How to...align a telescope mount

Follow our guide to setting up your telescope allowing you to accurately track celestial bodies.

Written by Adrian Ashford.

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For prolonged astronomical observation and most forms of astro imaging, there is a preferred form of telescope mount: the ‘equatorial’. You will encounter it in one of two principal flavours. The ‘German’ is the preferred form for refractors and Newtonian reflectors, where the instrument is attached to the mount in a cradle balanced by a counterweighted shaft on the other side. The ‘equatorial fork’ doesn’t require a counterweight and is the norm for squat-tube mirror-lens systems like Schmidt- and Maksutov-Cassegrains. Both of these forms of mount have two axes of rotation, one of which – the polar axis – has to be made parallel to that of the Earth (see box, ‘Equatorial mounting’).
    
Polar alignment
Northern hemisphere observers have a convenient marker to the north celestial pole in the form of Polaris, the Pole Star. For very casual viewing with a portable mount, merely sighting along the polar axis to Polaris will suffice. Some equatorials possess a hollow polar axis equipped with a small sighting telescope to assist you. Unfortunately, the true celestial pole lies nearly a degree (just under the width of two full Moons) from the Pole Star.
 


Equatorial mounting
The equatorial mount possess two axes of rotation, one of which is designed to be aligned parallel to the Earth’s axis of rotation. This so-called polar alignment is not hard to achieve.
It’s also well worth the effort as the scope only needs to be rotated about the ‘polar axis’ to track a celestial object. This motion can be controlled by a single motor (a ‘right ascension’ drive), freeing you to fully enjoy the view or to photograph the object in question.

In contrast, to track a celestial object with an ‘altazimuth’ mount, you will need to turn the telescope about two axes simultaneously. This skill comes with practise, but rules out most forms of astrophotography.


Accurate photography
If a higher degree of accuracy is required for astrophotography, or if your observing location has an obscured horizon to the north and you can’t see Polaris, you need to follow the walk-through at the bottom of the page. This is a procedure you can do in daylight.

You need to make two adjustments to get your equatorial mount working as designed. The first turns the polar axis until it points towards true north. This can be done with the assistance of an orienteering compass, providing you make allowances for magnetic variation (see links, ‘Where to find out more’).

The second adjustment inclines the polar axis to the horizontal at an angle equal to your latitude. For someone in, say, Norwich, this will be 52.6 degrees. You can determine your latitude and longitude online at Multimap.com.

Now that you have accurately polar aligned your equatorial mount, you can track celestial bodies with ease. You may use the mount’s built-in setting circles to dial in catalogue coordinates of objects too faint to see with the naked eye.

FIND OUT MORE
Find your latitude:
www.multimap.com
Find your magnetic variation:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp


Step 1 – creating a template

Once you know your latitude, use the protractor to create a right-angled triangle incorporating this latitude angle. The shortest side should be about the length of the spirit level.


Step 2 – tripod position

Find some level ground offering a clear sky view, preferably away from streetlights. A concrete slab allows you to mark the positions of the tripod feet for subsequent speedy set ups.


 

Step 3 – tripod adjustment

Extend the tripod legs equally for a comfy viewing position. Partially lock each leg. With the level on the tripod head, check it is horizontal in north-south and east-west directions.


 

Step 4 – initial equatorial adjustment

Lock the tripod legs and draw a line across each to mark the exact height (to assist future set-ups). Attach the equatorial mount to the tripod head and turn the polar axis so it points north.


 

Step 5 – adjusting latitude angle

With an assistant, place the longest side of your card template against the polar axis and rest the level on the shortest side. Adjust the polar axis until the level is horizontal.


Step 6 – adjusting to true north

Put the compass against the polar axis mount. Adjust ‘in azimuth only’ until the needle reads the National Geophysical Data Centre value for your location. Lock the azimuth adjustment.


 

This article appeared in issue 1 (June 2005) of Sky at Night Magazine

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