How The Human Eye Works

Human EyeIf you are expecting to see wonderful vistas of nebulae or galaxies surrounded by beautiful delicate veils of colourful nebulosity like you have seen in astronomy magazines or on the TV then listen very carefully.

The truth is that those glorious photos involve very long exposure times, multiple images added together with special software to improve contrast and colour depth, and using cameras that are far more sensitive than the human eye.

With the naked eye, yes, you can see wispy details in prominent DSO's (e.g. the Orion nebula can even be seen without a telescope or binoculars), but for faint ones, unless the sky conditions are very good, even with a decent telescope they will often appear as a dim, hazy smudge or blur!

At certain times of the year (especially in damp, dreary old United Kingdom), and cruelly usually coinciding just after Christmas when many people buy or receive telescopes as gifts, even on apparently clear nights when you can see some stars, the skies can be quite terrible for viewing even the dimmer stars, let alone nebulae and galaxies! Take heart - later in the year, when the weather becomes drier and the jet-stream stabilizes, views will improve dramatically (more is explained in the sections on Transparency and Seeing in just a moment).

Nebulas and galaxies are very faint objects. And yet besides the moon and planets, many deep-sky objects can be seen from light-polluted skies! A 200mm Newtonian reflector such as the XT8i (the XT10's smaller brother) will allow you to seek out as many DSO's as you can for a given amount of money.

In daylight the human eye uses a region of the retina called the fovea, which is densely populated with "cones" that are colour sensitive, but they require high light levels to work well. The central one-degree of the retina is packed with cones for full colour, high-resolution viewing. Stars and Planets can often be seen in colour because they are relatively bright points of light.

Cones and Rods
Section through the human eye and retina.

In contrast the more sensitive "rods" are not colour sensitive and only see in monochrome. That's right - BLACK and WHITE. And this is why DSO's that can have beautiful colours when imaged photographically, tend to appear as shades of grey when viewed through a telescope. Away from the central region, the cones are fewer in number, and the rods are more densely packed, but less so than the cones, resulting in low-resolution viewing. There is an optimum, ultra-sensitive, rod-packed region of the retina that peaks roughly 15 degrees from the centre.

How to use Averted Vision

This is why you can see fainter objects by using "averted vision". This means to get the most out of your night vision you have to look slightly to one side of the faint astronomical object you are trying to see.

Try this the next time you go outside to look at the stars, look directly at a region of stars, then look to the side while still "concentrating" your vision on the same patch of sky. You should notice the area away from your central vision can detect much fainter points of light. This can be used to advantage when looking through a telescope eyepiece. Also because you are moving your eye slightly, this creates movement of the light source which makes the faint stars more noticeable to your eyes.

At first this will be incredibly difficult to do naturally, but it will improve with practice. This 15-degree (or so) offset should be arranged so that you appear to place the faint object nearer to your nose, to avoid the blind spot where the optic nerve leaves the retina, i.e. look a little way above the object. The eye is four astronomical magnitudes (40 times) more sensitive at this crucial point than in the centre, even if the resolution is very low.

What will I see?

Depending on several factors, i.e. where you live in the world, the time of year, use of a dark viewing location free of light-pollution, the "seeing" and "transparency" of the night sky, how good and dark-adapted your eyes are, it is just possible to see colour in DSO's under exceptionally good conditions as long as your scope has sufficient light-collecting ability to trigger your cones. Stars are no problem, and are pleasingly colourful! (Albireo, a gold and blue double-star at the bottom of the cross shape of Cygnus is a lovely example).

The more aperture you have, the more light the telescope gathers.

First Dark-Adaptation

When you first go outside into the dark, your pupils dilate in about 20 seconds (expand to their maximum diameter), and very soon you will see fainter stars. In young people this may be 7mm or so, but for astronomers in their 80's it may only be a couple of millimetres across. This age issue is not a big problem at the telescope as higher magnifications produce a narrower beam of light that can pass through a smaller pupil.

For faint objects it can pay to use a very high magnification, however strange that might seem since higher magnitude normally reduces the amount of light passed by the eyepiece. Contrast becomes just as important as brightness on the dimmest targets. As you increase the telescope's magnification, background sky-glow from light pollution becomes much dimmer and swollen point sources like faint stars (typically spanning a few arc-seconds in diameter) start to cover maybe just a few more rods, convincing the brain that something really is there. Also, as the field of view becomes narrower, there is less chance of any really bright stars encroaching into the eyepiece field and dazzling the observer.

Second Dark-Adaptation

Our second dark-adaptation mechanism takes about 30-40 minutes to come into effect, so if you're planning to observe a number of objects, save the faintest ones till the very last. Without bright light on the retina, vitamin A is converted into retinene, then rhodoposin (visual purple) which dramatically improves the sensitivity of the rods and cones, by many thousand-fold. The combined effect of rhodoposin, and of using averted vision, amounts to more than 100 thousand times the sensitivity your central vision had, in a fully-illuminated room, before you stepped outdoors.

But strong white light rapidly reverses this change, while red light is far less damaging, and this is why you should take every precaution when star gazing to use a red LED torch, or cover a torch with red celophane. If you plan on using a laptop computer near your telescope, cover up all white or blue LED's with red insulating tape, adjust the brightness of the monitor to its minimum, and set the computers colour theme to be predominantly red. Many astronomy software packages have a red or "night-mode" display option.

Spending a day outdoors in the sun is not a good idea if you are a deep-sky observer, unless you wear wrap around sunglasses all the time. The eye takes 24 hours to recover from a sunshine onslaught and you may lose almost a Magnitude of your sensitivity the following night.

