Transcript Slide 1
Astronomical Seeing The Project Students will be introduced to the concept of astronomical seeing and how it affects the quality of astronomical images. The causes of seeing are discussed. Students are presented with a selection of images taken under different conditions and are asked to chose which images have been taken under the best seeing conditions. A discussion is then initiated on how best to identify the effects of seeing in an image and how scientists take seeing into account. National Schools’ Observatory Setting Up the Experiment 1 Students are presented the data twice. Distribute copies of the image sets (.jpg format) for each object. Images can then be analysed on computers by loading the images in your default image viewer or viewed collectively via projection. Distribute a copy of the worksheet to each of the students. Allow the students to quickly rank the quality of the astronomical image. If the students cannot distinguish between an image, note this on the worksheet. National Schools’ Observatory Setting Up the Experiment 2 Deliver the lecture on seeing (concept introduction folder) , and allow the students to have a more detailed look at the images. Using what they have learnt from the lecture. See if they can now distinguish between some of the better images. Prompt the students to look for areas where two stars are close together; the better the seeing the more resolvable (distinguishable) the two stars will be. Look out for stars which are much dimmer in some images due diffusion through seeing effects. Look for small features e.g. Craters on the Moon, the rings on Saturn or dust in galaxy and nebulae images. The sharper and better resolved the detail on these features, the better the seeing. National Schools’ Observatory Measuring and recording 1 Rank the images in order of their quality on the work sheet. 1 = Best and 6 = Worst. If you are unsure, place a joint ranking with the image you think is the closest. National Schools’ Observatory Astronomical Seeing • Even the best ground based optical telescopes are restricted by the presence of the Earth’s atmosphere. • Light from distant objects must pass through the Earth’s atmosphere before we can observe it. • The atmosphere contains a layer of turbulent air. • As the light passes through this turbulent layer the light waves are perturbed, altering how they are detected on the ground. • This effect is called ‘seeing’. National Schools’ Observatory Image created by NSO Seeing and Meteorology • Variation in temperature, humidity and wind speed make the atmosphere very turbulent. • Turbulent air contains pockets or ‘cells’ which have differing density to the region of air surrounding it. • It is cells such as these which cause ‘clearair’ turbulence, which is often experienced when flying in aircraft. • These cells will vary in size and shape and tend to drift around in the atmosphere. • Due to their differing density, each cell will have a slightly different refractive index. National Schools’ Observatory Refractive Index • Light in a vacuum travels at a constant velocity, c. (3x108 ms-1) • When light travels in a medium, the velocity changes by a factor of 1/n , where n is the refractive index of the medium. • The refractive index depends on the characteristics of the medium. • Changing from one medium to another will cause the angle of the incident light to change. • This occurs as the light travels from cell to cell. Image created by NSO National Schools’ Observatory Seeing and Meteorology (2) Large temperature gradients cause turbulence. These arise if air masses of different temperatures mix. This will occur when hot air rises from the ground and meets the colder air at higher altitudes or when the wind drives in weather fronts from surrounding areas. This means that seeing is better when: Observations are recorded at high altitude. i.e. The light passes through less of the turbulent air. Observations are recorded during a period of high pressure, when wind speeds are low at all altitudes. Observations are taken close to the zenith, where there is less atmosphere for the light to pass through. National Schools’ Observatory Scintillation • If the cells of varying refractive index are far above the telescope, scintillation occurs. • Scintillation is observed as irregular changes in the brightness of the observed objects. • This is what makes stars ‘twinkle’ at night. • This will make dim objects and stars invisible on images taken during bad seeing. National Schools’ Observatory Oscillation • Light travelling through areas of differing refractive index will also change position in the focal plane. • This causes distortions in the recorded image. • This effect is called oscillation. • The distortion rate is very high, typically more than a 100 times a second. • Since exposure times are normally much longer than the distortion rate. Distortion is averaged over the time of exposure resulting in a blurry image. National Schools’ Observatory Seeing and Stars • The resolution limit of a telescope , or how well a telescope can see objects, is determined by the size of its main mirror. • However, telescope resolution is also limited by the diffraction of light. • The result of this is that distant point source objects, such as stars, spread out to a small spot known as the ‘Airy disk’. • Astronomical seeing causes this disk pattern to be disrupted into a speckle pattern. • This will cause stars next to each other to merge into a single object. • On larger telescopes the diffraction effects are very small due to the large size of the mirror. National Schools’ Observatory Pickering Scale • Due to oscillation, a point source such as a star will spread out and become speckled. • The Pickering scale is a method of quantifying how good or bad seeing is. • 1 – Perfect seeing 10 – Very bad seeing. 1 2 3 4 5 6 7 8 9 10 National Schools’ Observatory Measuring and recording 2 Rank the images in order of the quality of seeing on the work sheet. 1 = Best Seeing to 6 = Worst Seeing. Record the reasons for choosing the rank of each image. Record which part of the image has been used to identify bad seeing. Compare the best and worst image. What are the differences? National Schools’ Observatory Discussion After the Experiment Is bad seeing easier to identify on some objects more than others? What are the best methods for identifying bad seeing on an astronomical image? Pick the image with the worst seeing, what kind of weather might you have expected on that day? Are there any images which have artifacts that cannot be attributed to seeing? National Schools’ Observatory Questions, Exercises and Tasks What methods are there for overcoming astronomical seeing? Are some parts of the world affected more than others? Where is the best place to locate a telescope? Are small telescopes affected more than large telescopes? Will observations of more distance objects such as galaxies be more prone to seeing effects? National Schools’ Observatory