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Last updated: 17 May 2005

Subject:	PE Series LNT Summary
Sent:	Tuesday, May 17, 2005 14:38:48
From:	mhogansr@comcast.net (mhogansr@comcast.net)
The following is rather lengthy, as it covers the results of my 
non-observing activities over the last five months.  Many of the 
items have been addressed at one time or another on your site, 
but I've tried to sort of consolidate them.  Some are obvious, 
others are not.  Do with it as you wish and I hope it is useful.

Nearly five months after ordering and paying for it, I finally 
have an ETX125PE that works almost as advertised.  It took an 
update of the AutoStar software, one replacement AutoStar, and 
two LNT/SmartFinder replacements.

During all the "downtime", I had some questions and, thanks to 
your informative site and some research, found some answers.  
Here's my Q&A.

What is the LNT?
It is an electronic module that provides several related 
functions intended to reduce required user input during AutoStar 
initialization and `scope alignment:
1) A crystal controlled, battery powered clock to provide 
    time/date, similar to an internal PC clock;
2) A sensor to automatically detect Level and North;
3) An LED and optical fiber to project a dot onto the 
    SmartFinder lens.

It is securely attached to a bracket that is mounted on the OTA, 
and has an adjustable cover holding the SmartFinder lens.

How does the LNT work?
The module has a serial data interface with the AutoStar via the 
`scope wiring and the HBX port, and is powered by the `scope 
power source.  The clock has its own battery and maintains time 
when the power is turned off.  The sensor is an array of 
magnetic sensors that can determine its orientation with respect 
to the local magnetic field. 

When power is applied, the AutoStar goes through its own 
initialization, including checking whether or not an LNT is 
present. If so, the AutoStar will:
1) Initialize the magnetic sensor array;
2) Download and store the current time/date from the clock.  Note 
    that once the initial time is loaded into the AutoStar, it 
    maintains its own clock and no longer uses the LNT clock;
3) Drive the OTA until data from the sensors indicate it is 
    aligned with the local magnetic field in both the vertical and 
    horizontal planes, relative to the center of rotation of the 
    array.  In the absence of other influences, this would be 
    Level and Magnetic North relative to the Earth's magnetic 
    field;
4) Correct the data for True Level and True North using 
    previously stored values and save the results for the current 
    session;
5) Drive the OTA in the AZ axis while monitoring any variation of 
    Level and calculate a Tilt coefficient, then save the result 
    for the current session;
6) Determine the position of the AZ full CCW stop, set by the 
    user, relative to North; this is used to determine which way 
    to slew without hitting a stop during GOTO's;
7) Turn on the SmartFinder dot and proceed with a standard 
    alignment with star positions calculated using parameters from 
    the preceding steps and stored site information;
8) When alignment is completed successfully, turn off the Dot. 
    Any corrections obtained will be temporarily saved in the 
    AutoStar for the current session.

From this point on the LNT serves no purpose except to turn the 
Dot on and off via the AutoStar "0" (Light) key. Note that any 
corrections determined during the process are only saved for the 
current session. When power is turned off, they are lost unless 
a Park Scope is used.  The only semi-permanent corrections are 
those obtained during Calibrate Sensors, which are saved until 
the AutoStar is Reset, or the Calibration is performed again. 

What are the advantages of the LNT?
1) The user only has to set the internal clock once, the first 
    time the system is used.  After that, the AutoStar will have 
    the correct time whenever power is applied.  It should be 
    remembered that the internal clock will have some drift, 
    similar to a PC clock, and should be checked/reset 
    periodically for really accurate time;
2) Physical setup of the telescope is much less critical.  Since 
    the LNT will determine Tilt, the mounting surface or tripod 
    head does not have to be perfectly level.  Also, orientation 
    of the base and OTA can be arbitrary since the LNT determines 
    the "home" position for beginning a star alignment.  The AT 
    series required the user to orient the base with the power 
    panel pointing West and the OTA level as the "home" position;
3) The SmartFinder provides a much larger FOV so star alignment 
    is simplified.

Does the LNT have any disadvantages?
Although the concept of the LNT is good, its implementation 
leaves a lot to be desired, especially the SmartFinder.
1) In the original version of the LNT, the optical fiber that 
    projects the Dot onto the lens was not always oriented 
    properly so the Dot wasn't visible.  In a subsequent 
    modification, the fiber is anchored with an adhesive but its 
    position is still not very precise and there may not be 
    enough adjustment to center the Dot during Finder alignment;
2) Also in the early version, the Lens could be easily inserted 
    the wrong way, so again no Dot was visible.  This also was 
    corrected in the later version but the position of the fiber 
    can still cause problems;
3) The SmartFinder Lens mounting is very susceptible to external 
    forces.  A small bump can disturb the alignment and I 
    understand it will not fit into a standard Meade hard case 
    without modification of the insulating foam;
4) The SmartFinder requires an on-axis eye position as opposed 
    to the right-angle view finder on the AT series, and there is 
    some parallax shift;
5) The LNT mounting bracket interferes with proper installation 
    of the LPI camera so it can only be inserted +/- 90 deg. or a 
    full 180 deg. from the normal position.  The resulting image 
    on a PC screen can be confusing;
6) A shortcoming, rather than a disadvantage, is that the 
    Instruction Manual for the PE gives no comprehensive 
    instruction as to the prerequisites for using the LNT for the 
    first time, and there are several:
a) The local time/date/daylight saving must be set as 
    accurately as possible;
b) The observing site lat/lon/time zone must be set;
c) A Motor Calibration and Drive training should be 
    performed;
d) The Dec circle must be set to "0" when the OTA is level, 
    if not already there.  This should be done using a bubble 
    level or similar method;
e) Calibrate Sensors must be performed.  The previous steps 
    should be completed first, since they all affect the 
    accuracy of values obtained during the procedure.  These 
    values will become the correction coefficients used after 
    the LNT determines local magnetic Level/North.

Any corrections and/or additional info on this subject will be
welcome and appreciated.

Regards to all,
Mike Hogan

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Copyright ©2005 Michael L. Weasner / etx@me.com
Submittal Copyright © 2005 by the Submitter
URL = http://www.weasner.com/etx/buyer-newuser-tips/2005/magnetic_compass.html