Night Sky Observing Tips

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You don’t have to wait for someone else to show you the stars. Go out one night and look patiently at those distant lights, and ask questions. Are they all white, or do some show the faintest hints of color, bluish or reddish hues that might tell us something about their nature. Are some parts of the sky more crowded than others? Do certain bright ‘stars’ shine with a steadier light than the shimmer of the rest? – David Levy, The Joy of Gazing

Question How do we Plan and Do an Observing Session?

Answer Here’s a checklist:
Plan: DRESS WARMLY – Always keep a hat, scarf and boots around even in the summer dewy nights

1. Get a book from the library or a magazine that features a particular selection of objects visible from your location at the current date or use a planetarium program like ECU. We can do a lab showing how to set the time, place, information detail, catalogues…

2 You can use smartphone devices but plan what you are doing beforehand so that you don’t just stare at the iPod

Better to plan indoors first.

  1. Use a star map for the time of year, see Lecture  on Star maps
  2. Use a star atlas like Pocket Star Atlas or Star Atlas 2000
  3. See: http://www.mvc.on.ca/astronomical-catalogues-stars-star-clusters-galaxies/

Do:

  1. Make sure you are comfortable at the eyepiece, find a chair or stool to sit on. You can sit down when you get tired.
  2. Choose an area to work – some constellations that are easy to find –  and pick from a list of different things:
  • Binary Stars
  • Stars with colour or colour contrast if a binary system
  • Star nebulae and nurseries and novae remnants
  • Star clusters
  • Galaxies
  • Clusters of galaxies

3. Use a low power of magnification to find the objects

4. Choose the right eyepiece for your target,

If the object is very large, use a low power of magnification (large focal length on eyepiece – i.e. 45 mm)
If the object is very small i.e. you are trying to see a tight double star or a planet, use a high magnification (small focal length on eyepiece) – i.e 8mm

 How do I predict if it will be clear enough to do Night Sky Observing:

Answer Use a Clear Sky Chart web-page for your location – example: Fred Lossing Observatory – Mississippi Mills – Clear Sky Chart

The data comes from a forecast model developed by of the Canadian Meteorological Center.CMC’s numerical weather forecasts are unique because they are specifically designed for astronomers.Like the one above which summarizes CMC’s forecast images just for Fred Lossing Observatory and the surroundings out to about 15 Kms.

Question When I look through the telescopes, what typical magnifications are we using?

Answer It depends on the telescope and the eyepiece. We were using low-power eyepieces and small reflector telescopes.For example, the little StarGazer Steve red scope is a 6″ aperture (primary mirror) and a focal length muliplier of 8. This comes to 48″ or 4-ft. It is roughly the length of the tube assembly. So for a given eyepiece we measure:

Magnification:

 Focal Length of telescope (Aperture 15cm x FocalLength F8)
 ---------------------------------------
 Focal value given on the eyepiece(mm)

For a wide field view we used a 24mm eyepiece:
Magnification:

1200 mm
---------   = 50X
24 mm
The most pleasing view in this scope is the eyepiece designed for this scope which is 18mm
i.e. 2/3 or 66X

Wide fields are ‘rich fields’. We can take in more of the cluster or more than 1 galaxy if we are looking at clusters.To do close-up work,to split doubles, or look at the detail on the planet – that’s when we want higher power! For this we would use 8mm (150X) or smaller. A big field of view (right) shows more sky and means LOWER magnification (i.e. 30-50x) . A small field of view shows the object ‘close-up’ and means HIGHER magnification (i.e. > 100x)

FieldOfView

Question: Why is is it hard to keep  star fields and planets centered in the telescope eyepiece if we are using a simple telescope mounted on a tripod? They seemed to float out of view very quickly.

Answer What we are observing is our own rotation on the face of the earth

 

  • The earth rotates on its own axis in 24 hours. That represents a rotation rate of 15 degrees per hour or 60 minutes. Divide 60 minutes by 15 and we are turning at the rate of 1 degree every 4 minutes. If we are not paying attention, when we look into the eyepiece we will see that the object has indeed been carried along in the sky… or that the earth has rotated underneath our feet! All objects that we view in a telescope will drift out of the eyepiece according to the sidereal rate. Motorized mounts to counter the earth rotation are used when we wish to track the object for imaging and spectroscopic data.

