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Meade Magellan II alignment and tracking.

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#1 dansawyer

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Posted 12 October 2019 - 01:49 PM

The mount is a Mead Magellan II. When powered on it blinks 3 times which should indicate the revision level. 

The mount is reasonably well aligned to Celestial North. Using the drift method and camera taking a picture every 10 minutes there is almost drift on either stars close to the azimuth or horizon in a 30 minute period. Next the Magellan alignment procedure is completed. This should set the scope up for 'push to' operation. 

However it does not. After many cycles and tests I have some observations and the result of one experiment. 

The first observed issue is when an object is selected and the scope manually moved to what should be the position the object is no where to be seen. I decided to test the accuracy of RA and Dec changes separately. I set the scope on an object and then moved the mount either RA only or Dec only. When moving in the RA axis the the Dec value remained steady. However when moving in Dec axis the RA value changed significantly. I assumed that the mount was trying to adjust for a perceived misalignment in the mount orientation. Since the changes were extraordinary, on the order of hours over a 90 degree Dec change this was not the result of polar alignment. 

Now the experiment. I first aligned the scope on the first target star. I then rotated the Dec axis to 90 degrees North and proceeded with the alignment procedure. This produced much better results. The procedure above produced relatively much less interaction between isolated Dec axis or RA axis movements. 

Has anyone else had similar observations? 

Is this a known issue with this revision level? 

Are there later software revision levels available? Is so can someone proved a link? 

Thank you all in advance. Dan 



#2 jdupton

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Posted 12 October 2019 - 02:10 PM

Dan,

 

   Check that the fork arms are properly adjusted. If the OTA had ever been taken off the arms and then replaced, it could out of alignment. Visualize what would happen if one fork arm were longer than the other. Since the RA and DEC axes are no longer orthogonal, a change in one can affect the reading of the other once you have star aligned the Magellan II.

 

  One classic test for fork alignment is to set the drive base on a level surface about 10 feet from a wall with about three or four feet available around the scope. Take off the front cap of the scope. Level the corrector plate so that it is pointing exactly straight up. Fix a laser securely to a tripod so that it cannot move. Point the laser to a spot just to the side of the secondary so that it bounces off the corrector. Now orient the fork arms so that a line between them points to the direction of the laser. 

 

   Now mark the point on the wall at least 10 feet away where the laser spot hits. Spin the fork arms (RA Axis) 180° and note where the spot hits now. Adjust the DEC axis slightly to get the points to be one above the other as you rotate the RA axis 180°. (As you turn the RA axis, the spot seems to move up and down rather than side to side.)

 

   If you cannot get the laser to hit the same spot vertically on the wall when the forks are along the line of the laser, then they are out of alignment (different length). You must realign them to a point such that you can rotate the RA axis 180° and not have the spot on the wall move up and down.

 

   It takes a little ingenuity to readjust the forks if they are found to be out of alignment. The adjustment points are at the bolts where the fork arms attach to the drive base. You must provide a small repeatably accurate "jack" of sorts to hold the weight of one fork and the OTA while you loosen the bolts to adjust the arm position.  I have not looked but there used to be Websites and videos that detailed how to run the test and then realign the fork arms.

 

 

John



#3 dansawyer

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Posted 13 October 2019 - 12:15 PM

John,

Thank you. I have had the forks off; I replaced the nylon sleeve bearings with needle bearings. I reassembled the forks on a marble machinist block using a precision level. You are correct; that was a bit of a fussy process. 

The issue seems to be that the Magellan is trying to correct for what it perceives to alignment errors. The system is set up carefully; alignment procedure completed; and then a couple of additional stars checked and added. If the controller is set to display RA and DEC, one axis is locked, and then the mount is swung slowly through the second axis the value of the locked axis changes.

This does not appear to be an encoder issue. When the system is powered up and the display set to the raw encoders the results are as expected. There is no interaction between the axis values and after server smooth swings through plus or minus 60 degrees or so on both axis the values return to near 0.

This is likely a software issue.

Do you know of any software upgrades? 

I have noticed code segments in some of the Yahoo group posts. Is there a full code list somewhere? 

Thanks again, Dan 



#4 photoracer18

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Posted Yesterday, 05:51 PM

JMI's encoder setup doc says the all the mounts they made for use + polarity. Make sure in the Magellan setup neither encoder is set to negative polarity as in -4000 (tics) as an example. both yours should say + before the encoder count. Also if your gears are not 1:1, meaning the gear on the axis and the gear on the encoder shaft are the same diameter, then if not, you need to tell the DSC that the actual encoder tic value is the encoder label number x the gear ratio (4000 tic encoder plus 2:1 gears equal 8000 tics per axis revolution).




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