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RainbowAstro RST-135 Review - The ideal imaging mount?

astrophotography imaging
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#151 tolgagumus

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Posted 11 July 2019 - 07:38 PM

Ah, missed that part. So homing is behaving as expected. But that still doesn't explain the parking being off by 180 deg in some occasions.
And no, I don't trust myself recognizing the stars correctly. That's why I mentioned in my post above that I might have centered to the wrong star.


Did you see the park position for the Southern hemisphere? You are supposed to park at 0-90 instead of 0-270

#152 w7ay

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Posted 11 July 2019 - 07:53 PM

I agree with you on the Alt Az knobs - too much play.

I think everybody agrees about the inadequate alt-az bolts.

 

The play in the azimuth bolts can be improved a little by wrapping a couple of layers of teflon tape that you get from hardware stores for pipe fittings.

 

The altitude bolt is a different story :-).

 

Cheers,

Chen


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#153 Howard Gao

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Posted 12 July 2019 - 02:43 AM

Can anyone tell me the size of the counterweight shaft? I'd like to know whether I can use any of my existing ones. Many thanks!



#154 Tamiji Homma

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Posted 12 July 2019 - 09:03 AM

Can anyone tell me the size of the counterweight shaft? I'd like to know whether I can use any of my existing ones. Many thanks!

 

It is very close to Takahashi EM-200/EM-11/PM-1 CW shaft. 

 

I can use Takahashi CW and RST-150H CW interchangeably on both Takahashi and RST-150H mounts.

I think RST-135 uses the same thing.

 

The shaft has M8(I need to confirm that ?) male bolt and screwing into mount.

I'll upload a picture later if you like.

 

Tammy


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#155 Howard Gao

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Posted 12 July 2019 - 09:23 AM

It is very close to Takahashi EM-200/EM-11/PM-1 CW shaft. 

 

I can use Takahashi CW and RST-150H CW interchangeably on both Takahashi and RST-150H mounts.

I think RST-135 uses the same thing.

 

The shaft has M8(I need to confirm that ?) male bolt and screwing into mount.

I'll upload a picture later if you like.

 

Tammy

Many thanks Tammy. A friend just told me the shaft should be 18mm in diameter, 20cm long and with M8 male bolt. It should be same as your RST-150 shaft.



#156 Tamiji Homma

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Posted 12 July 2019 - 09:48 AM

Many thanks Tammy. A friend just told me the shaft should be 18mm in diameter, 20cm long and with M8 male bolt. It should be same as your RST-150 shaft.

 

Here it is PM-1 CW and RST-150H CW together :)

 

large.jpg

 

Tammy


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#157 josh smith

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Posted 12 July 2019 - 11:37 AM

Here it is PM-1 CW and RST-150H CW together smile.gif

 

large.jpg

 

Tammy

 

How can you actually tell it's balanced?  Did you just do the math and figure out how much is needed and where it is needed and why did you decide to use a counterweight?



#158 Tamiji Homma

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Posted 12 July 2019 - 12:16 PM

How can you actually tell it's balanced?  Did you just do the math and figure out how much is needed and where it is needed and why did you decide to use a counterweight?

 

It is a good question.   Since there is no way to un-engage harmonic drive gear to check, I would have to guess by trial-n-error, making it completely unbalanced (mount moving without power), from the touch of system moment or you actually image and compare the result.

 

Well, I haven't put load that I need to use the CW in EQ mode yet so it is kinda academic for now.

 

Having said that, when I use in alt/az mode, I feel a little more comfortable having the CW to get system balanced when I have 25lb+ load on one side to counter the weight on the other side.

 

Tammy


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#159 Astrojedi

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Posted 12 July 2019 - 12:52 PM

Actually I think it may be better if the mount is not balanced. Earlier in this thread Bob (lukasik) posted a reference to the paper on Kinematic error (periodic error) in harmonic drives. Makes for interesting reading.

