NexStar 8 SE
___________________________________________________________________________
NexStar 8 SE Technical Specifications
Design: Schmidt-Cass Optical Design
Aperture: 8” (200mm)
Focal Length: 80” (2032mm)
F/ratio of the Optical System: 10
Primary Mirror: 100mm diameter, Starbright XLT
Secondary Mirror: 2.5”, 35% by diameter, 12% by area
Corrector Plate: Optical Quality Crown Glass Starbight XLT
Highest Useful Magnification: 480x
Lowest Useful Magnification: 29x
Light Gathering Power: 843x unaided eye
Near Focus: ~25’
Magnification (standard eyepiece): 81x
Optical Tube Length: 17”
Weight of Telescope: 24 lbs
Weight of Tripod: 9 lbs
___________________________________________________________________________
Telescope Basics
Telescope: An instrument that collects and focuses light.
Refractor: A telescope that uses lenses to collect and focus light.
Reflector: A telescope that uses mirrors to collect and focus light.
Compound (‘Schmidt-Cassegrain’- ‘Schmidt-Cass’): A telescope that uses a combination of mirrors and lenses to collect and focus light. Schmidt-Cassegrain system consists of a zero-power corrector plate, a spherical primary mirror, and a secondary mirror. Once light rays enter the optical system, they travel the length of the optical tube three times.
Focusing: The NexStar's focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle tube. The focusing knob, which moves the primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close focus (~25’) to infinity.
Magnification: You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your telescope, divide the focal length of the telescope by the focal length of the eyepiece used.
Magnification = Focal length of Telescope (mm) / Focal length of Eyepiece (mm).
If you are using the 25mm eyepiece; Magnification = 2000 mm / 25 mm = 80x magnification.
Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The general rule is that 60 power can be used for every inch of aperture. For example, the NexStar 8 is 8" in diameter. Multiplying 8 by 60 gives a maximum useful magnification of 480 power. Although this is the maximum useful magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 160 to 280 times for the NexStar 8 telescope.
Field of View: Determining the field of view is important if you want to get an idea of the angular size of the object you are observing. To calculate the actual field of view, divide the apparent field of the eyepiece (supplied by the eyepiece manufacturer) by the magnification.
True Field = Apparent Field of Eyepiece / Magnification
Using the example in the previous section, we can determine the field of view using the same 25mm eyepiece. The 25mm eyepiece has an apparent field of view of 52°. Divide the 52° by the magnification, which is 80 power. This yields an actual field of view of .65°.
Celestial Coordinate System
The celestial equator runs 360 degrees around the Earth and separates the northern celestial hemisphere from the southern. Like the Earth's equator, it bears a reading of zero degrees. On Earth this would be latitude. However, in the sky this is referred to as declination, or DEC for short. Lines of declination are named for their angular distance above and below the celestial equator. The lines are broken down into degrees, minutes of arc, and seconds of arc. Declination readings south of the equator carry a minus sign (-) in front of the coordinate and those north of the celestial equator are either blank (i.e., no designation) or preceded by a plus sign (+).
The celestial equivalent of longitude is called Right Ascension, or R.A. for short. Like the Earth's lines of longitude, they run from pole to pole and are evenly spaced 15 degrees apart. Although the longitude lines are separated by an angular distance, they are also a measure of time. Each line of longitude is one hour apart from the next. Since the Earth rotates once every 24 hours, there are 24 lines total. As a result, the R.A. coordinates are marked off in units of time. It begins with an arbitrary point in the constellation of Pisces designated as 0 hours, 0 minutes, 0 seconds. All other points are designated by how far (i.e., how long) they lag behind this coordinate after it passes overhead moving toward the west.
Polar Alignment: The process by which the telescope's axis of rotation (polar axis) is aligned (made parallel) with the Earth's axis of rotation. Once aligned, a telescope with a clock drive will track the stars as they move across the sky. The result is that objects observed through the telescope appear stationary (i.e., they will not drift out of the field of view). If not using the clock drive, all objects in the sky (day or night) will slowly drift out of the field. This motion is caused by the Earth's rotation.
