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

[bendiddley]

1 hour ago, vlaiv said:

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

 

Ok so it seems that having more lines is better. But as  you go up in aperture, what's best in terms of the thickness/quantity of the grooves/lines? Should they still be really fine and more and more of them or if not how wide can they/should they go and how many is best? I've seen a bahtinov for a 12" scope with only 15 horizontal grooves at 1cm wide each! Is that OK for a scope that size?

[vlaiv]

4 minutes ago, bendiddley said:

Ok so it seems that having more lines is better. But as  you go up in aperture, what's best in terms of the thickness/quantity of the grooves/lines? Should they still be really fine and more and more of them or if not how wide can they/should they go and how many is best? I've seen a bahtinov for a 12" scope with only 15 horizontal grooves at 1cm wide each! Is that OK for a scope that size?

All of them serve the same purpose - behave the same. You have slight defocus and it will show as asymmetry in spikes.

If you can't see them properly - use brighter star, use longer exposure, or bin your image, whatever improves SNR.

I doubt that one needs 1cm wide grooves / supports to have rigid B mask. I guess few mm wide is enough even for large masks.

[bendiddley]

Another one. On the WO masks how come they have loads of thin lines as well as wide slots?

[TerryMcK]

3 hours 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?

They're not all black. Mine are transparent and are the integral ones built into the scope cover that come with William Optics refractors.

[symmetal]

I find the WO transparent mask the easiest to use as it produces very useable sharp spikes on relatively dim stars, where a standard mask does almost nothing. The WO61 mask also works well on the Zwo 30mm mini guider scope when only the centre grooved lines are pretty much all that's in view. I tried a standard black 60mm mask which i bought before WO made the transparent ones and that did nothing on the guide scope. 😀

Alan

[bendiddley]

1 minute ago, symmetal said:

I find the WO transparent mask the easiest to use as it produces very useable sharp spikes on relatively dim stars, where a standard mask does almost nothing. The WO61 mask also works well on the Zwo 30mm mini guider scope when only the centre grooved lines are pretty much all that's in view. I tried a standard black 60mm mask which i bought before WO made the transparent ones and that did nothing on the guide scope. 😀

Alan

Thanks Alan. So is that why they have fine lines as well as the larger slots? so you can use them on smaller scopes as well as larger ones? Trying to understand the reason for the thin lines as well as the wider slots on the WO masks.

[symmetal]

40 minutes ago, bendiddley said:

Thanks Alan. So is that why they have fine lines as well as the larger slots? so you can use them on smaller scopes as well as larger ones? Trying to understand the reason for the thin lines as well as the wider slots on the WO masks.

That seems to be the case, though I don't know if that was the intention. Whether having the whole mask with just grooves would be just as good as the grooved and slotted one I don't know, but I imagine WO did tests and found what worked best, and decided on a slotted and grooved mask as the best all rounder I suppose. 😀

Alan  

Edited November 4, 2020 by symmetal

[JeremyS]

3 hours 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:

 

Don’t you have a big hole for the mozzies to get in through now?
 

😬

[Louis D]

4 hours ago, vlaiv said:

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.

More of a theoretical question.  Someone could probably use a scanning electron microscope to move atoms into the desired positions to empirically check this.

I figured that once the slits are too narrow, the waves wouldn't be able to pass through.  At that point, they're either going to be absorbed or reflected.

[newbie alert]

A friend made a bahtinov out of a cd case and scratched in the pattern  for his camera lenses.. seemed to work very well

Edited November 5, 2020 by newbie alert
Damn phone