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The SkyCommander unit velcro'd to my base
The SkyCommander DSC's are "Digital Setting Circles", or some people call it a digital telescope computer. Hooked up to my dob's alt and az axes via encoders, they feed information back to the computer to tell it where your telescope is pointing.
This allows it to perform a number of features, such as being able to select the object you want to view, and it will tell you in which direction to move your scope. The readout changes as you move the scope, and when both the alt and az coordinates read zero, then the object should be in the field-of-view in your eyepiece.
It can also work in "Search & Identify" mode, where it will tell you what objects are visible within 0.5 degree of where your telescope is currently pointing. Very handy for identifying a mystery fuzzy or cluster that you stumbled across.
It's similar to a GOTO system on computerised telescopes such as the LX200 range, with the exception that this does not move the telescope for you. It's often referred to as a "push-to" system, because you're still pushing the telescope to the right position, and the computer is telling you when to stop pushing :) And of course there's still no tracking, unless you track independantly using an EQ platform or similar.
To use a digital telescope computer, you need encoders that feed movement information back to the computer, and mounting hardware to hook the encoders up to your alt + az axes. In the case of my dob, the altitude encoder fits on the alt hub on the OTA, and the azimuth encoder fits on the central pivot bolt that connects the ground board to the dob base.
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The azimuth encoder connected to the dob base
I bought my unit second hand from IceInSpace Forum member westsky, and it included the computer, the encoders and the mounting hardware for a 10" dob - so it suited me perfectly.
Installation was fairly quick and painless, as westsky had machined up the mounting hardware to fit a 10" GSO dob and so there was little for me to do.
The azimuth hardware had to fit on the central pivot bolt on the base, so westsky made up a new bolt that screwed into the ground board from underneath and had a hole in the top for the encoder to fit into. I had to drill a 20mm hole through both the ground board and dob base, and attach the stainless arm holding the encoder to the inside of the dob base via another screw hold near the handle of the base.
The altitude hardware screwed into the alt hub on the OTA, replacing the screw that held the spring. The encoder simply fit into this, and the stainless arm holding the encoder was secured at the other end onto the screw that held the other end of the spring. I had to cut out a new notch in the stainless steel bracket to reach the bolt, but that was all that was required.
The installation required the removal of one of my springs, but that was no problem since I only ever use one anyway. It also means that to take the OTA off the base, I need a screwdriver to detach the encoder from the alt-hub.
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Setting up and using the unit is quick and painless. When you turn it on, it prompts you for todays date, and then you have to do a 2-star alignment so that it can work out where in the world it is. There is no need to point the OTA horizontally or vertically during initial setup.
For the 2-star alignment, it's best to choose 2 stars that are over 30 degrees apart. Once they're centered in a high-mag eyepiece, you press "Enter" on the keypad to align the second star, and repeat. Once aligned, you simply choose the object you want to look at and hit enter to get the coordinates to push to.
One annoyance with the alignment star catalog of 40 stars, is that at least half of them are northern hemisphere stars, meaning they're not visible from my location. That leaves only 20 stars to choose, and given that only half of those will be visible at any one time in the year, it really only leaves 10 bright stars in which to choose your 2 to align on.
When you push to an object and look through the eyepiece, if the object is not dead centre in the FOV, you can centre it in the eyepiece and then re-align on the object to update it's pointing coordinates, so the next object should be centered.
The unit has 9 catalogs and over 9000 objects in its database, including all of the Messier, NGC and IC catalogs, planets, double-stars etc. You can also program in your own objects with their RA/DEC coordinates for things such as meteors and comets.
Using the keypad, you can also find out the constellation, the type of object and the magnitude of the object you're looking at. There are lots of other features too, but you can read the manual if you're interested :)
You can also hook the unit up to a laptop with planetarium software, and if the software has drivers for the SkyCommander, you'll see a circle on the chart showing where your telescope is currently pointing. This gives you access to a full suite of objects with more information about them, and so you're not limited to the objects stored in the unit itself. Just move the telescope until the circle is pointing to the object on the chart that you're interested in.
The DSC's are the best investment I have made to-date on my scope. Instead of spending an evening finding half a dozen objects by starhopping - looking in charts, looking in the finder, back to the charts, look in the eyepiece etc, I can now spend the evening viewing 5 times as many objects simply by punching in what I want to view, and push the scope until it tells me to stop pushing :)
I used my DSC's extensively at the 2005 SPSP and it allowed me to view over 30 faint galaxies and other objects that I've never seen before, when I could've spent the night starhopping struggling to find a handful of them.
It doesn't mean you don't get to learn the sky, it simply means you get to view the objects you want to view. Learning the sky isn't done by starhopping, learning the sky is a personal choice that someone makes and can be done whether you have a dob, a push-to dob or a fully computerised goto scope.
I recommmend DSC's (whether it's SkyCommander, Argo Navis or any other brands/models) to anyone with a dob, it's really changed my observing experience for the better.