The Bresser Messier AR102XS is a 102mm (4”) f/4.5 ED doublet refractor with a focal length of 460mm. There exists a vigorous debate on various online platforms regarding the authenticity of the ED glass used in the AR102XS’ objective lens. Bresser ardently promotes the telescope as being equipped with an ED lens, although perplexingly, there is no trace of this information on the telescope’s barrel (it says achromatic on the label) or within the relatively vague documents accompanying the product. The precise nature of the glass utilised remains shrouded in mystery, but there is a strong possibility that it is FCD1 or FPL51.
Being a larger refractor and with a fast focal ratio of f/4.5, the AR102XS would have some aberrations even with superior glass and optical specifications. Being a fairly inexpensive FPL-51 doublet, the AR102XS unsurprisingly has quite a lot of chromatic aberration and field curvature at this aperture and fast focal ratio. Thanks to the ED glass in the objective chromatic aberration is in line with that of a slightly slower – say f/5.5 – achromat; large purplish halos surround the Moon, planets and stars brighter than second magnitude but overall they still appear fairly sharp. Field curvature is a problem for both imaging and views at the eyepiece as well; a field flattener solves the former but eyepieces more often than not can either reveal or even amplify the field curvature of the scope. Some eyepieces are worse in this regard than others – don’t be surprised if the view looks a little like a fish bowl if you attempt to steer the scope manually at low power. Nonetheless, with the right eyepiece the low-power views with the AR102XS are magnificent.
The AR102XS’ robust 2.5” hexagonal focuser takes centre stage as an unexpected boon – it boasts a swiveling feature, is secured firmly by three resilient screws, and possesses sturdy, wide angled rack and pinion teeth. In this instance, ‘hex’ alludes to the hexagonal internal draw-tube, as opposed to the more conventional cylindrical design. The downside of the hexagonal focuser is that it tends to get stuck on tiny bits of dust and dirt more easily, but this is of little concern with careful storage. Its spacious 2.5″ clear aperture drawtube prevents vignetting even in the presence of full-frame DSLR cameras or wide-angle 2” eyepieces. While it is a single-speed design, the 2.5” Hex focuser can be seamlessly upgraded to dual-speed for more control over fine focus – particularly useful for higher magnification observing.
To attach to a mount/tripod, the AR102XS is directly bolted to a Vixen-style dovetail plate attached to a “foot” on the back of the tube. While the telescope cannot be rotated in this configuration, the 2.5” Hex focuser can be rotated independently, so positioning of the star diagonal or a camera assembly are not a problem. The dovetail plate allows the scope to be attached to any compatible astronomical mount or a photo tripod head with a ¼ 20 stud.
Accessories
The accessories accompanying the Bresser Messier AR102XS leave much to be desired in terms of quality, although it is fair to concede that there are inferior options available. The supplied 1.25”, 25mm Super Plossl (26x) eyepiece is passable; however, the extensive use of plastic coupled with an unusually recessed eye lens considerably diminishes the comfort level whilst peering through it. It is also plagued by glaring and internal reflection issues. Boasting an apparent field of view of 50 degrees, this translates into a true field of 3° at 18x, which is relatively expansive, clocking in at just about six times the angular diameter of a full Moon or the Sun in the sky. Nevertheless, this is far from the widest field you can get with this scope’s 460mm focal length and 2” focuser.
The Messier AR102XS also comes equipped with a 1.25” mirror diagonal encased in a plastic housing, which inserts inside the contrastingly high-quality 1.25” compression ring adapter provided with the scope. The optical quality of the internal mirror of this diagonal is rather subpar and there is the potential for the introduction of mild astigmatism due to the irregularities in the mirror surfaces often found in these diagonals. The rough surface of this mirror exacerbates chromatic aberration and scattering from the telescope, which is further compounded by internal reflections within the plastic casing and the 25mm Plossl eyepiece.
The finderscope accompanying the Messier AR102XS is a cheap 6×30 magnifying unit. It has an inverted (upside-down) field of 7°, or about 14 times the angular diameter of the Moon or sun in the sky. This is actually not that much bigger than the 5.5° maximum field of view of the scope, however. While this finder can show you stars fainter than your naked eye alone can see, it is uncomfortable to look through and the view is rather dim compared to a 9×50 unit. However, given the cost, compactness, and wide field of the Messier AR102XS itself a 9×50 would be overkill. A red dot sight is more convenient and simple to use than the 6×30, but finding a replacement to fit the Bresser mounting shoe may not be worth the cost or effort.
At least from some retailers, a white light “safety film” solar filter is included with the Messier AR102XS. However this filter does not securely fit to the front of the telescope with anything other than a pair of plastic tabs, and as such it can easily fall off of the front of the telescope. Thus, is not the safest filter to use, and we would recommend either finding a way to secure it better with some sort of strap or clamping mechanism or simply purchasing a solar filter with a more secure attachment mechanism from another third-party vendor.
