therefore by applying from time to time yellow, red, blue and geen filters, the 150mm is capable of delivering sharp and well defined views of Venus up to at least 250x, showing cusp caps, limb brightening, terminator shading and some vague and indistinct markings. The objective is not corrected for all these colors, as it has been said, but Venus details are large e diffused and resolving power is not a critical factor. Even in integral light the planet remains well defined and only in twilight, when the sky starts to darken, a colored halo appear. Instead, the violet filter, the most used to highlight the so-called “UV markings” of Venusian atmosphere, can’t be used with such a short achromat because in this color the glass is really too much aberrated.
Notwithstanding the low altitude for northern observers this year, my 150 showed a beautiful and detailed Mars at powers as high as 270 – 300x, a limit determined more by the seeing than by the instrument. Dark regions, the polar cap, clouds and white-bluish terminator mists were all clearly visible without any difficulties. Looking with attention at the planet’s limb I could notice a faint red halo due to the light of this color that the objective doesn’t precisely focalize (the same I have seen around Mars looking into the Skywatcher 120mm f/8.3 refractor) however it did not affect image sharpness.
As I said before this planet is disappointing when observed through fast achromats due to both its brightness and its whitish color that seem to have exactly the purpose of highlight the secondary spectrum. My contrast filters – Baader Semiapo and Contrast Booster – could do nothing to improve the view. The image can be filtered more heavily, of course, by means of a yellow or green filter but this sacrifices the variety of colors and tints that makes Jupiter so attractive. The planet appears still detailed, however, the equatorial and temperate bands are clearly visible as well several festoons in EZ, the polar regions and the GRS, but magnification has to be kept relatively low in order to preserve contrast, and there is no comparison with the images that a good reflector or apochromatic refractor can deliver of this planet.
Here we have a completely different music. On the whole Saturn has a yellowish-greenish tinge that closely match the color for which Fraunhofer achromats are best corrected: at 300x and without any filter the planet appeared magnificent, sharp and contrasty. The globe showed several belts and zones as well as the north polar region, the Cassini division was visible all around the ring and the Encke minimum (not to be confused with Encke division) was clearly seen at both ansae. A slight yellow or yellow-green filter further improved the contrast but I found it to be unnecessary.
Having very good spherical correction in yellow-green, my 150 reach its Dawes limit (0.8”) without any difficulties, showing textbook perfect Airy discs surrounded by a faint diffraction ring. Doubles with large magnitude difference between components are slighlty more difficult to split in comparison to long focus achromats or apochromats due to the light diffused by the secondary spectrum. Star colors, moreover, are less pure compared to what can be seen in an instrument free of CA: white stars appear white-yellowish, blue stars are less “blue” and orange stars as Arcturus, Izar or Antares show a reddish “border”. It is a minor handicap, however, probably most observers would not even notice it, and I spent many funny hours splitting close doubles in some autumn constellations.
DON’T STOP ME DOWN, PLEASE!
I know that many owners of this telescope use to stop down the objective for high magnification observing with the purpose to make the CA less annoying, but my experience with both the f/5 and f/8 version of the 150mm achromat is that this practice is useless, it results in wasting brightness and details and shall therefore be avoided. Much better is to filter the telescope at the eyepiece, an old practice followed by most observers of the past. Giovanni Schiaparelli, for example, once pointed out that with his Merz-Repsold equatorial of 49 cm diameter (CA index of 0.74) he did not found any advantage in stopping down the aperture, preferring instead to make use of yellow and orange filters without losing in resolution (the type of color correction of that refractor made that filters particularly effective). Eugene Antoniadi, who was active at Paris-Meudon observatory, always made use of the 83 cm refractor (CA index of 0.6) at full aperture, criticizing Percival Lowell who had the habit to stop down the lens of his instrument at Flagstaff.
Personally I have found useful to stop down only when the objective has a defective edge that it is necessary to mask, or for observing the Sun with monochromatic filters that require a specific focal ratio.
RECOMMENDATIONS FOR USE
As known to its users, the 150 f/5 is not very heavy and can be safely used on a GP-like mount, but the tube is unbalanced toward the objective where lies most of the weight due to the glass and the dew cap. However putting some weight at the rear end – for example an extension tube, diagonal, 50 mm finderscope – and removing the dew cap allows a better balance.
The glass is held in a non-adjustable cell, therefore little can be done if the instrument is miscollimated. If one wishes to use it for high resolution observing a perfect collimation is absolutely required because a f/5 objective has no tolerance to misalignments. Moreover the focuser shall also be mounted on-axis and the mobile tube shall not tilt when moved in and out even at the maximum extraction. This is the reason why the original focuser should be replaced, and another reason is that at f/5 it is hard to focus precisely at high magnifications, particularly in presence of CA which makes more difficult to find the point of best definition: many unfavorable reports I have read of this instrument are probably linked to collimation and focusing problems.
The original black tube warms up under the Sun, therefore it would be better – unless the telescope is used only during the night, to re-paint it in white or to cover it with a white film.
I used the telescope with several diagonals. Mirror diagonals, irrespective of their quality, don’t work well because of the light scattering that makes CA even more evident. Prism diagonals are far, far better and this is generally true, in my opinion, for all refractors (glass likes glass…). My preferred one is the Baader-Zeiss but other prisms as Baader T2, Takahashi or Vixen work fine.
As for Barlows, only the best shall be used, cheap units shall be avoided even if labeled as “apochromatic” or so because only few of them has been designed to work well with a f/5 objective without introducing further aberrations. Tele Vue Barlows and Powermates, Baader VIP or Baader Zeiss, long Barlows as Parks, Orion, Vixen, Clavé all are fine.
It should be clear from the above that this Skywatcher 150 f5 refractor is not the instrument of choice for the planetary observer, yet this doesn’t mean it can’t be used for high resolution work. Of course if CA is the main problem in such an instrument, it shall also be the ONLY problem with which to deal, this is a very important point: it means that the objective shall be collimated, free of spherical aberration in the yellow-green, free of astigmatism, tensions and all other aberrations that if added to the CA that is inherent in the design would make the telescope really useless except for low power views of the Milky way It is often be read that Chinese optics of low focal ratio are of mediocre quality, but my experience indicates that it is indeed possibile to find a good sample capable of working at high powers.
ADDENDUM – After the article was published I have been asked about what would be, in my opinion, the filters to be considered for using the telescope for high resolution. Jupiter and the Moon are the subjects that could benefit more from filtering CA while in all other cases it could not be necessary, except for Venus that requires filters anyway. The less invasive one is the Baader Semiapo, it does not suppress completely the residual CA but reduces it to a widely tolerable level. Furthermore it sacrifices only little light and alters colors only slightly. Based on my tests the Contrast Booster filter has proved to be almost totally ineffective and even detrimental. Compared to the Semiapo, the CB is a poor performer both in terms of brightness and sharpness. I don’t understand why, based on its specification it should be better, may be it doesn’t work well with a f/5 light cone, in fact inserting it after a Barlow lens I noticed a slight improvement. I have not tried the Fringe Killer filter.
As for the classic colored filters – of which I recommend to use only the best on the market – yellow and yellow-green give the highest contrast in the observation of the Moon and Saturn, especially the yellow-green (or W11). For the Sun the Semiapo is enough, if one wishes to take images of the photosphere the Baader Continuum, or alternatively a green W56 or W58, is the filter of choice provided a full aperture safety solar filter is installed in front of the objective (alternatively a Herschel wedge + neutral filters may be used).
IMPORTANT UPDATE: the above test has been updated on April 30, 2017 (please follow this link).