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Mesu 200 RA Guide Speed Correction


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I previously had problems with my RA guide speed being too slow and as a result PHD was correcting RA always in the same direction and could not guide efficiently.

I decided to use Artemis (my ccd capture software) to track a star for 10 minutes and determine how many pixels it moved .

I made sure that my RA axis corresponded with the up and down lines of my cross-hair in Artemis and then slewed a star to the middle of the cross-hair.

I then placed my mouse pointer on the middle of the star and read the pixel number shown at the bottom right of the Artemis software window (for RA the pixel number is the second one)

After 10 minutes I placed the mouse pointer on the middle of the star again and jotted down the new pixel number.

I subtracted the first number from the second.

In my case the star moved up and moved 91 pixels.

I then multiplied 91 x 1.11 (the pixel scale of Atik 383 and Quattro 10")

This gave me 102 Arc-seconds of movement in 10 minutes.

Lucas Mesu Kindly helped me out with the next bit and I eventually managed to understand the Maths.

I launched Servo Config and  clicked " Edit Parameters" to get the data from my controller.

My number of ticks per scope revolution in the Ra Motor was 7949474.

In 10 minutes the Ra makes 9000 Arc seconds.

This is the Maths as explained by Lucas:

In 10 minutes the Ra makes 9000 Arc seconds.

Divide the error (102) by 9000=0.0113333333333333

0.0113333333333333 x 7949474=90094

7949474 - 90094 = 7859380

7859380 is the new number to put in the Ra Ticks per scope Rev window in servo config.

In my case this made it worse so instead of subtracting 90094 from my original ticks per revolution of 7949474, I added it to obtain my corrected number of 8039568.

Now my mount is tracking efficiently in Ra too and my guiding problems have been sorted too.



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I have tracked your progress with great interest. Although my equipment is very modest, I am also having a year of transformation, from camera on a tripod to Newtonian on guided EQ mount with integrated ASCOM control and plate solving. I hope to be ready to really use as intended for the winter season. Clear Skies, and keep looking up.--Jack

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Phew, Simon, this is a relief to me so Lord knows how relieved you must be! A simple incorrect parameter, in the end. Such is life.

How long are your guide subs? Our Mesu thrives on about 4 seconds.

All well after your op, too?

Clear skies,


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Thanks Olly,

My guide subs are 3 seconds at the moment.

Once I corrected the Ra guide speed I was thrilled to see the star stationary in the middle of my cross-hair in Artemis.

I went straight to the fridge of the Bar and bought up a beer to the Observatory and gleamed at my Mesu.

My hip is feeling really great, I walked for an hour yesterday. I have a bit of tendinitis of the tendon in the groin, I have to take it a bit more easy.

Here is a video clip 3 weeks after the op.

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But you mean RA "tracking speed" not "guide speed", correct? This may be very helpful for me and my Avalon M Uno. I've also noted the my mount tracking speed is slightly off in RA and I get westerly star drift consistently, even after accurate polar alignment. Although it still guides very well, it has always bothered me. There is a tracking rate adjustment slider in my Avalon Stargo software that I have played with that significantly reduces this drift, but I didn't know how to calculate the amount of error. Thanks!


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  • 2 weeks later...

I have replaced my center spot on my main mirror and have been able to collimate with more accuracy.

It was a Cats Eye Hot spot and It was a bit ragged around the edges.

My stars are beginning to look better.

Before my stars in the upper right corner of my images were elongated.

I found that a more precise collimation resolved this and the tilt issue I had before too.

Here are two screen shots of the CCD inspector results of a 15 sec sub.






Attached are two screen shots of my CCD Inspector results of a 10 min sub






Attached is the link to the fits of the 10min sub of M13.



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  • 2 weeks later...

I made a mistake in the last CCD test by pointing the scope at m13 and the image was overexposed.

Below is a better effort with a 1200 sec sub of the Pellican. CCD Inspector shows that my Focuser is aligned and collimation is ok.

Attached is also the 1200 sec pellican sub to check the stars for guiding errors.




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