In praise of encoders:
Review of the SkyWatcher EQ8-Rh
In late 2019, I took an opportunity to upgrade the SkyWatcher EQ8 equatorial mount in my dome (see full disclosure below). In my case, I chose the EQ8-Rh option with high-resolution optical encoders on the R.A. axis, rather than the new ‘vanilla’ EQ8-R.
Despite my astrophotography experience, I’m not an expert in the construction, mechanics or operation of equatorial mounts. But let me start with my take on the changes in the the base version EQ8-R compared to the first generation of this mount before discussing the remarkable encoders in the Rh version.
Built-in cable management
This is the most significant upgrade with the EQ8-R and a notable change to user experience. Cable snags on the EQ8 were one of several issues I faced in my automated dome. In practice it wasn’t hard to fix once I identified the issue, but anytime I wanted to change equipment or replace a suspect cable it was a tedious process of unwrapping the cable from the bundled loom and retying everything together afterwards. Having built-in cable management makes life a heck of a lot easier. Here’s what is included:
- USB 3.0 hub (4 ports)
- 2.1mm DC power (3 ports)
- Serial (3 ports)
Having first heard about this change as ‘cable management’, I initially expected a mount where the user could run cables through the internals of the mount (Paramount ME style) but in fact the mount has its own USB, power and serial cabling and the user connects power and one USB lead to the hub of the mount, and then individual hardware to the ports on the saddle of the mount.
I was concerned that I would find a device that would not be compatible with the provided port options but in practice it has worked flawlessly. My QHY367C is USB 3.0 (for power and transfer of large files) and operated with no issues through the mount hub.
I’ve never tried nor seen it myself, but I understand the process of pulling a Paramount apart to push and pull a new cable through the mount can be fairly tedious. As long as the available ports are compatible with your equipment, the EQ8-R approach is very convenient. To improve my setup, I now need to replace the long 3m or 5m cables with short cables as I currently have a lot of unncessary length from the old cables all bundled up beside the scope.
The mounting of the EQ8-R motors has been improved to create a better balanced mount and one that looks more symmetrical and better designed, although perhaps this never mattered much if you were carrying large loads. Ideally this would have been achieved in the original version but is a welcome improvement.
Belt Drive & Backlash
The new mounts feature belt drives on both the RA and Dec axis. It is claimed that this achieves “smoother guiding and reduced backlash” but I’m going to question this claim. The backlash that you could feel in the EQ8 due to meshing between the worm (screw) and the worm gear (worm wheel) is still present in the EQ8-R. This is likely the source of most backlash in these mounts and I’m not sure that changing from spur gears to belt drive from the motors really changes that much (although it is quieter).
Some Astrophysics mounts for example (among others) include spring-loaded worms which the EQ8-R does not have. I have not noticed any issues with guiding these mounts, either on the original EQ8 or EQ8-R, but neither have I had experience with spring-loaded worms to see or feel the benefits they provide. Regardless, these SkyWatcher mounts are widely used and I know that many others successfully guide these mounts with heavy rigs and long focal lengths.
The EQ8-R mounts feature built-in homing sensors which allow you to set a repeatable ‘start’ position and recover the park position. However this feature cannot currently be accessed via the EQMOD control application. It can be activated from the hand-controller (which would rarely be connected in most observatories) or by opening a simple third-party application called “EQ8 Tools” which requires disconnecting EQMOD or other mount control applications first. There is a relatively new alternative to EQMOD called Green Swamp Server which apparently can access the homing function directly and other capabilities of these Synta mounts, although it does not currently have modelling (pointing error correction) capability.
Improved Clutches & Saddle Locking Knobs
The EQ8-R has larger clutch handles on the mount and larger locking knobs for attaching the telescope to the mount saddle. In both cases these are welcome improvements to small but meaningful deficiencies in the original EQ8. In my mind, the new locking knobs have stilled erred a little on the side of aesthetic design rather than going for chunky knurled knobs that you can really wrap your fingers around in the sometimes tight space between the mount and the scope. The clutches themselves are claimed to be improved as well.
PC Direct Connect
The mount can now be connected directly via standard USB cables rather than needing to purchase “EQ Direct” adapter/interface cables for this mount. Another welcome and also perhaps overdue improvement.
In praise of encoders
The big addition to the new mount is the option of having a high-resolution encoder on the R.A. axis. This is the EQ8-Rh mount, where the ‘h’ stands for ‘high-resolution’, but the addition of that one small letter adds close to AUD 4,000 to the purchase price of the mount. I had little prior knowledge of these encoders but I have been blown away by what they deliver in practice.
While switching mounts, I was able to conduct the test above comparing the two. The timespan for the plot is nearly ten minutes, and guiding was turned off after around 2.5 minutes. Polar alignment on the original EQ8 hadn’t been adjusted in quite some time resulting in significant declination drift, but the more obvious feature is the periodic error in R.A. of around 15″ peak-to-peak.
With the freshly installed EQ8-Rh, I achieved good polar alignment with no measurable drift (at least for this part of the sky). But most remarkably, not only was there zero measurable periodic error, there did not appear to be any benefit gained by guiding in Right Ascension at all. It boggles my mind that the R.A. axis could be encoded with markings so tiny that the mount could monitor and adjust its own tracking to sub-arc second level, below the threshold of seeing conditions. This will not be news to users of other premium mounts with high-resolution encoders, but was certainly a surprise for me to see in practice.
