Visual Astronomy at the Telescope's Eyepiece

Mel Bartels

As a small child I remember being driven back to Portland, Oregon at night after a visit to relatives in the countryside. I lay in the back of the station wagon, peering up at the sky through the rear window. The stars were so brilliant against the darkest black of skies. It hurt my eyes to look at the brightest stars. What a contrast to the washed out city skies of Portland, even in 1960.

The Greek astronomer Hipparchus in the 2nd century BC invented the magnitude system, where the brightest stars are of 1st magnitude and the dimmest are of 6th magnitude. I suspect that this system was in use beforehand: it’s common for humans to divide groups of things into sixes and it would have been natural for us to call the brightest stars “first class”.

The magnitude system is logarithmic, not linear. This no doubt because our eyes work logarithmically (or at least semi-logarithmically).  For example, a star that is 1 magnitude brighter is 250% brighter; conversely a star that is 0.1 magnitudes dimmer is 10% dimmer.

The first lesson then is that we cannot get hung up on linear percentages, instead we must think in logarithmic magnitudes. This is difficult because discussions today are almost universally in percentages, which is completely misleading. Illumination drop-off at the edge of the eyepiece? Stated in percentages (e.g. 15% sounds terrible), should be in magnitudes (e.g. 0.06 mag, unnoticeable visually). Mirror coating reflections? Stated in percentages (e.g. 92%) should be in magnitudes (e.g. 0.04 mag loss). It is very difficult to see differences of 0.2 magnitude or less. And when the view is dimmed, both object and background are equally dimmed, leaving the contrast unchanged. Unless the view is grossly dimmed, the unchanging contrast means that the object does not lose visibility. I will be using magnitudes exclusively just as charts and observing manuals.

I've been enthusiastically observing for a number of decades. Here are the factors that influence what I can see that night through the eyepiece of my telescope.

• Aperture is the biggest factor. More aperture increases visibility not only because of greater light gathering power but also because the greater magnification brings the object in closer.
• Seeing the object in a larger scope then returning immediately to your smaller scope can result in a half magnitude gain.
• Observer experience is worth 2 magnitudes (I have a series of sketches of M31 from childhood onward).
• Observer variation is a half magnitude or more.
• Age matters a magnitude: young kids can see very faint stars; as we get older, our lens yellows and ability to detect fades.
• Knowing where to look and what to look for worth a magnitude.
• Averted vision is worth a magnitude.
• Dark adaption continues to produce increasing benefits for hours, ultimately worth maybe a half a magnitude.
• Field baffling is an overwhelming factor: the difference between nonexistent and fully baffled views can be worth magnitudes.
• Covering your head with a black cloth also yields improvements, perhaps on the order of a fraction of a magnitude.
• Time at the eyepiece is worth a magnitude (objects gradually become recognizable or detectable over a period of time, and then they fade after a prolonged period of continuous observing).
• Comfort at the eyepiece is worth a half magnitude.
• Rested eyes are worth half a magnitude. I often take short breaks throughout the night. Upon returning to the eyepiece I can see more until my eyes tire.
• Sky transparency is such an overwhelming factor; on rare perfect nights I’ve seen scopes perform as if they had almost unlimited aperture; let’s call superb sky transparency worth a magnitude or two.
• Filters are worth a magnitude.
• Visibility appears to correlate most with aperture, then apparent size (the greater the aperture, the greater the apparent size, limited by the full field of view).
• True binocular or two eyed viewing results in a half magnitude gain in stellar limiting magnitude and about a magnitude gain for extended objects.

Make these factors work for you and you can gain magnitudes in observing prowess. It’s like having a much larger scope on hand.

Why do amateurs ignore these factors in favor of obsessing over minutia like their telescope’s diagonal coating quality? Sometimes we humans become superstitious and engage in myopic inquisitions when the situation is difficult or fuzzy. Have courage, don’t obsess over some detail of your telescope and instead focus on the factors that matter.

Given a reasonable mix of these factors, how faint can you expect to see? The following chart is based on my decades of observing experience using scopes up to 40 inches [1M] in size.