 

Restating after reading again: 

 

If the load is very close to balanced the kinematic error will be higher. After the torsional load reaches a certain threshold the error (PE) seems to fall quite quickly.

 

https://www.semantic...5d8d5688ea37a52

 

Couple of more observations from the paper:

 

- The PE curves (Pure Component) are not smooth - there is a much more rapid change at the beginning of the cycle.

- Lower angular speeds give the lowest PE - good for imaging

 

Again this is not my area of expertise but the paper seems to be written quite clearly.


Edited by Astrojedi, 12 July 2019 - 01:08 PM.

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#160 josh smith

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Posted 12 July 2019 - 01:27 PM

Actually I think it may be better if the mount is not balanced. Earlier in this thread Bob (lukasik) posted a reference to the paper on Kinematic error (periodic error) in harmonic drives. Makes for interesting reading.

 

Restating after reading again: 

 

If the load is very close to balanced the kinematic error will be higher. After the torsional load reaches a certain threshold the error (PE) seems to fall quite quickly.

 

https://www.semantic...5d8d5688ea37a52

 

Couple of more observations from the paper:

 

- The PE curves (Pure Component) are not smooth - there is a much more rapid change at the beginning of the cycle.

- Lower angular speeds give the lowest PE - good for imaging

 

Again this is not my area of expertise but the paper seems to be written quite clearly.

It is an interesting read and I actually remember specifically running into this paper a number of years ago, too.  

 

I agree that an imbalance may be better for smoother tracking and also could help reduce vibrations of the mechanical system by a balanced system on long moment arms regardless of the drive system.

 

As far as I understand the way a PEC curve is recorded, it doesn't necessarily need to be a perfectly smooth and can offset changing rates of error as well.  From what I could tell, there would be 2 components making up the error and it would be specifically system dependent with one of the terms being environmentally dependent which could include the loading.  To me that says, in this system, for the most accurate correction curve, you'd want to measure one for each unique setup.  Whether or not that is really necessary would be up for debate.  I doubt it.  I doubt one really would help much even if you are guiding, but your data isn't backing that up yet.  You don't want to be limited in choosing the length of your guide exposures up to 5-10".  The portion not based on the flexibility of the drive system or environment would eat up the majority of the amplitude of the error and would probably be enough to take care of most of the work being done by the guiding.  

 

I know I've read somewhere that somebody mentioned the period is too long on these drives to apply a correction curve, but I'm not sure I understand or agree with that input.  Maybe I just misunderstood the point.



#161 tolgagumus

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Posted 12 July 2019 - 01:39 PM

To balance the RA you can make it paralel to the floor and turn off the power. If the power is off it will slowly creap. You can adjust the weight until it does not. It's not perfect but it will be close enough. For the DEC, put the whole thing together and place it on a round pencil on a table. Mark where it balances on the dovetail and place that mark as close to the center of the saddle as possible.

#162 deepanshu29

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Posted 12 July 2019 - 01:40 PM

Did you see the park position for the Southern hemisphere? You are supposed to park at 0-90 instead of 0-270

Yes, I saw that and I tried both while I was in Chile. My point, if I am telling it to park at 270, it should park at 270, not 90. Same goes when I tell it to park at 90, it should park at 90 not, 270. It did some wonky thing with parking. But it parked correctly every time If I did homing first. 

 

Tolga, 

Any chance that manufacturer can add PEC to this mount? 



#163 tolgagumus

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Posted 12 July 2019 - 01:48 PM

Yes, I saw that and I tried both while I was in Chile. My point, if I am telling it to park at 270, it should park at 270, not 90. Same goes when I tell it to park at 90, it should park at 90 not, 270. It did some wonky thing with parking. But it parked correctly every time If I did homing first.

Tolga,
Any chance that manufacturer can add PEC to this mount?


