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CEM25EC: Do I need to re-calibrate the encoder?

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#1 mehdymo

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Posted 22 August 2019 - 03:48 PM

Hi,

 

I have a CEM25EC. Last week I tested the mount and found that it works very well if I guide (despite the very bad seeing on that night), however when I use the encoder, it almost does not do anything. I am wondering if my expectation is high, I am doing something wrong, or I need to re-calibrate the encoder as I have upgraded the firmware, RA, and DEC (manual does not say anything about re-calibration). My setup was WO GT71+ASI1600+Filterwhell+60mm guidescope (I guess the whole setup is well below 10 lbs).

I have attached the PHD2 results for 4 different options:
1- Guiding on and Encoder on
2- Guiding on and Encoder off
3- Guiding off and Encoder on
4- Guiding off and Encoder off
Please note that:
- I was turning off the guiding by skipping output in PHD2
- I was turning on the Encoder by "Filtering RA" on hand controller

 

Any help is appreciated.

Thanks

Attached Thumbnails

  • guiding off+Encoder off.jpg
  • guiding off+Encoder on by Filtering RA.jpg
  • guiding on+Encoder off.jpg
  • guiding on+Encoder on by Filtering RA.jpg
  • guiding on-Encoder off-2.jpg


#2 OldManSky

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Posted 22 August 2019 - 04:13 PM

Possibly.

Have you looked at your PHD logs to see what PHD thinks your polar alignment error was?

Was the star it was tracking below about 35 degrees above the horizon, or within 20 degrees of the NCP?

 

The dirty little secret of RA tracking is that a constant-speed motor -- no matter how accurate -- will not accurately track the movement of a star across the sky.  Because that star's apparent motion is not constant.  So there are places in the sky where, if your PA is nuts-on, an encoder-equipped mount can track very, very well for quite some time.  But there are also places where that won't happen, no matter how accurate the encoders are...


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#3 Chuckwagon

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Posted 22 August 2019 - 04:30 PM

You should calibrate after a firmware update.  Here is what I posted in another thread.

 

https://www.cloudyni...-rms/?p=9361391

 

Then try your tests again.


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#4 mehdymo

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Posted 22 August 2019 - 04:32 PM

Unfortunately I didn't check the PA as the guiding result was good and I thought it is good enough. The tracking star was close to M33 which I think it was below 30 degree from horizon and far enough from polaris. 

Possibly.

Have you looked at your PHD logs to see what PHD thinks your polar alignment error was?

Was the star it was tracking below about 35 degrees above the horizon, or within 20 degrees of the NCP?

 

The dirty little secret of RA tracking is that a constant-speed motor -- no matter how accurate -- will not accurately track the movement of a star across the sky.  Because that star's apparent motion is not constant.  So there are places in the sky where, if your PA is nuts-on, an encoder-equipped mount can track very, very well for quite some time.  But there are also places where that won't happen, no matter how accurate the encoders are...



#5 mehdymo

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Posted 22 August 2019 - 04:34 PM

Charles,

I saved your post yesterday and actually that triggered this post :) thanks for your useful post.

You should calibrate after a firmware update.  Here is what I posted in another thread.

 

https://www.cloudyni...-rms/?p=9361391

 

Then try your tests again.


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#6 mehdymo

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Posted 22 August 2019 - 04:37 PM

I am wondering if you have tried this and have you seen any change after re-calibration.

Charles,

I saved your post yesterday and actually that triggered this post smile.gif thanks for your useful post.



#7 Chuckwagon

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Posted 22 August 2019 - 05:04 PM

I didn't compare any results from before I calibrated to after.  So I can't say if it had much impact.

 

However, if you read through the rest of that thread from my previous link, you will see where I did some testing to try and get data for Andrew to examine to look for SDE.  My logs indicate guiding performance of around RA RMS of 1.68" and DEC RMS of 1.20" with a Total of 2.07".  That's from an urban location with pretty poor seeing most of the time.  And I was using a much longer FL guide scope and much shorter exposures.  Your results are much better than that.  Some of them are showing a total RMS of only .51", which isn't bad.  smile.gif

 

I'm a bit confused by the pictures though.  There are 5 pictures and only 4 scenarios. 

 

1- Guiding on and Encoder on
2- Guiding on and Encoder off
3- Guiding off and Encoder on
4- Guiding off and Encoder off

 

And since you cannot actually turn the encoder off, just filter whether it allows RA guide commands or not, 3 and 4 are the same, no guiding, so the results should be the same since the Filter RA setting has no impact if no guiding is done.

 

Do your calibration, then redo your guiding tests.  Use the same star, guide settings, etc. for each test.  But change your scenarios to be;

 

1- Filter RA setting on and Guiding off

2- Filter RA setting on and Guiding on (the guide commands will get filtered, so they should not have an effect)

3- Filter RA setting off and Guiding on

 

Test 1 and 2 should give the same results.  And if the 25EC behaves as the other EC mounts have been recently reported as behaving, test 3 should give the worst results.