The encoders on the original EQ8 were a significant weakness, with advice from EQMOD users generally to disable them as they caused more issues than they solved (which was my experience also). I presume but cannot confirm that the standard low-res ‘Freedom-Find’ encoders on the base level EQ8-R have solved these issues, but these are really only relevant for manual use where you want to release the clutches and manually slew the scope without losing sense of position, which is not how most people will use these mounts.
The high-res encoder on the EQ8-Rh is something else entirely. On the Sidereal Trading website, they are listed as “on-axis Renishaw TONic incremental, 11.8 million tick, encoder on the RA axis. With the encoder built onto the RA axis this removes periodic error to nearly zero in RA with an accuracy of 0.11 arc-seconds”. You can find further info about these TONIC relative optical encoders on the Renishaw website also.
I conducted another test at a later time with Maxim DL with similar results. On this occasion, I stopped guiding in Right Ascension but continued guiding in Declination (Y direction) to correct any drift. Again there was no change in measured errors in X-direction after turning guiding off. This could become a standard way of operating this mount, guiding in one axis only or at least dialling the aggressiveness on R.A. guiding down to minimum. This was on a reasonably good night, with guiding RMS errors around 0.4″ with and without guiding. My intepretation is that the monitoring via the high-res encoders must be close to the claimed 0.11″ level for there to be no perceptible increase in RMS error when switching the guiding off.
Only having used short focal length refractors in my observatory to date, I don’t have the equipment to really challenge the mount and see what it can deliver (a 12″ Newt is on the way which should push it a little harder). The image below was captured early in 2020 while testing the Canon EOS Ra camera. As the camera did not have an SDK released, I could not automate the observatory operation with it.
So while not intended as a mount test, this was captured with no guiding (and also no periodic auto-focus, so the image is a little soft). 530mm focal length is not particularly taxing, but 5 minute unguided exposures with no trailing evident is still an achievement. And every equipment review needs at least one pretty picture (click through for high-res on SmugMug).
The high-resolution encoder provides some interesting new capabilities. I spent many tedious hours measuring and correcting for movement between frames in my 2017 solar eclipse sequence. Now, as long as I have polar alignment well sorted, solar and lunar eclipse images could be tracked without needing subsequent alignment during post-processing. Similarly for comets, the mount could be set to track the comet and a sequence of images could be stacked without further adjustment.
While I don’t have any experience with high-res planetary imaging, I can only assume that having sub-arc second tracking ‘out-of-the-box’ (no PEC training required), while the frame is filled with a fuzzy and dancing planetary disc at very high effective focal lengths, would have to be an advantage, if only for the convenience of having the highly magnified image stay tightly centred in a small field-of-view while capturing a video sequence.
Automated and Remote Observatories
I have been operating the EQ8 and now this newer mount fully-automated via ACP in my Sirius observatory dome since 2016. I have not invested sufficient time in the overall system to really get it singing, but aside from the cable snags which were easily solved (and a lazy user issue more than any fault of the earlier EQ8) I have had few issues with the original EQ8 mount itself. Most of the issues I have faced have been with a highly respected telescope brand that was sitting on top of the mount, or with the automation and control software and my Windows PC. This has on a couple of occasions led to the mount tracking round into the pier but with no damage to scope or mount that I’m aware of.
While I have the advantage of being able to walk out to the dome to rectify issues, in practice I am not out there often at all. As long as I had a webcam to ‘see’ where it is positioned in the event of a PC crash etc., then I would be comfortable operating a fully remote observatory with this mount. The webcam would be enough to manually slew back to a rough centre and then use the homing sensors to reset to the park position. I don’t have experience with premium mounts 2-4x the cost to know what I might be missing out on though, and I don’t expect I ever will!
The new SkyWatcher EQ8-R corrects some deficiencies with the original EQ8 and adds capability to the mount, particularly through the built-in cable management. In Australian dollars, the price has stepped up from the $5500 I paid in 2015, to around $8,000 now or $7,000 (USD 4000) for the equatorial head with no pier/tripod. (There may be some currency variation embedded in that change).
Adding the high-resolution encoders increases the price of the EQ8-Rh by 40% to around $11,000 (USD 7100). You’ll need to make your own assessment of whether that capability will translate to better outcomes for your intended use of the mount – autoguiding without the encoder may be perfectly adequate for most. It’s a big step up from here to other mounts with similar payloads and high-resolution encoders. I doubt there are many unhappy owners of an AP 1100GTO mount, which has a similar rated payload, option for high-resolution encoders on both axes and spring-loaded worms to reduce mechanical backlash (which the encoders claim to eliminate in practice), but it comes at nearly double the cost.
The SkyWatcher EQ8-R remains an attractive value proposition for those looking for an equatorial mount with large carrying capacity. The high-resolution encoder on the R.A. axis is remarkable and lives up to its claims, but the high-tech wizardry is also rather expensive. I hope this review may help you decide which mount is right for you and your budget.