Skywatcher 102 f/5 review and first light
by John Baars
Skywatcher Startravel 102 f/5 First Light.
In December 2022, there was a number of times when I grabbed my grabngo 102mm Maksutov on the AltAZ mount. However, I wanted just a little more impact, so I put my 140mm Maksutov, the OMC140 on it. That was on the edge, it vibrated quite a bit. A nicer image, though. Then I got the bright idea to put the Azimuth arm of the mount upright, so there was much less vibration coming through to the telescope, a huge difference. Focusing on the planets Jupiter and Mars thus became a lot easier. Still not satisfied because of the increased weight and the small field of view ( focal length of the OMC is no less than 2000 mm) I looked forward to another solution for the grabngo thing.
Coming up with a solution
My old Vixen ED f/9 was an option, but then I had to put the Azimuth arm at a 45 degree angle again, otherwise there was no room for observations near the zenith. Given the mass and length of the telescope, the vibrations would return. And well, as an old refractor-adept ( although I have had a lot of reflectors) there is really only one way to go, if it has to remain affordable. Skywatcher's 102 mm f/5 Startravel came to mind. It would be short enough, have little mass and have almost as much impact on deepsky as the OMC140 ( so it had once appeared to me in a direct comparison between my OMC140 and my 102 f/9 Vixen ED). Above all, a huge field of view, in which, for example, the Veil Nebula fits in entirely.
Ah yes, that C.A.
Anyone who knows even a little about it immediately shouts, "Chromatic Aberration!" As if that is all you think about. Some then. The chromatic error of an achromatic refractor has never bothered me, secretly I even find it pleasant. Even enjoyed the color in the 150mm f/5 I once had, but had gotten rid of again because of the weight. Well, that CA is there, less prominent than reflector-adepts would have me believe, but perhaps more prominent than common user-reviews predict.
Bright stars of magnitude 1 or 2 show it as a blue blur, at magnitude 3 you already have to look harder for it. On deepsky objects and large star fields you don't see it at low magnification. At high either. Not so. In that case it is not inferior to my 102mmf/9 ED Vixen.
On the Moon you see at magnifications from 36X a blue edge, but on the terminator not or hardly. Also not at higher magnifications. Details on the terminator would "drown" in a blue haze. Well, not really much. In Plato, I saw the familiar proprietary distortions on the crater floor and one small crater. My Vixen ED shows two. Granted, the shadows in the craters in Clavius were deep deep dark blue and not black. Nor does it really reach the maximum of 2XD in magnification. Above 167X, though, it was over with more lunar detail.
On planets, it is not a high flyer. I did see a loose dark structure on Mars, but the red of the planet "bled" a bit into space. Typical of doublets. On Jupiter two bands of clouds with some nodes in them, the whole thing surrounded by a slightly colorful fog ring. Not convincing as a planet telescope. An understatement.
Since the telescope can also be used during the day, as evidenced by the Amici prism supplied and not used (by me), I subjected that to a little investigation as well. In backlight and as the only subject black branches or crows, the blue is obvious. In normally lit subjects, it is not so noticeable. The manufacturer provided the lens cover with an additional 52 mm lockable hole. Especially for daytime use. That way it becomes an f/9.6 system. Couple that with a 52mm aperture and you have virtually eliminated the color error. During the day, at low magnification of say 20X, the exit pupil then matches the entrance pupil of the naked eye and you will not see any decrease in brightness. However, it does become slightly sharper and chromatic abberation-free.
Because all Skywatcher telescopes that had come to me so far suffered to a greater or lesser extent from axial coma, I also subjected this telescope to a star test. It surprised me positively, it was better than all Skywatcher telescopes I have tested so far. The German company that supplies it then also checks it to see if the instrument is diffraction-limited. In and out of focus proved fine, near a standard 0.25 P/V. Quantifying that by eye is tricky. Near focus the Poisson point was found to be not exactly in the center, but slightly off. Which produces a coma error in the order of 0.2 wave. That little error nibbles off several hundredths of the total Strehl. See the picture. The picture serves as an illustration, not a star test; all values are estimated from the star images. Perhaps in the future I will do something about it, like I did with my old f/5 150mm, which, by the way, was in worse shape at the time.
