Visual Astronomy and Dark Adaptation

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Glad you could join me for my next article!

Last blog, I wrote about the seven biggest mistakes people make when getting a new telescope.   In this blog, we are going to look at and earn about Visual Astronomy and Dark Adaptation.  One of my readers (thank you Carlton!) actually suggested that – he recommended expanding on the initial list, and it’s a good idea, as they are bigger topics than a hundred words can do justice to.

Many of us see magnificient pictures of deep space objects and want to see it things for ourselves that way – but don’t.  In my last blog, I wrote about a lovely deep space object – a nebulae – called M42, or the Orion Nebula.  It’s fantastic – we can talk about it some more.  On the left below, I have inserted a picture of what I saw it depicted as in a book I was reading.

Magnificent Capture of “M42” (Messier Object Number 42)
Whhaaaaaaaat??

Before we discuss it, it’s best to understand why we see in colour anyway.  We see my means of the retina, which is one of the most incredible tissues in the body.  It’s designed for versatile sensitivity.  Unlike a camera, our eyes don’t need flash equipment, nor does the light we need have to fall within a tight range like in a camera.  This is thanks to, in part about 125 million rods.  These are very sensitive to just small amounts of light, making vision possible at night. Then you have about 5.5 million cones that respond to brighter light and make detailed color vision possible. Some cones are most sensitive to red light, others to green and others to blue. 1g88 6/8 The Eye – The Envy of the Computer Scientist Do those colours sound familiar?  Yes, it’s RGB.  When all three types of cones are excited equally, the color you see is pure white.  Let’s discuss another amazing feature of your eyes.

Did you know that that picture genuinely shows much of the object?  Really!  It’s not half bad either.  The problem is your eyes are not seeing it yet.  The blog here is white.  So that your eyes can cope, the pupil narrows (it can narrow right down to 1.5mm2.06 inches, or expand to 8mm3.3 inches letting in 30 times more light!) and becomes very small.  It lets in only a tiny amount of light.  We have all experienced this.  Let’s use an example.  Imagine yourself trying to find your way to the kitchen after having just read a book in bright light – you stumble over everything.  While your pupils immediately dilate to help, initially, we see black as our cones (we use both cones and rods to see in our eyes) stop functioning in low intensity light.  The rods are not helping either right now as our rod pigments have been bleached out – but slowly, a molecule called rhodopsin regenerates allowing you to see more and more.

Cones?!!  Rods??!!  What am I talking about?  Well, if you type that into an anatomy app, your not going to find anything.  Both of those are contained in the retina, but the cones and rods are harder to find.  Keep going smaller.  If you could both see the rods and cones you have in your eyeballs and had the patience and time to count them, you would find you have about 6,000,000 cones and about 120,000,000 rods.  Some more beathtaking information here 4Notice the “figure from the textbook”, it shows a top view of the cones and rods, helping you to see how both are used at the same time!).

The larger images shows blood vessels, the choroid layer, the rods and codes, the base of the cone fibres, the nucleus and inner surface of the retina.
The colour display is a picture of the rod and cone photoreceptors in a human retina. Fluorescent probes were used to identify rod photoreceptors (green) and cone photoreceptors and horizontal cells (red). Color image courtesy of Dr. Robert Fariss, National Eye Institute, NIH

So, what happens is that when there is insufficient light for the cones to work, your internal systems switch over to the rods, which activate and get increasingly sensitive!  So, the longer you sit in the dark, the more your rods will kick in!  Have a look at this table, which shows the benefits of just waiting a bit.

As this table shows, cones have a sensitivity limit. After about 10 minutes, rods kick in as the main way to see in the dark. Without this design, we would be limited to a far more restrictive vision and would not see as well under bright or very dark conditions.

That’s about an hour to get maximum night vision, but you will see benefits after just ten minutes.  The point is this – to see more detail in deep-space objects at night, you need to be dark adapted.  If your not – you will see very little – only the brightest stars.  Visual Astronomy is all about dark adaptation and “playing the game” to see as much as possible.

