Hobym Crux-140 Initial Review

[Dan-el]

Edit: Jul 11, 2021. Added testing results for unguided performance.

Hello everyone,

This will be my first review on this site. I don't usually review anything as I consider myself to be relatively new to the hobby. Making an exception here as there is very little information out there about this mount and what is available is mostly related to a defective run back in 2020. Hopefully this adds to the conversation and helps out the next person who is looking. The same post is also over at CN forums.

 

First of all, a disclaimer: I am in no way affiliated with Hobym or any of the sales parties involved. This mount is purchased with my own money after doing my own research because I thought it fits my needs. I do want to thank Stephen over at Telescopes Canada for being patient with answering my questions and for sending the unit in stock back to Korea to be inspected for fault.

 

This review will go through from background specification to my preliminary testing and imaging. Sometimes I talk a lot so if you would like to skip straight to guiding test in part 4 it wouldn't hurt my feelings at all

 

1 - Introduction

Up to now I have been using a belt modded HEQ-5. It served me very well and has been nothing but reliable. I'm at a stage in my life though where space is a premium and the quicker/easier I can transport my kit the more I will be out imaging. The Crux-140 has quite a price tag but is also quite interesting/unique in design. It is comparable to the Rainbow Astro RST-135 which also uses strain wave gearing (aka harmonic drive) to achieve substantial torque for its size and good backlash performance. This mount is supposed to be capable of 8kg loads without counterweight and 15kg with, that is very impressive given its diminutive size.

 

My parcel arrived well packaged. All the items are in a soft travel case with plenty of foam lining. Basic items include the mount itself, an adapter to dual-mount a camera in place of the counterweight, TiTaN TCS controller module, mounting base for a polemaster, all necessary cables, allen wrench, and a USB memory card with firmware and drivers. I opted for the 2kg counterweight and shaft. If you choose you could also get a camera ball mount and polemaster.

 

The fit and finish is excellent. The unit itself is a powder coated blue. All aluminum plates, knobs are nicely anodized silver and black. Screws and knobs are smooth to use and there is no noticeable wobble anywhere. RA/Dec axis are tight, my imaging train did not cause it to rotate when powered off and I am unable to rotate them in any way with reasonable applied force, I did not Hulk on it. It fits onto a tripod using a standard 3/8 UNF, not much hassle there. One thing to know is that it does not come with a saddle. The mounting spacing is 35mm with provision for M8 or M6 screws. I have a WO saddle on there but 35mm seems to be pretty common so there are plenty of very well machined ones available on eBay if you are so inclined to wait a bit longer. I for one... couldn't.

 

Here it is mounted up on my Report 112 Astro. The mount itself weighs 3.5kg, wigh a 2kg counterweight. The entire unit weighs 7.5kg all inclusive and folds down to 32" for transport.

 

 

Here it is side by side with my HEQ-5

 

 

2 - Connectivity and software

The mount is controlled through its TiTaN TCS. The cable connecting the controller to the mount is a quick detach RS232 however I do not believe you could bypass the controller for computer control. The controller has provision for ST4 cables, USB, and with the most recent firmware update both bluetooth and wifi. The firmware update instructions said to use the RS232 cable for firmware upgrades, my laptop happens to still have one so that worked out well. The manual does NOT state that you could do this over USB. An adapter may be needed as serial ports are increasingly uncommon on current computers.

 

I had no issues with connectivity, firmware took about a minute but the UI update took nearly an hour. Make sure you have plenty of battery left or are plugged in. Driver installation is pretty straight forward. Everything is ASCOM compatible and NINA had no trouble at all recognizing and controlling the mount.

 

Packed up and ready to travel. Again side by side with packed HEQ-5

 

3 - Features

This mount could be used in both EQ and Alt-Az modes. The mount itself has a threaded screw hole to lock it in the alt-az position. There is a modest object database containing all the bright stars, Messier, NGC, and IC catalogues. It is not as extensive as most mounts would boast nowadays but it covers everything you would be able to see visually, beyond that you likely have computer control anyway.

