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Tutorial News

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My astronomy project:
EQ6 Mount, dismantling / rebuilding


  1. Introduction and Disassembling
  2. Internal gearbox alternatives
  3. Needed belt length, measurements
  4. New timing belt and pulleys has arrived
  5. Lids to worm gear adjusted
  6. Lathe of 60 teeth pulley
  7. Installing 60 teeth pulleys
  8. RA and DEC shaft shims
  9. Test installing timing belts and motors
  10. Installing RA timing belt and motor
  11. Installing DEC timing belt and RA shaft into chassis
  12. Testing and adjusting
  13. Power up and EQMOD gear ratio setup
  14. Hang up the telescope, balancing and calibrating parking point
  15. Fine adjusting RA and DEC worm gear play
  16. Drift align and auto guide calibration
  17. Setup EQMOD and PHD2 (moved to tutorial section)
  18. Test of auto-guiding
  19. Deep into the RA drift compensation
  20. Replacing the stepper motor driver
  21. New design of latitude adjuster
  22. During work

I take no responsibility or liability for what are written here, you use the information on your own risk!

16, Drift align and auto guide calibration:

Away with my girlfriends flowers from the balcony and out with the mount and telescope. It will be a clear sky this night, highest priority! Gunilla loves astronomy too :-)

Drift Align, altitude adjustment:

EQ6 Mount, Drift align and guide calibration Star map Credit CdC.

From our balcony I can't see the polar star, still I have to do a polar align. I use the drift align method. The free auto guiding software PHD2 has one implemented. I control the EQ6 mount over the EQMOD driver through the star chart software CdC or Skychart.

I start to adjust the altitude, from the star map I choose a star close to the celestial equator (DEC = 0), in my case in Eastern direction. But the Western direction work as good too, depends in which direction you have a clear sight. I found a star 3 degrees above the celestial equator and ten degrees above the horizon. The star must be bright enough to hold down the S/N ratio. In my case stars of magnitude 8 or brighter will do.

More to read about polar align at Wikipedia:

EQ6 Mount, Drift align and guide calibration Mount controller Credit EQMOD.

In the EQMOD window I can see the coordinates the telescope is pointing to.

EQ6 Mount, Drift align and guide calibration Drift Align, credit Open PHD2 Guiding.

After I have started PHD2 and initiate the mount and guide camera I open the drift align window, choose altitude adjustment. These software and drivers communicate over the standard ASCOM routine.

EQ6 Mount, Drift align and guide calibration

When drift align in work I can follow the DEC drift, the red line in my case. Look at the trend, is it upwards or downwards. When it's perfect the red line will be horizontal. In the window you can also see the polar alignment error, it's expressed in arc minutes / seconds or pixels. You must also do the azimuthal adjustment, that influence this altitude adjustment. Redo these two adjustments one or twice until it's stable.

I noticed how the guiding try to slow down the RA tracking. It's like the RA axis rotate to fast, strange, did I setup the gear ratio wrong, or? I don't think I have set it up wrong, because when I move the RA axis 12 hours or the DEC axis 90 degrees the scale of the mount indicate it's correct, at least not any big error.

Drift Align, azimuth adjustment:

EQ6 Mount, Drift align and guide calibration

Switch over to azimuth adjustment, this image is before I move the telescope to the Meridian at the celestial equator. Note the notes I have written down in the window. It can be very confusing in which direction to do the adjustment, you need some support in the form of these notes.

EQ6 Mount, Drift align and guide calibration

In the end I got the error come down to one arc minute, much better than I have achieved before. Still this strange behavior of RA.

You are always reading the DEC trend, both for altitude and azimuth adjustment !

EQ6 Mount, Drift align and guide calibration

I noted later that the latitude scale pointed to 68 degrees, that's not where I live, I live at 59 degree.

EQ6 Mount, Drift align and guide calibration

Checked with my spirit level, found it correct, I must have placed the scale in wrong direction.

Now it's time for a setup of the EQMOD and PHD2 that control the mount. It was in this project but now moved to a tutorial:

Auto guide calibration:

EQ6 Mount, Drift align and guide calibration

With polar align correct I can start with the auto guiding calibration. Take a star in Southern direction close to the meridian and at the celestial equator (DEC = 0).

EQ6 Mount, Drift align and guide calibration

After a while PHD2 gives an alarm and complain about the orthogonality between the RA and the DEC axis. Maybe something related to the strange behavior of the RA rotation speed I noticed earlier. This is the first time I have it setup with a non standard gear ratio, maybe some mistake I did.

EQ6 Mount, Drift align and guide calibration

More information from PHD2, the orthogonality error is 14 degrees, the mount is made with the axis 90 degree apart with high precision so the error should be almost zero. But there are also other things that can cause this error, see the message above.

EQ6 Mount, Drift align and guide calibration

RA rate and DEC rate looks ok, close to the equator the speeds should be equal.

Drift compensation:

I remember that it was a setting for drift compensation. I think it was something about some error or bug in the driver for the stepper motors in EQ6 mounts.

EQ6 Mount, Drift align and guide calibration

In my old configuration I have to set the drift compensation to 6 to get the correct speed of RA. Because I now have a different gear ratio this drift compensation is maybe different. I try with new values and got it almost correct with -18. I let it track without auto guiding and looking in the the guide window of PHD2 at which drift compensation speed the star stay at the same position. It's easier to see if you place a hair cross in the window. Let it track some minutes and you will see. You also see it on the guide pulses when auto guiding, if it has to send guide pulses all the time in one direction, then RA moves with wrong speed. A wrong polar align can also cause this.

Auto guiding:

EQ6 Mount, Drift align and guide calibration

Even if the calibration wasn't perfect I want to do a test with auto guiding, I'm exited to see how it perform. Any difference ? No, after all these works! But I see that the RA behavior is correct, the guiding pulses is symmetric.

EQ6 Mount, Drift align and guide calibration

I redo the calibration but got even worse orthogonal error. The drift compensation I added, is it included or not in the figures above ?

EQ6 Mount, Drift align and guide calibration

I have increased the RA Agr. to 75% and zoom in on the time axis to have more details, I get down the tot RMS error to 0.94". Maybe not bad but I could achieve that with the old spur gears too.

EQ6 Mount, Drift align and guide calibration

Expand the time axis to see how the RA guiding perform at a longer time scale. There's a clear periodic pattern, slowly variations should be possible to auto guide and then cancel the error. I have to try out better guiding parameters. Already now the auto guide tot RMS error is below 1" so there is hope.

PHD2 setup parameters:

EQ6 Mount, Drift align and guide calibration

A short look at the algorithms of PHD2. The backlash compensation is interesting, it have found it to be just below a half second. Just wonder how it has found this value, I take a guide picture with two seconds apart. Later when I used the mount for a while I will check the tension of the timing belts if they still are correct. Too loose and it will introduce backlash.

New problems to solve, why is the RA and the DEC axis not orthogonal? And the guiding error I have, can it be better with some adjustment of the guiding parameters?

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To page XVIII, Test of auto guiding

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