2247 replies to this topic

[Astrojedi]

The RST-135 is a new mount from a Korean company called Rainbow Astro which seems to be a subsidiary/ sister concern of Rainbow Robotics.

 

Rainbow Robotics is known for their endeavors into Robotics and more specifically their HUBO2 robot. So these folks seems to have a background in robotics which is quite relevant here.

 

The RST-135 is a different kind of mount to most you or I have used in the past. It uses Strain Wave Gears which are a type of Harmonic gear. This video explains the functioning pretty well: https://www.youtube....h?v=bzRh672peNk. These gears are used extensively in industrial and robotics applications and are incredibly robust as they are designed for 24/7 365 day operation. With the company's background in robotics I think this was probably a natural transition for these guys.

 

This is not the company's first mount. They have a whole line of mounts but started using harmonic drives in their last mount the RST-150. That mount was a bit more expensive as it used (more expensive) drives from a Japanese company. For the newer mount they have managed to source a less pricy drive which was brought the price of the mount down quite a bit.

 

So, what is special about this mount? Well quite a bit due to the harmonic drive used in the mount.

 

Harmonic drives are incredibly strong so they can be really small yet drive very large loads. You also don't need counterweights and there is almost no backlash. This yields a small lean mean machine when it comes to making a mount.

 

The one downside is that the drive has meaningful periodic error. But it is slow and very easy to guide out. But it also means you are limited in the length of the unguided exposures that you can use.

 

Here is the value proposition as I see it:

• Only 7.3lb
• 30 to 40lb payload capacity
• No counterweights required up to 30lb - saves dead weight and eliminates another of my pet peeves...
• No need to balance (Absolutely love this - I can add remove accessories or change OTA without worrying about counterweights. Balance issues in AP are one of my pet peeves. For visual in Alt Az this is ideal as well)
• Can be used in Alt Az and EQ mode
• Even if your polar alignment is slightly off it can compensate
• Little or no backlash - this is under appreciated but is a critical factor for guiding
• With guiding total rms error can as low as 0.40” which is very impressive
• Unguided - lets do some math. I saw the manufacturer state some where that the PE was about 20” in 420 or 430 seconds. Assuming it is 20” peak (not peak to trough) in 400s then with a sampling of 2” you should be able to get ~40s unguided exposures. If peak to trough then double that... approx 80s.

If I think about it, this is very close to the definition of my ideal imaging mount.

 

In this running thread / review I post my evaluation of this mount. Goal is to tease out max capacity, tracking accuracy, ease of setup / polar alignment etc. Here is what I will cover:

• Unboxing & accessories / cables etc.
• Ease of Setup & Polar alignment
• Guided and unguided imaging:
  • Widefield: using AT60 and the RedCat 51
  • Long FL imaging: using EdgeHD 8 and EdgeHD 9.25 scopes
  • Tease out maximum real world load capacity for imaging
• Visual
  • Will evaluate it in EQ and Alt-Az modes
  • Go to accuracy using 1, 2, 3, 4, 5  point alignment models
  • Max weight for visual e.g. can I push it to a C11 (~27lb OTA / 30lb with diagonal and EP) with counterweights? - this will likely come later as I don't have a C11 on hand
  • Tracking at higher magnifications (this is somewhat more dependent on the tripod  but I have sometimes seen oscillations due to motors as well but I don't expect that from this mount)

 

So lets begin with the unboxing pictures... 

Edited by Astrojedi, 01 July 2019 - 01:11 PM.

[Astrojedi]

Unboxing...

 

Here is the shipping box from Tolga. This thing is small. When I picked it up from UPS I thought I got the wrong box. The UPS employee was confused by the expression on my face. Usually when I receive mounts I expect to pull a muscle or two in my back moving it to the car...

 

More boxes. Inside is a even smaller box with stickers. I really liked this touch.

 

Comes with a nice case. Next to the ASIAir for scale. I could lose this mount in my astro stuff if I not careful!

[Astrojedi]

Supplied accessories

 

The mount comes with a hand controller and some accessories:

 

A tripod adapter

Polemaster adapter

Allen wrench to adjust latitude

USB Cable to connect to a PC

DC power cable

Cable for the hand controller

Screws to attach a dovetail to the Dec Head

 

The DC cable has alligator clips on the battery end which was completely useless for me as all my battery packs have 2x5mm DC jacks. Luckily over the years I have collected so much astro junk that is was easy to find a DC cable that worked. They should really supply a regular DC cable.

 

 

A close up of the tripod adapter:

 

I also ordered the optical Vixen dovetail saddle with the mount.