 

There is no provision for polar alignment if you did not spring for the polemaster and this will be a problem for some. I have enough imaging equipment that I use my guide camera and sharpcap long before I owned this mount but if you are exclusively a visual observer you are stuck. I have cheated in the past by asking the mount to slew to a known star and aligning my mount using the polar axis adjustments initially with acceptable results but that requires at least one precise polar alignment to have any chance of working. Consider springing for the polemaster or have some alternate polar alignment method. Alignment in Alt-Az is done through its equivalent of a Level-North (except you tell it which direction it's pointing). It works acceptably, nothing special. Then again I've never had much luck getting that type of alignment to be as accurate as I like.

 

Some of you will be concerned regarding smashing your scope into the tripod and the motor having enough torque (with no clutch) to ruin everything. This can happen. The first thing I did was to mount up my usual equipment and move it through the range and noted the limits. I then set the safety limiters to avoid having a pile of bent OTA and shattered glass. The slewing rate control is done through an analogue knob on the top left of the controller. There is also a distinct click toward the off position which would also trigger the mount to park itself. Parking position can be user defined once the initial sync is complete.

 

There is also the usual as far as inputting observing sites, time, date, so on. You could save a number of observing sites to be accessed later. An important feature is periodic error correction and training, I will get to that later.

 

4 - Testing

Mount testing is done with my usual imaging rig as follows:

Sharpstar 76EDPH + reducer

ZWO EFW Mini

ZWO ASI294MM

William Optics 200mm guide scope

ZWO ASI290MM guide camera

Everything included weighs 6.8kg

 

The seeing conditions that night was poor, a storm had just rolled through. I live at the foothills of the Rockies so this is not unusual at all.

 

Polar alignment is done with SharpCap. Double checked with rotation back to home. It bounced within a 15-20 arc second range that night due to seeing but still considered excellent otherwise.

 

 

Test 1: Unguided, no periodic error correction

 

Something went wrong immediately. I have never had this error before and my immediate thought was the backlash fault had crept into my current unit as well. Polar alignment was verified and it was not the problem.

I did not spend too much more time on unguided without PEC. It showed a low frequency oscillation over the course of roughly 5 minutes but superimposed is a mid-frequency oscillation with a period of roughly 10 -15 seconds.

 

Test 2: Guided, without PEC

 

 

I tried to guide anyway, just to see how it looks. Note that my PHD2 settings are carried over from my HEQ-5 and only tweaked slightly at this point. When I let it run for long enough the long period oscillation expected of strain wave drive becomes very evident. The higher frequency oscillations are still very clearly visible here.

 

Test 3: PEC trained single star guiding

 

Here we have some good news, after PEC training and enabling the feature the orthogonality error is gone. I brliefly played around with PHD2 settings a bit as well and it appears that it favours a more aggressive, higher frequency correction. Please note that even at the old settings I did not see the same oscillation that I saw previously. The more aggressive guiding pulses just helped get the RMS error down a bit more.

 

Test 4: PEC enabled, PHD2 multi-star guiding

I wanted to be fair to the mount. 0.66-0.8 is pretty average for the seeing that I get here with the HEQ-5 as well. Note that given my guide ratio this amounts to less than 1/4 of a pixel error but I wanted more.

 

For those who aren't aware yet, the multi-star guiding in PHD2 really is a revolution. Here is my guiding when I switched it on.

 

I can probably get it to look better if I really dialed in the settings. For the moment though this is more than what I need for my image scale with a healthy margin for when I start messing about with my mak. I will save those precious clear weather for imaging for now.

 

Test 5: Unguided, with PEC

 

Seeing tonight is nothing short of terrible. Very gusty winds and unsteady seeing. The overall function and setup is verified by guided performance which was 0.9" tonight, acceptable given that this is not even weather I'd normally image in. This should also be evident by the quite good Dec performance.

The graph says everything I think. Unguided tracking is unacceptable for imaging. There is barely noticeable actual star movement in 1.7x1.0 arc minute FOV in the 5 minute test run so it should be fine for visual use.