Mounting Recommendations
Given the lightweight nature of the doublet objective and stubby tube of the Messier AR102XS, the mounting requirements for this telescope are not nearly as stringent as those for a typically longer and/or heavier instrument of this aperture. As such, mounting requirements are more in line with those of a typical 80mm instrument. Since the AR102XS is primarily a low-power, wide-field instrument, we would recommend you consider a sturdy alt-azimuth mount such as the Vixen Porta or Explore Scientific Twilight series, for this telescope.
We generally wouldn’t recommend an equatorial mount for the Messier 102xs for visual use as it’s just not particularly comfortable or intuitive to pan around the sky, generally weighs more, and is likely to be less stable and less compact. This makes one a poor match for a wide-field, lightweight, and “grab n’ go” telescope. On the other hand, you’ll certainly want a GoTo equatorial mount for deep-sky astrophotography with the Messier AR102XS or any other telescope. A German equatorial mount used for imaging needs to be able to carry the AR102XS telescope and associated equipment such as a camera and an autoguider; for this purpose, we would recommend something in the EQ5 class range such as a Sky-Watcher HEQ5 Pro or Celestron Advanced VX.
Should I buy a used Bresser Messier AR102XS?
Being a simple doublet refracting telescope, a used Messier AR102XS is not that likely to be problematic because there simply aren’t that many components to be subject to any issues. This telescope is also not been on the market for very long, and as such you’re not going to run across an old unit in a barn, attic, or something of that nature. However you should of course avoid a unit with a damaged or broken objective lens, and you should make sure that the focuser on the back end moves smoothly, as the hexagonal drawtube is a little more prone to wear and tear than a conventional model. Likewise, the teeth of the rack and pinion should also be clean and free of any damage.
Alternative Recommendations
There aren’t actually that many direct comparisons to the Bresser Messier AR102XS, as extremely fast focal ratio refractors are typically more expensive and specialized Petzval refractors, generally for designed for imaging purposes only. Besides the Sky-Watcher StarTravel refractors, the most direct comparison to the Messier AR102XS would probably be one of the various small Newtonian reflectors usually sold as beginner telescopes, such as one of 114/130 mm f/3.9 and F5 Newtonian reflectors respectively. A 130 mm f/5 Newtonian with a 2” focuser is able to achieve roughly the same field of view (3-5°) that the AR102XS can before aberrations dominate the field. However, these telescopes do have diffraction spikes on stars, require some cooldown time, and of course need to be collimated.
Various longer focal length options, both doublet and triplet, achromat or apochromat, exist which can perform similarly to the Messier AR102XS for imaging with the proper reducer-flattener, and for visual use can achieve a fairly large field of view with less field curvature. However, the best of these are often vastly more expensive than the AR102XS, and as such you may be better served by a Newtonian reflector if your goal is simply a cheap wide-field instrument for looking through. If your goal is deep-sky astrophotography, many of these more expensive refractors are probably a better way to go, though they may require a bigger equatorial mount on account of their weight and heft.
Aftermarket Accessory Recommendations
Though certainly a bit of an expensive add-on, Bresser’s dual speed focuser upgrade for the Messier AR102XS vastly improves the ability of the focuser to provide smooth motions at high magnification, should you be using this telescope for high-power planetary lunar or double start work. If you’re doing deep-sky astrophotography you may be better served by purchasing a motor focuser rather than the dual-speed upgrade for finer control remotely via your PC for Imaging.
While it may seem like replacing the 6×30 finder attached to the Messier AR102XS is a top priority, in actuality the cost of replacing the bracket and associated hardware, then integrating a new red dot sight, is probably not worth it for a telescope that, when properly equipped with a two-inch diagonal, is basically a finder and of itself anyways on a manual mount. In any case, you should have little trouble using the provided 6×30 or even no finder at all with a wide-angle eyepiece. Of course, you’ll need to get that 2” diagonal first and we would recommend one of the various options such as Stella Lyra for this purpose.
Eyepieces as add-ons or replacements for the Messier’s stock 25mm Super Plossl are primarily going to be decided by your budget. What you should keep in mind, however, is that many 2” wide-angle eyepieces – particularly cheaper wide-angle designs such as “SuperView” or “SWA” Erfle-derived units – are probably going to have a lot of field curvature and maybe edge-of-field astigmatism with the AR102XS which could ruin their usefulness and account of achieving a wider field if most of this field is aberrated. However even Ultra wide angle eyepieces can have issues with field curvature with this telescope. We would recommend trying before you buy and consulting other amateur astronomers for their own personal thoughts on a good 2-inch wide angle eyepiece that will provide the sharpest image possible with this telescope.
For higher magnification with the Messier AR102XS, almost any eyepieces will do. Plossls, UWAs, goldline/redline eyepieces – you name it. The AR102XS tops out at around 100x for useful magnification, or about a 5mm eyepiece.
A UHC and/or oxygen-III nebula filter is certainly worth picking up for the Messier AR102XS and viewing large nebulae such as the Veil or North America nebulae. Get a 2” version.
What can you see?