We are working to see if it's possible.
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#164 EFT

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Posted 12 July 2019 - 03:22 PM

I agree that perfect balance is not necessary (it usually isn't even desirable on most mounts), particularly when most scopes are not terribly unbalanced (e.g., a finder is generally not going to be something to worry about when it comes to the scope not being balanced along the RA axis).  The best way to balance things at this point is going to be off of the mount.  DEC balance is easy this way but RA is difficult unless you are using a wide system like an SBS.  The reality is, this mount is never balanced in RA unless you are using counterweights and unless you are very far off, it is not going to make much of a difference because of the power of the drives.  The biggest risk of not being balanced to at least some extent, is the increase in the likelihood of back driving when the mount loses power or is turned off.  Using backdriving to balance the RA is very imprecise and simply unnecessary.  With lighter loads, it would not even work.

 

There is a "balance" section I the hand controller but it is not used at this time.  It could be that they will add a balance routine in the future, but again, what good would it do in RA without a counterweight.


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#165 Astrojedi

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Posted 12 July 2019 - 04:59 PM

Actually based on the paper above, I think this mount needs to be meaningfully (not slightly) unbalanced otherwise your Kinematic error (and PE) will likely go up significantly. Balance I believe is required only to ensure the tripod does not topple over. Makes no difference to tracking accuracy at all - in fact will hurt it. 

 

I will measure the PE with my EdgeHD 8. Will be interesting to see if there is any difference vs AT60. I am guessing that the AT60 setup provides enough torsional load (assuming I am understanding that correctly) to get into the orange zone (see picture) since the 31" P-P coincides very well with other data points.

 

Capture.JPG


Edited by Astrojedi, 12 July 2019 - 05:08 PM.

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#166 josh smith

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Posted 12 July 2019 - 05:09 PM

Actually based on the paper above, I think this mount needs to be meaningfully (not slightly) unbalanced otherwise your Kinematic error (and PE) will likely go up significantly. Balance I believe is required only to ensure the tripod does not topple over. Makes no difference to tracking accuracy at all - in fact will hurt it.

I will measure the PE with my EdgeHD 8. Will be interesting to see if there is any difference vs AT60. I am guessing that the AT60 setup provides enough torsional load (assuming I am understanding that correctly) to get into the orange zone (see picture) since the 31" P-P coincides very well with other data points.

Capture.JPG

I think we’re all in agreement it’s better unbalanced.

I am curious if a significantly heavier load impacts the secondary error due to the flexibility of the drive system.

Edited by josh smith, 12 July 2019 - 05:11 PM.


#167 Astrojedi

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Posted 12 July 2019 - 05:30 PM

I think we’re all in agreement it’s better unbalanced.

I am curious if a significantly heavier load impacts the secondary error due to the flexibility of the drive system.

 

My point was a little different to Ed's and maybe did not come across well.

 

I am saying that I would not use a counterweight to balance the OTA at all unless you feel the OTA is about to topple over. Even then I would not waste time "balancing" the OTA or do any of the gymnastics that Ed and Tolga are suggesting. I think it is completely unnecessary. Just slap on enough weight that you feel stabilizes the tripod. You are not balancing the OTA - you are stabilizing the tripod.


Edited by Astrojedi, 12 July 2019 - 05:31 PM.

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#168 psandelle

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Posted 12 July 2019 - 05:33 PM

My point was a little different to Ed's and maybe did not come across well.

 

I am saying that I would not use a counterweight to balance the OTA at all unless you feel the OTA is about to topple over. Even then I would not waste time "balancing" the OTA or do any of the gymnastics that Ed and Tolga are suggesting. I think it is completely unnecessary. Just slap on enough weight that you feel stabilizes the tripod. You are not balancing the OTA - you are balancing the tripod.

Agreed! The balance to tripod is the vulnerable thing if you push the rig/moment arm too much. But...I'm NEVER going to push the weight on this mount; it defeats the purpose for me. This is all about going light. Heaviest thing is going to be a 6" Sharpstar hypergraph with an OAG, which is plenty light. If I wanted heavy, I'd have kept the 10Micron. This is all about lightweight load outs for me.