It was also tested without counterweight. With my imaging rig coming in at a hair less than 7kg it showed no difference in guiding performance.

 

Conclusions

Overall I am very happy with the Hobym Crux-140 for my intended use and circumstances. It lives up to its claim as a compact, light weight mount that punches way above its weight class. I do not intend to push the mount to its limit, that is why my payload is still roughly 50% the stated capacity per established dogma. I would be quite comfortable pushing it to around the 10kg mark and expect it to hold up just fine. Beyond that the performance will likely degrade with more wear and tear on the components.

 

My big surprise is how poorly it performed without PEC. If anyone more familiar with the engineering aspects of these could chime in I would appreciate it. Practically it doesn't matter so much since I don't see a reason to leave it disabled. I am just curious as to what I'm seeing here.

Edit: I dug up some engineering reports from a few decades ago which discussed exactly the type of behaviour. I also noted on Rainbow Astro's page that the reason they have not implemented PEC is because of the periodic error is variable with different loads and orientation. I wonder if this means it will do better if I re-train PEC with each target I'm aiming at. It MAY further improve guiding performance however if this is necessary it would be practically useless for unguided tracking anyway since.... if you already brought the equipment for PEC training you might as well just use it to guide.

 

I would not get this if you only intend to do visual observations. In my mind it lacks the very important feature of polar alignment. If you intend to use a polemaster or some other set up though it will work fine, just a few more things to manage that you otherwise wouldn't have needed to.

 

In the end I see it as a very niche product. It is a premium mount that provides a very unique combination of traits in payload capacity, precision, portability, interesting engineering (which I personally enjoy) but you have to pay for that combination and not everyone needs all of those characteristics together. Something like my HEQ-5 is equally capable in every way I can see at 50% the cost. The only thing you sacrifice is weight and bulk. Would I recommend this mount? On the merits of the mount itself I absolutely would. Whether you think what you get justifies the cost though, that is up to you to decide.

 

Hope this helps a few people who might be looking. If there are any questions let me know I'll try to help.

 

In the mean time this is my first image with the setup. Go easy on me...

 

Edited July 11, 2021 by Dan-el

[Highburymark]

Excellent report on a mount that’s had little coverage on SGL. I’ve got the Rainbow RST-135 and so far am also very happy with it. Agree with you about not wanting to push the payload too far. I currently run mine with a 7kg scopes - might go up to 8-9kg in future but not more. Also agree that traditional mounts are the way to go if size and weight aren’t an issue. These mounts are all about compactness and incredible performance - and not having to be too precise with balance.

[Deadlake]

23 hours ago, Highburymark said:

Excellent report on a mount that’s had little coverage on SGL. I’ve got the Rainbow RST-135 and so far am also very happy with it. Agree with you about not wanting to push the payload too far. I currently run mine with a 7kg scopes - might go up to 8-9kg in future but not more. Also agree that traditional mounts are the way to go if size and weight aren’t an issue. These mounts are all about compactness and incredible performance - and not having to be too precise with balance.

How do you go about aligning either of these two mounts if you want to use in visual in ALT-Z mode?

Seems a little overkill to have to use a pole master or sharp cap.

In fact I'm not sure why its not built tin to the device??? 

[Highburymark]

19 hours ago, Deadlake said:

How do you go about aligning either of these two mounts if you want to use in visual in ALT-Z mode?

Seems a little overkill to have to use a pole master or sharp cap.

In fact I'm not sure why its not built tin to the device??? 

I can only speak for the RST-135 - It takes a little while to familiarise yourself with the controller, but alignment in alt az is very easy. I park the mount in a south-facing position, and it’s a 2-3 minute job aligning on a target. Most of the time so far I’ve been using it for solar. Without any other alignment objects in the sky it’s usually fairly accurate with the first go-to - then just centre, confirm, and it tracks for about 10 minutes before the solar disc begins to move out of view, then just repeat the process and it’ll track for hours. 
Don’t want to veer too much from the OP’s report though. Would be interesting to know how the Crux performs in alt-az. The hope is that the idiosyncrasies of these mounts have now been ironed out and they prove reliable

[Dan-el]

Hi all,

 

Sorry it took a while. The second COVID shot had me floored.

