Focusing Tips

Discussion in 'Image Capture' started by Mike Hambrick, Oct 27, 2018.

  1. Mike Hambrick

    Mike Hambrick Cyanogen Customer

    Jul 13, 2016
    I am looking for some tips on how to achieve the best focus using manual methods. The best way I have found to do this so far is to get to approximate focus using a mask or other aid, then I measure the distance that my focuser is racked out with a micrometer. Then I take a series of short (5 - 10 second) exposures changing the focuser position in increments of 0.001" inside and outside of the initial focus point. Then I visually compare the images and pick the one that "looks" best, and reset the focuser to that position. Surely there has to be a better way. Are the super sharp and detailed images only possible with automatic focusing ?

    Here is an example of what I have been able to achieve using the manual method I have described above, but I still don't think it is optimum.

    FYI, This LRGB image was taken using a TeleVue NP101is with a ST2000XL camera. I recently purchased a STXL 16200 camera, but I have not yet been able to use it due to a number of reasons.

    Attached Files:

  2. Doug

    Doug Staff Member

    Sep 25, 2014
    Having an accurate focus position measurement capability, however achieved, is an important tool. Since you have to hunt around for focus, you won't realize you found it until you've passed it, at which point getting back to the correct position is extremely important. The micrometer works.

    There are several measurements that MaxIm DL's camera control window can show you. Make sure you've got the bottom panel showing. You will get peak star brightness, half flux diameter, and full width half maximum measurements. Maximize the first number and minimize the second two. With some experience you will find which one works best for you. (Maximum is only useful if the stars are well sampled at best focus; otherwise it will bounce around a lot - especially when you're in perfect focus.)


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