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How To Focus a Telescope: Four Easy Ways



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We will teach you how to focus a telescope, in a few steps, using the most powerful tools out there.

Considering how much automation is currently used in astronomy, it is no surprise that these processes are more and more powerful. One that is used more often is autofocus. To obtain the sharpest images possible, astronomy software packages employ a set of algorithms to identify the smallest star size and Prism is no different. It can also use focuser encoder step count or millimeters as units, simply choose which you like to use in Settings > Software settings >Telescope tab >”Use focuser with encoder steps only”.

Prism employs two distinct methods to help you focus a telescope:

1. Focus a Telescope Using a Star Centroid

This method utilizes star centroid calculations HFD (Half flux diameter) and FWHM (Full width half maximum) to identify the width of a star, hence judging the sharpness of the image. It is broken into four different tools:

A. Manual focus:

You can move the focuser using the knobs, hand controller or the software counterpart to displace the camera in and out. Prism provides graphs to aid you in finding the sharpest focus possible. Manual focus is especially handy for a first time setup where one needs to find the best focus position possible by hand. It is also helpful when you using “AnyObject focusing” to achieve the best details on a sunspot of a crater on the moon.

manual focus

B. Automatic focus

This method requires the user to take an image and select a star to focus on. Head over to Observatory > Focusing Actions > Automatic focusing, then Prism displays red cross hairs on the image. Click on a star that is not too bright so it does not saturate at its sharpest point during the focus run.

Before figuring out what is the appropriate range of focus your system needs, one needs to find their critical focus zone or CFZ. A good reference that I use personally is our Astronomer Tools page to find out what your CFZ is. Once this number is acquired, you need to determine what your focuser’s step size in microns is. I have an Optec TCFS ( Excellent products) and my step size is 2um.

This tells me that my critical focus is 100um or 50 encoder steps. If you choose to use millimeters as a unit of distance to gauge where you focuser physically is, you can press the “optimum step” and Prism will calculate your step size in millimeter and use it. Pressing the “Calculate gap” will ask you for your best focus position and then establish 6 iterations of focus on either side. This will give a total of 13 steps of focus which should give you a nice V-Curve.

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C. Direct Focus:

This method is based on automatic focus and uses the same focus parameters but has a huge advantage. The process is as follows.

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  • You specify what magnitude star to use, search radius, exposure time, binning, star catalog to use, and what system to focus

Prism then:

  • Extracts candidate stars from the included Skychart
  • Sorts by magnitude and distance
  • Slews to the best star
  • Plate solves and centers the star
  • Perform autofocus
  • Slews back if the user chooses to.

This method is also included in the automatic observation with a few more features, like periodic focus and distance based triggers.

Also Read: How to become rich in Pakistan.

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At the end of both the automatic and direct focus, you are presented with a graph that displays the star measurement. The best focus will result in what we call a V-curve as displayed below.

D. Spectral Line Focusing

This feature allows a manual focus on a slit. Before using this feature, you must acquire an image with spectra, with the dispersion axis along the X-axis of the camera. Take an exposure (2 to 5 seconds), and select this function from the menu Observatory > Focusing actions > Spectral line focusing. A cursor will be displayed and used to select a line for focusing. Make sure not to use a line that is saturated. The software will compute the X-axis FWHM of this line.

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2. AnyObject ( FFT ) Focus:

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Prism utilizes FFT (Fast Fourier Transform) focus in addition to Star focus, it allows the user to find the sharpest images possible of objects like sunspots, craters on the moon, planets, etc. This method is performed manually in the same manner as manual Focus a Telescope.

Video tutorials

Please view the video tutorials that explain the three different methods in detail. Do not forget to expand the video to fullscreen and take advantage of the higher resolution.

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  1. your system using the automatic focus or direct focus routine in PRISM. Or, you may use a focusing aid such a bahtinov

    1. you can simply use the bahtiniv mask with a normal looped exposure.

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