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Bahtinov mask: useful?

Planetesimal

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Planetesimal

Planetesimal

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I've been seeing the Bahtinov mask discussed on occasional threads, and I understand it's used to help achieve better focus...

For purely observational use, with my set-up (F/8 newt with supplied R&P focusser), is there a benefit to using one of these?

And out of interest, can anyone explain how they work / how they are used?

Thanks! :smiley:

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Planetesimal

Planetesimal

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Thanks Bryan - from reading the wiki, it looks like it's used more for AP focus, but would probably be useful for visual. My concern is that the R&P focusser supplied with my 'scope is a bit on the clunky side, so I'd probably not be able to get the best out of a Bahtinov anyway!

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steppenwolf

steppenwolf

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I use one all the time for imaging but I don't think I'd use one for visual work.

They work by producing a set of diffraction spikes similar to those produced by a Newtonian reflector or a Ritchey-Chrétien - these two produce a simple cross. The Bahtinov mask has both horizontal and angled edges so the diffraction spikes it forms are a more complex shape, namely a cross and a horizontal line. Simply lining the horizontal line up with the cross so that the line passes through the centre of the cross ensures that you have reached focus on a point source like a star.

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Tim

Tim

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Useful? No. Essential? YES!

For imaging the mask is perfect, so intuitive, but visually I use mine to make totally certain that focus is perfect when observing planets. As you know these are tricky with the seeing coming and going, but the mask removes the guesswork and you know the focus is spot on and that any blurring will be the skies themselves. (Focus on a star for planets)

You need not spend a fortune, you can make them out of cardboard quite easily to test out how useful they might be for you :)

Cheers

Tim

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Astrosurf

Astrosurf

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I use a B mask for AP and visual. I have pretty awful eyesight with tons of floaters so need the mask! There's a B mask generator which someone will post if you decide you need one and would like to make one (it's fun, and much cheaper!). I made one from the plastic backing of a large, WH Smith drawing pad! I would be lost without my mask now.

But as people said, if your eyes are OK you probably don't need one for visual.

Alexxx

EDIT: I think this is the generator site, just in case you're interested: http://astrojargon.net/MaskGenerator.aspx

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Planetesimal

Planetesimal

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Great, thanks for all the advice! I think I'll go with making a home-made one, even though so far my only home-made item is a foam dew shield :)

Getting a lot more practice with the old craft knife - now there's something I didn't realise I was signing up for when I "walked into astronomy"!

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Planetesimal

Planetesimal

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Thanks Stu - while you're there, I've been asking on another thread about kit for AP. The broad consensus was the SW Evo 80ED Pro, which I see you've got in your equipment list. Do you find it works well for AP? And also, I can't find one for sale at the moment (in GBP) - how much should I expect to spend if I wanted one new?

Thanks! :smiley:

I'll leave it at that after this minor OP derail! :grin:

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StuW

StuW

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Thanks Stu - while you're there, I've been asking on another thread about kit for AP. The broad consensus was the SW Evo 80ED Pro, which I see you've got in your equipment list. Do you find it works well for AP? And also, I can't find one for sale at the moment (in GBP) - how much should I expect to spend if I wanted one new?

Thanks! :smiley:

I'll leave it at that after this minor OP derail! :grin:

The ED80 is good for semi-widefield photography, especially with the HEQ5 due to it's low weight. I haven't done much with it to be honest, apart from Solar and daytime wildlife photography, but I've found the biggest downfall is definitely the focusser, and that can be improved with an oil-stone and some patience.

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glowjet

glowjet

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Interesting. I'd never heard of the Lord mask before. Does it work on reflectors as well as fracs I wonder, given that you already get diffraction spikes from the spider arms?

You really should have read of http://www.brayebroo...ofer-primer.pdf Chris Lords primer does well to explain how the mask works. Then go and make yourself a Y mask out of a Black plastic document cover and try it. As a guide to dimensions/angles. Pavels formulae gives the angle as 20° for each leg of the Y or 40° between both. leg width is based on your scopes focal dim, some latitude is given here by the use of two figures : 150 and 200, divide either into your focal length and you will come up with two results, you can use one or the other, there is a little maths to do before you finally reach the the size for your bars, as you now have to divide your chosen figure by 2, as this figure represents a bar and space width and you only want half of this to give you the width of your bar. if things get to small, then simply multiply your results by 3, this is known as a third order mask. I should imagine the mask works on almost any telescope or even camera lens, which is capable of receiving a waverfront of starlight :)

John.

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