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MAS Frequently Asked Questions

MAS FAQ

MAS Frequently Asked Questions

Frequently Asked Questions about astronomy (see also MAS Beginners page)

This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).

Much of the material here has been taken from topics presented at out monthly meetings. If any member has subject they would like to present for 15-20 miniutes as a 'second half' topic, please contact the Meetings Secretary (to contribute directly to this page, please contact the Webmaster

(+) 0001 How do I find a local Astronomical Society ?

(+) 0002 How is Star brightness measured ?

(+) 0003 What are the Constellations ?

(+) 0004 What is the shape of the Earth ?

(+) 0005 What equipment do I need to start astronomy ?

(+) 0007 What telescope should I buy ?

(+) 0008 How much Magnification can I get ?

(+) 0013 What are Equatorial and Alt Az mounts ?

(+) 0100 How do I use my first telescope ?

(+) 0109 What is a finder ?

(+) 0110 What is an eyepiece ?

(+) 0111 What is a Barlow ?

(+) 0112 What is a focal reducer ?

(+) 0114 What are Nebular filters ?

(-) 0116 How do I use Setting Circles ?

Using Setting Circles Ever wonder what those numbers are, arranged in a nice circle around your mounts RA and Dec axis ?

Essentially, they are the Latitude and Longitude of the sky which, before the days of 'goto', was the only way to find faint objects too dim to see easily (or at all)

Just as terrestrial Lat/Long 'stick to the Earth' (so London, for example, is always at 52 / 0 degrees, no matter where you are on earth), RA and Dec 'stick to the sky' (so Sirius, for example, is always at RA 06h 45m 08.917s / Declination -16� 42' 58.02")

Of course, just as you have to 'align your map' to North and work out 'where you are on Earth' before you can 'goto' London, you have to do the same for the sky (i.e. align you RA and Dec circles for 'where you are now' relative to the sky) before you can 'goto' Sirius ...

Aligning the Dec axis is easy, you just have to 'point it North' (actually, the Dec axis is fixed to the mount, and you point the mount North).

Declination is measured northward or southward from the celestial equator, however we don't have to care about that since as soon as we 'point North' the Dec grid is correctly 'aligned' (Polaris is at +89� 15' 50.8" i.e. the Dec grid will show about 90) and won't change.

A stars 'Dec' measurement is given in degrees. A 'negative' value such as -10 means 'count 90 away from the Pole and then go another 10'. So '-10' means 100 degrees 'away from' Polaris (so down the Dec grid form (+)90 to 0 then another 10 past 0 (i.e. toward the (-)90).

One thing to note is that the mount has to be exactly level (i.e. you will need to actually use the spirit level to adjust the mount legs to get the mount head flat) because (unlike a 'goto') there is no way to 'adjust' the RA/Dec to take into account a non-level mount :-)

The RA is the one thats going to cause you a headache == remember the stars 'stay in the same place', it's the Earth that moves, so aligning the RA means knowing 'exactly where you are now' and that means 'how far the Earth has turned' i.e. the exact time (and date). With that information you can more the RA grid until it's in the correct position 'now'. Hopefully this explains why a stars RA position is given in hours/minutes/seconds (i.e. as a time) and why your RA grid is marked from 0-24 Hrs :-)

Fortunately we can 'cheat'. Just as your 'goto' uses 2 or 3 bright stars to 'align' itself, you can use the same trick to align your RA grid. Unlike the 'clever' computer, however, there is no point in choosing 'different' stars (the computer does this to cope with non-level mount). Since your OTA is likley aleady 'pionted' at Polaris (so you can adjust the finder scope to point in the same direction as the main scope), and since Polaris is (always) at RA 02h 31m 49.09s, all you have to do is twist the RA grid until the OTA 'pointer' is aiming at 2h 32m and then lock the RA grid to the mount axis !

To check you have 'got it right', you could swing your OTA to (say) RA 6h 45m 08.917s and Dec -16� 43' and take a peek through the eyepiece (if you find Sirius, you are spot on !) So, no need to know the date/time (or remember if it's British Summer Time and, if so, does that add or subtract from the time now ..)

You can now dig out your favorite Star Atlas, read off the RA/Dec of the object you want to find, swing the OTA to that position and (if your mount is level) the object will be right in the center of your eyepiece !

The problem is that without a motorised RA axis, the RA 'grid' will drift out of alignment at a rate of 1 minute every minute. This need not be a problem if you noted the time when you first aligned the grid since, when you come to seek a new object, if (say) 30 minutes have passed since the last 'alignment adjustment', all you have to do is twist the RA grid by 30 minutes ... and it' will then be 'aligned (for now)' again (of course knowing 'which way' to twist the grid comes with 'experience', or, in my case, 'trial and error' :-) )

Actually, you can even cheat here, so long as you are still observing the first object. If that object is (say) the Andromeda Galaxy (M31), this is (always) at RA 00h 42m 44.3s. So, whilst it's still in view, just twist the RA grid to 0h 42m before leaving M31 for the new object (for maximum accuracy, do this just before moving the OTA = remember the 1 minute per minute drift) !

My final comment relates to the planets - unlike stars, they don't 'stick to the sky' i.e. planets 'wander'. This means the RA/Dec has to be 'worked out' (I suggest eg. using Stellarium) before the start of your observing session (finding Uranus or Neptune without knowing their RA/Dec 'tonight' is not as easy as you might hope :-) )

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MAS Frequently Asked Questions