Note that this is not true if the telescope were mounted on a tracking mount. An equatorial telescope is built so that you an adjust  Right Ascension axis  parallel to the earth’s rotational axis. If this is done accurately enough, all that is necessary to keep the telescope pointing at a planet is to move this one axis at a rate equal and opposite to the rate the earth rotates.

eqMount1

A simple example of a tracking mount is called an Equatorial Mount. Aligning the telescope to a fixed point in the sky which isn’t moving allows you to track objects using the Right Ascension control (knob). The Right Ascension movement compensates for the earth’s movement and allows the telescope to ‘track’ an object. The part of the sky which doesn’t move is of course the North Celestial Pole ( if your in the Northern hemisphere ) which is rather like a hub around which the stars appear to rotate. Adding an electronic ‘clock drive’ to the mount so move the scope ever so slightly at the sidereal (or earth rotation ) rate of 15 deg/hour or equivalently 15 arc-sec/sec.

Note: If you wish to use  manual setting circles:

After you complete your polar alignment, (where the declination is at 90), slew the telescope to a bright star for which you know the Right Ascension and Declination (celestial equivalents of Longitude and Latitude on earth).

  • Set your RA setting circle to the RA of the star.
  •  Set your DEC setting circle to the Declination of the star .

Question: What is an A.U.?

Answer: The A.U or  Astronomical Unit is the average distance between the Sun and Earth.

au

Incidentally  – Historical significance of  Transit of Venus observations; attempts were made by astronomers to determine the A.U distance by observing the disk of Venus travel across the sun (at  Inferior Conjunction).  This method required that the baseline of the earth be used requiring an expedition by two teams of observers. The science of the day was to measure the Earth – Venus distance, and knowing the relative measures of planetary orbits around the sun, one could work out the Earth-Sun distance:

http://millstonenews.com/2012/06/observing-the-transit-of-venus-what-we-learned.html

Sun-Venus-Earth distances at Inferior Conjunction

Its value is 149,597,870 km (~150 million kilometers. ) It is RE in the diagram.

Question: What is a Parsec

Answer: A parsec (symbol: pc) is a unit of distance used to measure the astronomically large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is equal to about 3.26 light-years (31 trillion kilometers) in distance. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun.

200px-Stellarparallax_parsec1.svg

Question: What is a light year?

Answer: The light year (or light-year or light year) is a measure of distance that light travels in space in one year.

lightyear

  • Since the speed of light is about 300,000 km per second, then a light year is about 10 trillion kilometers (about 6 trillion miles). A light year is the distance light travels in one year.  it represents distance in the same way you tell your friend that the distance is  “60 minutes to the town center” . (It’s 60 minutes by car to the town center assuming you are going at 100 kms per hour, which makes it a distance of 100 kms).

The light year is used in astronomy because there are objects in space several orders of magnitude further out than the planets! Space objects such as stars and galaxies may be hundreds, thousands or millions of light years away. Even though the light year is a measure of distance, it is also a timekeeper – an object observed at 100, 1000, 1000,000,000 light years away is sending us photons from that ‘look-back’ time.

 

  • Example: Think of a star at a distance of 100 light years from us on Earth. Light leaves the star and takes 100 years to get to us. When we see the starlight, we are seeing that star as it was 100 years ago. Similar distance measurements:

Light minute – The distance that light travels in one minute (about 18,000,000 km per minute, or 11,160,000 miles per minute).

  • The Sun is about 8 light minutes from Earth Therefore Jupiter which is roughly 5 A.U. is about 40 light minutes from Earth so that light leaving the planet arrives at our telescope 40 minutes later.
  • At a larger scale, the galaxies in the Leo Triplet are 35 Million light years distance. Light that we see from these objects originated 35 million years ago (in time) from that distance. Light second – The distance that light travels in one second (about 300,000 km per second, or 186,000 miles per second)

We see ‘ghosts’ from the past . The time that the light left the galaxy was 35 Million years ago (after the dinosaur extinction)

chart3Annotations
chart 3 detail – courtesy NightWatch by Terrence Dickenson

Learning how to interpret the information in the charts:

  • Lum: Luminousity – in units of Solar Luminousity (3.828×1026 W – nuclear fusion output primarily)
  • Ly – Fundamental unit of distance (~10 Trillion km)
  • Mass – in units of Solar Masses
  • ‘Double’ Stars: A , B etc – Primary, Secondary Magnitudes:
    • Example Algeiba: A=2.3 m B = 3.4 m where m stands for visual magnitude
    • Sep = 4″ – Separation in arcseconds of sky – Know your Magnification and Field of View!