Observing the Moon
The moon is usually the first astronomical object we learn to observe. It is far larger than the stars or planets, and is bright enough to be visible during the day. Unlike, most astronomical targets, the moon doesn't require a large amount of magnification like the planets or large telescope apertures like dark sky objects like nebulae, gallaxies or star clusters. Simple binoculars are usually enough to greatly heighten the view of the moon. Obviously telescopes can greatly enhance lunar views.
During a month the brightness of the moon varies greatly from very bright, magnitude -11 at Full Moon, to virtually invisible at New Moon. Most astronomy clubs schedule their star parties and other events to coinside with the weekend closest to the New Moon, since that New Moon offers the opportunity for the best dark skies for viewing subtle dim objects like nebulae and gallexies.
Where To Look
Ecliptic
If you were to trace the path of the Sun across the sky, the Sun’s path is a line called the Ecliptic. The Ecliptic rises and falls during the year: The highest point is the Summer Solstice and at the lowest point, 6 months later, occurs on the Winter Solstice. Once you get a feeling of where the Ecliptic lies, you might discover that the moon and all the planets, with the exception of the former planet Pluto lies within a few degrees of the Ecliptic. The Ecliptic represents the edge view of the Solar System.
Scanning the Ecliptic will help you locate the moon and planets.
The Terminator
Once you've located the moon, and aimed your binoculars or telescope at it you will quickly realized that the best area to view is the area between the lit and dark sides of the lunar surface. That area is enhanced by shadows of the craters, mountains and canyons on the surface of the moon. The area between the lit and daark sides of the moon is called "The Terminator" and the cool thing is that the terminus moves to a different area, each night, as the phases of the moon change. Every night is a different and interesting view of the moon!
Equipment Needed
Telescopes
Because planets are bright, though tiny in size, a large telescope isn’t necessarily required for viewing planets like Mercury, Venus, Mars, Saturn and Jupiter. Large aperture telescopes are very beneficial to make dim things bright, like nebulae, galaxies, star clusters and the far outer planets: Uranus, Neptune and Pluto.
Almost any telescope capable of magnifying upto 200 times is great for viewing the moon.
Eyepieces
Eyepieces control magnification, field-of-view and eye relief. You can consider the eyepiece half of your optical system. Typically you will want high magnification eyepieces (100x-200x) for the moon and planets, while low power, wide field eyepieces are used for deep sky objects.
Calculating eyepieces
Each telescope is designed with a focal length. Eyepieces also have a focal length. This value is usually printed on the side or top of the eyepiece. If you divide the focal length of the telescope by the focal length of an eyepiece that will give you the power or magnification that eyepiece will give with that telescope.
For example: An 8-inch Schmidt-Cassegrain telescope has the focal length of around 2000mm. If you use a 10mm eyepiece with that telescope you will have 200 magnification (2000/10). A 30mm eyepiece in the same telescope will produce 67 power (2000/30).
So the lower the focal length of an eyepiece, the higher the power.
Sometimes eyepieces are also specified with Apparent Field-of-View measured in degrees. If you were to divide the Apparent Field-of-View by the power you will calculate the Actual Field-of-View that that eyepiece would have with the telescope.
To compare various eyepieces click here.
Planetary Filters
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Because they reflect light directly from the Sun, the moon and most planets are very bright. If you think of a telescope as a light amplifier, then most telescopes will produce an image of the moon or a planet that is too bright to pick out subtle details. There are three ways to adjust the light: planetary filters, polarizing filters and off-axis masks.
Looking at a full moon through a large telescope won't damage your eyes, but it will effect your night vision and you will step away from the telesocpe with "spots" in front of your eyes. |
Wratten System
The Kodak company developed a numbering system to specify color filters for use with black and white film. This is known as the Wratten System. Astronomy uses the same numbering system to specify planetary filters.
Because observational astronomy lacks color in the views of astronomical object until one gets into very large aperture telescopes (greater than 10 inches), using a planetary filter is like using a color filter with black and white film. They will reduce the brightness and enhance various features seen on the lunar or planetary disk.
Suggested Filters for the Moon
| Wratten Number |
Color |
Feature |
| 8 |
Light Yellow |
Contrast of features |
| 12 |
Yellow |
Contrast of features |
| 15 |
Dark-Yellow |
Contrast of features |
| 56 |
Light-Green |
Lunar Details |
| 82A |
Blue |
Low contrast features |
| ND |
Neutral Denisty (Grey) |
Light Reduction, no false color |
Polarizing Filters
Another popular choice for filtering lunar light is a polarizing filter. These filters consist of two polarizing filters that can be rotated to each other to adjust the amount of light that is passed through. This allows you to adjust the light level to a comfortable level for viewing the particular phase of the moon. Another advantage to polarizing filters are that they are "neutral-density" filters, grey in color and do not add any false color to view of the moon.