Whilst a 4″ telescope such as the Messier AR102XS might initially appear to be somewhat wanting in comparison to larger reflectors in respect to its small aperture – which would indicate of course a comparably puny capacity for light gathering and resolution, it is imperative not to underestimate its prowess. The unimpeded optics of a 4” refractor almost contend on an equal footing of light-collecting power with a 6” Dobsonian, particularly when the mission at hand is to scrutinize deep-sky celestial objects.
The Messier AR102XS can achieve a field of view of well over 4° with even a modest 2” wide-angle eyepiece (A 30mm SuperView yields 4.7° and 15x, for instance) and at eight times the angular diameter of the full Moon this is quite the field indeed. Think of the AR102XS as somewhat like a superpowered 15×70 binocular with some leeway for magnification with regards to performance and usage.
The Messier AR102XS demonstrates this proficiency in seizing the extensive breadth of larger star clusters. There are numerous exemplary clusters one can see such as the Hyades, M35, Pleiades (M45), Beehive (M44), and Double Cluster. Multiple globular clusters often can be seen dotting the southern summer Milky Way. This is quite a fun scope – and compact open clusters like M11, M36, M37, and M38 yield plenty if you bring the magnification up a notch too. All of these clusters exhibit colorful, pinpoint stars.
Turning the Messier AR102XS towards the brightest and most prominent nebulae will not leave one disappointed. Bright emission nebulae such as Orion (M42), the Lagoon (M8), and the Swan (M17) are all accessible with this scope even under light-polluted skies. However, it is of significance to emphasise the necessity for dark skies and a UHC nebula filter for an optimum experience. Light pollution can significantly diminish the intricate details of these celestial marvels, and observing them without a filter might lead to a less contrasted view. In the case of dimmer objects like the Veil or North America Nebula, you will need dark skies and a good filter to see them at all due to their low contrast.
Galaxies are relatively lackluster through a mere 102mm telescope, especially if you have any light pollution at all to contend with. Under dark skies the Messier AR102XS can show dust lanes in a handful galaxies like M31, the Andromeda Galaxy, or M82 the Cigar, and you’ll be able to see a few galaxy groups and clusters, including the famous Virgo cluster. However many of these objects will appear faint if they’re even visible at all under city skies, and in general are not exactly interesting or detailed through this telescope even under the best conditions possible.
Like galaxies, other small and faint deep-sky objects are a bit of a letdown with the Messier AR102XS, though mainly due to insufficient aperture than anything specific to this scope. Smaller planetary nebulae such as the Cat’s Eye and Ring are hard to even resolve with the AR102XS (even the comparatively huge Dumbbell Nebula is small with a telescope that is best used below 40x). Globular clusters remain unresolved “faint fuzzies” with the possible exception of M13 on the best of nights.
The rather high amount of chromatic aberration that the Messier AR102XS exhibits means that it really can’t show you much detail on Mars especially since Mars’ reddish hue is especially affected. Jupiter shows its main equatorial cloud belts and perhaps even the Great Red Spot but the four Galilean moons are likely to remain as pinpricks or blurs. Their Shadows may or may not be visible when they transit in front of the planet. Saturn’s rings are of course easily visible at any magnification with this telescope and a few of its moons such as bright Titan and dimmer Rhea are visible – however the Cassini division in the rings is not clearly delineated if at all, and the cloud belts on the planet may be hard to discern. The outer ice giant planets Uranus and Neptune little to the ar102 access apart from appearing as fuzzy bluish dots; this telescope simply lacks the resolving power to show either as a clear disk. Likewise, the AR102XS simply lacks the light-gathering ability to show either ice giant’s moons, nor distant and dimmer Pluto.
Astrophotography
Obviously, for lunar planetary astrophotography, the Messier AR102XS is probably the worst option imaginable, given that it’s only a 4” telescope and has a lot of chromatic aberration, not to mention the need for a 5x Barlow lens even get close to the requisite image scale. However, for deep-sky astrophotography, the Messier AR102XS is an acceptable choice. Its fast focal ratio of f/4.5 makes it excellent already with a suitable 1x field flattener. You could also get a reducer flattener and with most smaller chips this will still result in sharp and vignetting-free stars at the corners but could speed the telescope up to as fast as f/3 depending on the reduction amount. Unfortunately, you will be handicapped by this scope’s chromatic aberration due to the cheap ED glass and extremely fast focal ratio.
Thanks to its ED glass, the Messier AR102XS is on the fringes of being acceptable sharpness-wise for visual use. However, any astronomer knows that the blue-violet halos around brighter stars, a telltale display of chromatic aberration, are far more pronounced in long-exposure images than at the eyepiece. Unsurprisingly, the chromatic aberration of this telescope is sufficient that it will show up on stars in your photos at its native f/4.5 focal ratio, as well as with a focal reducer. This is similar to what can be expected when trying to use a medium focal ratio achromat for astrophotography – the results are certainly usable and not any worse than many telephoto lenses that people start out with, but if you’re investing serious money into an astrophotography, then the Messier AR102XS probably not the telescope for you – unless you are doing narrowband imaging with a very good autofocus routine and filter wheel, in which case chromatic aberration is of little importance and you can go right along shooting unimpeded with the AR102XS.
