Astronomy 302

Lecture 5

How to Observe

1.0 See outline here:

2.0 detailed notes from M. Bolte are here:

3.0 Some extra background RA DEC and Airmass:

To locate any object on the Sky we need to use 2 orthogonal axis
Right Ascension (RA) and Declination (Dec)



These are both really just angles on the sky.

see here (Prof O'Connell of Virginia) for more info


since the Earth is spinning but RA and DEC are fixed there
is always a relation between the airmass of the target
and the location of the observer and the time.

Indeed the local mean sidereal time (LMST) is equal to the RA of
Objects passing overhead (also called crossing the Meridian, or transiting!)





So if a star is transiting and it's DEC is at the latitude
of the Telescope (like DEC=30 degrees) it is at Zenith!

4.0 How Do I figure out Airmass for My target?

airmass = sec(z) =1/cos(z)

So if a star is at zenith it has an airmass of 1.0
if a star is 30 degrees over (z=30) it has an airmass of 1.15
at z=45 AM=1.41
at z=60 AM=2.00 --- now this is getting too low for the scope
at z=70 AM=2.9 --- Time to stop observing this object!
at z=80 AM>5 never go there!

High airmasses (AM>2.0) are bad because the:
 seeing gets worse,
the extinction increases,
more light pollution,
Atmospheric dispersion...


Atmospheric dispersion in the optical

 
The best airmass is *always* at transit (LMST=RA)
AMmin = 1/(cos(DEC -latitude)) (watch signs)

If you know the date of your observing run and the RA and DEC of your target find out how AM
changes through the night with a tool like "

example: I wish observe Vega Tonight  -- when do look for it?

 

then hit the "get airmass table" button
and you get (for tonight)



So clearly Vega is rising (negative Hour Angle, HA) at
sunrise. but Maybe I could get it at around 6AM
at an airmass of 1.664.

It is also clear that targets with an RA of ~7 hours are best (transit at midnight) for this time of year
targets with an RA of ~9 hours are then the best...