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10 micron performance?


dan_adi

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Hello,

Recently I stumbled on this video of a beautiful 10 micron 3000 HPS mount.

 

But when I reached the 9:55 minute mark, I was kind of disappointed with the numbers reported on 'guiding'. Since there were no corrections made, everything was running on a model. Considering he was using a C11 Hyperstar config, at a forgiven focal length ... 0.7 " RMS is quite disappointing. 

It could be the SCT quality. These scopes should not model well.

Bottom line, do you think it's the scope at fault? or is this a regular performance of this kind of mount?

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That’s normal according to 10 micron specs…see below. But I suppose what you’re paying for is being able to guide 100kg of payload at that accuracy. 

Also I guess there’s a lower limit on guiding accuracy imposed by your seeing blur. No point guiding at 0.3” RMS when your seeing is 1.5”?? 

That’s my thoughts anyway… Will be interesting to hear comments from mount experts. 
 

6BBCDD8F-A840-4A38-A4EF-D02D5FF6777D.png

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2 minutes ago, andrew s said:

I think it depends on the seeing.  No model can compensate for that. Regards Andrew 

If I remember correctly, seeing has a big impact with regular guiding. A model takes into account temperature, pressure, refraction .. so the seeing has less impact on a model. Mirror flop is random, so it cannot be modeled out. This could explain the deteriorating performance as time passed. 

Still, I didn't expect this.

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5 minutes ago, CraigT82 said:

That’s normal according to 10 micron specs…see below. But I suppose what you’re paying for is being able to guide 100kg of payload at that accuracy. 

Also I guess there’s a lower limit on guiding accuracy imposed by your seeing blur. No point guiding at 0.3” RMS when your seeing is 1.5”?? 

That’s my thoughts anyway… Will be interesting to hear comments from mount experts. 
 

6BBCDD8F-A840-4A38-A4EF-D02D5FF6777D.png

Indeed these are the specs. Have to say I am not impressed. 

"No point guiding at 0.3” RMS when your seeing is 1.5”??"

- Yes you should! Your stars fwhm is determined by a combination of factors like seeing, guiding error, image scale, angle from zenith etc. I regularly guide at 0.3" with seeing above or at 1.5" .

To test go here: https://clearskies.go.ro . Submit the default values in the exposure time calculator, then, on the results page go to the Image FWHM tab. Leave everything as is, except  increase the guiding error from 0.35" to 0.75". The FWHM jumps from 2.18 " to 2.68". That is an additional blur of 0.5 " in 1.5" seeing. 

 

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12 minutes ago, CraigT82 said:

Also I guess there’s a lower limit on guiding accuracy imposed by your seeing blur. No point guiding at 0.3” RMS when your seeing is 1.5”?? 

You want as good guiding as possible regardless of the seeing.

Two compound to produce resulting blur - lowering any component of the two is beneficial (it does not behave linearly and in principle - one can swamp the other if they are significantly different in magnitude - but tracking/guiding is reported in RMS error while seeing is reported in FWHM - two different measures. RMS is roughly FWHM / 2.355 - for Gaussian type distribution).

39 minutes ago, dan_adi said:

But when I reached the 9:55 minute mark, I was kind of disappointed with the numbers reported on 'guiding'. Since there were no corrections made, everything was running on a model. Considering he was using a C11 Hyperstar config, at a forgiven focal length ... 0.7 " RMS is quite disappointing. 

Ok, so you need to understand difference between mount tracking performance, mount guiding performance and tracking/guiding measurement precision.

What you see in the video is mount tracking performance as there are no guide corrections made.

image.png.bdb539bc8d240971cb064898d5f0be4d.png

we see three numbers here:

RA has 1.98" RMS value

DEC has 0.44" RMS value

Total is at 2.03" RMS.

But we have to note several things:

1. Mount tracking is flawed - probably because of poor model. RA has significant drift / downward trend on the graph. This means that mount is not tracking at correct rate. My guess is that the model is somehow flawed (probably because that "moving mirror compensation" - mentioned in the video).

We don't have accurate RA RMS once this linear trend is removed / model is fixed

2. There is quite a bit of seeing introduced error. If we look at DEC stats / graph - one thing is obvious. Polar alignment is very good and there is virtually no drift in DEC - which in turn means that DEC variation in position is mostly due to Seeing - we have in this case error in position measurement introduced by seeing for given exposure time (longer exposure times reduce this error).