Did some unguided testing tonight in terrible weather. Short version: it is effectively unusable for imaging unguided. The guide curve went off the graph at 16 arcsecond scale. I will update the original post after I sleep.

 

Best,

Dan

[Dan-el]

On 04/07/2021 at 12:10, Highburymark said:

Excellent report on a mount that’s had little coverage on SGL. I’ve got the Rainbow RST-135 and so far am also very happy with it. Agree with you about not wanting to push the payload too far. I currently run mine with a 7kg scopes - might go up to 8-9kg in future but not more. Also agree that traditional mounts are the way to go if size and weight aren’t an issue. These mounts are all about compactness and incredible performance - and not having to be too precise with balance.

Thank you for sharing your experience.

 

Out of curiosity how does the RST compare when it comes to unguided performance or guiding without PEC. I think the RST has encoders with the Crux does not. As you can see from my testing the Crux is limited to visual use without guiding as the error really appears to be very significant. I do understand this is a quirk of strainwave drives. I've even found engineering assessment from 40 years ago which describe this type of long and short period error and their source; I'm very curious how much the encoders help though.

[Dan-el]

On 05/07/2021 at 11:17, Deadlake said:

How do you go about aligning either of these two mounts if you want to use in visual in ALT-Z mode?

Seems a little overkill to have to use a pole master or sharp cap.

In fact I'm not sure why its not built tin to the device??? 

Alignment in alt-az is done exactly the same as an RST. You level it as best as you can (tripod, mount, scope). Using a compass point your scope at one of the cardinal directions and then tell the handset what orientation it's pointed at. Slew to a star, adjust until it's centered, push "sync". That has gotten very good alignment for me when it comes to re-slewing, the new targets are nearly at the center of my FOV and you can re-sync if you choose to with the new target.

 

The difficult part is definitely with EQ. Currently I have my ASI290 camera mounted up with a cheap Svbony 120mm guide scope that I carry around along with my mini-PC. I thread the camera onto the 1/4 UNC bolt that I put on there in place of the polemaster mounting and align with sharpcap. I currently have it set up such that I have 1 bag for "unguided observing" - It includes the TiTaN TCS, filter wheel/filters, flip mirror, eye pieces, and I toss the polar kit in there. I have an extra bag that holds spare batteries, 200mm guide scope, primary imaging camera/spacers, etc. I still carry the same number of bags anyway.

 

 

[Highburymark]

4 hours ago, Dan-el said:

Thank you for sharing your experience.

 

Out of curiosity how does the RST compare when it comes to unguided performance or guiding without PEC. I think the RST has encoders with the Crux does not. As you can see from my testing the Crux is limited to visual use without guiding as the error really appears to be very significant. I do understand this is a quirk of strainwave drives. I've even found engineering assessment from 40 years ago which describe this type of long and short period error and their source; I'm very curious how much the encoders help though.

Actually I haven’t used it in eq - I use it in alt az for visual and I’m learning (slowly) about solar imaging and only need alt az for that, so haven’t experienced the technical issues that affect imaging. There’s a long thread on CN about this, most of it above my head as a rookie imager…..

[Dan-el]

4 hours ago, Highburymark said:

Actually I haven’t used it in eq - I use it in alt az for visual and I’m learning (slowly) about solar imaging and only need alt az for that, so haven’t experienced the technical issues that affect imaging. There’s a long thread on CN about this, most of it above my head as a rookie imager…..

Understood.

 

I hopped over and found the thread. Looks like the RST-135 shares a lot of the same problems as the CRUX, however with PEC enabled the CRUX is capable of similar guiding performance as the RST-135E (the version with encoders) while costing much less. Unguided performance is still unacceptable for imaging but seems like a fair cost-performance tradeoff.