Equipment Review (Preliminary)

Photography

Because of a phenomena known as "field rotation", Az-Alt mounts are in principle not suitable for imaging. However, if exposure times do not exceed 30 seconds and not too many frames taken, then it is possible to obtain remarkable results with the AZ-GTi. After only a 1-star alignment, the image below has been taken with a DSLR through a Ø71mm/450mm APO, a stack of 18 frames each 15 seconds exposed at ISO 1600 alongside a nearby bright waning moon on October 18, 2019. No filters, no flats, no darks, no tricks.

The result is quite rewarding, considering that a more precise alignment would allow to increase exposure time and frame count, especially when using smaller APOs, such as 250mm to 420mm focal length. Longer focal length demands higher tracking accuracy, but the effect of field rotation is smaller because of the narrower field of view.

 

After a 'North-Level' alignment, these images of the Andromeda Galaxy and the Pleiades were taken through a 360mm focal length APO (with 1x flattener) consisting of 45 lights and 25 darks, 20 seconds each at ISO 3200.




Since star clusters do not require that many exposures, this image of the Double Cluster comprises 25 lights and 25 darks, 20 seconds each at ISO 3200.



All image stacks showed a notable amount of field rotation, but no issue for the 'Deepsky Stacker' software.

 

This excerpt shows a moving violet dead DSLR sensor pixel. The track of this single pixel is a result of image stacking and demonstrates the direction of tracking errors in mounts, which is very little in this image taken after merely a 1-star alignment. Dead pixels can be removed by adding dark frames when stacking. The brightest star at the right is 'Maia' in the Pleiades.

Physical Considerations

The Sky-Watcher AZ-GTi is specified for a payload of 5kg maximum. As a rule of thumb, no more than 80% of this capacity should be burdened. We obtain a reasonable payload of 3kg (safe limit) which includes Ø60-80mm refractor and Maktsutov OTAs with short tubes. The AZ-GTi with its GoTo function is specifically designed for visual, not for imaging, although many managed to take surprisingly good stacks of images with exposure times up to 30 seconds each. Sky-Watcher is so fair as to expressly exclude the mount for imaging. The mount can of course be used for timelapse photography. A sturdy mount with a standard photo 3/8 or 1/4 screw will contribute to enjoyable observation events. Note, however that a small APO with a DSLR may hit the tripod legs when the mount is slewing to high altitude objects. For this reason, Sky-Watcher is offering an optional tripod with a 217mm tall pillar.

Control

The mount's most prominent feature is a built-in WiFi module establishing its own independent WiFi network with a smartphone, tablet or desktop with free applications provided for download by Sky-Watcher. The control softwares takes care of critical location dependent settings and includes nearly all utility functions that a hand control unit for a large mount provides. Alternatively, being a Sky-Watcher brand, an optional wired SynScan hand control, version 3 or 4, can be connected to the mount's 6-pin RJ12 socket (ASCOM compliant). Well thought-out!

Remarkable

For increased observation convenience the AZ-GTi employs Sky-Watcher's Freedom-Find™ Dual-Encoders, which allows the telescope to be moved manually in either axis without the mount losing its positional information. In other words, when releasing the axis clutches and turning the scope away from a just slewed object, the primary encoder retains the last slewed position while a secondary (auxiliary) encoder records the manually caused displacement. To return to the object, simply press GoTo again. This method eliminates need for re-alignment. The auxiliary encoder needs to be turned on in the app. However, hitting the tripod entails an illegal motion which the encoders cannot compensate for. Also remarkable for its class, the control app allows backslash setting.

NOTE: The typical pointing accuracy of the mount is about ±10 arc minutes which is more than good for centering an object in a low-power eyepiece. The engaged auxiliary encoder actually erodes the pointing accuracy to about ±20 arc minutes. Yet it is a fair trade, especially when observing in a group with children.

In Operation

The AZ-GTi has been tested with a Ø60mm/360mm APO (1.5kg) and a Ø71mm/450mm APO (3.2kg). Both OTAs including 2-inch diagonal and eyepiece (together 850gr) are handled with ease. While tracking in both axes, the DC servo-motors are absolutely silent. Slewing to objects at the highest rate of 800 times sidereal naturally produces noise, but nothing distracting.

Users experienced with Celestron NexStar™ and Sky-Watcher SynScan™ hand controls will find the basically knowledge-free operation extremely intuitive and appreciate the same large database of objects. The control app allows various methods of mount alignment. The more 'alignment stars' involved the hight the GoTo accuracy. With the quickest '1-star alignment' the accuracy for finding bright objects is sufficiently accurate. 'North-Level Alignment' requires that the mount is roughly levelled and pointing north (RA clutch location). It is important to center alignment stars well because all moves after alignment will depend on the star centering accuracy.

Once aligned you simply select an object of interest to slew to. If the object should not be placed exactly in the center of the eyepiece view, the app offers you to manually center and sync on it. This helps the mount update its position, as a mechanical system tends to gradually loose position while suffering from backslash.

Dual Mode

The AZ-GTi is perfectly suited for visual work with a small telescope which is tracked to keep an object in the center of the eyepiece view. Minor tracking errors are largely neglectable. The wedge and ball head adapter from the Sky-Watcher StarAdventurer Pro Pack can be used for the AZ-GTi while, unfortunately, the counter weight shaft does not fit. The AZ-GTi needs a shaft with an M12 screw. Unlike an equatorial mount (or the AZ-GTi in EQ mode), an Alt-Az mount eliminates need for a counter weight. Sky-Watcher also offers an informal firmware version which adds control of equatorial mode when the AZ-GTi is placed on a wedge. You can always roll back the authorized firmware version. Equatorial mode requires the SynScan PRO app version.

Other Applications

Both, the SA and AZ-GTi mounts are ideal for increasingly popular EAA*. With either mount, a small telescope, a sensitive CMOS camera in place of an eyepiece, a computer, and finally with the popular "SharpCap" software with its impressive "Live Stack" feature, a system for EAA is easily and quickly set up at reasonable expenses.

 

* EAA: Electronically Assisted Astronomy, basically group or public viewing.

Positive

• Payload of 5kg good for small APOs and Maks
• Well thought-out, versatile sturdy concept in only 1.3kg mass
• Accurate Sidereal, Lunar, Solar Tracking
• A telescope can be moved in both axes without the mount loosing sync
• Fully fledged GoTo functionality
• Most satisfactory GoTo accuracy
• Control via WiFi and App or optional wired hand control
• Long battery life, up to 12 hours of continious tracking
• Intuitive firmware updating

Negative

• Some mechanical play in both axes
• Poor battery compartment construction
• Insufficient documentation (minor issue)

 

Summary

Definitely worth a consideration for mobile observation activities with lowest possible gear count. Attach to a sturdy tripod, insert batteries, fix the telescope and use a smartphone for easy 1-2-3 control of all built-in features. There are good reasons for why this mount is one of the most popular among portable GoTo mounts in its class and price domain.