Actually, error in DEC can be due to several things - shake of the setup due to wind, or mechanical shake from mount tracking - or seeing. Given that this is 10 micron mount - I think that we can exclude first two. It is mounted on a pier and it is a heavy duty mount - wind influence is unlikely. It is also very good mount mechanically and heavy duty mount - so I don't believe that motor tracking in RA would cause shake in DEC.

This leaves with seeing.

If you look at two lines - green is just a bit rougher than red (exclude downward drift) - this tells me that most of the error in RA is also due to seeing - but mount is not perfectly smooth. It will have some roughness and I don't think that this roughness can be guided out as it is too fine (encoders are already doing the work).

 

 

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18 minutes ago, dan_adi said:

If I remember correctly, seeing has a big impact with regular guiding. A model takes into account temperature, pressure, refraction .. so the seeing has less impact on a model. Mirror flop is random, so it cannot be modeled out. This could explain the deteriorating performance as time passed. 

Still, I didn't expect this.

 Not so. They account for the average refraction they can't account for the image shift due to random local "seeing cells". Your correct hysteresis can't be modeled out.

Regards Andrew 

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7 minutes ago, vlaiv said:

You want as good guiding as possible regardless of the seeing.

Two compound to produce resulting blur - lowering any component of the two is beneficial (it does not behave linearly and in principle - one can swamp the other if they are significantly different in magnitude - but tracking/guiding is reported in RMS error while seeing is reported in FWHM - two different measures. RMS is roughly FWHM / 2.355 - for Gaussian type distribution).

Ok, so you need to understand difference between mount tracking performance, mount guiding performance and tracking/guiding measurement precision.

What you see in the video is mount tracking performance as there are no guide corrections made.

image.png.bdb539bc8d240971cb064898d5f0be4d.png

we see three numbers here:

RA has 1.98" RMS value

DEC has 0.44" RMS value

Total is at 2.03" RMS.

But we have to note several things:

1. Mount tracking is flawed - probably because of poor model. RA has significant drift / downward trend on the graph. This means that mount is not tracking at correct rate. My guess is that the model is somehow flawed (probably because that "moving mirror compensation" - mentioned in the video).

We don't have accurate RA RMS once this linear trend is removed / model is fixed

2. There is quite a bit of seeing introduced error. If we look at DEC stats / graph - one thing is obvious. Polar alignment is very good and there is virtually no drift in DEC - which in turn means that DEC variation in position is mostly due to Seeing - we have in this case error in position measurement introduced by seeing for given exposure time (longer exposure times reduce this error).

Actually, error in DEC can be due to several things - shake of the setup due to wind, or mechanical shake from mount tracking - or seeing. Given that this is 10 micron mount - I think that we can exclude first two. It is mounted on a pier and it is a heavy duty mount - wind influence is unlikely. It is also very good mount mechanically and heavy duty mount - so I don't believe that motor tracking in RA would cause shake in DEC.

This leaves with seeing.

If you look at two lines - green is just a bit rougher than red (exclude downward drift) - this tells me that most of the error in RA is also due to seeing - but mount is not perfectly smooth. It will have some roughness and I don't think that this roughness can be guided out as it is too fine (encoders are already doing the work).

 

 

Very good points,

but I wonder how seeing can influence the model. While tracking there is no guide camera to influence the tracking, no camera, no guiding corrections. 

Does the seeing error creep in when we build the model? Does the seeing affect platesolving?

I do agree that the model is flawed due to the SCT mirror movement.

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11 minutes ago, andrew s said:

 Not so. They account for the average refraction they can't account for the image shift due to random local "seeing cells". Your correct hysteresis can't be modeled out.

Regards Andrew 

Hmm, but these mount have the option to update the temperature and pressure all night, so the refraction is computed at every zentih angle. 

With regard to seeing ... if we don't use a guide camera and issue no guide corrections, how can the seeing influence tracking?

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5 minutes ago, dan_adi said:

Hmm, but these mount have the option to update the temperature and pressure all night, so the refraction is computed at every zentih angle. 

With regard to seeing ... if we don't use a guide camera and issue no guide corrections, how can the seeing influence tracking?

It won't but it will shift the images on the camera. This will impact the reported tracking.  Regards Andrew 

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1 minute ago, CraigT82 said:

So what’s the consensus then? Is it a hunk of junk?! 0.7” rms does seem pretty underwhelming

My guess is that the model is flawed due to using an SCT with the mount. I bet with a high quality Planewave CDK the results would be much better. Or better yet, a refractor.

This is why you need both, good scope + good mount.

What is impressive to me, is that my Mesu mount has better performance than this, at more than half the price ... makes one wonder about overpricing.

