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Old 08-05-2013, 07:54 PM
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DSLR and a Bahtinov mask - seeking some advice

I'm having trouble figuring out how far from the front of the lens to put the Bahtinov mask. I've tried it right up against the lens and about 2 inches from it.

On both occasions I get the mask pattern on the stars rather than the rays.

Am I missing something simple here?

Thanks everyone

Niko
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Old 08-05-2013, 09:38 PM
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Are you using the dslr on a telescope or a camera lens? If your using a telescope can you focus on anything in the distance? You may need an extender to reach focus.

Tell us a bit more about your set up and we may be better able to help.


Thanks
Sandy
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Old 09-05-2013, 10:56 AM
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Thanks Sandy

It's just me, the DSLR, a tripod and the mask.

I use a mask on my ED80 with no problems at all. It sits at the end of the dew shield - so, a good few inches from the lens of the scope.

I've made up a little shield to hold the mask an inch or so from the DSLR lens figuring it needs a gap between the lens and the mask in the same way the scope works.

All I get is the baht mask lines on the star instead of the 3 rays to aid focussing.

Then I tried the mask directly on the lens with the same results.

???

cheers

Niko
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Old 09-05-2013, 04:52 PM
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Some time ago I picked up a stick on mask to go on a filter so from that I assumed that the mask goes where you would normally put a front of lens filter.

That being said I never had much luck with it, I am not sure it works terribly well on photographic lens and view finders / live view.
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Old 09-05-2013, 06:23 PM
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The diffraction pattern is very small with a short focal length lens. Try shorter exposures and zoom in to see it.

Bert
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Old 09-05-2013, 08:02 PM
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Thanks guys

Bert will try your suggestion

I wonder if a Y mask might work better for DSLRs?

Niko
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Old 10-05-2013, 04:57 AM
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I wasn't aware that the distance in front of the lens makes any difference. If the object is at infinity then the light rays would be parallel, so I can't really think of how the distance in front of the lens would change anything.
Anyhow, I find that my mask works (usefully) on lenses down to around 100mm focal length. Below that, the focus range that you see spikes instead of an out of focus blur with the B mask pattern, is too small to be useful.
For focal lengths less than 50mm, I focus on something terrestrial that is further away than the hyperfocal distance like a street light or red led on a power supply.

Steve.
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Old 13-05-2013, 08:13 PM
Garbz (Chris)
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The only time the star should take the picture of the mask is when it is HORRENDOUSLY out of focus.

Can you achieve focus without the mask? Then when you put the mask in front of the lens you should end up with diffraction spikes regardless of where you put it. At these focal lengths the distances shouldn't matter.
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