Observation Assignment 2: Field of View. Joshua Thomas ID: October 30, 2015

Transcription

1 Observation Assignment 2: Field of View Joshua Thomas ID: October 30, 2015

2 Part A: Field of View of Small Telescopes This first part of the experiment was performed at the GAO observation site ( W, N). The goal being to measure the size of the field of view for multiple eyepieces of varying focal lengths with the same telescope. This will allow is to know precisely how much of the sky we can see with each eyepiece and to choose an appropriate focal length for the viewing of any particular object. It also allows for easier comparison with a star chart since you would know exactly how much of the sky is being observed. Apparatus: In this experiment we made use of the Schmidt-Cassegrain, Celestron C8 telescope. Our particular telescope focal length of 80 and a lens diameter of 8. The maximum useable magnification was 480x with a resolving power of The particular telescope used for this experiment had the serial number of In the experiment we made use of two different eyepieces for the telescope, a 40mm and 15mm celestron piece. Procedure: In order to complete this experiment we made use of the drift method of determining field of view (FOV). First a specific star was chosen to observe as it moves across the sky at a certain speed. This was determined by taking the time it takes for stars to make a full revolution (t=23hr, 56m) as well as the declination of the star in the sky. For this experiment Altair was chosen as the target star. While keeping the telescope stationary we measured the time elapsed for the star to move completely across the diameter of the telescopes field of view. This time was then used in the calculations along with the velocity to find the FOV. Conditions during observation: Location: GAO ( W, N) Date/time: October 13/14, 2015 at 20:40 CDT Cloud Cover: Clear Transparency: Average Seeing: Average Sunset: 18:42 CDT, 10/13/15 Sunrise: 7:48 CDT, 10/14/15 Moonset: 19:14 CDT, 10/13/15 Moonrise: 9:20 CDT, 10/14/15 Moon Phase: 1 day after new moon Twilight start: 20:19 CDT, 10/13/15 End: 6:09 CDT 10/14/15 Wind: 22km/hr WNW Temp: 12 C Humidity: 55% Sky Glow: Slight glow along northern horizon, rest of sky quite dark

3 Observed Data: Time Elapsed (sec) Eyepiece: 15 mm 40 mm Trial 1 80 ± ± 4 Trial 2 82 ± ± 4 Trial 3 83 ± ± 4 Average 81.7 ± ± 4 Declination for Altair = 8 55 = 8.92 (Royal Astronomical Society of Canada, 283) Calculations: Angluar Field of View = Elapsed time x cos(declination) x 360 Time for revolution Angluar Field of View (40mm) = Angluar Field of View (15mm) = 186 cos(8.92) cos(8.92) = = ± 0. 5 = = ± 1 Uncertainty: R = kx R = k x ( Angle) cos(8.91) 360 FOV (40mm) = = Part B: The last part of this experiment is to find the approximate FOV of the telescope based at the ECO observing site ( W, N). This is useful for all the same reasons as part A such as finding objects in the telescope as well as comparison with star charts. Apparatus: This portion of the experiment was made using a Meade Lx200 telescope. This particular telescope had a focal length of 400cm and a maximum magnification factor of 960x. The eyepiece used at the time was a 26mm Series 4000 Super Plössl. Procedure: In order to determine this FOV rather than using the drift method as before we instead simply centered the telescope and a desired object (Vega in this case) and then compared the observed field of view with stellarium in order to get a good estimation on the observable field of view.

4 Conditions during observation: Location: ECO ( W, N) Date/time: October 22/23, 2015 at 20:40 CDT Cloud Cover: Clear Transparency: Above Average Seeing: Average Sunset: 18:24 CDT, 10/22/15, Sunrise: 8:03 CDT, 10/23/15 Moonrise: 16:00 CDT, 10/22/15, Moonset: 2:03 CDT, 10/23/15 Moon Phase: Waxing 70% Twilight start: 20:10 CDT, 10/22/15 End: 6:16 CDT 10/23/15 Wind: 5km/hr NW Temp: 12 C Humidity: 15% Sky Glow: Feint glow all around the horizons

5 Observations:

6 Analysis: Based on observations in stellarium and my sketch of Vega through the telescope the approximate FOV is about 15 ± 5 Discussion: The drift method of calculating the field of view of any telescope is quite simple. All you need is to observe a known star as it drifts across the telescope field of view while the telescope itself is held stationary. The time taken for the star to transit across the diameter of the view (t) in seconds must be recorded. Once this is accomplished the stars declination (dec) must be found in degrees either by measurement, or looking it up by some other method. These values can then be inserted into the following formula: Angluar Field of View (degrees) = tcos(dec) As shown by the data in this experiment where a 40mm eyepiece has a transit time of 186 seconds and a 15mm eyepiece has a time of 82 seconds the larger the eyepiece the longer the time it takes for the star to move across the field of view. According to the formula above where t is proportional to the FOV the higher the value of the eyepiece the larger the field of view.

7 According to the Observers Handbook 2015 (Royal Astronomical Society of Canada, 49) the angular magnification of a telescope is given by the following formula: M f o f e Where f o = focal length of objective lens f e = focal length of eyepiece For the Celestron 8 telescope with the two eyepieces of 15mm and 40mm and a focal length of 80 (2032mm) this results in the following levels of magnification: M(40mm) = = 50.8x M(15mm) = = 135x Conclusion: After performing this experiment we determined values for multiple FOV s for different eyepieces and telescopes. This yielded the following results: Telescope field of view Eyepiece Celestron 8 FOV Eyepiece Meade Lx200 Celestron Plossl 15mm ± mm Series ± 2.5 Super Plössl Celestron Plossl 40mm ± 0.05 Other measurements: Telescope field of view Eyepiece Celestron 8 FOV Celestron Plossl 32mm 44.6 ± 0.8' Celestron Plossl 24mm 40.47±0.49' As a general trend in all of these measurements the larger the eyepiece focal length the larger the field of view. This seems to be consistent across the entire data set with the largest being the 40mm eyepiece with ± 0.05 FOV and the smallest the 15mm eyepiece with FOV ± The only exception is the rough estimation of the Meade telescope however this is easily understandable as it s a different telescope then the rest.

8 References Cited: Chapman D Observer s Handbook pp. 49, 283. Toronto: RASC. Meade Lx200-ACF 16 with Super Giant Field Tripod. Meade Instruments Corp.