With absolute encoders this mount should deliver 0.2-0.3" RMS constantly! Otherwise it's hard to justify 20K EUR

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4 minutes ago, dan_adi said:

but I wonder how seeing can influence the model. While tracking there is no guide camera to influence the tracking, no camera, no guiding corrections. 

Seeing can't influence the model - but it can influence measurement of current position.

In order to measure how well the mount is performing we need to measure its motion against earth's rotation, right? In order to do that - we assume star must be stationary in the FOV.

Seeing will cause star to "bounce" around even if we have a perfect mount (there is a tilt component of wavefront aberration that is causing shift in apparent star position due to seeing).

6 minutes ago, dan_adi said:

Does the seeing error creep in when we build the model? Does the seeing affect platesolving?

No, or at least minimally.

Both of these depend on longer exposures than few seconds. In theory - tilt component of wavefront aberration should average out in few seconds. Seeing is measured as 2 second exposure star FWHM - that suggests that for the most part, after two seconds star profile is gaussian in shape - which is due to averaging different wavefront aberrations (central limit theorem).

In reality - I've found that sometimes exposures up to 8s help to stabilize seeing. I usually guide at 4s exposures because of this.

There is tradeoff - you need good mount (mechanically) to be able to guide with long exposures - longer the guide exposure, more actual tracking error builds up and larger correction one needs - guide RMS rises, but if guide exposure is too short - then RMS rises due to seeing introduced measurement error.

 

Bottom line - seeing is only important in sense that it obscures true tracking / guiding error and there are tricks that one can perform to minimize that:

1. longer guide exposure

2. multi star guiding

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3 minutes ago, vlaiv said:

Seeing can't influence the model - but it can influence measurement of current position.

In order to measure how well the mount is performing we need to measure its motion against earth's rotation, right? In order to do that - we assume star must be stationary in the FOV.

Seeing will cause star to "bounce" around even if we have a perfect mount (there is a tilt component of wavefront aberration that is causing shift in apparent star position due to seeing).

No, or at least minimally.

Both of these depend on longer exposures than few seconds. In theory - tilt component of wavefront aberration should average out in few seconds. Seeing is measured as 2 second exposure star FWHM - that suggests that for the most part, after two seconds star profile is gaussian in shape - which is due to averaging different wavefront aberrations (central limit theorem).

In reality - I've found that sometimes exposures up to 8s help to stabilize seeing. I usually guide at 4s exposures because of this.

There is tradeoff - you need good mount (mechanically) to be able to guide with long exposures - longer the guide exposure, more actual tracking error builds up and larger correction one needs - guide RMS rises, but if guide exposure is too short - then RMS rises due to seeing introduced measurement error.

 

Bottom line - seeing is only important in sense that it obscures true tracking / guiding error and there are tricks that one can perform to minimize that:

1. longer guide exposure

2. multi star guiding

I understand now, thanks!

 

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2 minutes ago, dan_adi said:

With absolute encoders this mount should deliver 0.2-0.3" RMS constantly! Otherwise it's hard to justify 20K EUR

0.2-0.3" RMS of what? Tracking precision or Guiding precision?

You compare it to Mesu - but two are very different in their use case - at least the way I see it.

10 Micron is to be used without guiding.

Mesu 200 absolutely needs guiding to be used.

If you want to compare the specs on the two that we have (we don't actually have guide precision of 10 Micron mount) - then compare this:

image.png.91e635dacac85387f6e7178667ae2e07.png

vs

image.png.421e23fb5c54803bac62b0bf2190d923.png

10 Micron is at least x2 more accurate in tracking (if we assume that P2P error is representative of smoothness of the mount) and can maintain that level of performance without needing to auto guide (they also state less than 0.7" RMS without guiding with accurate model).

On top of that 10 Micron mount has several other things that Mesu lacks.

1. It can track really fast (servo motors - specs say up to about 10 degrees per second)

2. It is probably not as sensitive to balance as Mesu is. Friction drives work the best when you have very balanced setup

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By the way - I would not get 10 micron mount for myself (although it might look like I'm the fan from above discussion).

It is indeed pricey in my opinion - and I would not have use for it.

I like friction mounts better, and I'm actually eyeballing this one:

https://www.geminitelescope.com/efric-friction-drive-mount-german-equatorial/

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10 minutes ago, vlaiv said:

0.2-0.3" RMS of what? Tracking precision or Guiding precision?

You compare it to Mesu - but two are very different in their use case - at least the way I see it.

10 Micron is to be used without guiding.

Mesu 200 absolutely needs guiding to be used.

