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Bahtinov Masks


gonzostar

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Hi 

I was wondering about buying a bahtinov mask for my imaging

First off all are they easy to use?

Also is it possible to focus a star (which magnitude is better?) using the live view on a DSLR camera with the mask?

Dean

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They work like a charm and only require you to look carefully at the position of the middle spike relative to the other two. It should be central.

The star cannot be too bright, though it can be too dim. It will work on the camera screen though bigger is better.

Excellent devices.

Olly

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Just so.

And not just for testing focus either. I'm using one to determine how much tilt I have in the system, and in what direction. Something that FWHM won't tell you.

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52 minutes ago, DaveS said:

Just so.

And not just for testing focus either. I'm using one to determine how much tilt I have in the system, and in what direction. Something that FWHM won't tell you.

Indeed! You've just given me a "Eureka" moment! The direction that the central spike moves away from the centre tells you if you are inside or outside focus!! :) Thank you!!! :)

I just need a reliable way to set up my mask now..... lol

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One thing I have found is that live view simulates long exposures by turning the gain up to silly levels and this can make the critical centre part of the pattern fuzzy. Once I get close to focus, it can help to take a short exposure and inspect it closely as I find it is generally much sharper. 2 or 3 such exposures are usually enough.

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I take the test in live view, about 10S and zoom in. Slight blurring of the small stars as I didn't wait for the mount to settle after focusing. This is about 10 secs exposure on Arcturus.

Bahtinov.jpg

For a bit of amateur spectroscopy, compare this which is Sirius on a webcam:

Bhatinov.jpg

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1 hour ago, Stub Mandrel said:

I take the test in live view, about 10S and zoom in. Slight blurring of the small stars as I didn't wait for the mount to settle after focusing. This is about 10 secs exposure on Arcturus.

Bahtinov.jpg

For a bit of amateur spectroscopy, compare this which is Sirius on a webcam:

Bhatinov.jpg

Think i understand now :smile: How long does thos process take? Does it make a diiference if you place  the mask on  dew sheild or not? 

Cheers for posting the pics

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18 minutes ago, gonzostar said:

Think i understand now :smile: How long does thos process take? Does it make a diiference if you place  the mask on  dew sheild or not? 

Cheers for posting the pics

There was a debate about this, then someone pointed out that using one on a reflector was like putting it at the end of a metre long dewshield - so the answer is a reosunding no!

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On 5/9/2016 at 12:01, PeterCPC said:

I put my mask on the end of my Astrozap dew shield on my C9.25 and it works fine.

Peter

You just need to make sure that the mask is large enough so that only light coming through the gratings gets to the objective. :) i.e. no gaps around the outside, otherwise you'll lose contrast in the spikes.

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1 hour ago, gonzostar said:

Good point, hadnt thought of that! Does "atmospheric seeing" also affect the contrast of the spikes?

I've not found seeing to impact that much - just pick a bright target, Vega, Sirius and Capella are my favourites - I can usually see one of them; if the sun ever sets :)

Using either live view on Canon D90, or MKI eyeball through an eye piece, or taking an image all seems to get me decent focus.

 

I am a noob.

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1 hour ago, gonzostar said:

Good point, hadnt thought of that! Does "atmospheric seeing" also affect the contrast of the spikes?

Not so much the contrast in real time*, but it will make the whole image jump around, just like a star would.

*If focusing by camera image, any exposure would be smeared about by the seeing.

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