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Focussing

Focussing with a Bachtinov mask

A Bachtinov mask is a piece of material with a patern of slits which will be placed in front of the telescope (see below images as example).



Mask for my Megrez72

Mask for my 15cm Newton

Click on the image for a bigger version


As the patern of slits depends on the apperature and focal lenght of your scope you need to make (or buy) a mask for each of your scopes. You can make your own design through this webpage and make one yourself, as I did.

When you move into focus you see the central spike moving in respect to the cross-spikes. When the central spike aligns exactly between the cross-spikes you are in focus (see below image). These images are made with my Canon 10D DSLR through my 150mm Newton scope and the corresponding Bachtinov mask.


Click on each image for a bigger version

The accuracy of the mask depends on the telscope used because of it's "depth of focus". Within the depth of focus, the spot size of the Airy disk stays the same and the telecsope can be considered focused (See below image)



This "depth of focus" is depending on the wavelenght of the light and the Focal Ratio of the telescope according to the following formula :

Depth of Focus = 4,88 x N2 x Lambda


For my 150mm Newton (N=6,3) the "depth of focus" would be 106μm, I used a wavelenght of 550nm as it lies somewere in the middle of the visible spectrum.

I use an 1:10 microfocuser with a resolution of 3,4μm/degree. This means tht the "depth of focus" in this example is 106/3,4 ~ 31° rotation of the microfocuser which gives you enough freedom to focus.

Be aware that the faster your scope is the smaller the "depth of focus" will be.

Examples :


Telescope at N= 4 --> Depth of focus= 43μm --> 13° rotation of the microfocuser.
Telescope at N= 8 --> Depth of focus=172μm --> 50° rotation of the microfocuser.
Telescope at N=16 --> Depth of focus=687μm --> 202° rotation of the microfocuser.


Above I considered the "depth of focus" only for 550nm as a good compromize for a DSLR color camera but it's clear that the "depth of focus" depends on the wavelenght of the imaged light. When you are imaging with a monochrome CCD camera through RGB interfrence filters the "depth of focus" is different for each filter.

For example we use a telescope with a focal ratio of 5 for imaging. The "depth of focus" for each color will then be :


Red (Lambda = 650nm) --> Depth of focus = 79μm.
Green (Lambda = 550nm) --> Depth of focus = 67μm.
Blue (Lambda = 450nm) --> Depth of focus = 55μm.


You see that the "depth of focus" in the blue part of the visible spectrum is the smallest which means that the focussing is most critical through this filter. If your imaging system is well focussed through the blue filter than it's also well focussed through the green and red filter.

See below a graph "Depth of focus" versus de wavelenght for several focal ratios.




Focussing with a Hartmann mask

A Hartmann mask is a piece of material with two or more holes which will be placed in front of the telescope. When the telescope is out of focus you see multiple star images (as much as there are holes in the disk). When you move into focus you see the multiple star images moving towards each other. When there is only one star image left you are in focus (see below image).


No focus --> Focus


It's very difficult to reach perfect focus because the star is several pixels in diameter.


Focussing on spikes

If the telescope is pointed on a bright star and it is in perfect focus you will see the so called diffraction spikes. When your telescope is not in focus you also will see these spikes but than dubble (see below image).


No focus --> Focus


In order to have your telescope perfectly focussed you should move the focustube until you see the four spikes each ending in a pinpoint.
© Copyright Rob Kantelberg
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