SOFTWARE AND HARDWARE AUTOGUIDING …

1 -1- SOFTWARE AND HARDWARE AUTOGUIDING ISSUES WITH THE LX200 GPSIf you ve had problems getting K3 CCDT ools, GuideDog or other AUTOGUIDING SOFTWARE to successfullycontrol your LX200 GPS through the RS-232 port, it may not be your mistake or faulty SOFTWARE that iscausing the problem. This paper explores a number of issues related to AUTOGUIDING through the RS-232 port, and suggests SuderNovember 2004 IntroductionMany LX200 GPS owners have been frustrated intheir attempts to perform remote guiding of theirtelescopes using serial commands directed to themount through the RS-232 port. This paperexamines some of the issues potentially causingthis frustration, and presents some solutions tothese problems. Information presented here has been developed bytesting my 10" LX200 GPS with several of themost widely-used freeware packages, with serialport monitoring programs, and with somediagnostic programs I wrote with Visual Basic.

2 Ialso learned a great deal from information postedon the ASCOM-Talk, Meade LX200 GPS, MeadeLX200, Meade Uncensored and QCUIAG Yahoodiscussion groups, as well , the MeadeAdvanced Products Users Group( ), the Meade Instruments webpage, a number of other LX200 GPS users, andauthors of some popular AUTOGUIDING paper should be considered a work inprogress. Meade continues to release new versionsof firmware for the LX200 GPS, and softwarecreators continue to develop new drivers andapplications - all of which may affect theinformation presented paper does not address guiding through theLX200 GPS autoguider port. Guiding in thismanner is an entirely different matter, generallyusing HARDWARE specifically designed for the task( , purpose-built CCD autoguiders).

3 Theautoguider port of LX200 and other telescopesand mounts expects, rather than serialcommunications, contact closures to commandguiding movements. While not exactly plug andplay, there are a number of readily available(although expensive) and relatively problem-freesolutions for AUTOGUIDING through the autoguiderport. Many LX200 users have reported goodsuccess with these Control of the LX200 GPS viathe RS-232 ConnectionMeade computer-controlled telescopes (such asthe LX200, LX90 and ETX models) can becommanded remotely by serial commandsdelivered to the telescope through the RS-232port. (See Figure 1.) A connection between theLX200 GPS and the serial port of a personalcomputer (or other device such as a PDA) can bemade using Meade s #507 connection cable.

4 Theconnection cable, which has a RJ-11 (telephone)connector at one end and a female DB-9connector at the other, can be easily put togetherin a few minutes using the instructions posted and elsewhere. Meade Telescope Serial Command Protocol(MTSCP) is a series of commands, sent to thetelescope from a personal computer or otherdevice, that duplicate many of the capabilities ofthe Autostar II handbox. Meade has published alisting of the serial command protocol These commands can be generated bysoftware run on the personal computer or otherdevice. Not all of the commands available on theAutostar II handbox are included in the MTSCP;however, some commands are available throughthe MTSCP that are not available via the handbox. Most commands simply cause the LX200 GPS to-2-Figure 1.

5 Use of the RS-232 portdo something, but some return information ( ,current pointing position or OTA temperature) tothe external SOFTWARE packages such as Meade sAutostar Suite, as well as shareware and freewareapplications ( , Cartes du Ciel, Iris andGuideDog) can generate commands for theLX200 GPS, sent over the serial connection. Forinstance, in Cartes du Ciel, a user can select anobject or point in the sky with the cursor, andcommand the telescope to slew to that object. Unfortunately, not all of the MTSCP commandsare implemented correctly in all versions of theLX200 GPS firmware. For instance, the command:SBn# (where n is a single ASCII digit between 1and 9) is supposed to set the baud rate of theconnection between the LX200 GPS and theexternal device.

6 However, I have found thatissuing this command to a scope running destroys the connection with the telescope(that had been working perfectly). Once thiscommand is issued, communication with thetelescope cannot be reestablished until thetelescope is rebooted by powering it off and MTSCP commands that are needed forguiding applications are those that set the trackingrate and slewing rate, and those that move or stopthe telescope. These are presented in Table with MTSCPAs indicated by the fourteen-page listing of theMTSCP offered by Meade, the command set isrobust, offering the opportunity to perform mostof the handbox commands remotely. If allcommands worked as they are described byMeade, the MTSCP would be adequate fordeveloping guiding and AUTOGUIDING SOFTWARE .

7 Unfortunately, such has not been the case. WebDove, Dan Johnson, I and others have testedmany of the commands with firmware , and and found that a number of thecommands either don t work as they are describedin the MTSCP, cause complete loss ofcommunication with the telescope until it isrebooted (power down and power up again), ordon t work at all. Following is a description of theproblems that may affect guiding operations(slewing and guiding commands).East and West Movement Commands ReversedIn at least one firmware version ( , I think), thecommands to move east or move west at thecurrent slew rate (:Me# or :Mw#) are reversed. Commanding an eastward slew produces awestward slew Tracking Rate Doesn tWorkAs described in the MTSCP, the commands :T+#and :T-# are supposed increase or decrease thetracking rate of the telescope.

8 Based on a modelwhere a Hz synchronous motor causesperfect sidereal tracking, these commands areintended to increase or decrease the tracking rateby Hz. In my testing of firmware , these commands had no effect. In fact, noneof the commands below the horizontal line inTable 1 have any discernable effect on aLX200 GPS running either firmware or Pauses After Slew CommandIn versions and (and probably earlierversions), there is a slight pause of the siderealtracking (the RA drive) immediately followingexecution of the :Me# or :Mw# Thepause is short, just a few hundred milliseconds. Nonetheless, it causes an apparent eastward driftof the target of several arcseconds. (This effecthas been described as ruberbanding because aguide star behaves as if it is attached to a rubberband stretched from the east side of the FOV.)

9 This is not a concern for visual use, but is a big1 Because each :Md# command is followed by a :Qd#command at the end of the intended slew interval, it is likelythat the pause actually follows the :Qd# for imaging. Not only does the pausemove the target off center (blurring the image), itmakes it extremely difficult to accurately return thetarget to the desired position. Returning the targetto center with MTSCP commands can only beaccomplished by purposely overshooting theeastward slew to account for the followingtracking pause. Obviously, this makes guidingwith MTSCP commands during imaging Do Problems with FirmwareImplementation of MTSCP AffectAutoguiding?In order to understand how these problems affectattempts at SOFTWARE -driven AUTOGUIDING usingMTSCP commands sent via the LX200 GPS RS-232 port, it is necessary to consider howautoguiding is performed.

10 Briefly, autoguidingsoftware performs the following the centroid of a guide star in an imageframe (from the guide camera) how far the centroid is (in the x- andCommands that work correctly in firmware and :Mn#Move telescope north at current slew rate*:Ms#Move telescope south at current slew rate*:Me#Move telescope east at current slew rate*:Mw#Move telescope west at current slew rate*:Q#Halt all current slewing*:Qn#Halt current northward slewing*:Qs#Halt current southward slewing*:Qe#Halt current eastward slewing*:Qw#Halt current westward slewing*:RC# Set Slew rate to Centering rate (2nd slowest):RG# Set Slew rate to Guiding Rate (slowest):RM# Set Slew rate to Find Rate (2nd Fastest):RS# Set Slew rate to max (fastest) #Set guide rate to +/- to arc seconds per second.)