Equatorial mounts - Part 3
Dec. is equivalent to north-south; RA is equivalent to east-west. Learning which direction is which helps you understand how your mount's axes move
By Anton Vamplew
In the first two parts of our guide to equatorial mounts, we’ve looked at how to set up an equatorial mount so that it will do its job properly, making it easy to find and follow objects out there in space. A star, planet or nebula can be found by using its co-ordinates on a great imaginary sphere projected onto the night sky, with the Earth at its centre. This is called the celestial sphere. Finding a galaxy this way is almost identical to the way you locate places on Earth using the latitude and longitude system; you just imagine the grid projected onto the starry realm. The only difference is that on the celestial sphere, latitude is known as declination (or Dec. for short) and longitude is known as right ascension (or simply, RA).
Both of these systems work in exactly the same way as they do for locations on Earth. Declination (latitude) lines run parallel to the equator from east to west, while right ascension (longitude) lines run ‘up and down’, from north to south. Every object in the sky has Dec. and RA co-ordinates, and by using the Dec. and RA setting circles on your equatorial mount, you can point your scope to find anything in the sky with just these two figures.
Find a bright star
Assuming you’ve already polar aligned your scope as detailed last month, the first step to finding that galaxy is to make sure your right ascension setting circle is set correctly. For this you’ll need the RA co-ordinates of an easily found bright star, like Vega in the constellation of Lyra (see page 66). Vega’s co-ordinates can be found from a planetarium program like Stellarium or a star atlas.
Loosen the locks on both the RA and Dec. axes and move the scope until it is more or less visually aligned with the star, then use the slow-motion controls – and your finderscope – to zero in on the target. Now take a look at the RA setting circle dial. If this is your first setup, it might not be reading the exact RA position you found from an atlas or software. If this is the case, fear not: simply rotate the RA setting circle’s dial until the pointer reads the correct co-ordinate. The Dec. setting circle’s dial is fixed in the correct position, so you never need worry about this going out of alignment.
Now you can use the setting circles to find that galaxy, simply by moving the axes so that the setting circles match the galaxy’s Dec. and RA co-ordinates. You can find objects below naked eye visibility this way too.
The beauty of the equatorial mount now comes into play: as you gaze in wonderment at your galaxy, you only need to adjust the RA axis with its slow-motion control to keep it in your eyepiece as it moves from east to west across the sky. And if you find the occasional twiddling of the RA slow-motion control a little tedious, you can get a motor to attach to this axis, which will do the tracking for you automatically.
As for the declination axis, you don’t have to touch that or its slow-motion control until you want to look at a different object. Then you just look up the co-ordinates of your next quarry, and move the Dec. axis and the RA axis until the setting circle dials give the right readings.
So, a well-handled equatorial mount is pretty much the perfect solution to hassle-free stargazing. Well, almost; there is one thing it can’t do, and that’s to track an object all the way across the sky. There will come a point when the bottom of the telescope’s tube will bump into the tripod leg, especially if it’s a long tube. Luckily, there’s an easy way around this called a ‘meridian flip’, which is described below.
When the tube bumps the tripod…
To keep track of your quarry as it moves from east to west, you might need to do a ‘meridian flip’
If your telescope’s tube bumps into the tripod as you’re tracking an object moving with the night sky, rotate the telescope tube by 180º in right ascension.
Next, rotate the declination axis so that the telescope tube is pointing at the object again. You can use the declination axis setting circle to get back to the original spot.
You’re ready to begin observing again. The meridian flip, as it’s known, is often needed on objects that are at their highest in the sky, so the tube is pointing straight up.
This article appeared in the August 2010 issue of Sky at Night Magazine