Why are all bahtinov masks black? Can they be coloured?

[bendiddley]

As the title says I'm interested to know why bahtinovs are black. Is it because they always have been or is there some science behind it. I guess it helps to show the diffraction spikes if the mask is dark, right?  But in a dark telescope in a dark outdoors surely even a coloured mask would look dark and capable of providing the contrast to show the spikes?

[vlaiv]

Color of mask has absolutely nothing to do with operation of mask - it is the edges and their angles that create spikes.

There is no reason why one could not use colored Bahtinov mask. There is however issue of light reflection that could potentially lower the contrast. Most accessories that are in light path are died black for this reason - to minimize reflections and stray light, however B mask is not kept on during imaging (at least not by choice , but sometimes it may stay on due to forgetfulness ) so it really does not matter and one could use colored masks without much issues.

[tooth_dr]

32 minutes ago, bendiddley said:

As the title says I'm interested to know why bahtinovs are black. Is it because they always have been or is there some science behind it. I guess it helps to show the diffraction spikes if the mask is dark, right?  But in a dark telescope in a dark outdoors surely even a coloured mask would look dark and capable of providing the contrast to show the spikes?

I heave black masks, blue masks and a shiny gold one.  All work perfectly well. 

[wormix]

I printed a grey mask for my Meg72 and it works absolutely fine

[bendiddley]

35 minutes ago, vlaiv said:

Color of mask has absolutely nothing to do with operation of mask - it is the edges and their angles that create spikes.

There is no reason why one could not use colored Bahtinov mask. There is however issue of light reflection that could potentially lower the contrast. Most accessories that are in light path are died black for this reason - to minimize reflections and stray light, however B mask is not kept on during imaging (at least not by choice , but sometimes it may stay on due to forgetfulness ) so it really does not matter and one could use colored masks without much issues.

Cool nice one guys, I figured it didn't matter. Re the reflections, so is that not an issue because the B mask is removed after use? Or are you saying that reflections of some sort affect the contrast of the spikes? I've seen shiny black masks made of acrylic, will they cause issues with reflections?

[vlaiv]

12 minutes ago, bendiddley said:

Cool nice one guys, I figured it didn't matter. Re the reflections, so is that not an issue because the B mask is removed after use? Or are you saying that reflections of some sort affect the contrast of the spikes? I've seen shiny black masks made of acrylic, will they cause issues with reflections?

I was just explaining why most of things are black - that is just a "convention" because there is a need to suppress stray light which lowers contrast if not suppressed. In the case of B mask - difference is not noticeable in most cases as most people use relatively strong star to focus on and yes, B mask is removed later when imaging.

[nephilim]

4 minutes ago, vlaiv said:

I was just explaining why most of things are black - that is just a "convention" because there is a need to suppress stray light which lowers contrast if not suppressed. In the case of B mask - difference is not noticeable in most cases as most people use relatively strong star to focus on and yes, B mask is removed later when imaging.

Or in my case a few years ago, not removed therefore ruining over an hour of precious imaging time under extremely rare clear uk skies 😒

[vlaiv]

Just now, nephilim said:

Or in my case a few years ago, not removed therefore ruining over an hour of precious imaging time under extremely rare clear uk skies 😒

I personally don't use it - I've found that FWHM/HFR much more suits me and is more precise in my case, however I do know that it often happens that people forget it on the scope.

[andrew s]

Mine is white. Just the material I had to had. Worked just fine.

Regards Andrew 

[Louis D]

What about being made from clear acrylic?  Would there still be enough diffraction at the edges to work?

[wormix]

2 minutes ago, Louis D said:

What about being made from clear acrylic?  Would there still be enough diffraction at the edges to work?

The william optics ones are clear acrylic i believe

[vlaiv]

Just now, Louis D said:

What about being made from clear acrylic?  Would there still be enough diffraction at the edges to work?

William optics seems to think so

However, their rationale that it will help with narrowband and make spikes stronger / more contrasty is wrong. Strength of spikes depends solely on length of diffraction edge - in this case, density of the grooves.

 

[bendiddley]

1 minute ago, vlaiv said:

William optics seems to think so

However, their rationale that it will help with narrowband and make spikes stronger / more contrasty is wrong. Strength of spikes depends solely on length of diffraction edge - in this case, density of the grooves.

 

What do you mean by density of the grooves?

[nephilim]

5 minutes ago, vlaiv said:

William optics seems to think so

However, their rationale that it will help with narrowband and make spikes stronger / more contrasty is wrong. Strength of spikes depends solely on length of diffraction edge - in this case, density of the grooves.

