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Uranium235

Focusing on stuff you cant see

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Ive been having a think today about how to capture galaxies such as M81/M82, objects too feint for the eye but well within the range of a DSLR.

The problem is, how do you make sure its going to be in focus when you take the sub? When looking through the viewfinder at objects mag 8+, I see nothing. I dont have the benifit of live view so I find myself squinting through the viewfinder desperately trying to focus very feint objects (not much fun for the neck & back as im quite a tall bloke). Its easy to get M31, 42 & 45 as they are already so bright, but the feinter stuff.... thats proving a litte more tricky.

Will focusing on the nearest, bright star, then slewing back to the target be good enough? Whats the best tactic for achieving correct focus on feint objects?

I wish someone would invent a low light cam that you can attach to a viewfinder to bypass all this annoyance and enable me to do it from a comfy chair :) damn... wish id brought a cannon now.

Edited by Uranium235
typo

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The easy way is as you say point the cam at a nearby bright star then slew back to your target. A Bahtinoff mask and some shortish exposures (say 4 seconds or so) will allow you to get precise focus.

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Cool, thanks :) just as I suspected.

I did build a bahtinov mask, but I havent tested it yet. Not sure if it will work though as Ive only cut it from light card with a scalpel (a long job indeed) so its a bit wobbly ;)

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As far as celestial objects are concerned - ALL of them can be considered to be at "infinity". Therefore you can focus on any convenient one and know that focus will be OK on another.

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Cool, thanks :) just as I suspected.

I did build a bahtinov mask, but I havent tested it yet. Not sure if it will work though as Ive only cut it from light card with a scalpel (a long job indeed) so its a bit wobbly ;)

I made one out of thin card and it works a treat, I even accidentally cut out one of the wrong slits so it goes slit, card, slit, card, card, slit, card, slit etc.. and it still works. All it needs is to set up some nice diffraction spikes for you.

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