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Hartmann Mask holes

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#1 dhkaiser

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Posted 20 September 2021 - 03:49 PM

Years ago I used a Hartmann Mask to focus my C14 and it worked well. 

Now I want to make one for a guide scope,

but can't remember if the placement of the holes is critical. 

Don't think so but am unsure.

 

Follow up question, does the light source need be at infinity?

 

Anyone know?


Edited by dhkaiser, 20 September 2021 - 03:56 PM.


#2 Oregon-raybender

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Posted 20 September 2021 - 10:26 PM

Depending on the size of the guide scope, I would guess 4 to 8 holes would be fine, you could have one hole

that is larger in size to check for focus shifts. If the holes are within a 1mm it should be fine. You could make a few masks with holes displaced just to see if there is issues. It would be best at infinity

 

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#3 dhkaiser

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Posted 21 September 2021 - 05:35 AM

Oregon-raybender, thank you for the response.  Ok I will do 4 holes and see how it works out.



#4 MitchAlsup

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Posted 21 September 2021 - 11:53 AM

If the holes have different shapes, it is a bit easier to see when all have reached focus.

 

Triangle, Square, and Circle.


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#5 KLWalsh

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Posted 23 September 2021 - 07:01 PM

I have a Hartmann Mask that uses 3 equilateral triangles arranged in a equilateral triangle.

One advantage to this type is that, when the scope is right at focus, the triangles creates a Bahtinov type diffraction pattern. There’s a central diffraction spike that gives a good indication when it’s dead center, or not.

(Image from the internet; no copyright stated.)

 

 

 

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#6 Oregon-raybender

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Posted 24 September 2021 - 01:04 AM

Do you have an image of this pattern?

 

Starry Nightswaytogo.gif



#7 KLWalsh

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Posted 24 September 2021 - 09:30 AM

Do you have an image of this pattern?

Starry Nightswaytogo.gif


No, sorry.
You’d need to configure it for your own scope anyway, taking into account the size of any secondary obstruction and the size of your objective. I’d suggest making the triangular holes as large as possible to get as much light as possible.

#8 Oregon-raybender

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Posted 25 September 2021 - 12:52 AM

I was hoping to see a star image produced by this pattern. I can make one on my laser and design different patterns of shapes.

 

Starry Nightswaytogo.gif




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