Subject: PE Series LNT Summary Sent: Tuesday, May 17, 2005 14:38:48 From: email@example.com (firstname.lastname@example.org) 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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