Some of the sky trackers available in the market (image courtesy of respective manufacturers or from the author).

• reduce the exposure time and increase the sensitivity. This will produce stars with minimum trails, but noise will be evident. See below for a table of maximum exposure times.
  
• track the stars, by using an equatorial mount.

	Very wide field imaging (e.g. with 8 to 15 mm focal lengths) Virtually any product is suited, as the needed tracking accuracy is not high. Also a polar finder is not necessary, and the mount can by pointed to the celestial pole only approximately such as by looking in a hole on the mount. A 5D classic camera with a 14 mm lens has a image scale of 120", so even large tracking errors can be tolerated and a few minutes of exposure are possible. Minimum weight For traveling on airplanes or trekking up to the highest mountain peaks, weight is a very important specification. However don't expect maximum quality if this is your requirement, so try to limit the maximum focal length to the ones stated above. Minimum cost Also in this case, with a cheap mount, don't expect to get perfect quality, so keep to very short focal lengths.
	Medium-wide field imaging (e.g. up to 100 mm focal lengths) If this is your preferred imaging field, than a decent guiding accuracy must be selected. I advise to avoid the simplest and cheaper models, and a true optical polar finder is needed. Anyway, don't expect very long exposures or focal lengths, because many factors concur to limit it, such as flexure in the tripod, mount, head, and polar pointing inaccuracy or periodic errors. In example, with a 400€ class mount you can expect a maximum exposure of 3 to 5 min with a 100 mm lens, and about 50% of untrailed images. Mixed tracking of stars and landscape Some photographers like to image the night sky together the terrestrial landscape. If a long exposure is made on a fixed tripod, star trails are present but landscape is still; instead if a long tracked exposure is made, stars are (hopefully) pinpoints but the landscape is moved. To get the maximum exposure time and both subjects fixed, a 0.5x tracking speed is needed (respect to sidereal 1x). Many newer mounts have this feature. The maximum exposure time for minimum trailing is exactly the double of the one reported in the table above.
	Maximum tracking quality (e.g. up to 300 mm) If you wish to image with medium telephoto lenses than tracking quality is a must. Moreover also the tripod and heads must be very solid (and expensive!). Long exposure (e.g. 10 min) astrophotography with a 300 mm lens is very demanding, and if this is your willings, I really advise to look for a traditional astronomical mount, such as a Synta HEQ-5, a Kenko NES, a Vixen GP-DX or GP2, or similar.

