Why does this bahtinov mask produce 8 spikes?

[bendiddley]

I found this bahtinov mask for DSLR's. Watched the video on the listing and have noticed it produces 8 spikes rather than 6. Does anyone know why this is? Is there something about the line pattern that might cause this? Link and photos below. 2nd photo shows screen shot of the video.

https://www.aliexpress.com/item/32978148314.html?

 

Edited December 3, 2020 by bendiddley

[Astrobits]

Every angle of lines on the mask will produce two spikes at 180 degrees to each other. Therefore if there are three different line angles you will get 6 spikes and with 4 line angles there will be 8 spikes.

Nigel

[bendiddley]

30 minutes ago, Astrobits said:

Every angle of lines on the mask will produce two spikes at 180 degrees to each other. Therefore if there are three different line angles you will get 6 spikes and with 4 line angles there will be 8 spikes.

Nigel

Thanks, I can only see 3 line angles on this bahtinov mask though, so not sure why it would create 8 spikes?

[vlaiv]

1 minute ago, bendiddley said:

Thanks, I can only see 3 line angles on this bahtinov mask though, so not sure why it would create 8 spikes?

You are using it on newtonian scope so other spikes are just regular newtonian spikes two of them being aligned with two spikes from B mask and hence hidden while other two being visible for total of 8 spikes?

[bendiddley]

Just now, vlaiv said:

You are using it on newtonian scope so other spikes are just regular newtonian spikes two of them being aligned with two spikes from B mask and hence hidden while other two being visible for total of 8 spikes?

I'm not using this mask, I haven't bought it. I saw it for sale and watched the video and am curious about why there are 8 spikes. I have attached some photos to the original post above, if that helps at all

[JamesF]

2 minutes ago, vlaiv said:

You are using it on newtonian scope so other spikes are just regular newtonian spikes two of them being aligned with two spikes from B mask and hence hidden while other two being visible for total of 8 spikes?

Ingenious :)

Easily tested too, I guess, by rotating the mask, say, 45 degrees.

James

[vlaiv]

2 minutes ago, bendiddley said:

I'm not using this mask, I haven't bought it. I saw it for sale and watched the video and am curious about why there are 8 spikes. I have attached some photos to the original post above, if that helps at all

Well, look at Nigel's post above ( @Astrobits ) - he explained why you see 8 spikes in the video.

What is misleading is mask in AliExpress product description - it shows regular B mask with 3 different edge orientations. One used in video is different - it has 4 different orientations:

As given in above answer - spike in the image is perpendicular to any straight edge in B mask and hence it creates two spikes - each one at 90° to edge but at opposite sides. 4 different edge angles will result in 8 spikes.

[steppenwolf]

1 minute ago, vlaiv said:

What is misleading is mask in AliExpress product description - it shows regular B mask with 3 different edge orientations. One used in video is different - it has 4 different orientations:

Perfect explanation

[bendiddley]

2 minutes ago, vlaiv said:

Well, look at Nigel's post above ( @Astrobits ) - he explained why you see 8 spikes in the video.

What is misleading is mask in AliExpress product description - it shows regular B mask with 3 different edge orientations. One used in video is different - it has 4 different orientations:

As given in above answer - spike in the image is perpendicular to any straight edge in B mask and hence it creates two spikes - each one at 90° to edge but at opposite sides. 4 different edge angles will result in 8 spikes.

Blimey! Good spot. That explains it then. I think that seller needs to sort his video out, might help to be videoing the item your selling or vice versa. Maybe he changed his design to the 3 angled one and forgot to update the video lol.