The Optics Performance
The telescope is a 102mm (4”) aperture 2-element achromatic refractor. Its focal length is 500mm (20”), resulting in a focal ratio of f/5. At f/5, the telescope should not be expected to perform well on planets or provide high power views of the Moon. This is a simple result of the fact that all forms of aberrations, including chromatic aberration, are made worse when the focal ratio is “fast” (short focal length compared to aperture). As a result, any object brighter than around 2nd Magnitude will show a noticeable blue-violet color fringe, especially when viewed at high power.
The typical rule for maximum useful magnification is about 50x per inch of aperture, and so we would expect to get up to 200x or so. However, because of chromatic aberration, the actual maximum is around half that, or around 100x. Beyond that, chromatic aberration blurs the image and prevents further details from being observed.
But to use this telescope on planets or for high-power lunar viewing would be a misuse of this telescope, which is designed to operate as a Deep Sky telescope. And at that, it performs beautifully. When used at low power, the brightest deep-sky objects pop out of the black sky. I observed several of my favorites, including the Double Cluster in Perseus, and found that they were almost as pretty as what I could see in my 6” Dobsonian reflector. The difference is that a refractor has no central obstruction, which improves contrast over a reflector, which has a secondary mirror obstruction.
Four inches of aperture is easily enough to observe every single one of the 110 Messier objects, and many many more after that. Charles Messier himself certainly liked his 4” refractor!
The Optical Tube Assembly
The achromatic lens is housed in a short and very sturdy metal tube, with a generous dew and light shield. The dew shield can be easily removed to reveal the beautifully large objective lens, though there’s not really a good reason to do this. The inside of the shield and the telescope is very black, preventing light from scattering. The back of the telescope reveals a great treat for a small telescope: a 2” diameter focuser. This allows the telescope to make use of ultra-wide-field 2” eyepieces when paired with a 2” diagonal (not included). The focuser is an ordinary rack & pinion unit, but it is very smooth and easy to use, in spite of the mediocre plastic knobs. The visual back on the focuser rotates and can be tightened with a locking ring, allowing any diagonal to be easily rotated to where it is most comfortable to use.
On the outer shell of the focuser assembly is a built-in Synta-style finder shoe, into which is placed a 6×30 Finderscope. (Though some versions of this scope have been sold with a red dot finder).
The Optical Tube Assembly is covered by a two-component dust cap. The middle section of the dust cap can be removed separately, acting as an aperture mask. Since Chromatic Aberration is related to focal ratio, not focal length, stopping down the telescope to around 50-60mm (essentially turning it from a 4” f/5 to a 2” f/10) means that chromatic aberration and light gathering both go down. This still restricts useful magnification to around 100x, but the view is a darker, more color-corrected view. It is useful when observing the Moon, planets, and some double stars. The darker view is more comfortable when viewing the Moon when it is full, or for darkening the extremely bright Venus to something easier to observe.
Reviewing the Mount’s Ability
There’s some good news and some bad news when it comes to the mount. The good news is that it is sturdy, not typically wobbly, and it stays where you put it. The bad news is that it’s a little stiff, and that its slow motions are both required for useful operation and unintuitive in operation.
The mount is an altitude-azimuth (up-down and all-around) system where the telescope rests on a sturdy hinge and that hinge rests on a swivel. In theory, this system can’t be perfectly balanced (compared to, for example, a fork or single-arm mount). However, in practice, the mount is stiff enough that even when pointed nearly at the zenith, there’s no problem with balance.
Both the altitude and azimuthal slow motion knobs interface with a worm gear/lever-arm system, which means they can run out of travel and stop working, at which point you’ll have to move the slow motion back in the other direction until the connector is at the center of the worm gear, and then manually move the telescope back to where you’d put it. When tracking an object for a long time with the slow motions, it is very easy to run out of travel, requiring a somewhat tedious reset.
In addition, the azimuthal motion can either be used manually or in slow motion. A locking knob allows you to tighten down the azimuth axis so slow motion can be used, or you can loosen the knob to allow the azimuth axis to be spun freely. The azimuth slow motion is not particularly responsive, requiring a half-turn before the mount responds to the cable being turned.
I normally consider an altitude-azimuth mount to be the most intuitive form of the telescope mount, compared to equatorial mounts. However, while it is conceptually intuitive, the equipment itself is somewhat unintuitive in this case. I would recommend this scope for any adult, but it’s a poor choice for a child due to the slow motion weirdness.
The slow motion cables are a standard cheap pair with small, hard plastic knobs on a bendy cable. The plastic knobs are uncomfortable in the hand, but they are usable.
The tube-rings are permanently attached to the mount, there is no Vixen Dovetail, so using the telescope tube with a different mount would require a new pair of tube rings.