Polar Axis: The axis around which the telescope rotates when moved in right ascension. This axis points the same direction even when the telescope moves in right ascension and declination.
The north celestial pole is the point in the northern hemisphere around which all stars appear to rotate. The counterpart in the southern hemisphere is referred to as the south celestial pole.
___________________________________________________________________________
Hand Control
___________________________________________________________________________
Celestial Observing
Lunar Observations: To increase contrast and bring out detail on the lunar surface, use filters. A yellow filter works well at improving contrast while a neutral density or polarizing filter will reduce overall surface brightness and glare.
Solar Observations: For safe solar viewing, use a Celestron solar filter that reduces the intensity of the Sun's light, making it safe to view. With a filter you can see sunspots as they move across the solar disk and faculae, which are bright patches seen near the Sun's edge.
___________________________________________________________________________
Telescope Maintenance
___________________________________________________________________________
Optional Accessories
Barlow Lens: A negative lens that increases the focal length of a telescope. Used with any eyepiece, it doubles the magnification of that eyepiece. Celestron offers two Barlow lens in the 1-1/4" size for the NexStar. The 2x Ultima Barlow (#93506) is a compact triplet design that is fully multicoated for maximum light transmission and parfocal when used with the Ultima eyepieces. Model #93507 is a compact achromatic Barlow lens that is under three inches long and weighs only 4 oz.
X-Cel Eyepiece: This 6-element design allows each X-Cel Eyepiece to have 20mm of eye relief, 55° field of view and more than 25mm of lens aperture (even with the 2.3mm). In order to maintain razor sharp, color corrected images across its 50° field of view, extra-low dispersion glass is used for the most highly curved optical elements. The excellent refractive properties of these high-grade optical elements, make the X-Cel line especially well-suited for high magnification planetary viewing where sharp, color-free views are most appreciated. X-Cel eyepieces come in the following focal lengths: 2.3mm, 5mm, 8mm, 10mm, 12.5mm, 18mm, 21mm, 25mm.
Filters: Celestron offers a wide range of colored eyepiece filter sets that thread into the 1-1/4" oculars. Available sets are: #94119-10 – Orange, Light Blue, ND13%T, Polarizing (#s 21, 80A, 15, Polarizing) #94119-20 - Deep Yellow, Red, Light Green, ND25% T (#s 12, 25, 56, 96ND-25) #94119-30 - Light Red, Blue, Green, ND50% T (#s 23A, 38A, 58, 96ND-50) #94119-40 - Yellow, Deep Yellow, Violet, Pale Blue (#s 8, 96ND-13, 47, 82A)
Moon Filter (#94119-A): An eyepiece filter for reducing the brightness of the moon and improving contrast, so greater detail can be observed on the lunar surface. The clear aperture is 21mm and the transmission is about 18%.
Polarizing Filter Set (#93608): Limits the transmission of light to a specific plane, thus increasing contrast between various objects. This is used primarily for terrestrial, lunar and planetary observing.
Solar Filter 8” (#94128): Covers the front opening of the telescope. View sunspots and other solar features using this double-sided metal coated filter for uniform density and good color balance across the entire field. The Sun offers constant changes and will keep your observing interesting and fun.
T-Adapter (#93633-A): Allows you to attach your SLR camera to the rear cell of your Celestron NexStar. This turns your NexStar into a 1250mm telephoto lens perfect for terrestrial photography and short exposure lunar and filtered solar photography.
T-Ring: Couples your 35mm SLR camera body to the T-Adapter. This accessory is mandatory if you want to do photography through the telescope. Each camera make (i.e., Minolta, Nikon, Pentax, etc.) has its own unique mount and therefore, its own T-Ring. Celestron has 8 different models for 35mm cameras.
Vibration Suppression Pads (#93503): These pads rest between the ground and tripod feet of your telescope. They reduce the amplitude and vibration time of your telescope when shaken by the wind or an accidental bump. This accessory is a must for long exposure prime focus photography
___________________________________________________________________________