The Sky-Watcher Az-GTi is an affordable, compact, lightweight, and fully computerized tracker for amateur observers and astrophotographers. And it was a game-changer in the panorama of entry-level and portable trackers.
But because the Az-GTI is built to a budget and the product quality control is variable at best, you may experience some minor difficulties when operating this mount, particularly in equatorial mode.
This article will help you tune your Az-GTI for the best performances.
Sky-Watcher Az-GTI: What Are The Most Common Issues?
The Az-GTI truly is a little marvel. But that does not mean there is no room for improvement, particularly when you do deep sky astrophotography with the Az-GTI.
Note: when you work in equatorial the Azimuthal movement is called Right Ascension, RA, and the Altitude is called Declination, DEC.
So, what are the most common issues reported by Az-GTI owners? They are two:
- Stiff Az / RA movement
- Severe backlash for deep sky astrophotography standards
If you still have warranty left on your Az-GTI, contact the seller to have it replaced or professionally repaired.
This maintenance guide is not supported by Sky-Watcher and you should proceed, under your own responsibility, only if you are comfortable with the procedures outlined here.
The author and the editor of NightSkyPix.com decline all responsibility for any damage to your Az-GTI resulting from following this maintenance guide.
What To Expect From These Fixes
Your Az-GTI will not be transformed into a pro-graded equatorial mount, but its performance and reliability may improve quite a bit.
After the tuning, I can consistently get very good unguided images with a Sky-Watcher Evoguide 50ED and ZWO ASI183MC.
The image scale from such a combination is a rather small 1.85”/px and, as you can see in the comparison below, there is not much difference in the stars’ size moving from 30s to 60s exposures.
Before tuning my mount, I had fairly common issues with guiding DEC in both directions and poor dithering performances.
After the tuning, I still have poor performances in DEC during calibration due to residual backlash in DEC. But after the initial settlement, guiding performances are very good for both axes and guiding in DEC in both directions is possible, as well as dithering in RA and DEC.
With my Celestron C5 reduced to 900mm is now a solid 75% on 120s exposures, only 15% less than my tuned NEQ5.
Check The Installed Firmware Version
Make sure you have installed the latest EQ firmware, version 3.37, or the older version 3.20. Everything in between will cause problems with third-party software.
Should you need help to update the firmware, you can refer to this article.
What Tools Do You Need To Tune And Maintain The Az-GTI?
Similar to the Star Adventurer PRO, maintaining the Az-GTI does not require particular tools and it is quite straightforward.
Here is a list of tools and materials you should have:
- A roll of paper towels
- A pair of gloves
- Degreaser Spray
- Multi-Purpose Synthetic (or Lithium) Grease
- Philips screwdriver PH0 x 60mm
- Small and large flat screwdrivers
- 1.5 Allen key
- Small pliers
How To Open The Az-GTI
The fixes proposed here require you to disassemble your mount, which, per se, should not invalidate the warranty as there are no seals to break.
The first step of any maintenance job consists in removing the plastic black cover:
- Disconnect the battery holder
- Unscrew and remove the clutch locking knob for the Alt/DEC movement
- With the Philips screwdriver, remove the 6 screws fixing the cover to the body of the mount. 2 screws are at the front of the mount, above the saddle and 4 are at the bottom
- Carefully lift the plastic cover and disconnect the two cables connecting the small circuit board on the cover to the motherboard on the mount.
Note: Not all screws are the same and you can refer to the image above to find which type goes where.
With the cover off, you can now see the interior of the mount.
The Stiff Azimuth / Right Ascension Movement
The Az-GTI has two clutches: one to engage the motor controlling the Az/RA movement and one to engage the motor controlling the Alt/DEC movement. The problematic movement is often the Az/RA one.
With the Az/RA clutch fully disengaged, you can manually rotate the mount and ideally, this rotation should be free.
A bit of resistance is normal but sometimes the movement gets really stiff for half of the rotation, while it is more or less free in the other half.
My Az-GTI was so bad that if mounted directly on the tripod, it would unscrew from the tripod rather than keep rotating in Az.