Off-axis Masks
Using an off-axis mask on the front of a telescope is another way to reduce the light gathered by a telescope for observing planets. An off-axis mask is a plate that fits in the front opening of the telescope with a smaller hole located between the center and the edge of the opening (off-axis). Frequently off-axis masks are built into the dust cover of some Newtonian reflector telescopes. The hole is placed off-axis to avoid being blocked by the secondary mirror, usually located in the center of the aperture.
Using and off-axis mask has two advantages of filters. They do not introduce false color and by reducing the usable aperture makes the telescope less sensitive to poor seeing conditions caused by turbulent atmosphere.
Kendrick Kwik-Focus
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One way to get an off axis mask is to purchase a Kendrick Kwik-Focus. The Kwik-Focus is an off-axis mask with three equally separated holes. It is a tool used by astrophotographers to assist in focusing. Basically, if your telescope is out-of-focus the mask will produce three separate images. The astronomer simply focuses the telescope until all the images are merged together and the telescope is focused, the astronomer removes the plate and starts photographing. |
To use the Kwik-focus to observe a planet, simply plug two of the holes with the conveniently supplied plugs supplied with the mask and return the plate to the front of the telescope.
For more information on the Kendrick KwikFocus go here.
Computerized Sky Guides
In the last couple of years a new class of astronomy gadgets have appeared. These handheld devices integrate GPS, Electronic compasses and motion sensors to create and integrate system that allow you to locate and identify visible objects in the sky without a telescope or other celestial aid.
These devices have three basic functions:
- Locate an object from the device’s database. Select the object from the database and follow the arrows to aim the device at the object in the sky.
- Identify an object in the sky. Aim the device at an object in the sky and press the “Identify” button to get a list from brightest to dimmest of candidate objects.
- Give visual and audio information about a select object in the Device.
These devices first must sync to the GPS satellites, so they work best when there is a relative clear view of the sky. Also, these devices are sensitive to electric and magnetic fields, so their battery compartments are shielded or separated from the rest of the mechanism and they work best when you step away from large metal objects like cars and electrical fields like high power lines.
Celestron SkyScout Personal Planetarium
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Announce in 2006 at the Consumer Electronics Show (CES), this is the first-of-its-breed device. The idea was so novel that it won the “CES 2006 – Best of Innovation Award”, the “Readers Digest – Best 2006 Award”, the “PC Magazine – Last Gadget Standing Award” and “Popular Mechanics- Editor’s Choice Award (CES 2006)”.
This device consists of :
- A black & white, backlit, liquid crystal display
- Audio port with “ear-buds”.
- Memory Card Port
- USB Port – for updating firmware.
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This device runs on two AA batteries that are place in metal shielded tubes before you install into the device to reduce electrical interference.
To aim the SkyScout, you look through the device that has two rings on either end of the chamber. The far ring has a ring of LED arrows to help you point your way
For more information on the Celestron SkyScout go here.
Meade mySky - Your Personal Guide for Sky Exploration
Announced in 2007, Meade came out with their mySky device. The mySky is a light weight, gun-shaped device which sport LED at the top side of the device for aiming and a 2 inch color video screen for visual output.
When you first turn this device on you get an option to watch an instruction video on how to use the device or simply start using the device.
The mySky consist of:
- A color video screen
- Audio port with “ear-buds”.
- Memory Card Port
- USB Port – for updating firmware.
- A cable port in interface the mySky with a Meade computerized telescope system. You can tell your telescope to move to a selected object from the mySky.
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This device runs on 4 AA batteries that are located at the bottom of the handle so they do not need to be shielded.
To aim the mySky simply look down the LED gunsite of the device and follow the “real time” star map projected in the video screen.
For more information on the Meade mySky go here.
Computer Software
One of the easiest ways to pinpoint the location of a planet or any celestial object for any given night is to use computer software to simulate the sky. Here are a few examples:
Starry Night
Available for Windows and Macintosh, this software is some of the most popular sky simulators. There is also a “PRO” version which allows you to control a computerized “GOTO” telescope.
More information for Starry Night can be found here. |
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The Sky
A beautiful program available for Windows is another popular choice for simulating the sky.
More information for TheSky can be found here. |
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