When I first got my XT10i, and once I had got everything set up correctly, during December while the night skies were quite good, I discovered just how much I could see with this scope. The Orion Nebula is quite an amazing sight. Through my 25mm eyepiece and 2xBarlow the main nebula fills the entire view, and it is fantastic to see the wispy veils, and the "Trapezium" - the four bright stars at its centre.

Star Clusters are another thing of beauty which I was mesmerized by. Looking at the thousands of stars so closely packed together in such a small part of the sky is just awesome. These are things I had never seen before, and seeing them yourself, "in the flesh" so to speak is quite remarkable.

When I viewed Saturn for the first time I couldn't believe not just the clarity and its stunning rings around it, but also the fact I could make out several of the moons circling Saturn. Likewise Jupiter.

The Leo Triplet was another nice discovery using the Tours feature of the Intelliscope, when it showed me three distant galaxies in close proximity to each other, so that you can see all 3 in a wide-angle eyepiece at once.

Not to mention the Moon. So incredibly bright (use a moon filter), and once focused on the surface, and using the standard 10mm EP plus the Barlow, position the view at the leading edge of the Moon and let it slowly traverse your view. You really feel as though you are flying in a spaceship just above the moons surface.

How Much Can I See?

People often ask when you show them your new telescope, "How far can it see?", which is a bit like asking the owner of a new vehicle how far it can travel. Even a small telescope can see objects millions of light years away, but this is not a meaningful figure and it is hard to be precise.

Telescopes have two important properties:-

  1. Light Grasp
  2. Resolving Power

Both of these depend on the aperture - the clear diameter of the mirror (or lens) that gathers the light.

Light Grasp

The light grasp depends on its area, which increases with the square of the diameter. A 50mm diameter lens/reflector has a collecting area of about 2000mm squared, whereas a 100mm diameter gives nearly 8000mm squared, or four times as much!

Light grasp is measured by the faintest star a telescope will show to the eye, the limiting magnitude.

A few observers do have truly exceptional night vision but, in the main, this is very rare. If you cannot locate the faint objects that others can see it is probably because of local light pollution, impatience, a poor night, or inadequate dark adaptation. Choosing the right magnification and, specifically, mastering the use of averted vision are all crucial.

Resolving Power

The resolving power is a measure of the finest detail your telescope can show. Traditionally this is measured by finding the closest double star that the scope will just show as two separate stars. There are many double stars in the sky, where two stars are in very close orbit around each other.

In astronomical groups, or magazine reviews you will often hear boasts of "splitting doubles", i.e. proving the resolution of a particular scope. Likewise when viewing the planet Saturn, its wonderful rings have a fine gap in the middle of the rings. This dark line is called the Cassini Division, and this too is regarded as a good indication of the resolving power of a telescope.

The XT10 has a 10 inch (250mm) aperture, and for this size the reckoning is a Limiting Magnitude of +14.7, and a Resolving Power of 0.5 Arc Seconds. Whereas a 2 inch (50mm) has Limiting Magnitude 11.2, and Resolving Power 2.3 Arc Seconds, and a larger still 16inch (400mm) scope has Limiting Magnitude 15.7, and Resolving Power 0.3 Arc Seconds.

To put this into context, some typical objects you might want to observe are;

  • Pluto, the most distant planet is a star like point of about 14th magnitude.
  • The brightest quasar (a type of starlike distant galaxy) is about 13th magnitude (which is brighter than 14th mag).
  • The more faint stars in globular and open clusters are usually fainter than 12th magnitude.
  • The brighter galaxies in the Virgo cluster are approx 9th or 10th Mag, so should be within reach of even small aperture scopes, although extended objects need another whole magnitude in hand.
  • Many comets are around 12 or 13 Mag, so a 200mm telescope is needed to see them.

For resolving power, the "seeing" plays a large part in what you can see, regardless of the telescope you own;

  • Professional astronomers measure seeing by the amount of blurring of a star image, by looking at double stars of known seperation, and aim for seeing better than 1 arc second, and consider 0.5 arc second seeing to be exceptional, even for a top quality mountain top observatory. For average sites 2 arc seconds or worse is typical, and yet this is the resolving power of a 60mm scope! So is there any point in getting a better scope for seeing fine detail?
  • Well, when seeing settles down, even from a city location it is possible to get sub-arc second seeing, and in moments of steadiness with your eyes you can see details with a modest telescope that are hard to photograph even with large instruments.
  • Mars, Mercury and Venus are only 3 or 4 arc seconds across at their most distant from Earth, yet when Mars is closest it is 25 arc seconds across, while Jupiter and Venus are more than twice that size at their closest.
  • Half an arc second on the Moon is about 1km, so if anyone asks if you can see the tracks of the Apollo Rover on the Moon, you will realise that even a racecourse would be only a tiny speck with the XT10.

Another thing which surprises me often, is how I can look up at the sky and think "Is it good enough to get the scope out tonight?". There will be some stars out, but mainly the brighter ones, so I think hmmm, maybe not worth it tonight. But then I remind myself that I have paid good money for this piece of kit, so I set it up. With just the basics I can be viewing in about 10 minutes, probably shorter if I don't have to wrap up warm. The surprising part is just how many more stars become apparent with the XT10.

Next section: Astronomy: What is Transparency?

Information Sources:

  • Article "Tricks of the Eye" by Martin Mobberley, Astronomy Now, Nov 2008 (dark-adaptation/retina)
  • Philip's Stargazing with a Telescope - Robin Scagell (light grasp/resolving power)

Other Topics in this XT10 Review: