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now i have made my bahtinov mask... how do i use it?


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okay so iv made my bahtinov mask... i will be imaging using a 5x barlow... and my webcam is an unmoded sp900nc... so i dont believe it can show 'live' stars on my laptop...

what do i do?

step 1: point at a bright star

step 2: put the mask on

step 3: put the 5x barlow in

step 4: ?

thanks

will

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Hmm, well I would tend to think that if you can't see Arcturus then you need a better camera or longer exposure. Not much is brighter than Arcturus.

The diffraction spikes are pretty bright and I think even a webcam should capture them. If not then you may need the long-exposure modification.

Doug

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I'm not sure people really think much about it, but once you have the mask on the scope then you're cutting out around 50% of the light, so it's not entirely surprising that it can be harder to see the image.

What I'd probably do is get the target star centred in the image without the mask, then put the mask on and push the gain/brightness up until you get a usable image again. As long as you get clear diffraction spikes it shouldn't matter that those settings aren't what you want to use -- you can return them to "normal" once you've sorted out the focus.

In my experience it takes a bit of practice though. It's possible to make a mask that's just a "Y" shape if you make it with the correct dimensions and obviously that blocks far less light, but it can still be an awkward job getting the focus sorted with the camera.

Another thing that can help if you have a "spare" eyepiece, is to remove the camera and drop in the eyepiece with a parfocal ring on once you do get the focus correct. If you lower and raise the eyepiece until it also achieves focus and then lock the ring, you should be able to get very close to the correct focuser position in the future using the eyepiece first.

James

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The mask, when placed over the end of the OTA, causes a very distinctive diffraction pattern to appear from bright stars. As you move the focus, only one of the lines in the pattern moves, when it is centralised within the other lines, you have achieved perfect focu; you then remove the mask.

see http://www.btinternet.com/~gfcarey/bahtinov.htm for some pics

Steve

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you can view live stars on your unmodded spc900 if you use a program called sharpcap and set the settings on the right to yuy2.....10 secs.....640x480.....leave the exposure at -4 and put the gain up full......you will see perfectly well

also get a program called bahtinovgrabber and when you use sharpcap run bahtinovgrabber aswell.....it tells you wether you are in perfect focus,you might think you are just using the mask but you wont be.

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