Astrophotography The Mask Bahtinov

Astrophotography The Mask Bahtinov

The mask is Bahtinov nell astrophotography widely used to a focus of their star-shaped objects as accurately as possible with a special filter (no glass, a simple mask of plastic or cardboard). Invented in 2005 (but only in 2008 that saw its realization in its current form) by Pavel Bahtinov (amateur Russian), is an improvement in the form of Hartman, another instrument widely used in photography of the stars of the sky.

In practice (please refer to the page relative to the mask of Hartman for more details), Bahtinov realized how increasing the number of linear edges allowed in the mask of Hartman to increase the amount of light distribution on the ray diffraction. The mask Bahtinov shows the half of the target (or better half of the mask) under the strips parallel to the central beam and the second half with other bands angle of + 40 ° and -40 ° (depending on the quarter of the area) as compared the first half. Structure of the mask Bahtinov
The mask Bahtinov shows the half of the target (or better half of the mask) under the strips parallel to the central beam and the second half with other bands angle of + 40 ° and -40 ° (depending on the quarter of the area) as compared the first half (image below).
The “pitch” of the slots (or the size of the coupling strip full + empty strip) is a function of the focal length in use and can be calculated using the following formula: pitch = focal length / factor Bahtinov.

The focal length is expressed in millimeters, while the factor Bahtinov, must be found empirically, is a number between 150 and 200 (typically using an average value, namely, 180). The value of the step, as you can well understand, influence not only on reaching the mask (there are-made templates, but many prefer to build the home, on the other hand a little cardboard!): The shorter the thinning step, it will for the strips (with a step 8 you have to 3mm open area intersperse with 3 mm strip of cardboard). In some cases, when the focal length of the lens is very short, “used” to step to multiply 3 in order to simplify the implementation phase.How to use the mask Bahtinov

Once built or bought the mask Bahtinov fits your perspective, you do not have to do it is to use.

It is a set of rays “created” exactly the mask can be applied to the lens: 6 rays that converge towards the central point, after which the two “supports” black and 6 more points of light around a further bright spot in the middle. When you see this then you will have achieved the best possible focus.To change the focus, you’ll notice how the rays will tend to move sideways in a very obvious. Less attention you have, the less light and less accurate the image will “target” of the mask you should get. Just to clarify the concept, that’s what happens in a situation of not focusing for perfect focus: