Celestron CR-150 HD instruction manual Viewing Conditions, Transparency, Sky Illumination, Seeing

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Viewing Conditions

Viewing Conditions

Viewing conditions affect what you can see through your CR-150 HD telescope during an observing session. Conditions include transparency, sky illumination, and seeing. Understanding viewing conditions and the affect they have on observ- ing will help you get the most out of your telescope.

Transparency

Transparency is the clarity of the atmosphere and is affected by clouds, moisture, andotherairborneparticles. Thickcumulus clouds arecompletelyopaque while cirrus clouds can be thin, allowing the light from the brightest stars through. Hazy skies absorb more light than clear skies making fainter objects harder to see and reducing contrast on brighter objects. Aerosols ejected into the upper atmo- sphere from volcanic eruptions also affect transparency. Ideal conditions are when the night sky is inky black.

Sky Illumination

General sky brightening caused by the Moon, aurorae, natural airglow, and light pollution greatly affect transparency. While not a problem for the Moon, planets, and brighter stars, bright skies reduce the contrast of extended nebulae making them difficult, if not impossible, to see. To maximize your observing, limit deep-sky viewing to moonless nights far from the light polluted skies found around major urban areas. LPR filters enhance deep-sky viewing from light polluted areas by blocking unwanted light while transmitting light from certain deep-sky objects. You can, on the other hand, observe planets and stars from light polluted areas or when the Moon is out.

Seeing

Seeing conditions refer to the stability of the atmosphere and directly effects the clarity of star images and the amount of fine detail seen in extended objects like the planets. The air in our atmosphere acts as a lens which bends and distorts incoming light rays. The amount of bending depends on air density. Varying temperature layers have different densities and therefore bend light differently. Light rays from the same object arrive slightly displaced creating an imperfect or smeared image. These atmospheric disturbances vary from time-to-time and place-to-place. The size of the air parcels compared to your aperture determines the “seeing” quality. Under good seeing conditions, fine detail is visible on the brighter planets like Jupiter and Mars, and stars are pinpoint images. Under poor seeing conditions, images are blurred and stars appear as blobs. Seeing conditions are rated on a five-point scale where one is the worst and five is the best (see figure 6-4). Seeing conditions can be classified in one of three categories which are based on the cause.

Type 1 seeing conditions are characterized by rapid changes in the image seen through the telescope. Extended objects, like the Moon, appear to shimmer while point sources (i.e., stars) appear double. Type 1 seeing is caused by currents within or very close to the telescope tube. These currents could be caused by a telescope that has not reached thermal equilibrium with the outdoor surroundings, heat waves from people standing near the telescope, or heated dew caps. To avoid the problems associated with Type 1 seeing, allow your telescope approximately 20 to 30 minutes to reach thermal equilibrium.

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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 Table of Contents Using the Lens Cap Aperture Stop4 Introduction I N T R O D U C T I O NIntroduction How to Use This Manual AWordofCaution6 Introduction The Refractor Optical SystemThe Mount Unpacking Your CR-150HD A S S E M B L I N G Y O U R C R - 1 5 0 H DThe CR-150HD Tripod Head Setting Up the Tripod Adjusting the Tripod HeightAssembling the CG-5Equatorial Mount 10 The CR-150HD Attaching the Accessory TrayAttaching the Equatorial Mount 12 The CR-150HD Attaching the R.A. Slow Motion KnobAttaching the Declination Slow Motion Knob 14 The CR-150HD 1.Lock the DEC clamp to hold the mount in placeTube Rings Dovetail Bracket Attaching the Telescope to the Mount16 The CR-150HD Balancing the Telescope in R.ACOMPLETELY Balancing the Telescope in DEC18 The CR-150HD Adjusting the Mount in AltitudeAdjusting the Mount in Azimuth Disassembling and Transporting Your CR 150 HD Technical SpecificationsStoring Your CR- 150 HD OPTICAL TUBET E L E S C O P E B A S I C S Attaching the AccessoriesThe Star Diagonal The EyepieceFocuser Tension 1 1/4 AccessoryAdapter Built-inT-Adapter22 Telescope Basics FocusingAttaching the Finder Aligning the Finder YourFirstLook Image OrientationDaytimeObserving 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 Astronomy Basics Motion of the StarsEquator Polar AlignmentDefinition 30 Astronomy BasicsFinding the Pole 32 Astronomy Basics Latitude ScalesPointingatPolaris 34 Astronomy Basics DeclinationDriftPolar Alignment Finders Aligning the R.A Setting CircleObserving the Observing the MoonPlanets C E L E S T I A L O B S E R V I N GObserving the Sun Using the Lens Cap Aperture StopSOLAROBSERVINGHINTS Celestial Observing38 Celestial Observing Using the Setting CirclesObserving Deep-SkyObjects StarHopping 40 Celestial Observing Transparency Viewing ConditionsSky Illumination Seeing42 Celestial Observing Celestial Photography C E L E S T I A L P H O T O G R A P H YLunar Phase Short Exposure Prime FocusCrescent 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 46 Celestial Photography PiggybackEktar 1000 color print Konica 3200 color print 48 Celestial Photography Projection 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 O P T I O N A L A C C E S S O R I E S 50 Optional AccessoriesDual Axis Drive System - #93523 Super Modified Achromatic SMA EyepiecesOptional Accessories Motor Drive System - #93518 52 Optional AccessoriesNGC# THEMESSIERCATALOGConst Type54 The Messier Catalog Const Epoch LIST OF BRIGHT STARSStar Name ConstellationAstronomy Texts FOR FURTHER READINGAtlases General Observational AstronomyCELESTRON ONE YEAR WARRANTY Item #21017-INST Price $10.00 Celestron International 2835 Columbia StreetTorrance, CA Tel Fax
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