Digital Setting Circles

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Magellan I Telescope Computer System for the LX10

magellan.jpg (124411 bytes)The Magellan I Telescope Computer System gives you the ability to locate astronomical objects much more quickly and accurately than just using the standard setting circles on the LX10.  The Magellan I computer comes with two optical encoder wheels for attachment to the RA and DEC axes of the LX10.  These wheels subdivide each rotation of the RA or DEC axis into 4096 units (i.e. a unit is 0.09 degrees).  As you move the LX10, the wheels inform the Magellan how many units you have moved the Telescope in RA or DEC.  This allows the Magellan to work out where the LX10 is pointing provided it knew the where the LX10 was pointing when it was powered on.  Telling the Magellan I where the LX10 is pointing when it is powered on is fairly simple.  The Magellan provides a list of bright, well known stars for use in alignment.  You pick a star from the list that is currently visible, centre the star in the eyepiece of the LX10 and then tell the Magellan that the star is centred.  You repeat this for one more star on the list and the Magellan is aligned.   From now on the Magellan will always know where the LX10 is pointing.

The Magellan has a database containing the positions of 12,218 celestial objects, including the Messier list and the complete NGC catalogue.  Once the Magellan is aligned you can select an object from the database and the Magellan will show you how far you need to move the telescope in order to reach the object.  The accuracy of this operation is highly dependent on a number of factors which are not fully explained in the Magellan Manual.  There is a discussion on the best alignment techniques below.  I wasn't getting very good results from my Magellan until I started using these techniques but am now getting very accurate results and find the Magellan an indispensable piece of equipment when trying to find faint objects for CCD imaging.

The Magellan has an RS232 port so that it can be connected to a PC.  This allows the position of your telescope  to be plotted against the night sky by certain astronomy software packages.  You cannot buy the RS232 cable to connect the Magellan to a PC and the manual that comes with the Magellan is not very clear on how to make one.  I have described how to build such a cable here.  I have successfully used the Earth Centred Universe planetarium software with my Magellan.  In my opinion this is an excellent program at a very reasonable price.

The Magellan uses batteries very quickly when connected to a PC through the RS232 link.   The main problem with this is not the battery expense (you could use rechargable batteries) it is the frustration of the Manegllan powering off in the middle of an observing session causing you to have to replace the batteries and repeat the alignment.   I have avoided this by connecting my Magellan to an AC/DC convertor that supplies a regulated 13V.  It is important that the convertor is regulated otherwise you risk damaging the Magellan.  I made the cable that connects the Magellan to the power supply simply by attaching some bell wire to a 9V battery connector. The battery connector at the end of the bell wire is plugged into the 9V battery connector in the Magellan and the other end of the bell wire is attached to the convertor (being careful to make sure that the polarity is correct).  I snipped the corner off the Magellan's 9V battery cover so that  that my power cord can come out of the unit when the battery cover is in place.  I could have left the battery cover off but I was concerned that the Magellan's electronics would be exposed to moisture in the atmosphere.

Aligning and Using the Magellan I

Two Star Alignment

The following techniques were developed by Nigel Burge.

First, accurately level and  polar align your telescope. Then you should give careful  consideration to the two alignment stars you will use.   The two alignment stars should be between 25 degrees and 75 degrees apart (Nigel usually tries for 45 degrees apart) and as close to the celestial equator as possible.  The first alignment star should be where you intend to first observe since accuracy will be best around this star.  If the separation approaches 80 degrees, accuracy suffers enormously.  For example, Hamal and Castor are 82 degrees apart and if these are used as alignment stars, the accuracy decreases as you approach the zenith and can easily be 30 degrees out.  On the other hand, if Hamal and Aldebaran are chosen which are 37 degrees apart, accuracy is very good throughout the sky.  Common sense would indicate that a wide separation would be better so why is this not the case? If the algorithm the Magellan uses is a cosine function, it would be inaccurate at a separation of less than 25 degrees (which is what the handbook says) and would also be inaccurate at a separation approaching 90 degrees (which would appear to be the case here).

