What we call "Digital Setting Circles" today are light years away from what they were just a few years ago. When first introduced, all the DSC's did was replace the manual setting circles and verniers with an electronic display and higher accuracy. Today's DSC's do that too, but, they also contain a large database of celestial objects, descriptions of those objects, the ability to identify where the telescope is pointed and a guiding procedure to help you move the scope to a selected object. Beyond that, they can be connected to a laptop or PC computer for manually moving the scope while looking at an electronic star chart and acting as a database for some GoTo telescopes.
So, what exactly, do you get with a complete DSC package? The components consist of a "module"(the computer), two encoders, a connecting cable and some sort of brackets to attach the encoders to your mount. Most are powered with a 9volt alkaline battery. Just like any other computer, these devices are not smart, the weak links are the battery and the operator (you). My experience indicates that the battery will operate the DSC reliably for a longer time than the operator will. Also, as with any computer, you must do things it's way, not your way. The following discussion is based on the DSC's produced by Tangent Instruments. These include the Advanced Astro Master (Celestron), Sky Wizard (Orion), Sky Vector (Lumicon), NGC Max (JMI) and others. The Sky Commander, Gemini, Skywalker etc are not discussed.
The "module" is a plastic box with a digital display, four large white buttons, one small white button and one small black switch. The switch does nothing but turn the computer on and off. The small white button steps the display through five brightness levels. The four large buttons are labeled MODE, ENTER, UP and DOWN. All of the functions are controlled with these four buttons, even with gloves on. Inside the case is a printed circuit board with four integrated circuits, two are in sockets and can be replaced when upgrades become available. You will probably never need to open the case unless you are naturally curious or a born tinker.
Information as to where the telescope is pointed comes from the two encoders. One is attached to the RA (azimuth) axis and the other is attached to the Dec (altitude) axis. The computer will work with virtually any mount currently in use, equatorial or alt/azimuth. The encoders sense movement of the two axes and send that information to the computer in the form of pulses. Standard encoders are marked 2160. This means that when the encoder shaft turns exactly one revolution, 2,160 pulses are sent to the computer and counted.
The connecting cable and/or the computer cable, plugs into a phone jack in the back of the module. The jacks are different sizes and the cables only go in one way. The battery simply snaps in under a removable battery cover. Hookup is very simple. The encoder mounting brackets are different for each type of mount, but most are simple to install and the appropriate tools come with the mounting kit.
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The moment of truth has arrived. You have installed everything and done the tests and setup procedures described in the Owner's Manual. But, how does the computer find anything if it doesn't know where it is in relation to the stars? There are two basic telescope mount types, Equatorial and Altazimuth. Within each type of mount there are several variations, but the basic principals apply to all variations.
The most popular type of mount these days seems to be the Altazimuth personified in the "Dobsonian" mounts. The "Alt" part of the name is the "Altitude" axis which allows the tube to swing up and down. The "Azimuth" part of the name is the horizontal platform which allows the telescope to swivel around parallel to the ground. If you have selected "AZ" (or similar) for an Altazimuth mount, the computer will display "Level Me" each time it is turned on. Turn the tube down until it is parallel to the base, not the ground. It helps to set up the telescope up indoors using a carpenter's level to level the tube. Make a mark on the side of the box near one of the side bearings and another mark on the bearing to line up with the mark on the box. This step will make it easy to line up the scope in the field. (If the computer asks for "Vertical", the optical axis of the tube must be set perpendiculr to the base and add the marks). Press "ENTER". The first mode will then be displayed asking you to aline the telescope on the first of two stars. Locate a star in the sky which is on the menu, center it in the eyepiece and press "ENTER", select a second star from the menu, center it and press "ENTER", (one star should be in the East and one in the West, separated by at least 45 degrees for the most accurate alinement).Your telescope now knows where it is as related to the night sky.
My computer is set up for a German Equatorial mount, roughly Polar Aligned. When I turn on the computer it displays "Dec=0". What does that mean? The North Celestial Pole (NCP) is at Dec=90degrees. The computer wants you to point the telescope at the celestial horizon, which is 90 degrees south of the NCP and therefore at "Dec=0". There is more than one way to determine this position. If your telescope has manual setting circles, just center Polaris (if you can see it, I can't) in the field of view, set the Declination setting circle to 90, then turn the telescope so that it points East until the setting circle reaches zero. Another, sloppier but just as effective, way of doing it is to simply turn the telescope eastward until the tube is level (similar to the "Level Me" step for Altazimuth mounts).
