Quote:
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Originally Posted by TrevorW
Focus: Not very well by eye
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Quote:
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Originally Posted by TrevorW
Unfortunately with the Pentax you don't have the ability to control and focus via a laptop.
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Trevor - hi
If your eyes aren't what they used to be, like mine, it does make focusing unnessesarily difficult. Without live view it can still be done very easily - and painlessly, with a Bahtinov mask. These are a bloke with fading visions' best friends ever. No laptop required.
Like you, I used to take a 3-second shot of a star, waited until the image appeared on my cameras LCD screen, and then judged how large the fuzzball was when zoomed in to 10x. I then took another shot after moving the focuser one way or t'other and compared the new fuzzball with the previous. I kept going until I could see a trend and then settle on a focus point where the fuzzball was the smallest. Very tedious and in this day and age of cloud. You'd most likely miss out on getting down to business for real as the clouds roll in. to ruin the night.
I now do it without any eye strain at all, and all in about a minute.
See the attached image. A Bahtinov mask creates a diffraction pattern with a cross of two static spikes - "L" left and "R" right. It then lays down a movable spike - "C" center whose lateral position is based on the quality of focus you have.
Set your approximate focus and place the mask in front of your ED80's objective. Take a 3-second exposure of a bright-ish star. If you're lucky you'll get a pattern on your zoomed LCD screen like the one in the attached photo, where the C spike is absolutely centred on the junction of the other two. If not, adjust focus a little one way or the other until you see it centred on subsequent shots. Stop when you're centred. The position is easy to see, and it's very obvious when it is not centred. Viewing a position of something like this is far more accurate than determining if a fuzzball looks tighter than it did in the last frame.
The beauty of this method is that it is super easy to see a definitive result - and you know that you have the besst possible focus you can get without spending an inordinate amount of time doing it.