As long as you accurately centre on ANY alignment stars, Magellan will give you an accuracy reading of about 100. This is purely a measure of how accurately you centred the stars and polar aligned the telescope.  It is NOT a reflection of the accuracy you will get when using the Magellan to find objects.  The Magellan can report an alignment accuracy of 100% when the alignment stars are close to 90 degrees apart and yet can be terribly inaccurate when it comes to finding an object.

Time Saving Tip: If you are performing a two star alignment you do not need to set the DEC to 90 degrees as requested by the Magellan alignment procedure.  When the Magellan asks you to set the DEC to 90 degrees simply press ENTER until you are requested to pick the first alignment star and then proceed with the alignment procedure as normal.

Once you have performed the two star alignment, accuracy will be very good in the region of the first alignment star chosen, to within ten degrees.  Anywhere outside that region and the accuracy will fall off. However it is easy to  synch on any bright star in the region in which you wish to observe to bring the accuracy back.  The Magellan manual only gives a few stars and implies that this is all that there are.  This is not the case.  Magellan has the same star catalogue as the LX200 comprising 251 alignment stars and 100 double stars.  You can download a copy of this list, which shows star number, position, Bayer letter and proper name, by selecting the format you require from the list below.

Before you begin a nights observation, make a list of the objects you want to find and look up each one's brightest star neighbour in the above list to determine its Meade star number.   Now use the following procedure to find each object with a high degree of accuracy:

  1. Enter the Meade number of the object's brightest star neighbour into the Magellan.

  2. Follow the Magellan's directions to locate the star.

  3. Centre the star in the eyepiece.

  4. Synch on the star by pressing and holding the ENTER key for a couple of seconds.   Then press ENTER again when it asks you to centre the star.

This procedure updates the position information in the Magellan so that it will be very accurate in the region around the bright star.  You can now use the Magellan to find your object as normal.  This technique invariably puts the object very near the centre of the FOV of a 32mm  eyepiece.

One Star Alignment

This is the alignment procedure that works best for me.  After following Nigel's advice on two star alignment I was still getting the occasional problem with alignment accuaracy.   I then found that the Magellan can be aligned very accurately and repeatably just by doing a one star alignment.  First you must accurately level and polar align your telescope.   Then you perform the same Magellan operations as for a two star alignment except when you are prompted to select the second alignment star you press the MODE key.   Alignment is then complete. So far, I have chosen a number of different stars for one star alignment.  They have been in the South or West and near to the celestial equator.  Doing a one star alignment seems to avoid the hazards of the cosine arithmetic that the Magellan must use on two star alignment.  I still use Nigel's advice on syncing on bright stars near the object I am trying to locate.  That seems to work very well with the one star alignment technique.  So far one star alignment and synching has put the target object close to the centre of the FOV of my 25mm eyepiece every time.

Unlike the two star alignment process you must correctly perform the operation of setting the DEC of your OTA to 90 degrees.  I believe the accuracy of this operaion is important and is dependent on how accurately your DEC setting circle is callibrated.   To check the callibration of your DEC circle remove the fork mount from the equatorial wedge and place the drive base on a flat surface such as a table top.   Make sure the OTA is pointing straight up when you do this so that the assembly doesn't topple over.  Check that the drive base is level using a spirit level.   If necessary place shims under the drive base to get it level.  Now, move the OTA until the sides of the OTA are truly vertical.  Again, use the spirit level to check this.  Now you have a vertical OTA and a level drive base, so the OTA must be at 90 degrees to the base!  At this point you should lock the DEC axis.  Check you DEC circle.  If it does not read exactly 90 degrees then you need to adjust it.   To do this, slightly undo the Meade logo knob that holds the circle in place.   You should now be able to rotate the circle until it reads exactly 90 degrees.   Once it is reading exactly 90 degrees tighten the knob.