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| en:ost:fokus [2016/11/03 23:53] – [Planet Mode] rhainich | en:ost:fokus [2016/11/10 23:12] (current) – [Aperture diaphragms] rhainich |
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| FIXME **This page is not fully translated, yet. Please help completing the translation.**\\ //(remove this paragraph once the translation is finished)// | |
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| /*This Page is Under Construction!*/ | |
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| ====== Focusing ====== | ====== Focusing ====== |
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| <code> Camera -> Focus</code> | <code> Camera -> Focus</code> |
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| [{{:ost:ccds:focus.jpg?direct|}}] | [{{:ost:ccds:ccdops:focus.jpg?direct|}}] |
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| The settings should be similar to the example shown on the left image. In this operation mode, the camera will repeatedly take images of a certain ''Exposure Time'' (in the left example 0.5s). The individual exposures will be automatically downloaded and displayed on the laptop display. The focus can then be coarsely adjusted by means of the focus knob. Subsequently, the fine focuser can be used to find the optimal focus. It is recommended to use a bright star for the manual focusing, while a globular cluster with its high stellar density is recommended for the next focusing step with the fine focuser. The low angular distance between the stars in a globular cluster facilitates very good focusing results, since the Airy discs of the individual stars can only be separated with a very well focused telescope. An optimally focused telescope operates with a seeing limited resolution, which for our site is often larger than 2". This is significantly worse than the diffraction limited resolution of our telescope, which is 0.33". The [[https://en.wikipedia.org/wiki/Angular_resolution#Explanation|Rayleigh criterion]] describes the theoretical limit at which tow Airy discs can be recognized as separated light sources. | The settings should be similar to the example shown on the left image. In this operation mode, the camera will repeatedly take images of a certain ''Exposure Time'' (in the left example 0.5s). The individual exposures will be automatically downloaded and displayed on the laptop display. The focus can then be coarsely adjusted by means of the focus knob. Subsequently, the fine focuser can be used to find the optimal focus. It is recommended to use a bright star for the manual focusing, while a globular cluster with its high stellar density is recommended for the next focusing step with the fine focuser. The low angular distance between the stars in a globular cluster facilitates very good focusing results, since the Airy discs of the individual stars can only be separated with a very well focused telescope. An optimally focused telescope operates with a seeing limited resolution, which for our site is often larger than 2". This is significantly worse than the diffraction limited resolution of our telescope, which is 0.33". The [[https://en.wikipedia.org/wiki/Angular_resolution#Explanation|Rayleigh criterion]] describes the theoretical limit at which tow Airy discs can be recognized as separated light sources. |
| ==== Planet Mode ==== | ==== Planet Mode ==== |
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| [{{ ost:ccds:planet_mode_info.jpg|}}] | [{{ ost:ccds:ccdops:planet_mode_info.jpg|}}] |
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| The planet mode offers the possibility to significantly accelerate the determination of the best settings. The readout time and the download time is significantly reduced in this observation mode, since only a small fraction of the CCD, which can be freely chosen, will be readout. The most sense makes the planet mode in combination with the fine focus. The planet mode can be activated by choosing the **Planet Mode** option for the menu item ''Frame size'' in the focus menu (see figure above). If the focus series is already running, the plate mode can be activated by pausing the exposures with a click on the ''Pause'' button in the status window (see right figure) and choosing the planet mode from the drop-down menu (''Frame''). Afterwards click on the ''Resume'' button to continue the focus series. | The planet mode offers the possibility to significantly accelerate the determination of the best settings. The readout time and the download time is significantly reduced in this observation mode, since only a small fraction of the CCD, which can be freely chosen, will be readout. The most sense makes the planet mode in combination with the fine focus. The planet mode can be activated by choosing the **Planet Mode** option for the menu item ''Frame size'' in the focus menu (see figure above). If the focus series is already running, the plate mode can be activated by pausing the exposures with a click on the ''Pause'' button in the status window (see right figure) and choosing the planet mode from the drop-down menu (''Frame''). Afterwards click on the ''Resume'' button to continue the focus series. |
