Last updated: 25 May 2006
Subject: LNT Summary Update Sent: Thursday, May 25, 2006 11:18:51 From: firstname.lastname@example.org PE Series LNT Q&A Summary 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 battery powered clock to provide time/date, similar to an internal PC clock; 2) Sensors to automatically detect Level and North; 3) A SmartFinder, consisting of a lens, LED, and optical fiber that projects a dot onto the lens. It is securely attached to a bracket that is mounted on the OTA, with a cover that holds the SmartFinder lens and optical fiber. The cover is attached with spring-loaded screws that allow it to be adjusted to align the SmartFinder with the 'scope optical axis. How does the LNT work? The module has a serial data interface with the AutoStar via the `scope wiring thru 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 sensors are an array of accelerometers and magnetometer that can determine orientation with respect to the Earth. 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) Download and store the current time/date from the clock. Note that once time is loaded from the LNT, the AutoStar maintains its own clock and no longer uses the LNT clock while powered on; 2) Modify the menu tree to include LNT functions and options. If the Auto align option is selected, Autostar performs the following; 1) Initialize the sensor array; 2) Drive the OTA until data from the sensors indicate it has detected North in the horizontal plane, and the point where it is perpendicular to the gravitational field, 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; 3) Correct the data for True Level and True North using previously stored values and save the results for the current session; 4) 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; 5) Determine the position of the AZ full CCW stop, which was initially set by the user, relative to North; this is used to determine which way to slew without hitting a stop during GOTO's; 6) 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; 7) When alignment is completed successfully, turn off the Dot. & nbsp; Any corrections obtained will be temporarily saved in the AutoStar for the current session. NOTE: If Autostar does not detect the LNT clock, it will default to 8:00PM on the last date that was set and request user input. Most of the LNT functions will not be available, with the possible exception of the Smartfinder dot. The most common cause, besides outright failure, is a dead or dislodged LNT battery. See Appendix D of the Manual for info on replacing it. 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. There is no defined "Home" position as with the AT series. Since the LNT will determine Level and Tilt, the mounting surface or tripod head and OTA need not be perfectly level. Also, orientation of the base can be somewhat arbitrary. I suggest the base be oriented so the full CCW stop is somewhere on the West side. This is so the LNT does not detect the South magnetic pole and give a false null as it slews CW during initialization. This has caused problems in some versions of the Autostar firmware. 3) The SmartFinder provides a much larger FOV, but no magnification, so star alignment is somewhat simplified. Does the LNT have any disadvantages? Although the concept of the LNT is good, its implementation leaves something 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. A later version appears to be a bit better, but its position is still not very precise and there may not be enough adjustment to align the Finder. 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 is very susceptible to external forces. A small bump can disturb the alignment and a sharp blow can break it off. I also 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 should be set accurately; b) The observing site lat/lon/time zone must be set; these value s are used to provide approximate correction for local magnetic deviation, which can be as much as 20 degrees depending on geographic location; c) A Motor Calibration and Drive Training should be performed; d) 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 correction coefficients used after the LNT determines local Level/North. After all the preceding setup steps are complete, scope power should be cycled off/on and time/date/site location parameters verified. There have been several reports of Autostar "forgetting" the values when shut down. If a reasonable amount of care is taken during the setup steps, GOTO errors should be minimized. Please note I am not affiliated with Meade and the preceding info was determined entirely through my own analysis. Any corrections and/or additional info on the subject will be welcome and appreciated. Regards to all, Mike Hogan
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