 

Cheers

 

 


Edited by Chuckwagon, 22 August 2019 - 05:05 PM.

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#8 mehdymo

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Posted 22 August 2019 - 05:26 PM

Charles,

 

Good point, yes 3 and 4 should be the same if we assume the encoder is always on (is it?). Two of the pictures with good results are for guiding only where the encoder is not used(=RA filter is off). If you click on the pictures, the title shows up below the image and tells you which one is which. Your tests #1 and #2 can be the same only if the polar alignment is 100% accurate and there is no need to correct the DEC because (when guiding is on and RA is filtered, the DEC is still being corrected). Currently, your #3 gives me the best result! and I expected to get the best result by your #2. I hope calibration fixes this. Also, the seeing was very bad and there was a very thin layer of cloud on the sky that can be seen in guidescope screen in some images.

 

Thanks again for your help.



#9 Chuckwagon

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Posted 22 August 2019 - 06:09 PM

Charles,

 

Good point, yes 3 and 4 should be the same if we assume the encoder is always on (is it?). Two of the pictures with good results are for guiding only where the encoder is not used(=RA filter is off). If you click on the pictures, the title shows up below the image and tells you which one is which. Your tests #1 and #2 can be the same only if the polar alignment is 100% accurate and there is no need to correct the DEC because (when guiding is on and RA is filtered, the DEC is still being corrected). Currently, your #3 gives me the best result! and I expected to get the best result by your #2. I hope calibration fixes this. Also, the seeing was very bad and there was a very thin layer of cloud on the sky that can be seen in guidescope screen in some images.

 

Thanks again for your help.

The encoder is ALWAYS on.  smile.gif  The only thing we can do is tell the mount to accept RA guide commands or not.  So even when you are guiding, and the mount is accepting guide commands, the encoder is still doing its thing.

 

The reason I listed only three tests is because test #1 is just your mount's native unguided performance.  Test #2 is really the same test, since the mount is going to filter and ignore any RA, though it will make DEC corrections.  It's test #3 that is interesting; does guiding help or hurt the performance of the mount.  Since there is no encoder on the DEC axis, you don't really care about the test results where DEC is concerned.  You can learn about if the DEC axis guides decently or if you have a lot of backlash or not.  But you can probably just look at the RA RMS results while testing the effects of the encoder.  You might even consider not guiding DEC at all just to limit variables.  And there is no Guiding off and Encoder off possibility, so that's not really a test, so only 3 scenarios.  smile.gif

 

PA will be the same for all tests, as will the drift induced by any PA error, so long as you use the same session to do the tests and use the same star. If you test near the meridian and celestial equator, and run about 10 minutes for each test, the conditions should be similar enough for all three tests as not to introduce significant variables.  

 

I know you were hoping to see that the guided test was the best, but results from quite a lot of other EC users has shown otherwise, so it wouldn't be surprising if the 25EC was the same.  If the CEM25EC behaves in the same fashion as the other EC mounts have been behaving, the guided performance results will be the worst of the three scenarios.  And aside from DEC results, the other two tests should be the same, unless DEC guide commands interfere with RA tracking, in which case the unguided test should be the best.  And if DEC commands interfere with RA tracking, that's a whole different can of worms I'm sure iOptron won't want to hear about.  smile.gif   But it would be nice if the CEM25EC didn't suffer in the same way the other mounts seem to be.  So I hope your guided test turns out best.

 

Good luck.


Edited by Chuckwagon, 22 August 2019 - 06:13 PM.

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#10 mehdymo

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Posted 22 August 2019 - 06:49 PM

Thanks, I will give it another try after recalibration if the FL weather permits. It is two months it rains every day (and night :) ).



#11 mehdymo

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Posted 23 August 2019 - 12:15 PM

I tried again last night. Trained the encoder and took multiple image (Thanks to Charles). I am not sure if it was because of the encoder training or the better PA, but it got a lot better. Here are the results:

1- Guiding was giving very good results: RMS < 0.5" (See the guiding results)

2- With polar alignment error of 2 arcsec, 3-5 minutes unguided exposure was possible. (See the PHD2 results [note that RA does not start from 0, it starts from 2"] and Veil Nebula images)

3- Polar alignment and balance play a critical role in the encoder performance.

 

Check the title of the images for more details.

 

My conclusion:

- The specified PE of 0.3" by Ioptron is for a perfect world and can not be achieved I guess. However, unguided exposure can be done easily.

- The better the PA, the better the unguided results. With a normal PA, unguided imaging up to 3 minutes is easy. My RMS was less than 1" in 3 minutes, so for larger pixel scales, longer exposure is possible. I guess unguided 5 minutes exposure with a small refractor and ASI1600 (pixel scale of ~ 2 arcsec/pixel) is possible. 

