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Whipping up a Y mask


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How do everyone! Inspired by a recent thread about making your own Y mask I've cut 3 bits of 4mm plastic, just need to find a protractor for getting the angles right (I assume this has to be 100% accurate as even a slight inaccuracy will render it useless?).

As I used quite blunt scissors and the plastic was quite thick the 4mm strips aren't completely uniform in width, they're quite straight but vary from their thinnest point to their fattest by about 1mm, will this make a big difference in the result?

When making a Y mask what are the absolutely crucial criteria, does each strip need to be completely straight, do angles need to be 100% accurate. Basically, how critical do I have to be to get a useable mask?

Edited by Mamsoth
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Speaking as someone who has made/used several, but has very little idea of the math behind them, I don't think there are ANY critical bits. Mine are just rough cut strips of card, stuck onto a ring that fits over the dewshield (also high tech - folded piece of cardboard, edges stuck together to form a cylinder) at angles that made a nice Y (<-those sort of angles) with a bit of black tape in the middle (bit of a 'splodge', really) to hold them together. Works fine.

HTH

Edited by Demonperformer
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Here is the absolute crucial criteria, DP has explained it in a nutshell, I do not know what scope you have but 4mm could be ok, what you will find in practice is that wide strips should form short bright diffraction spikes where thin ones produce fainter but longer spikes, as to the angle, any alteration will only make the X pattern wide or narrow but the function should stay the same, although it is nice to have even cut edges, from what I have read uneven ones work just the same, in the end experimentation is the order of the day :D

John.

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Thats fantastic, I really hate having to be overly fastidious. I would have thought though that if the angles weren't equal on each side of the horizontal then a perfect focus would produce a middle diffraction spike that was never perfectly centered within the other two spikes. But it sounds like that isn't the case?

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The basic principles that dictate from Chris Lord`s PDF concludes that any solid bar of narrow width placed across the aperture of a telescope will produce a diffraction spike, and further the resulting spike will always lie on the optical axis and be at right angles to the original position of the bar. Bearing these principles in mind you can create any diffraction pattern you want, as I have already pointed out the width of the bar should produce variation in brightness of the spike, so if your X is a bit dim then simply thicken the arms of the top par of the Y, hope this explains this a little better :D

John.

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That's such a relief, I was invisaging an obsessive night with the protractor. I have just bought a bahtinov for my refractor at £16 which will prove to be a shame if my Y mask works well but perhaps it will have saved me the £28 it would have cost for a B mask for my big Dob. Thanks for the explanation, now I just need a clear night for the testing

Cheers

Matt

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  • 2 years later...

I recently tried making a y mask for my 200mm dslr lens, and finally got enough clear sky to point it at a star last night. I got no diffraction spikes. I'm wondering what I did wrong, but the thought occurred that I used matt black card, does the mask need to be somewhat reflective on the inside edge to work?

I made the legs 4mm wide and used one of my bahtinov masks from my scopes as a guide for the angles (effectively using the bahtinov as a stencil).

Should I remake an identical mask in white card, or is it something about my angles or widths that is likely to be a problem?

Edited by SplintUK
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The info on the thread is ok. I've used nothing but a Y mask on camera lenses for a few years. It doesn't need to be reflective that I know of.

You didn't say which star or length of exposure or camera.

Try Vega for 5 seconds or so and re test. If it works then try a star in the region of interest. Don't go too dim.

Dave.

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What I learned tonight.

1 - wait till it gets dark - (no really, like after 11:30 pm in June)

2 - use ISO 6400 and fully open aperture f/3.5 on this 200mm lens, for about 6 seconds exposure

3 - skinny legs (3mm) aren't as good as medium (4mm) and thicker (13mm) is better still.

4 - colour doesn't matter, black is fine for the mask.

5 - bodge up works fine too.

I bodged up some thick legs about 13mm wide from the sticky strip of a post-it note and stuck them on the skinny legs and that worked best for me post-12157-13703862968_thumb.jpg

Pen included for size reference.

Tomorrow I will be making a better wide legged y mask out if card.

Thanks for the help guys.

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The Bahtinov and the later Lord Y masks are based on the criteria of focal length and aperture of a telescope, Pavel Bahtinov provided a formulae, so that bar and space widths could be calculated, which I will not go into here. As DP and James have already pointed out, there has been much experimenting with bar widths and varying angles, where positive results have been achieved, which would be far from the original intended specified design. It was not intended to use with lenses in photography, but has been quite successful, none the less, in helping to achieving an accurate focus.

The following points should be borne in mind when constructing your own...

1. Where a solid bar is placed across the aperture of a scope it will produce a diffraction spike at right angles to the original, which will lie on the central optical axis.

2. A thin bar will produce a long spike, whereas a thick bar, a corresponding short spike.

3. A grid of multiple bars set at the same angle, having a common width and space, is purported to produce brighter spikes. The jury is still out on this one.

4. Material used can be anything within reason, black and none reflective is recommended, 1mm tin sprayed black is a very good choice for a permanent mask, but black

plastic is more often used as it is readily available.

Experimentation is the order of the day, working within the guide lines given, until you come up with something that gives you a satisfactory result, have fun :)

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The Bahtinov and the later Lord Y masks are based on the criteria of focal length and aperture of a telescope, Pavel Bahtinov provided a formulae, so that bar and space widths could be calculated, which I will not go into here. As DP and James have already pointed out, there has been much experimenting with bar widths and varying angles, where positive results have been achieved, which would be far from the original intended specified design. It was not intended to use with lenses in photography, but has been quite successful, none the less, in helping to achieving an accurate focus.

The following points should be borne in mind when constructing your own...

1. Where a solid bar is placed across the aperture of a scope it will produce a diffraction spike at right angles to the original, which will lie on the central optical axis.

2. A thin bar will produce a long spike, whereas a thick bar, a corresponding short spike.

3. A grid of multiple bars set at the same angle, having a common width and space, is purported to produce brighter spikes. The jury is still out on this one.

4. Material used can be anything within reason, black and none reflective is recommended, 1mm tin sprayed black is a very good choice for a permanent mask, but black

plastic is more often used as it is readily available.

Experimentation is the order of the day, working within the guide lines given, until you come up with something that gives you a satisfactory result, have fun :)

thats a brilliant summary, thank you.

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  • 1 year later...

Just knocking one of these up before I go to Kelling tomorrow  for use with the DSLR lens.  

Am used to the Bahtinov mask with scopes, but not sure what i should be aiming for to know I have focus with a Y mask.

Reading above you're aiming for an X shape, but what does the shape look like when you're NOT IN FOCUS?  

Is is a question of the spokes transecting each other symmetrically or something? 

Thanks

Carole 

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  • 3 years later...

Just linked this thread for someone else and seen Carole's unanswered question from 2014. Therefore putting an answer on for anyone else looking:

When in focus, the bar across the X transects the X where the two lines of the X cross. When out of focus, this bar will be to one side (or up or down, depending on the orientation of the mask) of that point.

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