Observe galaxies, nebula and star clusters

Beginner astronomy: Deep sky observing

The pictures that the Hubble Space Telescope and other large, ground based instruments can show us are breathtaking. Color abounds, stars like swarming bees seem to fill these fantastic photographs of the night sky. Yet when we look through a telescope at many of these objects, the view can be... underwhelming.

Part of that is due to the fact that photographs can collect photons from those sources of light over long periods of time. Digital photography allows astronomers to “stack” these photos, meaning that several photographs over several nights can be “stacked” one on top of another to further increase the detail, color and resolution of what can be seen. The human eye has no such luxury, though. At best, it collects light for about a 1/5 of a second; this is hardly long enough to capture the light of a telescope photographing the same area of sky over many hours, stacking those photos to enhance it further. So what can we do to see more?

First, adjust expectations. Recognize that what you see will be non-photographic. If you expect to see less, you won't be disappointed. Secondly, and perhaps the greatest enhancement visual observers can make, is to get away from light polluted areas. Even small telescopes will show more detail of deep sky objects than a large scope will under washed-out skies. The star clusters, nebulae and galaxies we are trying to see have low contrast compared to the sky when that sky is lighter due to light pollution. But perhaps getting away from that light isn't possible – or practical most nights of observing. Not to worry, there's more you can do.

Fully dark adapt

Letting your eyes (or even one, with the use of an eye patch) fully dark adapt can significantly increase the faintness your eyes will pick up through the eyepiece. Waiting a full 30 minutes for your eyes to dark adapt will increase what you can perceive. Using red goggles to maintain this dark adaptation, or blocking extraneous light around you will help as well.

Use averted vision

This one seems counterintuitive, but makes sense when you think about the physiology of the human eye. Cones are the the spots on retina that pick up color, and they are concentrated in the center of the retina. Rods, on the other hand, are all around the rest of the eye, because when our ancestors needed to see at night for predators that may have been stalking them (or prey they may have been hunting), seeing “out of the corner of your eye” was a real benefit! So instead of looking “right at” the object you want to see, try looking slightly AWAY from the object, but concentrating on where it is in the eyepiece. Faint objects seem to “jump into view” using this technique, and it is an effective way to see more.

Eyez 'n the hood

Wearing a hooded coat or sweatshirt that is long enough to block extraneous light from the sides is a useful way to increase contrast by enhancing dark adaptation.  Some companies even sell apparel with longer hoods for this purpose.  Another alternative is to use a piece of dark-colored heavy fabric or a towel over your head while observing at the eyepiece.

Tap, tap, tap

A strategy some observers use to really seek out faint details is to “tap” the side of your telescope to let it “vibrate” just slightly. This goes ones step further with 'averted vision,' as not only did the human eye detect light from the sides better, it also detected motion. When you move the telescope, those very faint details can become evident when the object is being viewed with averted vision, and also moving. Try “tapping the tube” to tease out more detail at the eyepiece.

Aperture fever

Larger apertures may not show as much as small ones do under dark skies, but those larger scopes will still show more than small scopes will under the lights. So if possible, increase your aperture by using a larger telescope. The telescope will gather more photons that can be collected at the eyepiece, increasing your odds of seeing more. One thing that can help is increasing the magnification of the object you are viewing, if the size of the object allows you to do so and still keep it in the field of view. This darkens the sky background, and although it does not change the actual contrast, it increases the effective contrast that your eye perceives, which makes “faint fuzzies” more likely to stand out. A dark grey on a black background is easier to see than a medium gray on a medium-dark grey background.

Watch overhead

Not every object will ever be at the zenith (the point directly overhead), so ideally, look for objects when they are at or near the meridian (the line running from north to south) as they will be near their highest elevation at this time.  So by looking at deep sky objects when they are at the zenith or meridian, there will be less atmosphere to look through, reducing the dust and humidity light travels past to reach the eyepiece.

Filter it

With larger aperture telescopes (4.5" / 114mm to 6" / 150mm or larger), narrowband light pollution reduction filters, O-III and other filters can increase the contrast of the object viewed.  No filter will make the object brighter, but they do darken the sky background, which can provide an image that appears brighter by comparison, and perhaps more detail as well.  

Exit pupil

Calculating the best exit-pupil for your telescope / eyepiece combination will make a difference. The human eye does not dilate more than 7mm; even less for folks over age 40 or 50 (6mm or 5mm). So an exit pupil that is larger than this size will not allow your eye to see any more light. Also, you don't want to reduce the light your eye is capturing, so an exit pupil smaller than 1mm is going to make it difficult to see any more. Don't go larger than 7mm, and don't go less than 1mm.

How to calculate exit pupil:

Exit Pupil = Telescope aperture / Magnification provided by a give eyepiece 

- OR -

Exit Pupil = Eyepiece focal length / f/ratio of the scope 


A few other tips:

  • Cup your hand(s) around your head/eyepiece when you view, to block all extra light – even from dark-sky areas.

  • Use a chair to be comfortable; the more comfortable you are at the eyepiece, the more you are likely to to see

  • Avoid alcohol and drugs. These can reduce your senses, particularly sensitivity to low light levels.

  • Be well-rested; tired eyes don't pick up as much detail as sharp, attentive ones do.

  • Wait for nights of good transparency; haze and humidity in the air can block even the tiniest details. Nights where the stars are twinkling wildly can often mean bad planetary observing, but ideal DSO observing. Adjust your observing sessions accordingly.

  • Choose higher locations if possible; dew tends to form in lower areas more quickly

  • Observe later at night; often, business lighting is turned off after 10 to 12 pm, and there tend to be fewer cars driving at those times as well.

Above all, don't “force” things. If teasing out more detail just isn't working on a given night, it may be any one of a number of “gremlins” keeping you from seeing more – haze, humidity, fatigue, etc. Try again another night – the stars will still be there! The key is to have fun – make sure you do!

There's a lot more here on Eyes on the Sky. For example, every week on the homepage there is a new astronomy video about observing objects in the night sky. They're only 5 minutes long - why not check out the latest one right now? For those new to astronomy, don't miss Eyes on the Sky's Ultimate Beginner's Guide!