If you want to compare the specs on the two that we have (we don't actually have guide precision of 10 Micron mount) - then compare this:

image.png.91e635dacac85387f6e7178667ae2e07.png

vs

image.png.421e23fb5c54803bac62b0bf2190d923.png

10 Micron is at least x2 more accurate in tracking (if we assume that P2P error is representative of smoothness of the mount) and can maintain that level of performance without needing to auto guide (they also state less than 0.7" RMS without guiding with accurate model).

On top of that 10 Micron mount has several other things that Mesu lacks.

1. It can track really fast (servo motors - specs say up to about 10 degrees per second)

2. It is probably not as sensitive to balance as Mesu is. Friction drives work the best when you have very balanced setup

If the model is not quite good, as is the  above example from youtube, the 10 Micron should/could be guided also. There are lots of 10micron users, mostly examples on CN, that use guiding + model, simply because with lower quality scopes, the model is not enough.  

I was referring to RMS guiding accuracy.

The option for really fast tracking is useful for satellites, otherwise maybe for a fast model build? 

I had my fair share of headaches with the sitech control system on the Mesu, until I learned what was needed for a smooth and reliable operation. So I am not a big fan of the sitech system. But once you learn the software, the performance of a guided friction drive is quite impressive. 

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2 minutes ago, dan_adi said:

I was referring to RMS guiding accuracy.

Well, we don't have data on that from above video nor from specs sheet.

2 minutes ago, dan_adi said:

The option for really fast tracking is useful for satellites, otherwise maybe for a fast model build? 

Yes, satellites, ISS and so on ...

3 minutes ago, dan_adi said:

If the model is not quite good, as is the  above example from youtube, the 10 Micron should/could be guided also. There are lots of 10micron users, mostly examples on CN, that use guiding + model, simply because with lower quality scopes, the model is not enough. 

For some reason I don't like idea of encoders + guiding. It seems like waste of encoders :D - and I always think that two might "fight" if software is not properly implemented.

What is to be trusted? Guide command or encoder correction. In ideal world - guide command, but as we have seen - it depends on accuracy of measurement and seeing can disturb it somewhat. But if we had perfect guide corrections - then we would not need encoder at all, right? So it must be encoder that has "the upper hand" - but why guide then? And off I go in circles ... :D

 

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17 minutes ago, vlaiv said:

For some reason I don't like idea of encoders + guiding. It seems like waste of encoders :D - and I always think that two might "fight" if software is not properly implemented.

I view it as the best of both worlds. Let the encoders do most of the work for tracking, and correct an eventual drift  with a guide correction. I've seen examples of 1-2 guide corrections per minute.

I wonder how professionals are doing it. It's safe to assume they can  afford absolute encoders. Do they also guide ? Given those big scopes, and big metal structures, I guess flexure and mirror movement is a problem for them as well. 

 

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1 minute ago, dan_adi said:

I wonder how professionals are doing it. It's safe to assume they can  afford absolute encoders. Do they also guide ? Given those big scopes, and big metal structures, I guess flexion and mirror movement is a problem for them as well. 

Have no idea, but maybe we can get some info here:

https://planewave.com/product/l-600-direct-drive-mount/

direct drive?

Zero backlash / Zero periodic error - no need for guiding with good enough model?

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On 13/02/2023 at 21:41, vlaiv said:

By the way - I would not get 10 micron mount for myself (although it might look like I'm the fan from above discussion).

It is indeed pricey in my opinion - and I would not have use for it.

I like friction mounts better, and I'm actually eyeballing this one:

https://www.geminitelescope.com/efric-friction-drive-mount-german-equatorial/

@vlaiv

Apologies, a bit off topic here.

I am also very interested in the Gemini e-Fric, but with the sad news of Andreas passing away a while back, is the mount still in production and supported?

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4 minutes ago, oymd said:

@vlaiv

Apologies, a bit off topic here.

I am also very interested in the Gemini e-Fric, but with the sad news of Andreas passing away a while back, is the mount still in production and supported?

To be honest - I had not heard of the sad news and I have no idea if it is still in production and/or supported.

My level of interest was only up to "selecting" it as possible upgrade, but I'm really far away from upgrading the mount (maybe a year or two - need to first finish the observatory).

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I think this new offering from JTW looks fantastic. Friction drive with high resolution encoders as standard and a 35kg payload at £6k probably (when you include VAT and shipping). Expensive, certainly, but when you consider it’s basically a competitor to the 10 Micron GM1000 (priced > £10k) with 10kg more payload and a friction drive rather than worm gear, it looks like a bargain. Very positive reports on the Trident from what I can see so far as well, so bodes well for this one.

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