- Guiding will give excellent results which is fantastic in this price range. It gives me the RMS of below 0.5" all the time without any tweaking, I guess 0.3" is possible by working on worm meshing, lens and counterweight side unbalancing, PHD2 patameters, and mount calibration. 

 

I would say it is a good mount for unguided imaging with a small refractor and pixel scales of ~ 2". 

Attached Thumbnails

  • Guiding on Veil Nebula near Meridian.jpg
  • Guided on M31-30 degree over horizon.jpg
  • Unguided-RA Filtered+5minutes-On Veil Nebula near Meridian.jpg
  • Single__7_2019-08-22_23-40-31_Bin1x1_180s__-15C-small.jpg

Edited by mehdymo, 23 August 2019 - 12:25 PM.


#12 Chuckwagon

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Posted 23 August 2019 - 03:51 PM

Well that's great news!  Glad it worked well for you. 

 

I guess the guiding problems of the other EC models is less pronounced (maybe not an issue) on the CEM25EC.  Some of the speculation from some of the users having issues was pointing to load size.  So maybe because the 25 really isn't going to face large loads, that problem just doesn't pop up.  In any case, it's good to see that at least your sample of the CEM25EC guides without major headache.  :)

 

One question; did you notice if you were getting much RA drift?  Which tracking rate were you using, Sidereal or King?  Some folks have had issues with RA drift on the other EC mounts, so I thought I'd ask if you noticed any.  If you had drift, it reduces the level of unguided performance, so it would be interesting to know.  Plus, if it's an issue across all of the EC mounts' theniOptron has some work to do to get their tracking rates more precise.  (I know they've addressed some issues in firmware release previously, but if the problem still persists it'd be nice to know.)

 

The .3" spec is measured at the encoder, not in the real world.  And it's an RMS result, so it doesn't mean there won't be larger deviations from time to time.  For us to measure the accuracy requires a less precise method than just reading the encoder output.  And as soon as you introduce an atmosphere into the mix, all bets are off.  Some places never have <2" seeing, so trying to verify if the mount can actually track with .3" accuracy is sorta pointless in those places.  Besides, in places with poor seeing, slight tracking inaccuracies would be swallowed by the seeing.  I'm happy if it tracks well enough to be within the pixel scales I shoot at.  I never guide my 25, because I'm almost always using a pixel scale over 2" per pixel.  For example, last night I was shooting with a 24mm lens at 20.6" per pixel.  That's forgiving enough that the mount meets the need. 

 

But it is nice to see that if I do need to guide sometime, it seems like a decent possibility.  :)



#13 mehdymo

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Posted 23 August 2019 - 04:15 PM

I was using sidereal tracking rate. I saw it in discussions previously, but unfortunately I forgot to test it. I noticed that when I guide the DEC only, the result becomes better which mean that if the PA is good (no drift in DEC) and balance is well, it guides very good. I can say if PA is good enough and you balance on the target direction, 3 minutes exposure is quite possible. There was a low amount of drift which I think is less than the pixel scale on each exposure. What I liked about it was the very good guiding results. If guided, it gives RMS of ~ 0.5" all the time and as I said it becomes lower if you tweak it. I would say for pixel scales over 2" and small refractors, it is quite possible to do unguided exposure with this mount. 

Well that's great news!  Glad it worked well for you. 

 

I guess the guiding problems of the other EC models is less pronounced (maybe not an issue) on the CEM25EC.  Some of the speculation from some of the users having issues was pointing to load size.  So maybe because the 25 really isn't going to face large loads, that problem just doesn't pop up.  In any case, it's good to see that at least your sample of the CEM25EC guides without major headache.  smile.gif

 

One question; did you notice if you were getting much RA drift?  Which tracking rate were you using, Sidereal or King?  Some folks have had issues with RA drift on the other EC mounts, so I thought I'd ask if you noticed any.  If you had drift, it reduces the level of unguided performance, so it would be interesting to know.  Plus, if it's an issue across all of the EC mounts' theniOptron has some work to do to get their tracking rates more precise.  (I know they've addressed some issues in firmware release previously, but if the problem still persists it'd be nice to know.)

 

The .3" spec is measured at the encoder, not in the real world.  And it's an RMS result, so it doesn't mean there won't be larger deviations from time to time.  For us to measure the accuracy requires a less precise method than just reading the encoder output.  And as soon as you introduce an atmosphere into the mix, all bets are off.  Some places never have <2" seeing, so trying to verify if the mount can actually track with .3" accuracy is sorta pointless in those places.  Besides, in places with poor seeing, slight tracking inaccuracies would be swallowed by the seeing.  I'm happy if it tracks well enough to be within the pixel scales I shoot at.  I never guide my 25, because I'm almost always using a pixel scale over 2" per pixel.  For example, last night I was shooting with a 24mm lens at 20.6" per pixel.  That's forgiving enough that the mount meets the need. 

 

But it is nice to see that if I do need to guide sometime, it seems like a decent possibility.  smile.gif




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