Meade Instruments Corporation
Telescopes · Binoculars · Microscopes


 
Meade Apochromatic Refrators Instruction Manuals
Models 102ED, 127ED, 152ED, and 178ED
 C. TELESCOPE OPERATION
IMPORTANT NOTICE! Never use a telescope or spotting scope to look at the Sun! Observing the Sun, even for the shortest fraction of a second, will cause irreversible damage to your eye as well as physical damage to the telescope or spotting scope itself.
[ toc ] 1. Your First Observations

With the telescope assembled as described in Section B, and with the diagonal mirror and eyepiece in place, you are ready to make observations through the telescope. Even without the viewfinder (if not yet installed), terrestrial objects will be fairly easy to locate and center in the telescope's field of view with a low power eyepiece, by "gunsighting" along the side of the main telescope tube.

By unlocking the R.A. lock (8, Fig. 4), the telescope may be turned rapidly through wide angles in Right Ascension (R.A.). The reason for the terminology "Right Ascension" and its complementary term, "Declination" will be made clear further on in this manual. Fine adjustments in R.A. are made by turning the R.A. slow-motion knob (9, Fig. 4), while the R.A. lock is in the "locked" position.

Releasing the Declination lock (5, Fig. 4), permits sweeping the telescope rapidly through wide angles in Declination.

To use the Declination fine-adjust, or slow-motion control, lock the telescope in Declination using the Declination lock, and turn the Declination slow-motion knob. (7, Fig. 4).

With the above mechanical operations in mind, select an easy-to-find terrestrial object as your first telescope subject for example, a house or building perhaps one-half mile distant.

Unlock the Dec. lock, and R.A. lock, center the object in the telescopic field of view and then re-lock the Dec. and R.A. locks. Precise image centering is accomplished by using the Dec. and R.A. slow motion controls.

Note: The R.A. and Dec. slow motion control knobs cannot be used when the optional #1697 CDS is operating. When power is supplied to the telescope, use the CDS keypad to make fine adjustments in R.A. or Declination. Forcing the manual control knobs when the motors are powered can cause damage to the gear system.



[ toc ] 2. Focusing

The focus knob (16, Fig. 4) allows focusing the image. Focusing the telescope from its nearest possible focus point (on an object about 50 ft. to 150 ft., depending on the model) to an object at infinity requires a fairly large movement of the focuser drawtube. The focuser is designed to provide an extremely sensitive means of bringing an object into precise, sharp focus. After a specific object has been brought into focus, closer objects require moving the focuser drawtube outward; more distant objects require moving the drawtube inward.




[ toc ] 3. Magnifications

The magnification, or power, of the telescope depends upon two optical characteristics: the focal length of the main telescope and the focal length of the eyepiece used during a particular observation. For example, the focal length of the 4" f/9 telescope is fixed at 920mm; the focal length of the 7" f/9 telescope is fixed at 1600mm. To calculate the power in use with a particular eyepiece, divide the focal length of the eyepiece into the focal length of the main telescope. For example, using the SP26mm eyepiece supplied with the 4" f/9, the power is calculated as follows:

Power = 920mm/26mm= 35X

The type of eyepiece (whether "MA" Modified Achromatic or "PL" Plössl, "SP" Super Plössl, etc.) has no bearing on magnifying power but does affect such optical characteristics as field of view, flatness of field and color correction.

The maximum practical magnification is determined by the nature of the object being observed and, most importantly, by the prevailing atmospheric conditions. Under very steady atmospheric "seeing," the 4" APO may be used at powers up to about 400X on astronomical objects, the 7" APO up to about 700X. Generally, however, lower powers of perhaps 250X to 350X will be the maximum permissible, consistent with high image resolution. When unsteady air conditions prevail (as witnessed by rapid "twinkling" of the stars), extremely high-power eyepieces result in "empty magnification," where the object detail observed is actually diminished by the excessive power.