But having a stiff rotation does not mean your mount will not work properly, though: usually the motor has enough torque to keep tracking properly.
As they say, if ain’t broke, don’t fix it 🙂
There is a better, faster, and easier way for this than disassembling the mount.
The Best Way Of Balancing The Payload
If you are thinking the stiff rotation does not let you balance your gear, disassembling the mount to ease up the stiff rotation will not dramatically improve balancing.
In my experience, even when freeing up the az/RA rotation, the mount keeps opposing enough resistance that you still need to “feel” for the right balance.
A much faster and easy way to reliably balance the payload on the Az-GTI is simply balancing your gear on a table.
When done, mark the position of the counterweight on the shaft and where to clamp the dovetail in the saddle for future reference.
How To Fix The Az/RA Stiff Rotation
Time: 30 minutes
Tools: Philips Screwdriver, 1.5mm Allen key
Require Disassembly The Mount: Yes
Should the motor stall at the stiff point, you have no choice and must open your mount and ease up the stiff Az/RA rotation.
The Source Of The Problem And The Fix
The cause of the problem is the retaining nut for the Az/RA clutch assembly being over-tightened.
Couplings between parts in the clutch assembly may not be very precise and tight spots can stiffen the rotation when the retaining nut is overtightened.
The fix consists in loosening up this nut just enough so that the mount can rotate smoothly when the clutch is disengaged, having no play when the clutch is engaged.
Step 1: Remove the plastic cover from the Az-GTI
Refer to the section “How To Open The Az-GTI.”
Step 2: Disconnect And Remove The Motherboard And Its Support
Gently disconnect all the connectors you found on the motherboard. To do this, use your nails or a small flat screwdriver to disconnect the connectors rather than pulling them out from the cables.
To reconnect the cables in the proper order, refer to the photo above on the right.
The motherboard is mounted on a plastic support via the two black screws. Remove them to take out the motherboard.
Then remove the plastic support to reveal the Az/RA Auxiliary Encoder.
Step 3: Remove The Az/RA Auxiliary Encoder
The Auxiliary Encoder is a slotted disc used to improve the GoTo/Tracking accuracy.
To remove it, simply unscrew the four black Phillips screws at the center.
Step 4: Adjust The Retaining Nut
With the encoder removed, you can now access the Az/RA clutch retaining nut.
Undo the clutch and manually rotate the base of the Az-GTI to access the two grub screws on the side of the retaining nut.
With the 1.5mm hex key, unscrew the grub screws to free the nut. Fully engage the clutch and use two Allen keys to unscrew the retaining nut, as shown in the image below.
Warning: If you fully remove the nut, the base of the Az-GTI can fall off. Do this on a table or, better, with the Az-GTI on a stable tripod to easily check for unwanted plays.
Screw back the retaining nut by hand until you can, then, very gradually, thigh it a bit more using the two Allen keys. Then, lock it by tightening the two grub screws evenly.
Do all this with the clutch unlocked and keep checking how free the rotation feels.
Note: Losing the retaining nut may introduce a slight play between the body mount and the base. This is not an issue as long as this play goes away once the clutch is fully engaged.
As a rule of thumb, I prefer having some resistance rather than a very free rotation so as not to develop unwanted play in time with the use of the mount.
Step 5: Reassembly of The Mount
When you are happy with your tuning, reassemble the mount by following steps 1 to 3 in reverse order.
How To Remove The Backlash
Time: 30-60 minutes
Tools: Philips and flathead screwdrivers, pliers, 1.5mm hex key
Require Disassembly The Mount: Yes
If your mount is not tracking well despite good polar alignment and payload balance, if you have play in the mount when the clutches are properly engaged and/or you experience poor guiding performances, you may have backlash in the gears.
The Source Of The Problem And The Fix
To rotate the payload at the proper rate, the motor sets in motion a number of gears that have to be properly meshed together.
If the meshing is too loose, gears can play back and forth causing backlash. If too tight, the motor can stall and/or periodic error from gear eccentricity can reduce tracking and guiding performances.