The tripod is a sturdy aluminum unit that must be fully extended to be used, whether seated or standing. It could stand to be a bit taller, but it works well enough and I didn’t find that it shook or wobbled at all after I tightened up the screws. It includes a large triangular tray to be placed on the leg spreaders and is a good place to put eyepieces and dust caps when not in use.
Reviewing The Provided Accessories
The StarTravel 102 AZ3 comes with a set of mostly acceptable accessories. It comes with a 6×30 Finderscope (though at different times this scope has been sold with a Red Dot Finder instead), a pair of 25mm and 10mm “Super” (Modified Achromat/Kellner) eyepieces, and a 45 degree prism diagonal.
The Finderscope is actually a very usable unit. Though a straight-thru image-reversed finderscope is not ideal, I found that the ergonomics of using this finderscope were not too obtrusive, unless the telescope was pointed very high. It’s worth noting that, with a wide field eyepiece, the telescope is capable of acting as its own finderscope, and a red dot finder can be just as suitable. I found that the straight-thru finder did occasionally obstruct my ability to find stuff very high in the sky, but it works. I would replace it with a Right Angle Correct-Image (RACI) Finder, which will just be more comfortable to use.
The Eyepieces are standard fare on SkyWatcher entry-level telescopes. They are “Super” “Wide-Angle” Modified Achromats. By “Super,” they mean “buy our eyepieces please!” By “Wide Angle,” I’m not sure just what they mean, since the field of view is less than 50 degrees. With three elements, they provide decent correction. They’re fine, but for the price point, higher quality Plossls (4-element) eyepieces would have been nice. The focal lengths chosen are quite useful as all-around useful magnifications–25mm and 10mm provide 20x and 50x respectively.
The Diagonal is the real disappointment. No astronomical telescope should be shipped with a 45-degree diagonal. The 45-degree diagonal is useful only for terrestrial observing, where the telescope is pointed within about 45 degrees of the horizon. When pointing above that, you have to start looking up into the telescope, which rather defeats the purpose of a diagonal to begin with. The prism system creates a diffraction line running vertically through bright planets and stars at high power, but this is no more distracting than the four diffraction spikes of a typical Newtonian. It is imperative that the 45-degree prism be replaced with a 90-degree diagonal for astronomical use.
90-degree diagonals come in both erecting prisms and simple mirror systems, the latter being more useful for astronomy and the former being more useful for terrestrial viewing. Personally, I like the erect-image, especially on the Moon. Although an upside-down view (as in a reflector or a refractor without a star diagonal) is just as correct as the erected image, the mirrored image seen in a mirror diagonal is noticeably incorrect on the Moon and certain star clusters, and makes it difficult to get “scientifically correct” sketches. It is up to personal choice whether you choose to replace the diagonal with a erecting prism or a mirror, but replacing the 45 degree prism with a 90 degree one is necessary.
A diagonal is optically necessary, not just ergonomically necessary, as the focuser does not have enough back-travel to bring eyepieces into focus without the diagonal.
A few things can significantly improve the viewing experience. As mentioned, a replacement diagonal is almost a necessity for astronomical viewing, or else viewing above 45 degrees becomes very difficult. If you can afford to replace the diagonal with a 2” unit, you will have many more options for wide-field eyepieces, which also tend to be quite expensive.
In addition, a cheap moon filter can actually improve the view significantly. The cheap moon filters, which use dark yellow-greenish plastic (as opposed to the more expensive dark gray “neutral density” filters) actually block out the blue-violet color fringe and reduce chromatic aberration, resulting in a sharper view of the moon and the bright planets. Specially made yellowish color filters can also be used to fight violet fringing specifically.
If you get a new eyepiece, I’d actually recommend going for a lower magnification first. If you have a 2” diagonal, get a focal length of between 25 and 35mm with an apparent field of view of 60 degrees or more–which can only fit in a 2” diagonal. If you keep the 1.25” diagonal (45 or 90 degrees), get a 32mm Plossl eyepiece, which will maximize the field of view and surface brightness and make finding dim objects easier. The next eyepiece I’d get after that is a 9mm Goldline (66 degree ultra-wide-angle), which would replace the 10mm Modified Achromat the scope comes with, and will be both more comfortable to use and provide a wider field of view. This can be especially useful for observing smaller star clusters and nebulae, which can pop out of the darker background better at a higher power. If you’re not convinced you need to upgrade the 10mm, you could get the 15mm Goldline or an equivalent Dual ED (Paradimn/StarGuider) eyepiece, which would split the difference between 25mm and 10mm.
Finally, consider replacing the finderscope. The straight-thru unit (6×90 scope or red dot finder) becomes useless above about 45 degrees of altitude, while a RACI finder can be useful at any orientation.