The Planet Neptune

Observing Neptune

As an outer planet, Neptune is best observed around its Opposition. Neptune's opposition occurs about once a year. At opposition Neptune's brightness is around magnitude 7.8. Limits for naked-eye observation with clear dark skies is about magnitude 6, so at opposition Neptune would still not visible to the naked-eye and would require a telescope or binocular just to locate it. You will most likely need a 10 to 12 inch telescope to resolve this blue-green planet to a disk.

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. To pinpoint a specific planet at a specific time, you may want to: consult magazines like Sky and Telescope or Astronomy, or use software (see below), or one of the new handheld computerized realtime gadgets (see below), or consult a website like Astro Planet.[ :-) Ed.]

Optimal Times

There are certain times in a planets orbit when a planet is “optimal for viewing.”  For the outer planets: Mars, Saturn, Jupiter, Neptune, Uranus and Pluto the point of best viewing is at the Opposition.

Oppositions

For planets outside the Earth's orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around Oppositions are the best time to view these. An Opposition occurs when the planet is opposite from the Sun, relative to the Earth. At Opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation. Also at Opposition the planet comes physically closest to the Earth in it's orbit so it appears as large as possible.

Opposition

Neptune

Neptune is located in the constellation Capricornus until the year 2010. Neptune is at opposition around mid-September, so the best time to observe Neptune is between July and November.

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 100 to 200 times is great for viewing planets and our 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

Planetary Filters Because they reflect light directly from the Sun, most planets are very bright.  If you think of a telescope as a light amplifier, then most telescopes will produce an image of a planet that is too bright to pick out subtle details.  There are two ways to adjust the light: planetary Filters and Off-axis masks.

 

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 planetary disk.

For more information on Planetary Filters, click here.

Suggested Filters for Neptune

Wratten NumberColorFeature
12Yellowto enhance the blue-green atmosphere
15Yellowto enhance the blue-green atmosphere
30Magentato enhance the blue-green atmosphere
32Magentato enhance the blue-green atmosphere
57Greento enhance the blue-green atmosphere

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

KwikFocus

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:

  1. 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.
  2. 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.
  3. 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

Celestron SkyScout

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 :

  1. A black & white, backlit, liquid crystal display
  2. Audio port with “ear-buds”.
  3. Memory Card Port
  4. USB Port – for updating firmware.

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:

  1. A color video screen
  2. Audio port with “ear-buds”.
  3. Memory Card Port
  4. USB Port – for updating firmware.
  5. 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.
Meade mySky

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.

Starry Night
TheSky

The Sky

A beautiful program available for Windows is another popular choice for simulating the sky.

More information for TheSky can be found here.

Stellarium

For those who are into open source software. Stellarium is available from sourceforge.org.  Stellarium is available for Windows, Macintosh, Unix and Linux.

Stellarium


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