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Mesu 200 RA speed correction


tomato

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For a while now, I have thought that the RA speed on my Mesu 200 was not at the correct setting to achieve optimum guiding. I found this very helpful thread on SGL and have attempted to follow the method to correct this:

Mesu 200 RA Guide Speed Correction

However, I have encountered an interesting observation when trying to implement the correction. I aligned the camera so that a star tracks along the line of the reticle cross hair on the image when moving in RA only. I have then let the mount track in RA for 10 minutes to measure the length of the star trail and so calculate the error in the speed. However, when I examined the image the star trails are not parallel to the reticle line, but at a small angle to it. This means the mount has moved  both in RA and Dec, but how come this was not apparent when I did the test slew in RA? The only difference is the test slew was done at a much faster speed and for only 5 sec duration, vs the error measurement which was done at the sidereal rate for 10 minutes. Is this PA error or something else?
 

Note I never get Orthogonal errors when PHD calibrates.

Edited by tomato
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Where are you pointing when doing this?

Only thing I can think of is check you don't have any CalPoints (PointXP file loaded). 

There is a setting in the config under Mount Pams tab>Freeze Declination (Equitorial only) not sure if it does anything as the Mesu is set as a  German Equ. 

 

Peter 

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TBH I can’t remember if I had synced the mount or not prior to running the test. I think the scope was pointing at alpha Cass.

I never measured the length of the star trail as it threw me when it didn’t run parallel to the horizontal axis of the image. I was going to adjust the camera to make it so but yes, you’ve guessed it, it went cloudy and has been so ever since.

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Another explanation can be astronomical refraction.

As you track the star - it changes Alt coordinate - gets higher in the sky and its apparent position changes from the true position.

https://en.wikipedia.org/wiki/Atmospheric_refraction

(see section astronomical refraction)

Since you are tracking at an "angle" - if you are not tracking due south - some of motion goes into RA component and some goes into DEC.

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Perhaps not so accurate but you can test/adjust the ticks within ServoConfig select Edit Parameters (2nd down) & load from the controller, next select Tick Determination (5th down) this opens Assist Encoder Ticks Determination, use the lower half to record RA ticks... here you'll use the Scope Encoder location for adjustment of the Motor Encoder ticks. 

1/ set the Ending Deg's to 180.0000 default is 360.0000 (crash setting) 

2/ using the hand pad move the mount in RA until counterweight bar close to level with weights to the west 1st movement is east to west (clockwise) 

3/ click Step 1: Align (Start) this sets both RA encoder to zero

4/ click Step 2: this moves the RA 180 degrees & records both RA motor & mount encoder ticks, look at the Scope Encoder Location if not 10000 move mount with hand pad to make it so. 

5/ click Step 3: Aligned (saves motor ticks for averaging) 

6/ click Step 4: Move to Start, this time the Scope Encoder should be 0 if not adjust with hand pad to make it so. 

7/ repeat steps 3/ to 6/ another 4 times this gives an average of both directions that can be saved to the controller... alternatively copy the average from the clockwise side if more bothered about tracking than pointing.

Applies to Mesu mk1 only no stars needed daytime job no responsibility taken for errors in the above if in doubt please read sitech manual before proceedings! 

 

Peter

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Hi Peter,

Thanks for this, if we get some clear sky soon I’ll try the star drift method, as I can directly see the effect on the guiding performance, but if the grey skies persist I will give your method a go.

Steve

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19 minutes ago, tomato said:

Hi Peter,

Thanks for this, if we get some clear sky soon I’ll try the star drift method, as I can directly see the effect on the guiding performance, but if the grey skies persist I will give your method a go.

Steve

Hi Steve, 

 Ok first off the Scope encoders are 10000 ticks per 360 degs. section 4/ should be 5000 & not 10000 release this as in writing below. 

I've tried it in both RA & Dec, for some reason the Dec Scope 10000 tick encoder always went from 5000 to 0 with 180 degs. of movement but the RA always needed a little tweek... not sure if balance has any effect!

If you try the Tick Determination please would you make a note or screenshot/photo the results, I'll do mine again & post them here for reference. 