Notice that the lines are banded or thickened. You might fall slightly above or below these bands based on the factors discussed earlier. Beware of anyone or any calculator that states overly precise limiting magnitudes. These are at best guides and give a false impression that an object is either perfectly visible or perfectly invisible. Objects on the edge of visibility come in and out of view over a period of time. One night that object might be visible three times in a half hour (my standard for detectability). On another night it simply is completely invisible. On rare perfect nights not only can I detect it much of the time but there is detail too. Also if the galaxy or cluster or planetary is unusually large, then the detection limit will suffer. Note that as aperture increases, minor differences (say between a 20 inch and a 22 inch telescope) become insignificant, even undetectable except for rare edge cases.

At first aperture is everything, then it is nothing; eventually it simply is. At first we can't get enough aperture. Then almost like a boomerang we trim way back in aperture. Notice how many experienced amateurs own not only their big scope but also a smaller scope? Finally, aperture takes its place in the pantheon of factors, being traded for field of view and for convenience of viewing. A 6 inch [15cm] is a perfect aperture to learn how to observe. With it you can see thousands of objects from a dark sky. A 12 inch [30cm] will resolve almost all clusters and show galaxy groupings. If you think that you “need” large aperture to see the skies, that small aperture won’t work, then something has seriously gone amiss. Large aperture makes it more difficult to learn the art of observing. Do yourself a favor and spend a lot of time observing with smaller scopes too.

What magnifications should be used? I favor three strategies both based on exit pupil (the eyepiece's focal length in mm divided by the telescope's overall focal ratio [e.g., 24mm eyepiece on a F/6 scope produces a 4mm exit pupil]):

The first is based on Richard Berry's advice. Arrange your eyepieces so that they give exit pupils as following:
5-7mm Richest Field observing
3-5mm best deep sky observing
1-2mm best detailed observing (globulars, planetaries, lunar and planetary)

The second is based on Stephen O'Meara's comments (e.g., his Herschel 400 Observing Guide). He uses modest aperture (4 inches [10cm]) at low, medium and high powers. He takes his time studying the object carefully at each power. His low, medium and high exit pupils are:
If you are wondering who to look to for observing advice, pay attention to the top observers who use smaller scopes, like O'Meara.

The third is a strategy that I've developed in response to the super wide angle eyepieces available today. It allows me to see large scale objects otherwise too big for a given scope. I call this strategy “framing” or “composing” the view where the object is magnified to fill the eyepiece’s field of view as much as possible with a nice border around it for contrast. Increasing the apparent object size beyond this 'cut-off' results in a less pleasing more difficult view. Here, the widest possible field of view is important, even at the cost of more glass for the light to pass through. In this approach, I smoothly decrement the exit pupil. I use a set of exit pupils as follows (note that the typical set of eyepieces does not fit nicely):
5-6mm for largest scale objects
3-4mm for medium scale objects
1-2mm for small scale objects

Finally, poor seeing conditions especially with larger apertures will limit magnifications to 200-300x or 2-3mm exit pupil.

It helps to have an observing program and plan your evening's viewing. The Astronomical League has a number of observing plans. Or create your own, i.e., comparing the shapes of globular clusters in the Sagittarius region or colorful double stars in Bootes. Use a table for your eyepieces, tools, charts and texts or for your tablet and lightshield. Plan on 20 minutes per object. I strongly encourage you to sketch what you see. This hones your observing skills and brings out details in the object. Observe at all three ranges of power: low, medium and high.

For observing large scale regions of the Milky Way and more on organizing observing and sketching sessions, see my dark nebulae observing comments at

Counting the Pleiades, an exercise into extended limiting magnitude

Averted vision works best if you know where to aim your eyes in the field of view. Here's a chart to help.

For extended objects, things are not as simple as stars. For starters, it is not possible to increase the surface brightness of an extended object by increasing the aperture. An example: take an object of 10 magnitude/ square arcsecond as seen by the unaided eye at night, exit pupil open to 7mm. Now, look at the object through a 10" scope. If there is no magnification to the image, the surface brightness will increase by the ratio of the scope's aperture to the eye's aperture squared, or, (10"/0.3")^2 =~ 1000x. However, in order to fit all of the light from the 10" aperture into the eye's exit pupil, we must use at least 33x. 33x will dilute the image brightness by 33^2 =~ 1000x, so we are back where we started. In fact, because of mirror coatings not reflecting 100%, and the small obstruction caused by a diagonal, the image brightness per area will actually be a little less than with the unaided-eye.

