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Offline chris.bailey

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Focus Focus Focus
« on: 10:09:40, 25 February, 2013 »
Getting absolute focus is one of the key areas of making a fine astro image. Out of focus stars detract significantly. We have all done it, captured a pile of subs only to find that they are all slightly off focus leading you to wish you had spent a little more time getting it right before hitting the capture button. When it leads to a wasted imaging session it is a hard lesson to learn. Depending on the setup getting critical focus may need the finest of touches on the focus fine control, so here are some top tips to getting repeatable focus

Dont rely on your eyes alone - though getting rough focus by eye is fine, dont rely on it alone to get focus perfect. If you really have no other means of assisting focus a good tip is to take a 10 second exposure of a star populated area of the sky. If focus is good a zoomed-in image should show a number of single pixel stars (not to be confused with bright hot pixels) of varying brightness. If there are none, the chances are that focus is adrift.

Bahtinov Masks - uses a geometric grating to create a diffraction pattern that when used on a bright star acts as a focus aid
Bahtinov Mask
shows it in action. Several types of mask are available (you can even make your own) but the Kendrick solution is cheap and rugged and works well Using one in practice can take a bit of getting used to but they are a definate improvement over using your eyes alone to judge critical focus.

Star Measurement - Most capture programs have a fine focus routine that relies on using a sub-frame and continuous imaging coupled with taking a critical star measurement such as Full Width Half Maximum (FWHM) or Half Flux Diameter (HFD) to aid focussing. The focus star should be large enough to enable the measurements to be meaningful but not saturated. Over a series of exposures the fine focus adjustment can be adjusted to get the minimum FWHM or HFD readout. This takes a bit of practice as in the field, seeing conditions will mean the measurement wanders around even without touching the focusser so dont rush it and try to see the pattern rather than individual results.

Re-Check Focus - In long imaging sessions, re-check focus after aquiring a new target and/or if the temperature changes more than a couple of degrees. This is largely equipment specific but even refractors do drift a little if the temperature changes significantly. It is also worth re-checking focus after filter changes.

Practice - When you have chosen a method of focussing, practice it until you have the confidence to use it as part of your routine. A couple of short imaging sessions on an open cluster is a good way of honing those focus skills.
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