Nebulae and How to Observe Them

Newtonian owner then consider your ladder carefully. A ladder with a large area
on each rung is much more comfortable than a standard ladder. You want more
than just a narrow area that digs into your instep; a large step allows your entire
foot to stand comfortably on the ladder. One of the reasons that I owned the 13
inch f/5.6 Newtonian is that it did not need a ladder for most observing positions.
When I was looking near the zenith I needed a short step, so I carried a wide board
that allowed my whole foot to stand on that 2X10 (5cm by 25cm) inch board and
be comfortable. A.J. calls his a “human shim.”
Knowing the Magnitude of an Object
in your Telescope
The magnitude system is not linear. That means that the values given would not
form a straight line if plotted on a graph. This all started because your eye is not
a linear detector of light. If one star appears twice as bright as another it is giving
off about 2.5 times as much light as the fainter star. Also, a low number in the magnitude
system is a bright star and higher numbers are faint stars. I keep this straight
by remembering that a first-magnitude star is a “first-class” star; so it obviously
seems brighter than a second-magnitude (or second-class) star.
Let me give you an example using nebulae. The famous Ring Nebula (M 57) in
Lyra has a magnitude of 9.0 and it is easy to spot in both the Nexstar 11 and the 4
inch refractor. NGC 2346 is a planetary nebula in Monoceros and it has a magnitude
of 12.5. I cannot make it out in the 4 inch, but it is pretty bright in the NS 11.
After several years of experience with the 11 inch I can say that objects fainter than
14th magnitude are quite difficult with that telescope.
You need to determine the magnitude limit of your telescope. Most electronic
planetarium programs will give you the magnitudes of the field stars in a
particular area. Find a location that has a variety of magnitudes on the star
chart and use it to determine the limiting magnitude. The best way is to go draw
that field carefully and then compare it to the chart. There are also charts from
variable star observers that are available on the Internet. You can use those fields
as well.
There is one more complication here. It is called surface brightness. When we
are discussing the brightness of an extended object and not a star, the nebula is
spreading its light over a sizeable area and the star is a point. So the Ring Nebula
may have the brightness of a ninth magnitude star, but that comes from an object
that is much larger that the disk a star presents to the observer’s eye. Because the
star is more concentrated it is going to be easier to see than a nebula of the same
magnitude.Another excellent example is the NGC 7293, the Helix Nebula in Aquarius.
This object is given a total magnitude of 6.0, but it is spread over a large part
of the sky. So, even though a person with average eyesight can see a star of 6th
magnitude, there is no chance of seeing the Helix Nebula without at least a pair of
binoculars.
Improving
your Skills
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