Included were a 25mm and a 10mm eyepiece, a 2X barlow, a reddot finder and an Amici prism. I did not include the eyepieces or the prism in my tests. I just used my own higher quality stuff, including a Baader prism, a 24mm Panoptic, the Leica ASPH, a Morpheus eyepiece and a Baader Zeiss barlow. And as a finder my 50mm RACI with interchangeable eyepiece. The included Barlow was a disaster in terms of light management, huge light reflections by the inner walls. Really bad actually. I plastered it inside with flocking material from Protostar and after that improvement it turned out to be miraculously not that bad at all! It is quite usable now.
Focuser is a standard one from Skywatcher. Just one with a rack and pinion. Heavily maligned, but it is not too bad for me. Especially for my visual use. The German firm Teleskop-Specialisten adjusted it fine before shipping. I have no complaints about it. In case of any ingrained tilt, the two subtle Allen bolts are sufficient to eliminate it.
An f/5 system has only a very small depth-of-focus area. That's why I put a "focusing eyepiece holder" on the prism. A very fine helical focusser as a focus slow down, so to speak. That works well. From slightly out of focus to slightly in focus is an easily 0.4 rotation.
Extension tube for Zenit
To allow the telescope to also see into the zenith ( 80°- 90°) without the mount getting in the way of the back of the telescope, I had to mount an additional extension after the prism into the rotating helical. That way the rear end "slides" up a bit forward, deeper into the tube. This way it is possible to observe in the zenith too.
Finder and balance
It comes with a red dot finder. I can use that, but I prefer a Raci finder with interchangeable eyepiece. With an 11 mm Nagler, I achieve 19 X magnification at a 4 degree field of view. Very useful here in the city. Many Messiers then already loom in the finder. Mounting that viewfinder next to the main telescope presented some additional balance - and space problems. That has been solved, although the rear ring that holds the telescope in its cradle is in a somewhat unusual place around the tube.
On January 30, it was clear. In the afternoon I had already had the telescope outside for a while to view the Moon in daylight. In the evening, heavy clouds passed by, but occasionally it stayed open for a longer period of time. Seeïng was not top, neither was transparency and then there was the Moon! However, you can't have everything.
Once cooled down ( about 15 minutes) I aimed it at Aldebaran. A beautiful red "pit" welcomed me, complete with a small blue cloak. A quick star test. The results are above. Double star Castor was obviously a breeze, Rigel was a little trickier, while Alnitak's double was at the limit of seeing possibilities. Trying any closer made no sense. Seven days later, with better seeing, it was separated properly.
On to the Orion Nebula, which in the 14 mm Morpheus eyepiece could be seen neatly between NGC1980 and NGC1981 in one field of view. Breathtakingly clean. Now that's what you buy a telescope like this one for! The Trapezium was neatly depicted as four separate "needle prick" stars, something you can sometimes miss in large binoculars due to image errors.
And this was my own sketching, two years ago.
I barely recognized M35 at 20X, only at higher magnifications ( 90X) did I see how beautiful the open cluster actually is. Also briefly looked up NGC2392, the Eskimo Nebula. At first a very small ball of fog in the midst of a rich star field. A little sneeze. At higher magnifications, the central star popped up with averted vision. (125X) All in all, what I am used to for a 102 mm refractor.
The Moon, already discussed earlier, at higher magnification with a blue edge, however, not on the terminator. Plato with one crater pit in it as well as the uneven shades of gray on the crater floor. Clavius' small craters, however, did show a deep dark blue shadow. The level of detail in debris around craters is just a little tad below my Vixen ED. Magnifying higher than 167X made little sense.
The same was true for Mars and Jupiter. Mars did show at least one dark albedo spot, but the planet "bled" a little red light into space. My 120 ED Evostar does the same a tiny bit, my 102 ED Vixen does not. Jupiter was nothing to write home about. I did observe two bands of clouds with some nodules in them, but the slightly hazy fog in which this planet cloaked itself prevented further detail observations. I suppose an evening with less moisture in the air is at least a prerequisite for this telescope on Jupiter. Then again, for planets, one of my other telescopes really needs to go outside anyway.
And finally, the way it held up on the Alt Az mount. And that was neat. Because of its low weight and upright Azimuth arm, vibrations are minimal. A tap against the tripod leg is not even seen. Focusing does, but in such a way that you don't have to let go to see the result of your effort; it extinguishes almost immediately. The helical spacer allows very precise focusing.
A nice grabngo!
I hope you enjoyed the read.
Finally I did an update by more carefully repositioning the frontlens in its cell after all. Without extra spacers underneath the spacing-ring. And with a rather astonishing effect. I wonder why this wasn't done by the manufacturer in the first place.