| </WRAP> | </WRAP> |
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| ==== Lochblenden ==== | ==== Aperture diaphragms ==== |
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| | Aperture diaphragm proofed to be a very useful assistance in finding the best focus and in testing the quality of the telescope imaging-capabilities. Aperture diaphragm with two apertures are usually called Scheiner diaphragm, whereas aperture diaphragm with more than two apertures are denoted as Hartmann diaphragm. The aperture diaphragm are mounted before the Schmidt plate. |
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| | [{{ ost:telescope:scheinerblende_wiki.jpg?400|Scheiner diaphragm like it is available for the lab course}}] |
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| Lochblenden haben sich in der Astrophotographie als Fokussierhilfen und zum Test der Abbildungsqualität von Teleskopen bewährt. Lochblenden mit zwei Öffnungen bezeichnet man in der Regel als Scheinerblende wohingegen Lochblenden mit mehr als zwei Öffnungen als Hartmannblende bezeichnet werden. Angebracht werden Lochblenden vor der Schmidtplatte. Zur Fokussierung wird das Teleskop auf eine helle Lichtquelle (z.B. einen hellen Stern) gerichtet. Da die Lichtstrahlen von den unterschiedlichen Öffnungen der Lochblende an verschiedenen Punkten die Ebenen vor und hinter der Fokalebene passieren sind mehrere Abbildungen der Lichtquelle zu erkennen wenn das Teleskop defokussiert ist. Durch anpassen des Fokus können die mehrfachen Abbildungen zum überlappen gebracht und schlussendlich zu einer Punktquelle vereinigt werden. Hat man dies erreicht kann man davon ausgehen, dass man den optimalen Fokus gefunden hat. | For finding the focus, the telescope needs to be pointed to a bright light source (e.g. a bright star). Several images of the light source are recognizable in case the telescope is defocused, since the light rays from the different apertures of the diaphragm pass through different points of the focal plain. The distance between the different images of the light source can be reduced by adjusting the focus. The optimal focus has been reached when the different images are aligned. |
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| === Scheinerblende === | === Scheiner diaphragm === |
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| [{{ etc:scheinerblende_wiki.jpg?400|Scheinerblende wie sie für das Praktikum zur Verfügung steht}}] | For our telescope, we have a Scheiner diaphragm with rectangular apertures, where the apertures are rotated by 45° towards each other (see the right figure). The advantage of this kind of Scheiner diaphragm is that the images of the object are superimposed with spikes, due to the diffraction at the apertures. According to the rotation of the apertures also the spikes of the different images are rotated by 45° towards each other. The spikes provide a good assistance in focusing the telescope, since the spikes form a symmetric "starlet" only for a well focused telescope. A template of the described Scheiner diaphragm (A2 format) for the C14 from Celestron can be found {{ost:telescope:scheinerblende.pdf|here}}. |
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| Zur Verfügung für das Praktikum steht bisher eine Scheinerblende mit rechteckigen Öffnungen bei der einer diese Öffnungen um 45° gegen die andere gedreht ist (siehe rechte Abbildung). Diese Lochmaske hat den Vorteil, dass aufgrund der Beugung an den Öffnungen die Abbildungen des zu fokussierenden Objektes jeweils mit Spikes überlagert sind, welche entsprechen der Drehung der Öffnungen ebenfalls um 45° gegeneinander verschoben sind. Die Spikes sind eine gute Hilfestellung bei fokussieren, da diese nur bei idealer Fokussierung ein symmetrisches "Sternchen" bilden (siehe Abbildung unten). Eine Vorlage der beschriebene Scheinerblende im A2-Format für das C14 von Celestron ist {{etc:scheinerblende.pdf|hier}} zufinden. | [{{ost:ccds:scheinerblende_complete.jpg?815|Test exposure of a bright star employing a Scheiner diaphragm (improved focus from left to right)}}] |
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| [{{ost:ccds:scheinerblende_complete.jpg?1000|Testaufnahmen eines hellen Sterns erstellt mit der Scheinerblende (von links nach rechts zunehmend bessere Fokussierung)}}] | === Bathinov diaphragm === |
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| === Bathinovblende === | More to come...! |
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| Kommt noch! | |