 

Here is the hand controller. It is quite large relative to the size of the mount but very ergonomic.

 

Here is the mount installed on my Manfrotto 028B tripod.

 

  

 

 

 

[Astrojedi]

First light was purely visual with my Tak 76DCU - a warm up to understand the functions of the controller and get familiar with the mount.

 

Used it in EQ mode with a very rough polar align and leveling. Overall even after a 5 star align the gotos were slightly off. But tracking was superb. Observed Jupiter at 300x and no hint of vibrations.

 

The mount has zero backlash. After it stops moving there is no movement. And it is incredibly strong.

 

The noise while slewing is there but less than my other mounts (Celestron, MyT etc.).

 

 

 

[Astrojedi]

First light for imaging was with my AT60ED wide field imaging setup in my heavily LP backyard. Wanted to start with a shorter FL setup to get going before getting into longer FL imaging with this mount.

 

Attaching the OTA to the mount was a breeze. No counterweights and balance issues to worry about (in RA or DEC). Just felt so natural... just like how imaging should always have been. Took me all of 15secs!

 

Next I did a polar align with the Polemaster. Here is where I encountered a slight hiccup. On the first try after I got precise alignment of the two guides provided by the  Polemaster software, I went to lock down the Altitude knobs and as I moved the locking knobs the alignment moved. This is a design issue unfortunately. Moving the locking knobs moves the alignment.

 

So, I had to restart the alignment. The second time around I was much more careful and tested the movement due to the locking knobs before hand so that I could compensate. Alternated between turning the alignment knobs and the locking knobs such that when the locking knobs were tight I was in precise alignment. The Azimuth was a bit better - much less movement on locking.  

 

I have a feeling that as I load a heavier OTA on this mount for imaging this issue will only get worse.

 

On the positive side the gotos were much more accurate in this session. After polar alignment I did a 3 star align after which it put NGC6960 right in the middle of the sensor. The images below show exactly where the goto landed.

 

Overall I am really liking this mount so far.

 

Edited by Astrojedi, 01 July 2019 - 02:22 PM.

[Astrojedi]

I imaged a couple of objects with the AT60ED. M101 and NGC6960 (Veil). I did relatively short exposures with guiding ~1hr on each object (10 x 5min exposures) as the goal was to understand the mount better.

 

I also tried unguided shots but the most I could achieve was 45s without issues. So I will focus on guided imaging here.

 

M101 was in the western sky and NGC6960 was in the eastern sky when I imaged them. None of them crossed the meridian during imaging.

 

Here was the setup:

AT60ED with field flattener

ASI183MC Pro x1 bin

Opolong 2" CLS CCD filter

ZWO mini guidescope + ASI120MM miniguider

ASIAir

 

I connected the mount to the guider using a ST4 cable. Unfortunately ASIAir does not directly support the mount at this time - which means no gotos and platesolving using the ASIAir.

 

Overall impressions... the mount guided very well out of the box. I did not change any default settings on the ASIAir. Interestingly guiding was a little better on the Veil than M101. Not sure why this was. Another observation was that the tracking error (rms) went up and down periodically. For many seconds (30+) I would get 0.4" - 0.6" RMS then for many seconds (30+) I would a much higher 0.8" - 1" rms error.

 

I am not sure if this was seeing related or mount related. I don't have enough experience with this type of mount to comment. More experimentation should tease out the source.

 

Also the mount generally tracked better if I was less aggressive on Dec 70% and about 80% on RA.

 

The stars in each sub were perfectly round. No issues.

 

Here are some tracking graphs:

Edited by Astrojedi, 01 July 2019 - 02:23 PM.

[psandelle]

Enjoying the write-up.

 

I've found I've dropped my aggressiveness down to 50% with longer (5 sec) guiding exposures. I have an ADM saddle (as I wanted something a little more sturdy than the one that came with the mount).

 

Paul

[Astrojedi]

Thanks Paul. I will try that next time. I tired 2 secs but the RMS got worse so I reverted back to 0.5s. Maybe something else was going on.

[psandelle]

Thanks Paul. I will try that next time. I tired 2 secs but the RMS got worse so I reverted back to 0.5s. Maybe something else was going on.

Could be (also, we have different guide cameras, etc., so the mileages may vary...). Mine's been very consistent so far (knocks on wood multiple times). Got clouded out last Saturday, but trying to get out this Saturday. Also, I took care of any cable drag by taking out the screw that covers the optional counterweight bar hole and put an eye bolt in, so I velcro-tie my two-cables (USB & Powerpole power cable, sheathed in weave) to it, and it keeps things clean.