When beginning observations on a particular object, start with a low power eyepiece; get the object well-centered in the field of view and sharply focused. Then try the next step up in magnification. If the image starts to become fuzzy as you work into higher magnifications, then back down to a lower power, the atmospheric steadiness is not sufficient to support high powers at the time you are observing. Keep in mind that a bright, clearly resolved but smaller image will show far more detail than a dimmer, poorly resolved larger image.

Accessories are available both to increase and decrease the operating eyepiece power of the telescope. See your Meade dealer or the Meade General Catalog for information on accessories.


[ toc ] 4. Apparent Field and Actual Field

Two terms that are often confused and misunderstood are "Apparent Field" and "Actual Field." "Apparent Field" is a function of the eyepiece design and is built into the eyepiece. While not totally accurate (but a very good approximation), "Apparent Field" is usually thought of as the angle your eye sees when looking through an eyepiece. "Actual Field" is the amount of the sky that you actually see and is a function of the eyepiece being used and the telescope.

The "Actual Field" of a telescope with a given eyepiece is calculated by knowing the "Apparent Field" and power of an eyepiece with a given telescope. The "Actual Field" of a telescope is calculated by taking the "Apparent Field" of the eyepiece and dividing by the power.

The following table lists the most common optional eyepieces available and the "Apparent Field" for each eyepiece. The power and "Actual Field" of view that each eyepiece yields is listed for each basic telescope optical design.

Table 2: Optional Eyepieces

[ toc ] 5. Celestial Coordinates: Declination and Right Ascension

Analogous to the Earth-based coordinate system of latitude and longitude, celestial objects are mapped according to a coordinate system on the "celestial sphere," the imaginary sphere on which all stars appear to be placed. The Poles of the celestial coordinate system are defined as those 2 points where the Earth's rotational axis, if extended to infinity, North and South, intersect the celestial sphere. Thus, the North Celestial Pole is that point in the sky where an extension of the Earth's axis through the North Pole intersects the celestial sphere. In fact, this point in the sky is located near the North Star, or Polaris.

On the surface of the Earth, "lines of longitude" are drawn between the North and South Poles. Similarly, "lines of latitude" are drawn in an East-West direction, parallel to the Earth's equator. The celestial equator is simply a projection of the Earth's equator onto the celestial sphere. Just as on the surface of the Earth, imaginary lines have been drawn on the celestial sphere to form a coordinate grid. Celestial object positions on the Earth's surface are specified by their latitude and longitude.

The celestial equivalent to Earth latitude is called "Declination," or simply "Dec.," and is measured in degrees, minutes or seconds north ("+") or south ("-") of the celestial equator. Thus any point on the celestial equator (which passes, for example, through the constellations Orion, Virgo and Aquarius) is specified as having 0°0'0" Declination. The Declination of the star Polaris, located very near the North Celestial Pole, is +89.2°.

The celestial equivalent to Earth longitude is called "Right Ascension," or "R.A." and is measured in hours, minutes and seconds from an arbitrarily defined "zero" line of R.A. passing through the constellation Pegasus. Right Ascension coordinates range from 0hr0min0sec up to (but not including) 24hr0min0sec. Thus there are 24 primary lines of R.A., located at 15 degree intervals along the celestial equator. Objects located further and further east of the prime (0h0m0s) Right Ascension grid line carry increasing R.A. coordinates.

With all celestial objects therefore capable of being specified in position by their celestial coordinates of Right Ascension and Declination, the task of finding objects (in particular, faint objects) in the telescope is vastly simplified. The setting circles of the LXD650 and LXD750 mounts included with Meade apochromatic refractors may be dialed, in effect, to read the object coordinates and the object found without resorting to visual location techniques. However, these setting circles may be used to advantage only if the telescope is first properly aligned with the North Celestial Pole.


