Crescent observation with binoculars

Version of 24. July 2010, Martin Elsaesser

This page describes the use of binoculars as tools for the observation of the lunar crescent.

Instruments

Optics

Binoculars are usefull tools for crescent observation, as the use of two eyes generally improves human vision. The higher the magnification of the binoculars, the better the expected performance, as the human eye finds it easier to see low contrast detail when it is larger.
As a disadvantage high magnifications reduce the field of view, which makes pointing the binoculars more difficult. The field of view should not shrink to much, as the full diameter of the moon should still be visible.
A large lens diameter is usually not required, as these observations happen during bright twilight, so an exit pupil (EP) of 2.5mm is good enough.

The EP can be calculated by dividing the lens diameter by the magnification.
Example: 25x100 binoculars: EP = 100mm / 25 = 4mm

The sharpness and color correction of many binoculars is improved by stopping down the optics somewhat. As we generally have sufficient brightness available for crescent observation, this can be done without problems and also simplifies focussing. Dark cardboard with round holes mounted in front of the optics is sufficient for this task. The diameter of these holes is often chosen 2.5x larger than the magnification (in mm), for an resulting EP of about 2.5mm.

10x50 is a good general purpose size, that might still be used without a tripod.
15x70 might be somewhat better, but really needs a tripod to be of any use.

Tripod

Even small binoculars such as 7x50s should be mounted on a tripod for best performance. For larger instruments such as 25x100s, use of a tripod is absolutely necessary to do any usefull observation. The tripod should allow precise movement. Scales for altitude and azimut on the tripod are very usefull for locating the crescent, if they are acurate enough.

Preparation

Focus

Focussing the binoculars precisely to infinity is important. This can be done with some clouds or far-away terrestrial subjects such as trees. Focussing should always be done from closer objects towards infinity. Focus might differ from person to person and is a very important point for difficult crescents.

Alignment

The internal alignment of the two halfs of the binoculars is important. If this alignment is off it might be inconvenient, a strain on the eyes or even impossible to observe well with both eyes. You can check alignment on some bright star at night, when defocussing one of the optics. Alignment can be adjusted on most binoculars by tiny screws on the body, usually hidden below the rubber armor.
Technical instructions on alignment can be found at the Oberwerk company.

Filters

Filters can be used to improve contrast between crescent and sky. Filters are usually mounted on the eyepieces. Red filters are the usual choice, but might also dim the view to much. Experiment.
(No filters at the eyepiece can make binoculars safe for visual observation of the sun. Do NOT try this!)

Blocking stray light

Contrast is the main issue for crescent observation, so any stray light should be avoided. You can easily build dew-caps for your binoculars to improve things at the front.
Also, blocking distracting stray light from entering the eye from the side is important. The best approach here is to use a hood to cover your head during observation, but that is often inconvenient. Using both hands to cover your eyes at the side during observing is the easiest approach.

Cleanliness

The optics should be as clean as possible, to not loose contrast and avoid distractions. The eyepiece is generally more important here and dust inside the eyepieces can be extremely anoying. Avoid scratching the lenses during cleaning.

Safety

Daytime observation, with the sun still above the horizon is EXTREMELY DANGEROUS with binoculars, as these often have stray-light problems and both eyes are endangered at the same time. If you really want to try this, the only safe way is to stand in the shade of a building or similar which prevents direct sunlight from hitting the optics. A good idea is to do this with two persons, with one person checking for the sunlight issue while the other person observes.

Size and orientation

For a better chance to find the actual crescent, you need to know the expected size and orientation of the crescent relative to your field of view beforehand, so you know what to look for. Doing test observations with your binoculars will show you the easily visible field of view (which should also be written on the binoculars). Any good software will predict the orientation of the crescent for your observations.

shape and orientation of the crescent
A sample image from a planetarium software, showing the orientation of the crescent at a certain date.

crescent and field size
A sample image from a planetarium software, showing the size of the crescent in the field of view of the optics. The blue circle has a diameter of 2.5, which equals 44m at 1000m distance.

Actual observation

Looking at the correct spot

Thin crescents can be very difficult to see, so sweeping around wildly while searching might not work. You need to have a very good understanding where exactly to search for the crescent. You can get the coordinates of the moon from any number of software packages, with altitude/azimut values being most usefull. The crescent continously moves (taking only 2 minutes to move one diameter!), for which you need to compensate.

crescent and venus and coordinates
A sample image from a planetarium software, showing the crescent above the horizon for a specific location and time. Alitude and azimut coordinate lines are drawn and coordinate values are shown. Such charts allow precise pointing of the optics, if you have identified the coordinates of your local landmarks.

Seeing it

When you are sure of looking at the correct spot at the correct time, examine the field of view in detail, looking for the thin crescent arc or the entry of the crescent into the field of view. Really thin crescents can be extremely difficult to see and you might need to focus your atttention at the exact spot in the field of view.

Documentation

Whether you succeeded or not, a good documentation of your attempt is always usefull. When did you observe? From where? What were the weather conditions? What instruments did you use? How did you find the correct spot in the sky? Also, reporting your results to an organisation like ICOP is a good idea.

Photography

Taking pictures of the crescents you have observed is a good idea. You can try to do this through the binoculars, but using a DSLR and tele lens and separate tripod will probably give better results.

crescent and clouds
Simple image of the crescent above a cloud layer, taken with a DSLR and tele lens.

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