 

Paul

[w7ay]

I went to lock down the Altitude knobs and as I moved the locking knobs the alignment moved. This is a design issue unfortunately. Moving the locking knobs moves the alignment.

Hi Astrojedi,

 

Great set of photos.

 

You might find that making the mount slightly north heavy might help a little with the "altitude drifts when tightening altitute locking bolt" problem.

 

For example, add a small counterweight (I used a 3 lb Takahashi counterweight, which fits the 18mm RainbowAstro counterweight shaft perfectly) so that there is a small pressure while you lock the bolts.

 

I have found that it also helps if I only loosen the altitude lock to adjust the altitude while keeping the azimuth moderately locked.  The PA alignment procedure in ASIAIR is pretty good with showing independent alt and az adjustments as long as the tripod is somewhat level.  It is best to ignore the graphics of the ASIAIR PA and just let the alt and az numbers guide your adjustments.

 

I tired 2 secs but the RMS got worse so I reverted back to 0.5s.

I too have found that shorter (2 second, in my case) guide exposures/pulses yielded smaller errors than longer ones (> 3 secs), but I am sure that when seeing is bad, I will have to switch to longer integrations.

 

The mount is quite capable of 0.5-ish RMS arcsecond type guiding with the ASIAIR and ST-4 (best I got was around 0.35").  I get that when I use my indoor guiding tests with 1 second integrations.  But out in the real sky, 0.7" to 0.8" is more typical.  That being said, anything better than 1" RMS is plenty good enough for the plate scales that I use as long as there are no wild peaks in the guide graphs.

 

For my indoor guiding tests, I have a written a program (MacOS, natch) that displays a bunch of stars (just random points) whose movement is a function of the distance of the screen from the telescope, the alzimuth angle of the screen, and the elevation of the screen w.r.t. the guide scope. I have this "artificial sky" about 25 feet from the mount.  It was written to learn/test new guide algorithms and came in useful to quantify the errors of the RST-135.

 

 

I also ordered the optical Vixen dovetail saddle with the mount.

Very good -- you should be able to set the meridian limit a bit past the meridian (depends on the imaging train, of course) with that saddle.  I started with an ADM Vixen saddle, and found out that the saddle's knobs can easily hit the mount.   So, I had a 10mm thick aluminum puck with two M8 holes machined for me to lift the saddle a bit away from the declination plate to give me additional clearance.  I later found that the short Losmandy dual saddle (with both D and V jaws) that has a single clamping knob also clears the mount quite well; and I have been using that since.   I think the key to the better clearance is a single knob.

 

Have fun, Astrojedi.  

 

- chen

 

[Astrojedi]

Thanks Chen. I was able to achieve <0.4" rms guiding but the issue is what I highlighted above. That periodically (after every 200-300s) the rms error would inexplicably creep up and stay high for maybe 100s or so. The mount seems to be exhibiting two different personalities.

 

I am not 100% sure that this is related to seeing but with some more experimentation I think I should be able to tease out the source. 

 

 

 

[gotak]

You might want to do a unguided run to see native PE. Guiding works well with smooth long period errors but if the drive has rapid high amplitude errors it could cause problems.

[whwang]

Thanks for the report.  I agree with gotak.  A long unguided session with short guide exposure may tell you what's really going on.

 

I am also curious if unguided exposures (5 minutes, for example) with lenses shorter than 200mm are possible.

[josh smith]

Thanks for the detailed report. I will continue to follow. I hope to make an effort at first light this week. Assuming the tracking is smooth and periodic, it seems that a PEC curve would allow unguided imaging. I believe Tolga was going to talk to Ray about how and or whether PemPro could be used. Honestly, a little mini guider with a guide lens or oag is easy and fine to use anyways, but it sure would be nice to get to remove that one more piece of the rig if possible for widefield setups anyways.

Edited by josh smith, 02 July 2019 - 10:21 AM.

[Astrojedi]

Here is a (very) quick process of the NGC6960 stack (10 x 5min) in APP to primarily show the nice round stars.

 

There is quite a bit of noise in this image as I only used a single dark to remove the amp glow which added more noise. There was a mix-up with the darks - I took them at the wrong gain. Will reprocess once I have a chance to retake the darks.

 

Edited by Astrojedi, 02 July 2019 - 11:52 AM.

[Salty_snack]

Very interesting. I’m considering purchasing this mount and appreciate the time you put into this write up.

How is the tripod working out for you? An issue that I was anticipating is flex of the tripod with such an unbalanced load.

Based on the design it appeared the locking mechanism could cause shift in the mount. I’ve had the same issue in the past with equatorial mounts and my solution was just to leave it unlocked which actually worked just fine. I was wondering if leaving it unlocked is a feasible option for this mount?

