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This 3 page Class Notes was uploaded by Guido Casper IV on Wednesday October 28, 2015. The Class Notes belongs to PHYS 1040 at Weber State University taught by Daniel Schroeder in Fall. Since its upload, it has received 15 views. For similar materials see /class/230787/phys-1040-weber-state-university in Physics 2 at Weber State University.
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Date Created: 10/28/15
Photographing Star Trails Project for Elementary Astronomy You ve seen beautiful long exposure photographs of star trails7 vividly illustrating earth s daily rotation relative to the stars This project offers you an opportunity to make your own star trail photos For this project you need suitable equipment D Camera The ideal camera for this exercise is a 35mm SLR lm camera A fully manual camera is best7 but you can use an auto exposure or auto focus camera as long as you can gure out how to operate it manually Some digital SLR cameras may also work7 but their exposure times can be limited due to digital noise andor battery life Most smaller cameras lm or digital will not take long time exposures If you are in doubt about the suitability of your camera or how to use it7 bring it to your instructor preferably without lm in it V Tripod A tripod allows you to point the camera and hold it steady for long exposures In principle you could rest the camera on a large sack of beans on a sturdy table7 but it s really best to have a good tripod V Film Ordinary color print lm is best7 preferably with an ISO rating of about 200 Slower lm ISO 100 or so is ne if you have a reasonably fast lens f2 or less If you use ISO 400 lm with such a lens7 compensate by stopping down the lens somewhat Avoid lm speeds higher than ISO 400 You can also use color slide lm if you have a way of making prints from it perhaps with your own scanner V Locking cable release or the equivalent You need a way of keeping the camera s shutter open without holding your nger on the button continuously Some cameras have a built in shutter lock that works great7 but most require an add on cable release which you can lock With some digital cameras you can operate the shutter by remote control V Timer or watch You ll need a way of timing the length of your exposures typically 15 minutes to one hour or more A timer that you set to beep after a certain amount of time is most convenient7 but an ordinary wristwatch will do V Pencil7 paper7 and ashlight for making notes conduct this observing project7 follow this procedure D Start this project well before the due date7 in case your photos don t turn out the rst time D Test your equipment While indoors7 practice attaching the camera to the tripod and oper ating the shutter and cable release to take long exposures If you re using a lm camera7 do this before you put the lm in If you re using a lm camera7 put the lm in and take a couple of ordinary short exposure daytime photos to start the roll This will enable whoever processes the lm to tell where one frame ends and the next begins If you re using a digital camera7 try some 15 minute test exposures at night7 to make sure they turn out Figure out which aperture and ISO settings give the best results Determine how many 15 minute exposures you can take before your battery becomes discharged7 and plan accordingly to use extra batteries or to recharge between photos V Make your photos from a dark location with a clear View of the sky away from any direct light sources A suburban back yard may work ne if there aren t too many trees or streetlights A local park may be another option It isn t necessary to get completely away from civilization but you shouldn t be near any brightly lit areas either V Make your photos on a dark night when the sky is clear and the moon isn t near full Some moonlight is acceptable as long as it isn t too bright D Use your widest angle lens not counting highly specialized sheye lenses or a zoom lens on its widest setting V Set your lens to focus at in nity With some lenses this means simply turning the focus ring as far as it will go With others you ll have to point at a bright far away object and focus by eye V For each photo you take be sure to write down the time when you started the exposure the time when you ended your camera settings the direction the camera was facing and any other observations such as sky conditions or planes that ew through the eld of view V Make your rst photos with the camera pointed east Tilt the camera upward so the sky lls most of the frame but with the horizon or a tree or some other terrestrial object visible near the bottom Take a 15 minute exposure with the lens opened to its widest aperture Then close the lens one or two stops and take another 15 minute exposure then close the lens another one or two stops and take a third 15 minute exposure Try some longer exposures if you wish preferably with the lens stopped down somewhat If you re using a digital camera use whatever aperture and ISO settings give a reasonably dark sky while still showing plenty of stars V Repeat the same procedure for photos pointing south and west V Now point your camera to the north so the north star is visible in the photo Take a onehour exposure with the aperture 2 or 3 stops below its widest opening Try some other aperture settings or longer exposures if you have time D Check your camera lens for dew after each exposure Under humid conditions dew may form on the lens surface blurring your photos If this happens you ll need to warm up the lens indoors and perhaps rig up a lens hood to prevent dew from forming D Have your lm processed and prints made just as you would for ordinary daylight photos With digital photos you may wish to invert the colors in the image changing black to white etc in Photoshop or a similar program before printing For your report analyze and discuss your photos as follows D For each of the four directions east south west and north choose the best of your photos Be sure to attach these to your report labeling each with the direction time and camera settings In your report discuss which camera settings gave the best results and why D Discuss the direction in which the stars appear to move in each of your photos Explain these directions in terms of the earth s rotation and the latitude of your location D Pick one of your photos and use it to determine the rate at which the stars appear to rotate around us in degrees per hour You can do this in either of two ways Your east and west facing photos show stars rotating in circles with you at the approximate center By comparing one of these photos to a good star chart and identifying the stars that are shown you can determine the scale of the photo in millimeters or inches per degree Once you know the scale measure the length of any star trail to determine how many degrees of rotation occurred during the photo Then calculate the number of degrees of rotation per hour Alternatively you can use your north facing photo in which the center of the circles is approximately at the north star There s no need to determine a scale on this photo but you do need a protractor to carefully measure angles Either way explain your method and show all your calculations in your report Also compare your result to what you would expect based on what you have learned in this course V Find a star in one of your photos that is noticeably yellow or orange in color and one that is noticeably bluish Identify each of these stars by comparing your photos to a good star chart and look up the spectral type of each star and the corresponding temperature Discuss in general how you can determine the temperature of a star from its color D Discuss any other interesting experiences and observations
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