Figure 7: The Celestial Pole

[ toc ] 6. Lining Up With the Celestial Pole

Objects in the sky appear to revolve around the celestial pole. (Actually, celestial objects are essentially "fixed," and their apparent motion is caused by the Earth's axial rotation). During any 24 hour period, stars make one complete revolution about the pole, describing concentric circles with the pole at the center. By lining up the telescope's polar axis with the North Celestial Pole (or for observers located in Earth's Southern Hemisphere with the South Celestial Pole) astronomical objects may be followed, or tracked, simply by moving the telescope about one axis, the polar axis. In the case of the Meade APO refractor telescopes, this tracking may be accomplished automatically with optional electric motor drives.

If the telescope is reasonably well aligned with the pole, therefore, very little use of the telescope's Declination slow motion control is necessary; virtually all of the required telescope tracking will be in Right Ascension. (If the telescope were perfectly aligned with the pole, no Declination tracking of stellar objects would be required). For the purposes of casual visual telescopic observations, lining up the telescope's polar axis to within a degree or two of the pole is more than sufficient: with this level of pointing accuracy, one of the telescope's optional motor drives will track accurately and keep objects in the telescopic field of view for perhaps 20 to 30 minutes.



Figure 8: Finding Polaris

Begin polar aligning the telescope as soon as you can see Polaris. Finding Polaris is simple. Most people recognize the "Big Dipper." The Big Dipper has two stars that point the way to Polaris (see Fig. 8). Once Polaris is found, it is a straightforward procedure to obtain a rough polar alignment.

To line up the telescope with the Pole, follow this procedure:

    1. Using the bubble level located on the pier cap, adjust the tripod legs so that the telescope/tripod system reads "level."

    2. Set the equatorial head to your observing latitude as described above.

    3. Loosen the Dec. lock (5, Fig. 4), and rotate the telescope tube in Declination so that the telescope's Declination reads 90°. Tighten the Dec. lock.

    4. Using the Azimuth control knob (11, Fig. 5) and Latitude adjustment knob (5, Fig. 5), center Polaris in the field of view. Do not use the telescope's Declination or Right Ascension controls during this process.

At this point, your polar alignment is good enough for casual observations. There are times, however, when you will need to have precise polar alignment, such as when making fine astrophotographs or when using the setting circles to find new objects.

As an aside procedure, during your first use of the telescope, you should check the calibration of the Declination setting circle (6, Fig. 4). After performing the polar alignment procedure, center the star Polaris in the telescope field. Loosen slightly the knurled lock screw of the Declination setting circle. Now turn the circle unit until it reads 89.2°, the Declination of Polaris, and then tighten down the knurled lock screw, avoiding any motion of the circle.

Once the latitude angle has been fixed and locked-in according to the above procedure, it is not necessary to repeat this operation each time the telescope is used, unless you move a considerable distance North or South from your original observing position. (Approximately 70 miles movement in North-South observing position is equivalent to 1° in latitude change). The equatorial head may be detached from the field tripod and, as long as the latitude angle setting is not altered and the field tripod is leveled, it will retain the correct latitude setting when replaced on the tripod.



[ toc ] 7. Precise Polar Alignment

It should be emphasized that precise alignment of the telescope's polar axis to the celestial pole for casual visual observations is not necessary. Don't allow a time-consuming effort at lining up with the pole to interfere with your basic enjoyment of the telescope. For long-exposure photography, however, the ground rules are quite different, and precise polar alignment is not only advisable, but almost essential.

Notwithstanding the precision and sophistication of the optional computer drive system available for the Meade APO telescopes, the fewer tracking corrections required during the course of a long-exposure photograph, the better. (For our purposes, "long-exposure" means any photograph of about 10 minutes duration or longer). In particular, the number of Declination corrections required is a direct function of the precision of polar alignment.

Precise polar alignment requires the use of a crosshair eyepiece. Meade Illuminated Reticle Eyepieces are well-suited in this application, but you will want to increase the effective magnification through the use of a 2x or 3x Barlow lens. Then follow this procedure, sometimes better known as the "Drift" method:

    1. Obtain a rough polar alignment as described earlier. Place the illuminated reticle eyepiece (or eyepiece/Barlow combination) into the eyepiece holder of the telescope.