 

Peter

 

 

 

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Thanks for the tip, I have succeeded in measuring a star trail over 10 mins and have a value of  42.7 arc secs in 10 mins, 4.27 arc secs per minute. I was about to use the PHD Guiding assistant  as a second measure but  it has clouded over...😟

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On 28/10/2021 at 00:10, tomato said:

For a while now, I have thought that the RA speed on my Mesu 200 was not at the correct setting to achieve optimum guiding. I found this very helpful thread on SGL and have attempted to follow the method to correct this:

Mesu 200 RA Guide Speed Correction

However, I have encountered an interesting observation when trying to implement the correction. I aligned the camera so that a star tracks along the line of the reticle cross hair on the image when moving in RA only. I have then let the mount track in RA for 10 minutes to measure the length of the star trail and so calculate the error in the speed. However, when I examined the image the star trails are not parallel to the reticle line, but at a small angle to it. This means the mount has moved  both in RA and Dec, but how come this was not apparent when I did the test slew in RA? The only difference is the test slew was done at a much faster speed and for only 5 sec duration, vs the error measurement which was done at the sidereal rate for 10 minutes. Is this PA error or something else?
 

Note I never get Orthogonal errors when PHD calibrates.

What is you guiding RMS with the mount? When it's not windy outside, I can go as low as 0.2" total RMS with the Mesu, even 0.17". Do not attempt to change settings in Sitech Config like ticks/rev, without talking to mr. Mesu first or take a picture of your settings. I have the same settings since I bought the mount and everything is just fine. The guiding RMS will vary with seeing conditions, altitude (better near zenith than at lower elevations), balance, guide camera arc sec/pixel vs main scope arc sec/pixel. Check other factors before fiddling with Sitech Config. 

Posted a few guide graphs with my Mesu on the Mesu thread

 

For balancing I use a pressure dial gauge and I use almost the same image scale for scope/main camera vs scope/OAG/guide camera. As an example try to bin your guidecamare x2 or x3 so to be above your main scope-camera combo image scale. You will see a falsely improved guiding

Edited by dan_adi
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Thanks for your advice but I cannot see this being an irreversible change. If you read the imbedded thread in the first post you will see I am following the method as provided by Lucas. If the revised tick number doesn’t work I can always revert back to the original setting.
 

The giveaway is the RA is constantly correcting in the same direction and the guide points in PHD are well clustered but offset from the the centre in RA. I see this effect with two very different scope configurations on the mount (dual Esprit 150s, and RASA8/SY135 combination) so I don’t think it is balance related. I suppose I could  try running it with just a guidescope centrally mounted on the Mesu, but that’s a lot of work, quicker to just give the tick adjustment a try.

The mount performance is fine but if I can fix this error then I think it’s worth doing if nothing else it will cut down on corrections being sent to the RA motor.

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3 hours ago, tomato said:

Thanks for your advice but I cannot see this being an irreversible change. If you read the imbedded thread in the first post you will see I am following the method as provided by Lucas. If the revised tick number doesn’t work I can always revert back to the original setting.
 

The giveaway is the RA is constantly correcting in the same direction and the guide points in PHD are well clustered but offset from the the centre in RA. I see this effect with two very different scope configurations on the mount (dual Esprit 150s, and RASA8/SY135 combination) so I don’t think it is balance related. I suppose I could  try running it with just a guidescope centrally mounted on the Mesu, but that’s a lot of work, quicker to just give the tick adjustment a try.

The mount performance is fine but if I can fix this error then I think it’s worth doing if nothing else it will cut down on corrections being sent to the RA motor.

It is not an irreversible change for sure. To know the balance is ok, you need some measurement. One way is with a force dial gauge, measure the newtons, or measure the current draw for RA and DEC with a voltmeter. Otherwise you can't really be sure, but I found the friction drive is quite tolerant to imbalance ... 

From the old thread, after the tick correction, the total RMS was 0.66". That means without the correction, the error was worse. Even 0.66" for a Mesu is a little high in good conditions, but maybe the conditions were not great, or other factors were at play. 

