bahtinov mask

[sparkyjon]

ok what are these for , sorry noob question..

i know they are something to do with focusing,. but how do they work and are they left on whilst imaging? and are they needed for webcam imaging..?

[JamesF]

They create a diffraction pattern like an "X" with a vertical (or horizontal) bar through it. Out of focus one way the bar is to the left of the central cross of the X. Out of focus the other way and it's to the right. When you're perfectly in focus it goes straight through the middle. Ideally you need the telescope centred on a bright star to do this.

They're not "needed" for anything, exactly. They're an aid to focusing.

James

[JamesF]

Looks like there's a far better example than I could give right here:

http://en.wikipedia.org/wiki/Bahtinov_mask

James

[kyokugaisha]

Definitely don't leave it on when you image! That would be really dumb. I would never do that. Nope, I would never lose an hour of subs because I left it on...

[sparkyjon]

nice one james , thats a good link and explains it well.. these would be handy for collimation on sct i'm assuming..although i think mine is looking good now.

[mareprah]

I don't think it could be an aid for collimation. Just for focusing.

If you want to get your scope collimater, use a small metal ball, place it somewhere 20m or 30m away from the scope, but the ball must be lit with a nearby diode, flashlite or something. This makes a great artificial star - at least I was told so (never tried it yet, but it seems like a good idea)

As for kyokugaisha/Marc - common dude, there's a nicer way of telling that, the guy just asked a simple question, and if he doesn't know what bahtinov is/how does it look like, than you cant expect from him he'll be knowing how to us it... Wasn't a dumb question at all.

[sparkyjon]

appreciate the help mareprah but i think marc was being a little sarcastic as he has done it himself maybe ?? well thats how i read it , if he wasnt out side now

[kyokugaisha]

It was definitely meant to be a humorous comment - sometimes forget that doesn't come across well in text! I've done it twice now and it is amazingly annoying.

[stem1989]

They are a must have for any imagers kit.

[ollypenrice]

It was definitely meant to be a humorous comment - sometimes forget that doesn't come across well in text! I've done it twice now and it is amazingly annoying.

I thought this was clearly ironic (not sarcastic) and obviously intended to send yourself up, not the OP. I think most of us have the Bahtinov T shirt!

I'd call these masks all but essential and, at long focal lengths, absolutely essential because measuring the star size in Full Width Half Max just doesn't work.

Olly

[JamesF]

I thought this was clearly ironic (not sarcastic) and obviously intended to send yourself up, not the OP.

Absolutely agree. It didn't even occur to me that it might be interpreted differently. On the assumption that English is not mareprah's native tongue, I assume the self-deprecation was lost in translation. Easily done; I mean, how often does it happen between English and American?

James

[ollypenrice]

Absolutely agree. It didn't even occur to me that it might be interpreted differently. On the assumption that English is not mareprah's native tongue, I assume the self-deprecation was lost in translation. Easily done; I mean, how often does it happen between English and American?

James

Yes, very often! Our English ironies don't always come across as such.

Olly