    2. Point the telescope, with the motor drive running, at a moderately bright star near where the meridian (the North-South line passing through your local zenith) and the celestial equator intersect. For best results, the star should be located within ±30 minutes in R.A. of the meridian and within ±5° of the celestial equator. (Pointing the telescope at a star that is straight up, with the Declination set to 0°, will point the telescope in the right direction.)

    3. Note the extent of the star's drift in Declination (disregard drift in Right Ascension):

    Figure 9: Mount Too Far East

    a. If the star drifts South (or down), the telescope's polar axis is pointing too far East (Fig. 9).

    Figure 10: Mount Too Far West

    b. If the star drifts North (or up), the telescope's polar axis is pointing too far West (Fig. 10).

    4. Move the wedge in azimuth (horizontally) to effect the appropriate change in polar alignment. Reposition the telescope's East-West polar axis orientation until there is no further North-South drift by the star. Track the star for a period of time to be certain that its Declination drift has ceased.

    5. Next, point the telescope at another moderately bright star near the Eastern horizon, but still near the celestial equator. For best results, the star should be about 20° or 30° above the Eastern horizon and within ± 5° of the celestial equator.

    6. Again note the extent of the star's drift in Declination:

    Figure 11: Mount Too Low

    a. If the star drifts South, (or down) the telescope's polar axis is pointing too low (Fig. 11).

    Figure 12: Mount Too High

    b. If the star drifts North, (or up) the telescope's polar axis is pointing too high (Fig. 12).

    7. Use the latitude angle fine-adjust control (5, Fig. 5) to effect the appropriate change in latitude angle, based on your observations above. Again, track the star for a period of time to be certain that Declination drift has ceased.

The above procedure results in very accurate polar alignment, and minimizes the need for tracking corrections during astrophotography.

In addition to the drift method described above, two optional accessories can be used to achieve a precise polar alignment. The optional #814 Polar Alignment Finder is a borescope device which uses an internal etched reticle showing where to place the North Star to align the telescope. And the #1697 Computer Drive/Slew System has a computerized polar alignment routine that automates the complete process, allowing very fast, precise polar alignment.


[ toc ] 8. Setting Circles

Setting circles included with Meade ED apochromatic telescopes and equatorial mounts permit the location of faint celestial objects not easily found by direct visual observation. With the telescope pointed at the North Celestial Pole, the Dec. circle should read 90° (understood to mean +90°). Objects located below the 0-0 line of the Dec. circle carry minus Declination coordinates. Each division of the Dec. represents a 1° increment. The R.A. circle runs from 0hr to (but not including) 24hr, and reads in increments of 5min.

Note that the R.A. circle (17, Fig. 5) is double-indexed, i.e., there are 2 series of numbers running in opposite directions around the circumference of the R.A. circle. The upper series of numbers (increasing counterclockwise) applies to observers located in the Earth's Northern Hemisphere; the lower series of numbers (increasing clockwise) applies to observers located in the Earth's Southern Hemisphere.

With the telescope aligned to the pole, center an object of known R.A. in the telescopic field. Then turn the R.A circle, which can be rotated manually, until the R.A. coordinate of the object is correctly indicated by the R.A. pointer. As long as the telescope's motor drive remains "ON," the R.A. pointer will then correctly indicate the R.A. of any object at which the telescope is pointed throughout the duration of the observing session.

To use the circles to locate a particular object, first look up the celestial coordinates (R.A. and Dec.) of the object in a star atlas. Then loosen the R.A. lock and turn the telescope to read the correct R.A. of the desired object; lock the R.A. lock onto the object. Next, turn the telescope in Declination to read the correct Declination of the object. If the procedure has been followed carefully, and if the telescope was well-aligned with the pole, the desired object should now be in the telescopic field of a low-power eyepiece.

