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Achieving Focus

Focusing the camera is one of the most important steps in astrophotography. When I tried taking my first photographs I simply looked through the view finder on my Nikon D300, and tried to eyeball when the camera was in focus. Although this worked well enough for me to produce an acceptable image, it is far from perfect. There are a number of techniques which can be used to achieve better focus.
Software Assisted Focusing Live View

There's a lot of software out there which can be used as a tool for focusing. Most of the programs work by basically analyzing pictures from the camera, and having you adjust the focus until the average size of the stars is as small as possible. Some programs can even connect to a motor attached to the focuser and focus automatically. I have never used software assisted focusing, but here is an example of a program which does this. DSLR Focus

Many DSLR cameras allow you to use live view mode. This displays what the camera sensor is seeing on the camera’s monitor. Focus can be achieved by zooming in on a star as much as possible and trying to make that star as small as possible by adjusting focus. For this it is best to pick a star which is not to bright, but still bright enough to be visible on the screen. I particularly like stars which have dimmer companion stars since if the brighter star is as small as possible, and the companion is also visible you know you have good focus. I have used this method to capture many of the photos on this website.

Bhativov Mask

Although I used to focus through Live View, I now use a Bahtinov mask to focus. A Bahtinov is placed over the aperture of a telescope and creates a diffraction pattern. To focus the telescope the large central spike needs to be aligned between the two crossing spikes. I have found that the best way to focus is to put the camera in live view with the Bahtinov mask on, and point the telescope at the brightest star nearby. It is important to use a bright star as the diffraction spikes can be difficult to see in live view mode. Next, if you can see the spikes, line them up to focus the camera. If your camera does not have live view, or you cannot find a star which is bright enough, you can focus by taking short exposures and adjusting in between images of that star. This method of course is slower and harder to use. Another advantage of using the Bahtinov mask is that it is easy to check focus during an imaging session if you are worried your camera may have drifted out of focus. Simply put the mask back over the aperture for one of your exposures, and make sure the stars still have the correct diffraction pattern.

DeepSkyStacker Live

Although DeepSkyStacker Live is not a tool for focusing, it can warn you if your camera drifts out of focus. Each time your camera takes an image and sends it to the computer, DeepSkyStacker Live registers it and gives you statistics about that image (# of stars, score, FWHM, etc.). By monitoring the statistics you can be sure that your camera is still in focus. I think FHWM (Full Width at Half Maximum) is the best indicator since it is a measure of the average size of the stars in an image. Smaller FWHM values usually mean sharper images. Be aware that lower scores and FWHM values can also be caused by poor guiding so focus may not always be the cause of a drop off.

For more on focusing see Focusing Methods for Astrophotography - Astropix.com

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