[psandelle]

Very interesting. I’m considering purchasing this mount and appreciate the time you put into this write up.

How is the tripod working out for you? An issue that I was anticipating is flex of the tripod with such an unbalanced load.

Based on the design it appeared the locking mechanism could cause shift in the mount. I’ve had the same issue in the past with equatorial mounts and my solution was just to leave it unlocked which actually worked just fine. I was wondering if leaving it unlocked is a feasible option for this mount?

I haven't put super heavy loads on it yet (13 to 15lbs), but it's not flex with my tripod that would be a problem (there really isn't any at anything near the mount's capacity), it is actual unbalance that would get it eventually (so, if putting on a pedestal/pier, it can really hoist stuff around).

 

I found that I get close with the alt knobs loose, then get them somewhat tightened for the last little bit, so final tightening really isn't a problem. I don't think working with them unlocked would be a thing...but I'll have to look mine over (if I get some free time this evening).

 

Paul

[tolgagumus]

Here is a (very) quick process of the NGC6960 stack (10 x 5min) in APP to primarily show the nice round stars.

 

There is quite a bit of noise in this image as I only used a single dark to remove the amp glow which added more noise. There was a mix-up with the darks - I took them at the wrong gain. Will reprocess once I have a chance to retake the darks.

 

 

Excellent job. Don't worry about the noise. This is a mount review. All we care here is if the stars are round.

 

Also a comment about polar alignment adjustment locks: Do not loosen the locks to 100%. Just crack them loose a tiny bit to allow movement. This way, when you lock them back up, you are not moving it very much. To be honest, I found even the rough polar alignment step in Polemaster software sufficient to do guided imaging. I did 900 second exposures with no problem. 

 

Here are some more tips: 

Leave the altitude and azimuth knobs in their halfway travel before you start

Set the rough altitude adjustment with the supplied allen key

Leave the altitude intentionally lower so you lift the mount as you adjust for altitude. Only gravity pushes it down and if you have a light scope, it may be hard to come back down. I had to push it down by hand to make a down adjustment. This may not be an issue for lower altitudes but it was an issue at 40 degrees. This also works together with not loosening the locks 100%. 

 

For others who are considering this mount, I highly recommend the RT90C tripod from Amazon. 

[psandelle]

Tolga - do you ever tighten the az lock all the way (which takes some man-power)?

 

Paul

[tolgagumus]

Tolga - do you ever tighten the az lock all the way (which takes some man-power)?

 

Paul

Hi Paul,

 

Yes but enough to secure it, I don't try to push it to it's limit. 

[psandelle]

Cool. That's what I've been doing. Thanks!

 

Paul

[Astrojedi]

Very interesting. I’m considering purchasing this mount and appreciate the time you put into this write up.

How is the tripod working out for you? An issue that I was anticipating is flex of the tripod with such an unbalanced load.

Based on the design it appeared the locking mechanism could cause shift in the mount. I’ve had the same issue in the past with equatorial mounts and my solution was just to leave it unlocked which actually worked just fine. I was wondering if leaving it unlocked is a feasible option for this mount?

The tripod is rock solid. I have had it for many years. It is incredibly good value for money. But if I was buying new I might go for a carbon fiber version as it is lighter. 

[Astrojedi]

 

Also a comment about polar alignment adjustment locks: Do not loosen the locks to 100%. Just crack them loose a tiny bit to allow movement. This way, when you lock them back up, you are not moving it very much. To be honest, I found even the rough polar alignment step in Polemaster software sufficient to do guided imaging. I did 900 second exposures with no problem. 

 

 

That is a good tip although I have found quite a bit of movement in the last turn of the locking knobs as well. Having said that I don't think this is a deal breaker as it is not hard to work around. If it was preventing accurate polar alignment I would be less happy.

 

Honestly I am very happy with what I am seeing do far. The real test will be with my EdgeHD 8. But I am already finding it hard to go back to using counterweights and balancing. The ease of use is great.

[psandelle]

Yeah, this little mount is so small, I'm going to go OAG in the future...just to make things even tighter.

 

Paul

[calypsob]

I get that these harmonic gears dont need a counter weight to operate, but without a counterweight the entire unit is subject to mechanical flexure.

 

The point where this mount meets the tripod, the alt adjustment knobs,the bolt that affixes declination, and the saddle connected to the scope are all points of mechanical flexure under the load of a scope hanging off of the side.  I dont think PEC is going to correct some of the odd behaviors that may arise, but data seems pretty good so far and I think it would probably improve with a counter weight and a better tripod mount.