Ad Astra Observatory

TRICKS TO SOLVE THE DEC GEAR SLOP/SLOW MOTOR PROBLEMS:
DEC axis steps:
1)Before going any further, refer to Mike Dodd’s excellent article

over on the CGE group in his folder on Autoguiding Analysis, for
how to calculate your field of view and pixel guide rate for your
mount, based on your focal length. This information will be
required for the rest of this post to make sense unless you are
already familiar with these calculations. For mounts that
fail to successfully make it through a CCDSoft Calibration
routine, you can plug these numbers into the Autoguide parameters
for a workaround. For a quick and dirty overview:
Image scale for your scope in arc-seconds per pixel =

(206625/(Focal length of your scope x 1000) x Pixel size of your
CCD camera’s guide chip (not the imaging chip-different chip!!).

EXAMPLE: Using the ST-7 CCD camera (guide chip has a pixel size of
15–binned 2×2, this value would be 30–don’t forget this if
binning the guide chip during guiding!!:
206625/(1260 x1000) (1260 is the focal length with an F/6.3 reducer

attached) x 30 (the 2×2 binned pixel size of the guide chip) = 4.91
The image scale is therefore, 4.91 arc-sec/pixel ON THE GUIDE CHIP.

(Note, if you use the imaging chip and NOT the guide chip to
autoguide, you will have to redo these calculations. Example, the
imaging chip’s pixel size on my ST-7 is 9 pixels, while the guide
chip is 15).

Guide correction rate = rate of autoguiding/image scale

For the typical Celestron hand controller, you will see that the
Autoguide Rate default is 50% of sidereal. This works out to 7.5
arc-seconds per second guide correction rate. For our example
though, (remember, I set the DEC autoguide rate to 99%), it
is 14.85 arc-sec/pixel–this value represents 15 arc-sec
travel per second of time (which is sidereal rate) times 0.99 (from
the hand controller rate setting) for the value of 14.85.

Mount’s calculated pixel rate of correction on DEC (Y) axis:

14.85/4.91 (from the calculations above)= 3.02 pixels per second
2) Plug this number into both DEC (Y) autoguide parameters in
CCDSoft to start with when commencing autoguiding.
NOTE for the RA axis, I did not change the Autoguide rate from the
factory default of 50% (equals 7.5 arc-seconds movement per second
of elapsed time). So instead of using 14.85 for the value from
the above computation, I would use a value of 7.5 for the RA axis
calculations.

NOW, TO ADJUST FOR THE DEC MOTOR’S SLOW RESPONSE/GEAR SLOP: This

information applies if your situation is like mine, and you can’t
get a successful CCDSoft Calibration routine due to DEC backlash or
slow motor response. If you CAN get a good Calibration, forget the
below information!!
1) Put the CCDSoft onscreen mode into “crosshair mode”, then place
a target guide star into the center of the crosshair box.
2) With the CCD chip, follow the target star for around five

minutes on-screen (NO autoguiding, just watch it on-screen)to see
which way it drifts in the target box.
3) Using the CCDSoft direction buttons, take note of which buttons
you hit and re-center the target star. Example, if you had to
hit the “N” direction button in CCDSoft, then back over in the
Autoguide Parameters settings, you want to make sure you have
entered the above calculated numeric values for that direction,

so the mount will automatically move that direction when
autoguiding is enabled (ordinarily North is the same as the Y+
direction, if you have the CCD camera square to the scopes axes.
For the OTHER DEC direction (in this example, Y-, or South),put
the number “1000” in for it’s value. This has the effect of
turning OFF drive corrections in the opposite DEC direction, and
is not needed anyway, since the star will only drift in one DEC

direction for that session’s polar alignment, assuming you don’t
move your scope mount’s position or tripod legs after you’ve done
polar alignment for the evening. By turning off a non-needed
direction in Declination, you are making sure that the DEC motor
will never reverse itself during a guide session, so no backlash
can come into play. Also, by moving the target back to center
with the CCDSoft direction buttons (or the hand controller for

that matter), you are winding out the backlash in both gears
before you start guiding. This will give you much better
autoguiding. Always do this last, right before you start to
autoguide. The RA backlash is not a consideration with Celestrons
because the motor never reverses direction since you are making
guide corrections at less than sidereal rate, or by the motor
simply stopping or slowing down, but not reversing, in order to
make a correction during autoguiding. It’s a still good idea to

wind it out with the direction buttons last though, even in the
RA axis, to ensure that the backlash is pretty much gone when
autoguiding commences.
OPTIMUM PARAMETERS:
1) If there is some DEC drift, drift toward the south is the better
direction. This way, when you wind re-center the image as above
with the “N” direction button in CCDSoft, you are in effect

winding out the backlash in the DEC gears prior to autoguiding.
This is also in keeping with the “Up and to the Right” rule that
Celestron states in their manuals for final approach/centering of
the target prior to commencing autoguiding. Likewise, wind out
the RA backlash by pushing the “W” direction in CCDSoft just prior
to commencing autoguiding.

2) Keep the scope SLIGHTLY east-heavy, REGARDLESS of which direction
it is pointing, to keep backlash out of the RA gears. Note, this
means adjusting the counterweights slightly when crossing the
meridian for a new target.
3) Tube balance in the DEC axis is more iffy. Just set it slightly
rear-heavy and forget it, unless you have so many DEC tracking
problems that you suspect DEC gear backlash. You can repeat the
same procedures to get exact with the tube balancing if needed.

FINAL TIPS:

Sometimes you have to verify that your CCD camera and
CCDSoft “thinks” N,S,E,W are what they really are. If you are having
trouble getting this to work, look at the autoguider.log file that
CCDSoft creates on your hard drive and see which relays are kicking
in. Also, make sure to mount the camera square to the axes as
mentioned before for best results. Check the box in the CCDSoft
settings to turn “ON” the autoguider log file prior to commencing

autoguiding and study your results in the log to get a feel for
what the camera and mount are doing. Also, I can’t emphisize enough
the value of Mike Dodd’s article in the Files section of the CGE
group. This same information applies to the AS’s as well, and will
save you time and frustration if you have a mount that won’t
calibrate or autoguide well with CCDSoft or Maxim.