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-4-

y-directions

) from desired position

3. Calculate the duration of slews (x- and y-

directions) required at the current slew

(guiding) rate to return the star to the desired

position.

4. Issue command(s) to the telescope to slew in

the direction(s) required

5. Stop the slew(s) after the calculated

duration(s).

6. Repeat the process.

The duration of the slew may be altered (usually by

an “aggressiveness” setting) to over- or

undershoot the required slew in steps 4 and 5.

Generally, the position errors induced by improper

tracking (periodic error, inaccurate sidereal rate, or

poor polar alignment) are much smaller than the

sidereal rate. In other words, the errors that must

be corrected are much less than 15 arcseconds per

second. At a guiding frequency of 1 Hz, the errors

are typically a few arcseconds or so (depending on

seeing).

Typically in the past, MTSCP autoguiding for

LX200 telescopes has attempted to use the :Md#

and :Qd# commands (where d is the direction of

slew). For a LX200GPS using a guiding rate

setting of 66% (about 10 arcseconds per second)

and a guiding frequency of 1 Hz. This means that

the required slewing movements (at the guide rate)

are typically 300 milliseconds or less. The software

would calculate the duration of the slews (both x-

and y- directions), issue :Ms# and :Me#

commands (for a guide star that had moved

southeast of the desired position), then issue :Qs#

and :Qe# commands at the proper times.

In firmware version 2.0i and below, the :Qe#

command is followed by a pause of the sidereal

tracking, variously reported as 200 to 500

milliseconds. Imagine the effect this has on the

process. The star has just been moved back on

center (assuming an aggressiveness setting of

100%), but it is now allowed to drift eastward by

three to eight arcseconds, some of which may occur

while the guide camera is acquiring the image that will be

used to calculate the next corrections. This drift has not

been anticipated by the guiding software. When

the next frame from the guide camera is analyzed,

a large position error will be calculated and

appropriate commands issued to return the star to

the desired position. If the aggressiveness is set at

100% or higher, and if all of the tracking pause

occurred before the guide camera image for the

next iteration has been captured, the star will be

moved back to the desired position, only to be

once again immediately “knocked off position.”

The result will be oscillation of the guide star back

and forth with each guiding iteration. However, if

the aggressiveness is set to less than 100%, or if

tracking pause has not been completed before the

next frame is acquired by the guide camera, then

the guiding software will not be able to fully move

the star back to center. With each iteration, the

star will move farther east and will quickly leave

the frame.

This latter scenario is exactly what users of

K3CCDTools2 v. 2.1.3 and below have

experienced, as well as GuideDog users running

versions 1.0.4 and below. The software was

working correctly, but the telescope was not doing

as commanded. (Ironically, this problem does not

seem to have affected LX200 classic users.)

Neither Peter Katreniak (author of K3CCDTools)

nor Steve Barkes (author of GuideDog) have

LX200GPS telescopes (although Steve has a

LX200 classic), so they are both dependent on

user reports to assess the performance of their

software on our telescopes. Both Steve and Peter

are aware of the LX200GPS problems, and have

taken measures to address them (as described

below).

But the plot thickens.....

Undocumented Command

In an attempt to figure out what the heck is going

on with LX200GPS autoguiding, Web Dove used

PortMon to log the commands that were being

issued by autoguiding software through the serial

port of his computer to his LX200GPS. He did

this using Meade’s Autostar Suite and GuideDog

version 1.04. In doing so, he made an important

discovery. GuideDog was using :Md# and :Qd#

commands as described above to guide the scope.

However, he discovered that Meade’s Autostar

Suite uses an undocumented command.

Some autoguidng software defines “x” and “y” axes.

Normally, the x-axis is aligned with the RA axis, and the y-

axis is aligned with the declination axis.

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