To assist and speed up the process at the scope in the dark, I set the mount up in the house, leveled it and centered the weight shaft visually by sighting it against the tripod leg and locking it in position. I then placed a carpenters level in the dovetail slot and turned the Dec axis until the bubbles were centered. The two axes are now very close to being 90 degrees to one another. I then placed light colored tape over the separations between the movable parts of the RA and Dec axes and drew a straight line through the middle of the tape with a "Sharpie" and a straight edge. I then slit the tape where it bridged the fixed and moving sections of the mount. The tape is 1/2" x 1" yellow or white tape.
In the field, simply line up the marks and press "Dec=0". The marks make repeatability easy and remove one source of error in the nightly alinement process.
***** Many of the following steps apply to all mounts, so read through it even if you do not have an Equatorial Mount. *****
Press the ENTER button. You are now asked to select an alignment star from a list which is displayed when you press ENTER again. Let's choose "Alpheratz" (a Andromeda) which is to the East in early Fall evenings. Move the telescope and center Alpheratz in the field of view (FOV). Press the ENTER button once. A "Warp Factor" will be displayed. This represents the distance the telescope moved from the unknown Dec=0 position to the known position of Alpheratz (ignore this first warp factor). The computer now knows that it is pointed Eastward at Alpheratz.
The display will revert to "Align Alpheratz" which is your signal to choose the second star. Press the UP or DOWN button repeatedly until the name of the second star, Vega (a Lyrae), which is to the West, is displayed. Move the telescope to center Vega in the FOV. Press ENTER once. A much smaller Warp Factor will be displayed. The computer has determined the telescope position from two known locations. I repeat the process once more by recentering Alpheratz, selecting it from the list and aligning on it (smaller Warp Factor), then recentering Vega and aligning on it. The final warp factors should be between 0.0 and 0.5. Mine are typically 0.2-.0.3.
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You are now ready to ask the computer to guide the telescope to objects all over the sky. Press the MODE button, then the UP or DOWN button until "Catalog" is displayed. Press ENTER and ST*** will be displayed with the ST blinking. Because we want to find the Andromeda Galaxy we need to ask the computer for M-31. Press the UP button and the display changes to M001. Press the ENTER button until the second zero is blinking. Press UP until the second zero changes to three. Press ENTER again, the last digit is already a one, so press ENTER again. M031 and it's coordinates will be displayed. Press ENTER again and information about M-31 will scroll across the display. Press MODE, Press UP and GUIDE will be displayed. Press ENTER and you will be given directions and distances to move the telescope to place M-31 in the FOV. Move the telescope in both axes until all the numbers change to zero. M-31 should now be in the low power FOV. Center it, press MODE once, press UP or DOWN until ALIGN is displayed. Center M-31 and press ENTER to display ALIGN M031, Press ENTER again and a Warp Factor will be displayed, usually less than 0.5. You have just fine tuned the computer alignment to this part of the sky. After alinement, press MODE and the UP or DOWN button until CATALOG is displayed. This step will avoid accidental alinement on the wrong object.
How does it work? When you select an object from the catalog, center it in the FOV and press ALIGN, the computer stores the coordinates of that object in temporary memory. When you select a new object from the catalog, center it and press ALIGN, the computer subtracts the coordinates of the two objects, computes and displays the Warp Factor. I do not know what the Warp units represent. The GUIDE function works in a similar way. The computer has stored the coordinates of the last object you ALIGNed on and subtracts the coordinates of the new catalog selection. The difference in RA and Dec between the two objects is displayed as distance to move the telescope in degrees, the direction of movement is displayed as arrows rather than plus and minus values. As you move the telescope to the new object, the computer constantly reads the encoder data and subtracts the coordinates from where the telescope is pointed to where the telescope is going. When the coordinates of the destination are the same as the coordinates of where the telescope is pointing, the computer displays for RA and Dec are zero. The object should then be in the low power FOV.
Believe it or not, you will want to observe some objects not contained in the database. Press MODE, then the UP or DOWN button until "Mode RA DEC" is displayed. Press ENTER and the coordinates of where the telescope is pointed will be displayed. Move the telescope until the coordinates of the object you seek are displayed. It should be visible in the low power FOV.
If you stumble upon an object you like but can not identify, press MODE, then press UP or DOWN until "Identify" is displayed. Press ENTER. "Find ANY Mag10" will be displayed with the Any blinking. Press UP or DOWN to move through the menu until the type of object you want is displayed, then press ENTER again and the 10 will blink. The range is from mag-6 to mag-17, default is mag-10. Press UP or Down to change the magnitude. This will expand or reduce the number objects searched in the database. Press ENTER once, after a few seconds, the computer will display "Found ****". Press ENTER again for scrolling information about the object you have found. If you want to return to this object again on another night you will need to record the coordinates. Press MODE, then press UP or DOWN until "Mode RA Dec" is displayed. Press ENTER and the coordinates for the object will be displayed. Write them down for future reference along with any other notes you find useful.
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