Celestron CR-150 HD Calculating Magnification, Determining Field of View, Telescope Basics

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Calculating Magnification

Calculating Magnification

You can change the power of your Celestron CR-150 HD telescope just by chang-

 

ing the eyepiece (ocular). To determine the magnification for your telescope, you

 

would simply divide the focal length of the telescope (1200mm) by the focal length

 

of the eyepiece that you are using. In equation format, the formula looks like this:

 

Focal Length of Telescope (mm)

 

Magnification=————————————————

 

Focal Length of Eyepiece (mm)

Determining Field of View

Let’s say, for example, that you are using a 20mm eyepiece. To determine the magnification, simply divide the focal length of your telescope (1200mm) by the focal length of the eyepiece (20mm). Dividing 1200 by 20 yields a magnification of 60 power.

Although the power is variable, each instrument 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 CR-150 HD is 6" in diameter. Multiplying 6 by 60 gives a maximum useful magnification of 360 power. Although this is the maximum useful magnification, due to limiting atmospheric conditions most observing is done in the range of 20 to 35 power for every inch of aperture which is 120 to 210 power for the CR-150 HD.

Determining the field of view is important if you want to get an idea of the 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. In equationformat,theformulalookslikethis:

ApparentFieldofEyepiece

TrueField=———————————————

Magnification

Using the example we started with above, we can determine the field of view using the same 20mm eyepiece. The 20mm eyepiece has an apparent field of view of 52°. Divide the 52° by the magnification, which is 60 power. This yields an actual field of .87°, or a little more than three quarters of a degree.

For terrestrial viewing, field size is often referred to as feet at a thousand yards. To convert this to feet at one thousand yards, multiply the actual field of .87° by 52.5. This produces a field width of 45.7 feet at one thousand yards.

The apparent field of each eyepiece that Celestron manufacturers is found in theCelestronAccessoryCatalog(#93685).

26 • Telescope Basics

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Contents 6 Refractor Model #21017The CR-150HD Assembling the CG-5Equatorial Mount T A B L E O F C O N T E N T Sii Table of Contents Using the Lens Cap Aperture Stop Table of ContentsI N T R O D U C T I O N 4 IntroductionHow to Use This Manual AWordofCaution Introduction6 Introduction The Refractor Optical SystemThe Mount A S S E M B L I N G Y O U R C R - 1 5 0 H D Unpacking Your CR-150HDThe CR-150HD Tripod Head Setting Up the Tripod Adjusting the Tripod HeightAssembling the CG-5Equatorial Mount Attaching the Accessory Tray 10 The CR-150HDAttaching the Equatorial Mount Attaching the R.A. Slow Motion Knob 12 The CR-150HDAttaching the Declination Slow Motion Knob 1.Lock the DEC clamp to hold the mount in place 14 The CR-150HDAttaching the Telescope to the Mount Tube Rings Dovetail BracketBalancing the Telescope in R.A 16 The CR-150HDBalancing the Telescope in DEC COMPLETELY18 The CR-150HD Adjusting the Mount in AltitudeAdjusting the Mount in Azimuth Storing Your CR- 150 HD Technical SpecificationsDisassembling and Transporting Your CR 150 HD OPTICAL TUBEThe Star Diagonal Attaching the AccessoriesT E L E S C O P E B A S I C S The EyepieceAdapter 1 1/4 AccessoryFocuser Tension Built-inT-Adapter22 Telescope Basics FocusingAttaching the Finder Aligning the Finder DaytimeObserving Image OrientationYourFirstLook NEVER POINT YOUR TELESCOPE AT THE SUN UNLESSNighttime Observing 26 Telescope Basics Calculating MagnificationDetermining Field of View GeneralPhotographyHints Your Celestron telescope can be used for both terrestrial and astronomical photography. Your telescope has a fixed aperture and, as a result, a fixed f/ratio. To properly expose your subjects photographically, you need to set your shutter speed accordingly. Most 35mm single lens reflex SLR cameras offer through-the-lens metering that lets you know if your picture is under or overexposed. This is important for terrestrial photography where exposure times are measured in fractions of a second. In astrophotography, the exposures are much longer, requiring that you use the “B” setting on your camera. The actual exposure time is determined by how long you keep the shutter open. More on this in the section on “Celestial Photography.” 28 Astronomy Basics A S T R O N O M Y B A S I C SThe Celestial Coordi- nate System Motion of the Stars Astronomy BasicsDefinition Polar AlignmentEquator 30 Astronomy BasicsFinding the Pole Latitude Scales 32 Astronomy BasicsPointingatPolaris DeclinationDrift 34 Astronomy BasicsAligning the R.A Setting Circle Polar Alignment FindersPlanets Observing the MoonObserving the C E L E S T I A L O B S E R V I N GSOLAROBSERVINGHINTS Using the Lens Cap Aperture StopObserving the Sun Celestial Observing38 Celestial Observing Using the Setting CirclesObserving Deep-SkyObjects StarHopping 40 Celestial Observing Sky Illumination Viewing ConditionsTransparency Seeing42 Celestial Observing C E L E S T I A L P H O T O G R A P H Y Celestial PhotographyCrescent Short Exposure Prime FocusLunar Phase QuarterThe exposure times listed here should be used as a starting point. Always make exposures that are longer and shorter than the recommended time. Also, try bracketing your exposures, taking a few photos at each shutter speed. This will ensure that you will get a good photo. If using black and white film, try a yellow filter to reduce the light intensity and to increase contrast Piggyback 46 Celestial PhotographyEktar 1000 color print Konica 3200 color print Projection Photography 48 Celestial PhotographyTelescope Maintenance Care and Cleaning of the OpticsT E L E S C O P E M A I N T E N A N C E Dual Axis Drive System - #93523 50 Optional AccessoriesO P T I O N A L A C C E S S O R I E S Super Modified Achromatic SMA EyepiecesOptional Accessories 52 Optional Accessories Motor Drive System - #93518Const THEMESSIERCATALOGNGC# Type54 The Messier Catalog Type Star Name LIST OF BRIGHT STARSEpoch ConstellationAtlases FOR FURTHER READINGAstronomy Texts General Observational AstronomyCELESTRON ONE YEAR WARRANTY Item #21017-INST Price $10.00 Celestron International 2835 Columbia StreetTorrance, CA Tel Fax
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