This leads to the interesting conclusion that the brightness of the sky glow as seen in the eyepiece is entirely dependent on exit pupil.  At a given location on a given night, no matter the size of scopes, if they are giving the same exit pupil, then the sky glow brightness will be very similar.

So why then is aperture the dominant factor? If exit pupil or sky background brightness is kept constant, then as aperture increases so must the magnification. The object appears larger and is easier to see. It’s like moving in closer. If magnification is kept constant then the object and background brightness increase, also making the object easier to see.

Conduct your own experiments; I have. Find a large rock and walk away from it until you can't see it. Now walk towards it. Do this in dark skies and in a forest under dark skies. Try this with a small rock. Take a magazine page then shine a very dim flashlight on it. Walk away. Now walk towards it. At first it simply becomes easier to detect; eventually the largest shapes are discernable and finally large print. Walking towards the rock or magazine page is equivalent to increasing aperture.

Better yet, take a nice enlarged print of a galaxy or globular cluster or planetary nebula or dark nebula. Dimly light it. Walk away and towards it. Not only does the object become easier to see as you approach the print, individual stars and detail become more visible too. That's aperture and magnification at work.

Very wide fields of view at widest exit pupils allow for more aperture for a given field and also increased detail because the objects are spread out more. For more on this, check out my "Why Am I Seeing More" page.
What is sky glow brightness? The night sky, even at very dark sites, glows faintly due to zodiacal light and airglow. See Brian Skiff's discussion at You can measure the darkness (or brightness) of your night using a sky glow meter available at Dark sky sites have readings close to 21.5 magnitudes per square arcsecond. Observing through a telescope with your eye's pupil fully opened results in a sky glow in the field of view equal to that of the night sky. Magnifying the image results in smaller exit pupils, the useful maximum magnification or smallest exit pupil being close to 1mm. The sky glow brightness drops more than 4 magnitudes to close to 26 magnitude as exit pupil shrinks to 1mm.

There's a great deal of discussion about Blackwell's studies and Clark's presentation. Here's my take:

So how can we see the object in the scope? The eye is a marvelous detector of low contrast faint objects, but the light must fall on large numbers of rod cells so that the eye-brain can detect the slight contrast difference between object and background. The slighter the contrast, the more rod cells that the object's light must fall on in order to generate a signal difference between object and background. By increasing the telescope magnification, the object is magnified so that its light falls on many rod cells. There are two points to consider when an object is in the field of view of an eyepiece. The first is the object combined with the sky glow from the atmosphere that is directly between us and the object, and the second is a point away from the object, which is the sky glow only. The ratio of brightness between these two points is sometimes called the object contrast. This contrast value stays constant despite any  increase in magnification because both points are equally dimmed.

The seminal reference on visual astronomy is Clark's book, "Visual Astronomy of the Deep Sky". In it Clark explains and quantifies the visual detection of objects. Clark has added additional comments since the book's publication, at Clark uses data from a World War II study by Blackwell.

Here a brief presentation of the Blackwell data. The eye's detection ability with sky background brightness values from 21 to 26 is:

From the chart we can see that large exit pupils result in the best ability to detect objects over a wide range of apparent sizes. As the exit pupil shrinks, the ability to detect objects declines and becomes concentrated on apparent sizes of about a degree. We can see this by plotting best apparent detection size against declining sky background brightness. Here are two visualizations of the data:


The data and its interpretation has been the subject of intensive discussions between Prof Clark, Nils Olaf Carlin, Harold Lang and myself.

For Nils Olof Carlin's analysis of Blackwell's original data, see blackwel.html. Here, Nils shows that the best contrast comes when the background is dimmed below visual detection and the object is about one degree in apparent size.

Bill Ferris has generated a series of ODM matrices that compare the variables with each other:  

I wrote a visual detection calculator that presents the data by aperture and exit pupil. I believe that the whole issue of visual detection needs more observations and possibly a new model. The detector that I wrote uses the Blackwell data. Like any ground breaking study, there remains much to be done. The study was done with two eyes - how does a single eye do? Objects in with complex isophotes need to be studied, distractions of other objects in the field of view needs to be investigated and variations in the color of the objects need to be checked. Also needing observations is variation in the ages of the observers and especially telescope construction features like baffling and cleanliness of optics.

Greg Crinklaw has invested a great deal of time into improving his visual detection calculator based on empirical results at the eyepiece. See his SkyTools software and in particular his comet chasing page.