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Curly Diffraction spikes from a Bahtinov Mask.


MarsG76
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Hi All,

Has anyone seen this phenomenon while using a Bahtinov mask for focusing... for some reason my diffraction spikes are curly!

There was no scope/mount vibration that I could detect, no other obstruction in the optical path such as dew, dirt or anything other than a UV/IR filter. No trees or other obstruction in the line of sight.
There was a little bit of haze in the sky and the 88% full moon was about 25 degrees toward the east from the star I was focusing on at the time.

Also these curls rotate with the rotation of the Bahtinov mask...

So unless the curls are some kind of a negative moon phase reflection, I'm out of ideas at this time.

The diffraction spikes on my SCT are straight, so this seems to be happening on my BOSMA Beta-RE 80mm refractor, but I haven't seen this happen before tonight.
Camera is a QHY268M.

This has not stopped me from focusing on the star, and doesn't look like this effect is present in my test subs exposures, but I'm very curious to what could be causing these curls.

Clear Skies,

Mariusz

 

IMG_5110.JPG.JPG

IMG_5111.JPG.JPG

Edited by MarsG76
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3 minutes ago, Louis D said:

Looks like a low budget sci-fi alien invader's ship descending from the sky. 😜

Magnified, they appear to be overlapping semicircular arcs.  Perhaps there's a dominant reflection from one side only going on somewhere?

Theres nothing visibly obvious... 

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It looks like this might be effect of color correction of objective lens - imaged with full spectrum mono camera.

https://www.cloudynights.com/topic/665210-question-about-using-a-bahtinov-mask/

Bahtinov mask acts like very low resolution diffraction grating and spikes are in fact low resolution spectra.

image.png.68c7b99ad5526cbb256c194bcc49dcd7.png

In above image multiple orders can be seen in each spike.

Position of each spike will depend on focus, and depending on color correction of telescope - focus can shift with wavelength creating little curls or at least waves.

You are not seeing that colors are affected as you are using mono camera.

 

 

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3 hours ago, MarsG76 said:

There was no scope/mount vibration that I could detect

How do you focus? Could they be oscillations of the scope after focusing by hand? Presumably the amplitude of the oscillations wouldn't need to be large to give that effect.

Ian

Edit. On second thoughts, I don't think you'd get that! 🙄

 

 

 

 

Edited by The Admiral
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On 15/05/2022 at 06:26, The Admiral said:

How do you focus? Could they be oscillations of the scope after focusing by hand? Presumably the amplitude of the oscillations wouldn't need to be large to give that effect.

Ian

Edit. On second thoughts, I don't think you'd get that! 🙄

 

 

 

 

Definitely not oscillations.... I think Vlaiv is on the money...

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On 15/05/2022 at 06:05, vlaiv said:

It looks like this might be effect of color correction of objective lens - imaged with full spectrum mono camera.

https://www.cloudynights.com/topic/665210-question-about-using-a-bahtinov-mask/

Bahtinov mask acts like very low resolution diffraction grating and spikes are in fact low resolution spectra.

image.png.68c7b99ad5526cbb256c194bcc49dcd7.png

In above image multiple orders can be seen in each spike.

Position of each spike will depend on focus, and depending on color correction of telescope - focus can shift with wavelength creating little curls or at least waves.

You are not seeing that colors are affected as you are using mono camera.

 

 

I think you nailed it... thanks for that link....
Images in the link definitely resemble what I was seeing... and I think that I didn't see this phenomenon before because I was focusing at a lower resolution image BIN 2x2 or 4x4... now I did it at BIN 1x1 with the 1:1 view centered in the frame and it all became curly.

I did a experiment today and again I saw the culrs but not at BIN 4x4.

Nice work.

 

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1 minute ago, MarsG76 said:

I think you nailed it... thanks for that link....
Images in the link definitely resemble what I was seeing... and I think that I didn't see this phenomenon before because I was focusing at a lower resolution image BIN 2x2 or 4x4... now I did it at BIN 1x1 with the 1:1 view centered in the frame and it all became curly.

I did a experiment today and again I saw the culrs but not at BIN 4x4.

Nice work.

 

I did not realized it until I found the images on the link I provided.

Now I'm thinking that it can be somehow used to visualize / measure color correction of refractor telescopes?

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5 minutes ago, vlaiv said:

I did not realized it until I found the images on the link I provided.

Now I'm thinking that it can be somehow used to visualize / measure color correction of refractor telescopes?

Yes thats true... although if a Takahashi has this issue than it must not be a big deal... chromatic abberations are definitely of no concern when imaging with a mono camera and the telescope is refocused for each filter.

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3 minutes ago, MarsG76 said:

Yes thats true... although if a Takahashi has this issue than it must not be a big deal... chromatic abberations are definitely of no concern when imaging with a mono camera and the telescope is refocused for each filter.

Well, all refractors have this issue - question is - how much of it do each of them has :D

If we could measure it without expensive lab equipment like monochromators and such - I guess it would at least be interesting / educational.

I think that proper shape for measurement is simply half grating looking like this:

image.png.c6032e9cb8ada5c809c38ea0a28f9069.png

(just half of aperture covered by grating and other half clear).

This animation shows why spike moves from one side to other as focus is reached and passed:

Simulation_of_a_bahtinov_mask_diffractio

Further from focus image resembles more that of aperture itself and as it focuses - it comes to a point. Similarly - if grating is just to one side - it will switch sides in out of focus image - that means it should pass thru a point in focus - much like horizontal grating does in above animation (starts on one side and then moves to other as it defocuses).

 

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24 minutes ago, vlaiv said:

Well, all refractors have this issue - question is - how much of it do each of them has :D

If we could measure it without expensive lab equipment like monochromators and such - I guess it would at least be interesting / educational.

I think that proper shape for measurement is simply half grating looking like this:

image.png.c6032e9cb8ada5c809c38ea0a28f9069.png

(just half of aperture covered by grating and other half clear).

This animation shows why spike moves from one side to other as focus is reached and passed:

Simulation_of_a_bahtinov_mask_diffractio

Further from focus image resembles more that of aperture itself and as it focuses - it comes to a point. Similarly - if grating is just to one side - it will switch sides in out of focus image - that means it should pass thru a point in focus - much like horizontal grating does in above animation (starts on one side and then moves to other as it defocuses).

 

Now this looks like a experiment calling out for attention... have you got a 3D printer? If so, print the mask and give it a try.

 

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1 minute ago, MarsG76 said:

Now this looks like a experiment calling out for attention... have you got a 3D printer? If so, print the mask and give it a try.

 

No (not yet, expecting to get one any time now :D), but I do have unused B-mask that I can use by covering angled grating part with something and 4" F/10 achromat :D.

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18 minutes ago, vlaiv said:

No (not yet, expecting to get one any time now :D), but I do have unused B-mask that I can use by covering angled grating part with something and 4" F/10 achromat :D.

Do it... and tag me on your posts about it.

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