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en:ost:fokus [2016/11/05 04:02] – rhainichen:ost:fokus [2016/11/10 23:12] (current) – [Aperture diaphragms] rhainich
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 <code> Camera -> Focus</code> <code> Camera -> Focus</code>
  
-[{{:ost:ccds:focus.jpg?direct|}}]+[{{:ost:ccds:ccdops:focus.jpg?direct|}}]
  
 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. 
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 ==== Planet Mode ==== ==== Planet Mode ====
  
-[{{ ost:ccds:planet_mode_info.jpg|}}]+[{{ ost:ccds:ccdops:planet_mode_info.jpg|}}]
  
 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. 
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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.  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. 
  
-[{{ etc:scheinerblende_wiki.jpg?400|Scheiner diaphragm like it is available for the lab course}}]+[{{ ost:telescope:scheinerblende_wiki.jpg?400|Scheiner diaphragm like it is available for the lab course}}]
  
 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.  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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 === Scheiner diaphragm === === Scheiner diaphragm ===
  
-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 {{etc:scheinerblende.pdf|here}}. +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}}. 
  
 [{{ost:ccds:scheinerblende_complete.jpg?815|Test exposure of a bright star employing a Scheiner diaphragm (improved focus from left to right)}}] [{{ost:ccds:scheinerblende_complete.jpg?815|Test exposure of a bright star employing a Scheiner diaphragm (improved focus from left to right)}}]
  • en/ost/fokus.1478318540.txt.gz
  • Last modified: 2016/11/05 04:02
  • by rhainich