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Obtaining Focus on DSO


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Hello,

Hopefully a simple question. I have a small refractor with a Canon EOS that has LIve View. If I am wanting to start imaging on a DSO what do I need to do to obtain accurate focus. Do I simply point the scope at a bright star, focus on that and I am done ? - If that were the case then if I make a note of the position of the draw tube, then I would never need to focus again, as I could just keep returning to my reference point ?

Thanks

Alistair

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With my DSLR I also found the easiest way to focus was with a bahtinov mask on a really bright star. Rather than use live view, which was not really sensitive enough on my camera, I would connect the camera to my laptop and use tethered shooting software to take shots and view them full size.

You get much larger and clearer difraction spikes from the mask this way and that makes things easier. Anything that makes focusing easier is good, because it never seems that easy when it comes to astrophotography, as you have to be extremely precise.

I Don't use Canon and Nikon cameras, but it always sounded like the tethered shooting software for those was far better than for my Pentax.

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As above - the atmosphere and conditions will mean you need to adjust everytime, but if you at least know the ballpark you need to be in, that can save some pain.

The other thing you can do is connect to software like Astrophotography Tool (APT) and use the Focus Aid in that - if I remember correctly this is only available in the paid version, but the license is about £12 or something similarly cheap - and that helps immensely. I use it and, although it can still be a battle to get it right, it is a lot easier than using LiveView on the camera screen - especially if you screen is fixed like my 1100d!

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I find the Focus Aid easier to use than a Bhatinov mask, although that may be down to practice. However, the "image" you use to focus on with a mask is much more "stable" that with the Focus Aid in APT which uses the FWHM measurement which is fluctuating constantly. I like having "numbers" to refer to, but I find it can take a long time to settle as you are always chasing better numbers. I definitely need to get back into trying the Bhatinov mask - they are only £10-15 as well, so a great investment.

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Don't want to go too far off topic, but thanks for the software - makes life really easy, and I'm only scratching the surface, so know there is lots more I can do with it in the future when I graduate to EQMOD/ASCOM and CCD imaging!

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