Also if RA is correcting in the same direction, maybe the aggression in RA, in phd, is not set correctly. Try increasing the aggression and lower hysteresis. Best to post a guide log on the phd forums and see what others have to say about it. 

If in good conditions (seeing, wind, balance etc. ... ) you get below 0.5" RMS, I see no reason to touch the tiks/rev

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Yes, I thought the Mesu was quite tolerant to imperfect balancing, but maybe not so forgiving when the payload is pushed. @Tomatobro is looking at ways of measuring the loads on the mount to achieve a more precise balance.

With an OAG I have had guiding at around 0.5” RMS, but again with RA corrections all in the same direction, this is after using the Guiding Assistant  settings in PHD.

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7 hours ago, tomato said:

Yes, I thought the Mesu was quite tolerant to imperfect balancing, but maybe not so forgiving when the payload is pushed. @Tomatobro is looking at ways of measuring the loads on the mount to achieve a more precise balance.

With an OAG I have had guiding at around 0.5” RMS, but again with RA corrections all in the same direction, this is after using the Guiding Assistant  settings in PHD.

Guess you've already checked your drift is the same both sides of the meridian? 

Peter 

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22 hours ago, tomato said:

Yes, I thought the Mesu was quite tolerant to imperfect balancing, but maybe not so forgiving when the payload is pushed. @Tomatobro is looking at ways of measuring the loads on the mount to achieve a more precise balance.

With an OAG I have had guiding at around 0.5” RMS, but again with RA corrections all in the same direction, this is after using the Guiding Assistant  settings in PHD.

In my experience with the Mesu, the mount can go below 0.5" RMS on a regular basis with an OAG, if the conditions I mentioned earlier are met. Do you use a refractor on it? Or a mirror scope? In my experience the guiding is better with a refractor on the mount. I don't use phd anymore, so others can give you better advice to phd settings. What other causes can make RA corrections in the same direction? Also the RA drive tracks in the same direction, is it not?, and phd simply slows it down and/or speeds it up. So you are saying phd always tries to slow it down or speed it up, with no alteration in between?

 

Edited by dan_adi
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I can have 6” refractors on the mount, or an 8” RASA, depending on what I want to image. For small galaxies I’m imaging at 0.94” per pixel so I use the OAG, the RASA set up is more forgiving on guiding, where I use a guidescope.

The RA corrections are on the same direction,  both sides of the meridian, always  to slow the RA speed down.

@Tomatobro is making an electronic device to measure the forces required to move the mount and I will use this to improve the balance before implementing any tick count changes. Balancing a dual set up with a RASA on one side and a Samyang 135 on the other presents quite a challenge.

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8 hours ago, tomato said:

I can have 6” refractors on the mount, or an 8” RASA, depending on what I want to image. For small galaxies I’m imaging at 0.94” per pixel so I use the OAG, the RASA set up is more forgiving on guiding, where I use a guidescope.

The RA corrections are on the same direction,  both sides of the meridian, always  to slow the RA speed down.

@Tomatobro is making an electronic device to measure the forces required to move the mount and I will use this to improve the balance before implementing any tick count changes. Balancing a dual set up with a RASA on one side and a Samyang 135 on the other presents quite a challenge.

With dual refractors, balance is a bit harder indeed. All you need is a pressure dial gauge to measure the force needed to move the mount. They are accurate. Good luck, tell us what you find ... 

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3 hours ago, dan_adi said:

With dual refractors, balance is a bit harder indeed. All you need is a pressure dial gauge to measure the force needed to move the mount. They are accurate. Good luck, tell us what you find ... 

Can you please provide a link for the type of pressure gauge you have? 

 

Peter

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13 hours ago, whipdry said:

Can you please provide a link for the type of pressure gauge you have? 

 

Peter

Similar to this https://www.hans-schmidt.com/en/produkt-details/force-gauge-fb/

You can simply measure the newtons needed to move in RA and DEC. I needed one because I have around 40 kg on the mount and the moment arm of a 8 inch frac is important, so I need good balance. The fine adjustments are made with 2 or 5 kg sliding weights on the top Losmandy plate. 

Edited by dan_adi
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