	Sky-Watcher Star Adventurer Even if I've not tested this tracker, I've added to the list because it is very interesting and because I've seen used by many friends. By analyzing to the friends results I think this tracker has the best performances of all the list, mainly thanks to the robustness and good polar finder. However the robustness translates in bigger weight and less compactness. And a heavier and robust tripod is fundamental to exploit the good working at longer focal lengths. It can be used stand alone, thanks to the 4xAA batteries, or connected to an external 5 V battery (like smarphones USB power packs). At 0.5x it can be used for astro-landscape imaging. Moreover it can be used for panning time-lapses thanks to 2x, 6x and 12x speeds. I really recommend to by the "Astro Photo Bundle" so that all necessary accessories are present: polar finder illuminator, base head, L-bracket, counterweight bar and a counterweight. In conclusion, if you need the better performances also with longer focal lengths, this is the choice to do. Otherwise if you need the less weigth as possible, and you need to use only with short focal lengths, than one of the other trackers will better suite your needing.
	iOptron Skytracker I've tested this mount with my friend Alessandro Gambaro, that purchased it to have a very portable mount for wide field imaging. The first feeling is that this is a very well engineered product, provided with all necessary accessories such as a polar finder and a adjustable base head. And the price seems ok for such a bundle. Power supply is internal with 4 AA batteries, suitable for a few night of tracking. The polar finder reticle is illuminated by an internal red led and the reticle is provided with graduation; Polaris must be placed in the right place, and this place can be easily found using the iOptron Polar Scope app (or other compatible ones). Fine tuning movements are possible in altitude thanks to the screw on the base head, while for azimuth motions I found more easy to move a little a tripod leg because the azimuth motion of the head is not very accurate. The compactness of the mount make the mounting very easy and fast. To be ready for imaging, only a few minutes are needed even on the first night. The bad news come when we've measured the periodic error (PE), by using a very light and compact setup. A ~100" peak to peak PE was found, and this will severely limit the maximum exposure with long focal length. In example a 100 mm lens on a Canon 60D (4.3 micrometer pixel) has a image scale of 9", and the motion due only to the PE would be 11 pixels (period is 9.2 min). Fortunately motion is not sinusoidal and in a 3 min interval the tracking is much better. So, with this sample, when imaging with such a setup and limiting the maximum exposure to 3 min, about 33% of tracked images can be obtained. Click for full res image The periodic error graph of iOptron Skytracker, obtained with a Starlight Xpress Lodestar ccd with a 100 mm focal length scope (23 mm diameter). Total load on the mount, including a Manfrontto 496 RC2 ball head, was about 500 g.
	Vixen Polarie I've seen this mount in a shop and I was surprised by the compactness: it is much smaller than principal competitor, the iOptron SkyTracker. This mount is not provided with a base head, so a very solid one must be purchased. The optional polar finder (highly advisable) must be mounted in the same axis where the camera head goes, so it is not easy to periodically check the polar alignment. If needed, you have to dismount the camera and head.
	Astrotrac TT320X-AG I was able to play with a couple of these mounts owned by friends. From an engineering point of view I find this solution very intriguing to understand, and the good news is that it is working very well! The manufacturer state a maximum periodic error of 5", a really good value. For sure periodic error is not the only tracking error, and to minimize drifts a rock solid base head is necessary, such as the one manufactured by Astrotrac. The polar finder, optional but indispensable, attaches the the polar bracket with magnets. In the samples I've seen the fixing is not secure and the polar pointing can be hugely affected. I hope that in newer models this problem has been fixed.
	Sightron Japan NanoTracker This is the smallest mount of the list! I've seen this very small mount only on the internet, so I don't know how it performs. For sure I don't expect a great accuracy, only by the fact that no polar finder is available and only a hole with a ~9° FOV is present. The main unit must be connected using a cable to the hand-pad, that includes the batteries and settings. This solution is much less elegant than the single piece of the iOptron SkyTracker or Vixen Polarie mounts. However it can also be used for daytime time-lapses thanks to a 50x speed.
	Minitrack-LX (by Cristian Fattinnanzi) This is the cheaper tracker of the list, and the reason is that the motor is made with a simple kitchen timer, coupled to plastic gears. No batteries are needed! It is manufactured by hand by an Italian amateur and friend, Cristian Fattinnanzi, that is using it successfully for very wide field images. Even if I've never seen and used, by looking at the advices of Cristian I recommend this product only for less demanding tasks. E.g. the new balancing spring must be set for the camera weight so that the tracking can be smooth. Polar pointing is done with a small tube, so you cannot expect the accuracy of a mount provided with an optical polar finder.
	Kajers Astronomical camera drive Model 7700 I've inherited this all-brass mount by my friend Aldo Radrizzani, that used it in some travels to the Southern hemisphere in the '80s and '90s. This mount is so uncommon that only recently I discovered the name of the manufacturer, Kajers, because it is not written nowhere. I can only suppose it is a product of around 1980. If you have any information, please write me! The level of details is very high, at a level much higher respect to modern mounts: graduations are available on AR and DEC, and many labels help in the use. The tracking is given by a 45° sector of a geared wheel. Both the wheel and the worm gear are made of brass and with large diameters, much larger also respect many standard astronomical mounts for big telescopes. The motor is a synchronous AC motor, thus it needs a 220 V AC 50 Hz power supply, not very easy to be found on the field. But nowadays small and inexpensive 12V DC to 220V AC inverters are available. The 5x polar finder has a not illuminated cross reticle and a right angle mirror. The use is not easy also because the aperture is very small (diaphragmed to about 1 cm) and Polaris is very hard to be found. I found much easier to do a first alignment by pointing a green lases inside the finder eyepiece and then to look directly and fine tune (be careful with the laser, so that the light cannot harm anyone eye!). As with any other mount, I've found that a very solid tripod and base head is fundamental. Otherwise flexure will vanish any good polar alignment and tracking will be inaccurate. The tracking accuracy at first was not very good because some gears on the motor worm gear has been damaged by a shock. However I was able to move gears a little bit so that good gears are used. Now tracking is exceptionally good! From the test I've performed I cannot see any clear periodic error! If present it must be smaller that 10" pk-pk, a value that is not found even on 2000€ big astronomical mounts! That a great surprise. However from the test I've found an obvious problem: drift. This is mainly due to the inaccurate polar alignment, not possible to a better degree because the reticle in the polar finder is not graduated and Polaris cannot be offset from the celestial pole by the required 50'. Click for full res image Periodic error of the Kajers mount, obtained with a Starlight Xpress Lodestar ccd with a 100 mm focal length scope (23 mm diameter). Total load on the mount was about 100 g.

I hope to get more testing on these and other mounts in the future, and I'll update this page. Stay tuned!

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