If you do not immediately see the object you are seeking, try searching the adjacent sky area, using the R.A. and Dec. slow-motion controls to scan the surrounding region. Keep in mind that with the 26mm eyepiece, the field of view of the telescope is less than 1°. Because of its much wider field, the viewfinder may be of significant assistance in locating and centering objects, after the setting circles have been used to locate the approximate position of the object.

Pinpoint application of the setting circles requires that the telescope be precisely aligned with the pole. Refer to the preceding section on "Precise Polar Alignment" for further details.

The setting circles may be used with or without the optional computer drive system. As you track the object, whether by turning the R.A. slow-motion control knob or using the optional computer drive system, the setting circles keep position with the object.



[ toc ] 9. Observing with the Telescope

Meade APO telescopes permit an extremely wide array of serious observational opportunities. Even in normal city conditions, with all of the related air and light pollution, there are a good many interesting celestial objects to observe. But to be sure, there is no substitute for the clear, steady, dark skies generally found only away from urban environments, or on mountain tops: objects previously viewed only in the city take on added detail or are seen in wider extension, or even become visible at all for the first time.

The amateur astronomer is faced typically with two broadly defined problems when viewing astronomical objects through the Earth's atmosphere: first is the clarity, or transparency, of the air and, secondly the steadiness of the air. This latter characteristic is often referred to as the quality of "seeing." Amateur astronomers talk almost constantly about the "seeing conditions," since, perhaps ironically, even the clearest, darkest skies may be almost worthless for serious observations if the air is not steady. This steadiness of the atmosphere is most readily gauged by observing the "twinkling" of the stars: rapid twinkling implies air motion in the Earth's atmosphere, and under these conditions resolution of fine detail (on the surface of Jupiter, for instance) will generally be limited. When the air is steady, stars appear to the naked eye as untwinkling points of unchanging brightness and it is in such a situation that the full potential of the telescope may be realized: higher powers may be used to advantage, closer double stars are resolved as distinct points and fine detail may be observed on the Moon and planets.

Several basic guidelines should be followed for best results in using your telescope:

    1. Try not to touch the eyepiece while observing. Any vibrations resulting from such contact will immediately cause the image to move.

    2. Allow your eyes to become "dark adapted" prior to making serious observations. Night adaptation generally requires about 10 to 15 minutes for most people.

    3. Let the telescope "cool down" to the outside environmental temperature before making observations. Temperature differentials between a warm house and cold outside air require about 30 minutes for the telescope's optics to regain their true and correct figures. During this period, the telescope will not perform well. A good idea is to take the telescope outside 30 minutes before you want to start observing.

    4. If you wear glasses and do not suffer from astigmatism, take your glasses off when observing through the telescope. You can re-focus the image to suit your own eyes. Observers with astigmatism, however, should keep their glasses on since the telescope cannot correct for this eye defect.

    5. Avoid setting up the telescope inside a room and observing through an open window (or worse yet, through a closed window!). The air currents caused by inside/outside temperature differences will make quality optical performance impossible.

    6. Perhaps most importantly of all, avoid "overpowering" your telescope. The maximum usable magnification at any given time is governed by seeing conditions. If the telescopic image starts to become fuzzy as you increase in power, drop down to a reduced magnification. A smaller but brighter and sharper image is far preferable to a larger but fuzzy and indistinct one.

    7. As you use your telescope more and more, you will find that you are seeing finer detail: observing through a large aperture telescope is a required skill. Celestial observing will become increasingly rewarding as you eye becomes better trained to the detection of subtle nuances of resolution.

Next Page

Related Topics:


| home | about meade | product information | dealer locator | Meade 4M |
| customer support | investor relations | dealer support |
| employment opportunities | site map |

® The name Meade, the Meade logo, and ETX are trademarks registered with the United States Patent Office,
and in principal countries throughout the world.
Copyright © 2006 Meade Instruments Corporation, All Rights Reserved.
This material may not be reproduced in any form without permission.