 

I'm buying the Zenithstar 81 in a few weeks and the Optolong L-extreme narrowband filter. It'll be interesting to see how well the combination works as the L-Extreme has a very narrow band pass of 7nm.

Edited November 4, 2020 by nephilim

[vlaiv]

1 minute ago, bendiddley said:

What do you mean by density of the grooves?

Low diffraction - small spikes :

High diffraction - large and strong spikes:

or this one:

[Louis D]

5 minutes ago, vlaiv said:

William optics seems to think so

However, their rationale that it will help with narrowband and make spikes stronger / more contrasty is wrong. Strength of spikes depends solely on length of diffraction edge - in this case, density of the grooves.

 

Do you mean that by having more closely spaced, finer grooves, you will have stronger spikes?  If so, that sort of makes sense.  However, don't you reach a point where they are so fine they just look gray to the incoming light?

[vlaiv]

https://www.dpreview.com/forums/thread/4454760

 

[bendiddley]

4 minutes ago, vlaiv said:

or this one:

Never seen one like this. Is that clear plastic in the middle with black printed lines? Who makes these ones?

[vlaiv]

5 minutes ago, Louis D said:

However, don't you reach a point where they are so fine they just look gray to the incoming light?

You reach a point where mask starts working as diffraction grating. You can see that as one gets more line density - spectrum starts to form in different orders, like in this image:

With higher density you are both increasing length of diffraction edge and spacing of the grooves - second one just makes spectroscope of higher and higher resolution. Diffraction gratings have hundreds of grooves per mm and produce nice rainbow spectra - like StarAnalyzer.

This is 100 lines per mm - very stretched out central spike of bahtinov mask. B mask has 3 distinct angles of grooves and that creates three different spectra at an angle.

[vlaiv]

3 minutes ago, bendiddley said:

Never seen one like this. Is that clear plastic in the middle with black printed lines? Who makes these ones?

I just did image search on google and came up with that image among others. Have no idea who makes it.

[ursamajorro]

My home made mask is a Bahtinov simplified at maximum. I used mosquito net. It practically costed me nothing.  How it looks intra and extrafocal see video:

 

 

[Louis D]

9 minutes ago, ursamajorro said:

My home made mask is a Bahtinov simplified at maximum. I used mosquito net. It practically costed me nothing.  How it looks intra and extrafocal see video:

Reminds me a bit of my home brew apodizing mask made from black window screen.  That mask causes some really wild spectral artifacts in the outer field.

Edited November 4, 2020 by Louis D

[bendiddley]

7 minutes ago, ursamajorro said:

My home made mask is a Bahtinov simplified at maximum. I used mosquito net. It practically costed me nothing.  How it looks intra and extrafocal see video:

 

 

Brilliant!! That is ingenious!!! 

[Louis D]

17 minutes ago, vlaiv said:

You reach a point where mask starts working as diffraction grating. You can see that as one gets more line density - spectrum starts to form in different orders, like in this image:

With higher density you are both increasing length of diffraction edge and spacing of the grooves - second one just makes spectroscope of higher and higher resolution. Diffraction gratings have hundreds of grooves per mm and produce nice rainbow spectra - like StarAnalyzer.

This is 100 lines per mm - very stretched out central spike of bahtinov mask. B mask has 3 distinct angles of grooves and that creates three different spectra at an angle.

That's what I was thinking it reminded me of.

However, what happens when the line width is narrower than the wavelength of visible light?  For instance, say, greater than 10000 lines per mm?  Do you just get a really good spectral analyzer or does something else start to happen?

[vlaiv]

2 minutes ago, Louis D said:

However, what happens when the line width is narrower than the wavelength of visible light?  For instance, say, greater than 10000 lines per mm?  Do you just get a really good spectral analyzer or does something else start to happen?

Not sure how you plan to cut such thin lines . At some point I wanted to make very narrow slit - like 10µm slit (which 100 per mm) and that was simply beyond my skill by far. In any case, I suspect that it will start behaving like solid and reflect light. In fact - most of the things that we perceive as solids are very empty and we could argue that they are extremely fine gratings (crystals for sure) - some transmit light and others reflect it, but that depends on internal structure.

Even if material was transparent and had very fine "molecular" level "grooves" in crystalline structure, I think there would no longer be diffraction effects. Diffraction happens because of different distance between light paths. Once slits are too narrow - difference in distance between adjacent paths vanishes and so does phase difference for interference. Wavefront will just pass undisturbed (transmission) or get reflected back (reflection), but there won't be diffraction.