Have you ever really thought about just how big a nebula is? Here's some perspective: the longest
man-made structure in the world is the great wall of china and it's just over 21 000 kilometers
long walking every day it would take you about a year and a half to walk from one end to the other
the lagoon nebula is 312 trillion kilometers across walking every day it would take you over
22 billion years to walk from one end to the other but knowing the lagoon is 312 trillion
kilometers across will give you no sense of how large it will actually appear in the night sky
from earth the reason being is astronomical objects are at all different distances away
some like the lagoon are actually much closer to us in the milky way than others
for this reason as an astrophotographer i talk about deep sky objects almost exclusively in terms
of their apparent size meaning how large they appear in the night sky and the parent size of
objects can be quite non-intuitive for example the lagoon nebula is three times the width of the moon
and you might be thinking how could that be right i see the moon all the time i've never seen the
lagoon nebula with my naked eye doesn't that mean that it is smaller well no actually the moon does
appear smaller than the lagoon but the moon is very very bright which is why we can see it all
the time and it's bright because it's lit by our sun while the lagoon nebula is very dim not only
because it's many light years away but because it's a diffuse nebula which by their nature are
very dim being diffuse clouds of gas so the way um anyways the way we measure apparent
size is actually pretty simple we measure things in something called angular diameter
let me explain imagine yourself out in the desert no trees perfect horizons in every direction
you'll be seeing the entire night sky which can be thought of as a dome or one half of
a celestial sphere the full sphere being 360 degrees around so half a sphere is 180 degrees
from horizon to horizon with this in mind we can define anything that we see in the night sky
in terms of angular diameter for example the whole big dipper is about 25 degrees across
while the moon is just half a degree across and remember from earlier that the lagoon nebula was
three times the width of the moon so yes it's about one and a half degrees across
now a handy way to get to know these measurements and how they actually appear with their eyes
is to hold up your hand at arm's length in front of the night sky and if you make the hang loose
sign that's about 25 degrees the rock on sign is about 15 degrees a closed fist is 10 degrees the
three middle fingers is five degrees and a single index finger is one degree so how does all this
help us photographically well the first thing you have to understand is the field of view of your
system of your camera and telescope or your camera and lens once you know the width and the height
of your field of view in degrees or arc minutes uh we then know how different objects will fit
and before i show uh a tool here let me just explain arc minutes um because i've said that
term but i haven't yet explained it one arc minute is one sixtieth of one degree so half a degree
is 30 arc minutes we can break that down further into arc seconds and our second is 1 60 of an arc
minute typically when we're describing deep sky objects we use arc minutes because so many of them
are under a degree wide the lagoon nebula which i talked about earlier is fairly large for a deep
sky object at 90 arc minutes across or one and a half degrees just to give you an idea
for some other popular objects the orion nebula is one degree or 60 arc minutes the
andromeda galaxy is a whopping three degrees across at its widest point or six full moons
um back to field of view though i'm going to jump into one of my favorite tools for showing this
which is black water skies imaging toolbox it's a free web app i'll put the link in the description
to use it you're going to go over here and use these icons i'm going to start with this icon in
the top left corner and from here you can pick your camera and telescope and click calculate
and then it will update over here and this green box is your field of view and if we
zoom in here on the left hand corner i can show you that this field of view is 2.6 degrees wide
by 1.78 degrees high now let's say you went into this here and you couldn't find your combination
of camera or telescope or maybe you were using a camera lens what you can do is just click on
this second box if you don't see them represented in that list and enter these things manually so
these would be specs about the camera and then this is specs about the telescope or lens so let's
say i was using a 200 millimeter lens instead i just click calculate and just like that it updates
this has been another installment of five minute fridays next week i'm going to start
counting down some of the best nebulae in the night sky for astrophotographers
till then this has been Nico Carver from Nebula Photos. Clear skies!