Origin of backlash between gears. (Image Credit: en:User:GearHeads, User:Slashme on Wikipedia CC BY-SA 3.0)
How To Check For Backlash
The first step in fixing the problem is to find where the backlash occurs. In the Az-GTI, there are two possible sources of backlash:
- The coupling between worm and ring gears
- The gearbox
With the Az/RA clutch engaged, try to gently wiggle back and forth the mount while mounted on a sturdy tripod: if the mount wobbles, you most likely have backlash between the worm and the gear rings.
To check for backlash in Alt/DEC, engage the clutch and rock the mount saddle back and forth feeling for any play.
This type of backlash is often removed by tuning the spring-loaded mechanism that meshes the worm and the ring gears.
To check backlash in the gearboxes, you need to remove the plastic back cover from the mount body and rock back and forth the gears in the gearbox and look for a large play.
This type of backlash is reduced by tuning the motor position so as to manually mesh the gear in the gearbox.
Fix The Backlash Between Ring And Worm Gear
This fix is the same for both Alt/DEC and Az/RA axes.
The motor assembly includes the motor, the gearbox, and the worm gear. The assembly pivots at the large screw next to the gearbox and is kept meshed with the ring gear by a spring.
Spring loading the motor assembly ensures constant tight meshing despite gear eccentricity, improving on the design of the star adventurer where gear eccentricity combined with tight meshing can stall the motor.
But sometimes the whole assembly may get stuck after being pulled back under gear eccentricity, thus creating backlash.
To fine-tune the spring-loaded mechanism, remove the black plastic cover from the mount. Pull the motor assembly away from the ring gear and check it can spring back in position when released.
With the small flat screwdriver, hold in place the screw of the damping nut and, with a set of small pliers, gently unscrew the nut so that the motor assembly can freely move. Then, gradually tighten the nut to add some resistance to the movement of the motor assembly.
When you find the sweet spot, engage the clutch and check if the backlash is gone. It may take you some attempts before getting it right.
Fix The Backlash In The Gearbox
Warning: Manually meshing the gears too tightly may damage the motor and will reduce performances due to periodic error becoming dominant.
Note: Some backlash comes from the motor axle and can’t be fully removed.
Start by removing the black plastic cover from the mount then we need to remove the motor assembly.
Step 1: Remove The Motor Assembly
- Disconnect the cable from the motor
- Remove the pivot screw (A in previous image) with the large flat screwdriver
- Remove the screw holding the spring (B) with the Philips screwdriver
- Remove the damping nut (C) by unscrewing the screw with a small flathead screwdriver
Step 2: Remove The Gears In The Gearbox
By hand or with the head of the small flathead screwdriver, turn the gears of the gearbox until the grub screws are accessible.
With the 1.5mm hex key, remove the grub screws (2 per each gear) and the gear from their axles. Be careful not to strip the grubs’ heads.
This is the right time to check for play in the worm gear axle: grab the worm gear and try rocking it side to side: it should not move. If there is play, you have to remove the motor and tight the nuts visible in the image below.
With the gears off, you can access the three Philips screws holding the motor in place. By loosening those screws we are able to remove the motor or adjust its position.
The idea is to lose the screws and move the motor so that the two gears in the gearbox will mesh tightly. This is easier said than done as you need to test the meshing with the gear in place, but with the gear in place you cannot access the screws to adjust the motor position.
When you mount the gear back, mind that one of the two grub screws should bite on the flat part of the axle visible in the left image above.
A good mesh is such that there is not any large play between the gears, but you can spin the gears by hand relatively easily.
When you are happy with the meshing, and it will take you quite some time and trials, put some fresh grease on the gears before reassembling everything.
NOTE: At the pivot point for the mount assembly, you may note there are two washers: make sure you place one of those washers in between the motor assembly and the body of the mount.
Step 3: Reassembly of The Mount
When you are happy with your tuning, put back the mount cover.
As you can see, there is quite a lot we can do to improve the performance of our Az-GTI.
But one thing I didn’t describe was how to grease it. For this and more, have a look at this video, where I rebuild the mount from its base up, also discussing the working principles and some tuning.