How is all this fascinating stuff relevant to your attempts to see what’s up there?  There is no light adaption whatsoever needed to see the moon.  None at all.  But when you go to the really fascinating things, Deep Space Objects, you are going to want your night vision.  Without it – your in for some disapointment  – True Story to illustrate! – I was outside looking through my Telescope. I’d insisted that all lights are off, so that I could get dark adapted.  Soon (this was as I was more experienced), I was seeing the most amazing detail in M42.  Just then, my wife came out after watching TV for a while.  She couldn’t even find her way to me, poor thing, as she wasn’t dark adapted.  I showed her M42, truly a spectacular wonder, and she merely grunted.  A week later however, she saw the same thing, after being dark adapted and it blew her away.  The difference?  Dark adaptation – Her rods properly kicked in – at least a bit.

The next thing we need to grasp is why things are black and white when looking through a telescope.  Let’s visualise going for a walk at night with no moon for about 10 minutes.  You walk through a colorful flower garden.  Do you see the colors?  “Of course not” you say, and of course, your right – we just see the shades of grey.  What happens if you have brought a torch and shine it on the flowers?  Ah, now you see the color.  Why?  To see very well in the dark, our rods need to be used instead of our cones.  It’s the rods that are highly sensitive and help us – but they don’t allow us to detect color.  When you shone the light on the flowers however, the cones kicked back in and let you see in color.

There is more, while we need darkness to see the stars, to see faint deep space objects such as M42 (actually, it’s very bright for a nebulae, but we will let that pass…), we need light – In fact – lots of it.  What sort of light though?  What we really want isn’t “general light” which actually gets in the way, it is the light from M42 itself, which has traveled for 1340 years to reach you.  That light, as I explained, is faint.  And because it is so faint, you simply can’t see it in color with your eyes – ever (Mind you, I have read some Astronomers claim they have experienced faint color – I struggle to believe it, as it would mean there is sufficient light to excite the cones – but who am I to definitely question what theys saw, the eye is amazing).  Turn the light off, your pupil dilates letting in 300% more light compared to sitting at the beach squinting.  Wait 10 minutes, your rods kick in significantly and keep improving for up to an hour.  The effect?  Your more likely to see M42 like this:

Good Dark Adaptation, wide open pupils, good equipment and the right conditions will let you see spectacular views. Just don’t expect your rods to see in color! Image courtesy of Bob Familiar – Flikr, 2011.

Why then, do we see color pictures of nebulae everywhere?  Is it made up?  Pretend?  Fake visual artistry?!  No – it is not made up, it is real.  Although our eyes get accustomed to the dark after a few minutes, they require a certain minimum intensity of light to perceive color, and starlight is not that strong. Thus, while we can see faraway objects, we still find it difficult to distinguish their color5g88 9/8 Reach for the Stars. In his book Galaxies6Galaxies, Timothy Ferris, 1988, ISBN:0517631768, Ferris explains that the photos of faint distant objects, such as galaxies or most nebula, “are time exposures obtained by aiming a telescope at a galaxy and exposing a photographic plate for as long as several hours while starlight seeps into the photographic emulsion. During this time a driving mechanism compensates for the earth’s rotation and keeps the telescope trained on the galaxy, while the astronomer, or in some cases an automatic guiding system, makes minute corrections.”

But does it matter?  No, it does not, for several reasons.  Firstly, imagine you are watching a color movie in black and white.  Sure, it would annoy you for a few minutes, but then you are lost in the excitement of the movie itself.  Within a few minutes, you won’t be thinking about it anymore.  In the same way, visual astronomers appreciate the magnificent things they see – in color – or without!  Secondly, it isn’t like watching black and white pixels on a screen.  It is not like there are little blocks there – it is the most incredible high resolution “HD” that you can see – your limits are simply your eyes.  That is truly amazing and makes it better than any ipad screen ever invented.  It also looks a little different depending on the “seeing” (atmosphere interference etc..) and your technical skills in looking after your telescope.  This can be very enjoyable!

If you want to get into seeing things that way, you need photographic / digital equipment to capture what’s up there.  That topic is for another way.

In this blog we covered why we can’t see in color the way we expect and the importance of being night adapted to see more.  The next blog being prepared is on the magnification myth of telescopes!

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