How to master setting circles
Would you like to locate celestial bodies with greater ease and accuracy? Then follow this simple guide.
Written by Ade Ashford.
The plaintive call we’ve heard from many a novice telescope user is, “I can never find anything. What am I doing wrong?” Very often there’s a simple reason that is easy to rectify. First, always check that your finder – typically a small, wide-field-of-view optical instrument mounted like a gun sight on the side of the scope – is correctly aligned. Then you can be sure that what you see at the centre of the finder’s eyepiece will also be visible in the main telescope.
But what if the object you wish to locate is too faint to see in the finder, or maybe it’s in an area of sky that you’re unfamiliar with? There are three ways to track it down.
This is the process of following a trail of interlinked and easily recognisable patterns of faint stars starting from a readily identifiable naked-eye star in the general area of your target. You will need a detailed star atlas, or charts printed from a desktop planetarium package to a scale large enough to match the field of view of your finder/telescope.
A helpful hint to remember here is that at a magnification of about 50 times, your scope’s field of view is around 1º wide, or twice the width of the full Moon, while most optical finders have fields of around 7º.
The coordinate conventions we use in the sky are extensions of their familiar terrestrial counterparts, latitude and longitude. Just as latitude ranges between zero and 90 degrees north and south of the Earth’s equator, so too does declination range between zero and 90 degrees north (+) and south (-) of the celestial equator.
Whereas terrestrial longitudes range between zero and 180 degrees east and west of the Earth’s Greenwich meridian, the celestial counterpart – right ascension – is measured in hours, minutes and seconds over a zero to 24 hour range.
Still perceived by some old-school observers as cheating, the modest prices of entry-level computerised telescope mounts make them very appealing to beginners as well as experienced observers. After performing a brief alignment procedure at the start of an observing session, the mount’s computer can accurately model the sky above and will automatically drive the scope to an object selected from a pre-programmed menu and track it. However, for all their convenience, Go To telescopes are anathema for those with a dislike of computerised technology, whining motors, or a reliance on power sources in the field.
The third and time-honoured method of locating celestial objects is to make use of the setting circles built into an equatorial mount. They offer a convenient way of locating what you want to see by ‘dialling in’ their positions from the coordinates published in catalogues and, indeed, this magazine. In last month’s How To we showed you how much easier it was to track celestial objects once the equatorial mount’s polar axis was made parallel to the Earth’s axis of rotation. Accurate polar alignment is an essential prerequisite to the following procedure.
The best way to find a faint object from its catalogue coordinates is to offset from a readily identifiable nearby object like a naked-eye star. The reason for choosing a starting point reference object close to your intended target is that it minimises any errors you may have inadvertently made in setting up and aligning your mount. Use a low-power eyepiece to give you a wider field of view, too.
The rest of the procedure is best illustrated by the example described below and pictured in the step-by-step guide. It involves finding the popular summer deep-sky object the Dumbbell Nebula (M27), which is in the relatively obscure Milky Way constellation of Vulpecula. M27 lies about 24º from the brilliant blue-white star Vega in the constellation of Lyra. Vega, then, will be our starting point. Consulting a catalogue gives us the following set of coordinates:
RA = 20h 00m dec. = +22.7°
RA = 18h 37m dec. = +38.8°
Loosen the clamps locking the axes of the mount and swing the scope to point at Vega, carefully centring it in the field of view before locking the axis clamps once again. Note the declination reading for Vega; if it doesn’t read close to +39°, very carefully adjust the indicator pin or the declination scale until it reads the correct value (consult the manual that came with your scope if necessary). Note that checking and adjusting the declination scale is something you only have to do once with a new mount.
Check that Vega is still centred in the field of view and slip the right ascension scale until it reads the value of the star (again, consult your telescope manual for specific instructions). The coordinate reference frame of the mount is now ‘locked’ to that of the sky. Unlock the declination axis and move the scope down (south) by just over 16° until it reads the value of the Dumbbell Nebula: +22.7°. Lock the declination axis once more.
Now unlock the polar axis to adjust the right ascension. Since M27 has a right ascension greater than Vega, you need to rotate the polar axis by 1h 23m in an anti-clockwise direction (as viewed from the north) until the right ascension scale reads 20h 00m. Carefully lock the polar axis and look through the finder or the main instrument. You may need to use your slow motion controls to refine your pointing, but the soft glow of the nebula should be there to greet you.
A step-by-step guide to using your mount’s setting circles
Always follow the manufacturer’s instructions for aligning your scope’s finder with the main instrument. Choose a low-power eyepiece and sight along the tube to a distant and well-defined landmark to centre in the main scope’s eyepiece. Without moving the tube, adjust the alignment screws of the finder bracket until it shows the same feature at the centre of the finder’s view.
Accurate polar alignment is an essential before using setting circles on an equatorial mount. Once you are confident that the polar axis of your mount is pointing as close as possible to the north celestial pole, track a star near the celestial equator for 10 minutes. If the star slowly wanders north or south in the field of view you will need to refine your alignment.
Use the equatorial mount’s setting circles to find the Dumbbell Nebula by offsetting in declination and right ascension from Vega, a star that lies nearby. Once that’s centred in the field of view, check that the mount’s declination scale does indeed read the correct value. If you have a new mount, you may need to adjust the declination scale or pointer to read the true value of +38.8º.
With the polar (right ascension) axis and declination axis locked, check that Vega is still centred in the field of view. You may need to refine the scope’s pointing if necessary. Adjust the right ascension scale (see your mount’s instructions for precise details) until Vega’s catalogue value – 18h 37m – reads against the pointer. You may need to interpolate if the scale isn’t graduated so finely.
If you’re using a manual equatorial mount on which the right ascension scale moves with the stars, carry out the following procedure as swiftly as possible to avoid errors creeping in. Loosen the declination axis and move the scope down (south) by a fraction over 16º until the scale reads the value of the Dumbbell Nebula: +22.7º. Lock the declination axis once more.
Unlock the polar axis clamp and rotate the scope by the difference between the right ascensions of Vega and the Dumbbell Nebula. Depending on how the scale is calibrated on your mount, you need to rotate the scope about the polar axis by 1h 23m in a clockwise direction looking from the north until the scale reads 20h 00m. Lock both axes and you will be very close to your intended target.
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This article appeared in the July 2005 issue of Sky at Night Magazine