 

Paul


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#169 Astrojedi

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Posted 12 July 2019 - 08:04 PM

Installed the 135 on my concrete pier. Connection is pretty solid. Not even the slightest of vibration. Will see how it performs.

 

Will try some heavier OTAs as well e.g. C9.25 and C11. The only thing that makes me nervous with the very heavy OTAs is the 3/8” tripod connector. Maybe it will be fine but I just imagine the whole load hinging on that one little screw.

 

 

243CD198-8180-472E-A823-BC31C43D87BD.jpeg

 

 

 



#170 EFT

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Posted 12 July 2019 - 08:47 PM

Be careful to not let it back drive with the big OTAs.  They will come down fast due to their height and weight.



#171 Astrojedi

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Posted 13 July 2019 - 09:38 AM

Be careful to not let it back drive with the big OTAs.  They will come down fast due to their height and weight.

That is one thing I am concerned about with large OTAs. I will use a large DC battery pack to ensure no power cut out. If it does start to back drive is there a way to stop it?


Edited by Astrojedi, 13 July 2019 - 11:15 AM.


#172 EFT

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Posted 13 July 2019 - 12:24 PM

I'm not sure if there is.  If the power came back up it might be able to stop it but that would depend on how much immediate power it could apply to the servos.  But you won't be worrying about that it the time, you will be worrying about trying to catch the scope.  If it goes into a fall it will probably either be a complete loss of power from the supply or the cord getting pulled and in either case, you won't be rushing to fix that problem.  In addition, it could happen if you park the mount and turn it off if it turns out that the tall and heavy scopes isn't well enough balanced.  When mine went into a back drive, it seemed like it got a head of steam going and wanted to keep going but it could just be that I was a panicking a bit at the time.  In my attempt to stop it, I ended up lifting up the entire rig with the tripod. 

 

You installed yours on a pier and that will definitely help because the base won't move if you grab the scope.  I think that for now, you just have to keep this in mind to not be too surprised if it does happen.

 

I decided to go down just now and test things out.  With a relatively short (vertical and horizontal) setup weighing about 18 pounds, it started back driving at between 10 and 20 degrees off vertical.  Turning the power on stopped it, but I didn't wait for it to gain a lot of speed.  If you try to catch it, it's not like catching something that is just dropping freely.  The drive system does feel like it really wants to keep going.  In some respects this may be something that we have to get used to with this mount.  But I conversed with a friend who does a lot of automation work and the strain wave drive, like the direct torque motor drive, is often used with a electro-mechanical friction break to stop it from back driving under a variety of circumstances.  Of course, something like that is not going to help when it comes to keeping the system as small as possible.


Edited by EFT, 13 July 2019 - 12:26 PM.


#173 aneeg

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Posted 13 July 2019 - 02:57 PM

This thread has made this a stay-away- mount.

 

Arne


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#174 Astrojedi

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Posted 13 July 2019 - 03:28 PM

Don’t be too hasty in your judgement. While I am very honest and critical about all my equipment I can tell you that this mount is not far from meeting its value proposition. 

 

Also I don’t want comments like this to derail this thread. Such comments do nothing but inflame emotions with zero contribution.

 

If you don’t have any useful or constructive contribution or any genuine questions I very politely request you not to post here. I really want this thread to stay focused on this mount and our collective learnings. 


Edited by Astrojedi, 13 July 2019 - 03:29 PM.

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#175 Blue_Orca

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Posted 13 July 2019 - 03:29 PM

This thread has made this a stay-away- mount.

 

Arne

I disagree.  It's all about compromises and understanding your priorities.  All this thread has done, is showing us where the limits are on the RST-135.  The mount is no miracle, it may be a stay-away for some, but a very useful tool for others, such as myself, who primarily travel by air to do astrophotography on a 80/480 LOMO and telephoto lens.

 

Blue  


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