Full transcript
0:00remember we really want to worry about
0:02focusing the light we're gathering light
0:04in the telescope and focusing it to get
0:07a clearer image but where we focus that
0:10light can really depend on what we want
0:12to do and actually determine the type of
0:14telescope we want to design in the case
0:16of a so called prime focus the light
0:19would just be reflected off the mirror
0:21and to a specific point I kind of eluded
0:25this already this is not the most useful
0:27cuz great you got a nice beautiful image
0:29right here where the prime focus happens
0:33the land but unless you have a sensor
0:36there it's not that useful can't really
0:39stick your head there and see the image
0:40because guess what now you're blocking
0:42the light if you can put a smaller
0:43sensor you can actually get some data
0:45though and I mean you can have pretty
0:48small sensor there relative to the size
0:51of the overall telescope more often
0:54though what we're doing is deflecting
0:55the light somewhere else in the case of
0:58the Newtonian focus
1:00we're actually deflecting it to the side
1:02outside of the actual telescope itself
1:06right to an eyepiece so often you might
1:09have a sensor there to read it or you
1:11might just have someone using their eye
1:13to see it this is the classic design for
1:16most amateur telescopes because again
1:17you might have your eye there you might
1:19have a camera there to be able to take
1:21an image more advanced setups can
1:24include things like the Cassegrain focus
1:26you can see in this case the secondary
1:29mirror which I may not have alluded to
1:31that name before the primary mirror is
1:33the first mirror it reflects off the
1:35secondary mirror is meant to deflect
1:38these new light rays somewhere else
1:40and by the way it's worth mentioning
1:42this mirror the secondary one is very
1:44small this drawing is not to scale this
1:47is very small so it doesn't block much
1:48of the incoming light it's simply meant
1:50to deflect the light in this case to the
1:52eyepiece and then the Cassegrain wanted
1:54to flex is straight back down to a
1:56narrow gap in the mirrors and again not
1:59to scale so tiny little gap and thus the
2:02light ends up being focused at the base
2:04of the telescope the reason this is
2:07useful is you can have your equipment at
2:09the base of the telescope collect all
2:11the light and do whatever you need to do
2:13with that light
2:13there we're gonna see what some of the
2:16things you can do with the light are a
2:17little bit later but it's just sending
2:19it to a different part so it can be
2:21analyzed somewhere and there is other
2:23setups like nasmyth where it's actually
2:26sent off to a room especially in very
2:29advanced telescopes you might be sending
2:32this light to another room for it to be
2:34analyzed by a bunch of machines and to
2:37do a very our ideas things with it so
2:40now thing is are we gonna use refracting
2:45telescopes or reflecting and for a while
2:48refracting telescopes were actually more
2:50common but basically we're gonna see and
2:55I'm gonna just tell you right now
2:56reflecting telescopes are by far and
2:59away the most common most professional
3:02telescopes are reflecting telescopes
3:04especially any large ones are definitely
3:07reflecting telescopes now because
3:09there's a variety of limitations in
3:11issues with trying to do refracting
3:13telescopes compared to the reflecting
3:15case we want just this mirror boy well
3:19for one thing as the light passes
3:23through the lens in the refracting case
3:25some of that light can actually be
3:27absorbed we lose some of the light just
3:31refracting through the lens so right
3:36away that's an issue we're not gonna get
3:38as great an image some of the actual
3:41electromagnetic waves can be absorbed so
3:45already a problem but also even look at
3:50these two images the lens is way up here
3:52the mirror is way down there why is that
3:55matter well when we have a big heavy
3:57lens
3:58it can only be supported around the
4:00images if you tried to support it behind
4:03or in front you would actually be
4:05blocking part of your telescope so we
4:07can only support it around the edges and
4:09so a very very heavy lens is actually
4:12gonna bend your telescope now has a
4:15problem it actually starts to bend and
4:17cause issues with your telescope where
4:20is the mirror it's at the bottom we can
4:23brace this we can put a ton of things
4:25underneath this mirror to help support
4:27so it doesn't actually deform your
4:29telesco from the weight and thus we can
4:32have heavier mirrors but also again look
4:37at this light passes through the lens
4:39versus reflects off the mirror if it's
4:42passing through a lens that means both
4:44sides of this land has to be perfectly
4:47polished tiny little flaws are gonna
4:49make the light not actually refract
4:52properly get deflected in weird ways
4:54so both surfaces have to be polished
4:56very perfectly with the lens a mirror
4:59only one surface has to be polished so a
5:02lot of practical reasons to actually go
5:06with the reflecting case but there's one
5:09other one as well and that is the idea
5:10of chromatic aberration think back to
5:14when we talk about spectroscopy the
5:17whole idea was we put light through a
5:19prism that light actually spread out
5:21into the different colors because the
5:24different wavelengths bent at slightly
5:25different angles guess what the same
5:29thing can happen in our lens and we
5:31don't want that you can think great all
5:33that lights being focused but actually
5:35the blue light versus the red light will
5:37be focused to slightly different places
5:39creating a blurry image you can see it
5:41quite clearly in this one where we all
5:43of a sudden have some blue spots some
5:45red spots this chromatic aberration in a
5:48lens can really mess with our image as
5:50compared to if we use a reflecting case
5:52there is ways to try and deal with this
5:55in a lens but they're pretty expensive
5:57and we're better off just going to the
5:58reflecting case so most modern
6:02telescopes are reflecting telescopes
6:05because of all of these many advantages
6:07not the least of which chromatic
6:10aberration again you don't want to blur
6:12your image just because it's passing
6:14through a lens