So as to try a lens, I wanted to equip it with a Bathinov mask. Indeed, I am convinced by this technique, where the diffraction pattern generated by the mask is adjusted in order to focus.

A simple Internet search leads at the first place to the site 'astrojargon' and its mask generator:
http://astrojargon.net/MaskGenerator.aspx

After having introduced the parameters of the optic, the site generates a mask as a .svg file (Scalable Vector Graphics). The file can then be opened in a browser and printed; its size will be right to the millimetre.

Given the delicacy of the cutting work that I figure to provide, I created a second mask with a thicker structure. It is as simple as changing the settings of the generator.

Fig. 1 Bathinov Masks - Standard and with a wider structure.



The next step is to carefully cut out the footprint of the mask. Then it will be laminated, to ensure its good sturdiness.

Fig. 2 Standard mask after lamination



For mounting the mask, I used the same technique as to setup my previous Hartmann masks: a strip of plastic is bend in the form of the dew shield. Care should be taken to leave some play. Indeed, the assembly will be used in cold conditions and it will shrink by thermal contraction.

Fig. 3 A plastic (Laminated floor under-layer) is cut and adjusted to the form.



The pieces are fixed together by means of flexible glue, suitable for thermal expansion. After drying, refine the setup by beautifying.

Fig. 4. Mask gluing



At the first trial, the mask manufacturing technique has proved itself to be disastrous. Indeed, the plastic film turns translucent after being laminated. The mask is useless.

Fig. 5 View through a laminated mask.



Looking for an alternative, I printed then directly the mask on a transparent film.

Fig. 6 Masks directly printed on a transparent film.



A quick test of transparency shows that the new mask will fully play its role. It only remains to fix it on a support to be later slid on the dew shield.

Fig. 7 Transparency of the mask printed on film.



During testing, the new masks have proved to be fully functional. Whether the structure originally proposed by the generator or the larger structure, the masks were effective.

Fig. 8 First Lights. On the left, with the mask generated by default; right, with the larger structure mask.



A final note concerns the direct printing on plastic film: the mask is not uniformly black (Figure 7, left). Black painting it by hand is likely to increase the contrast during use, but actually the masks were shown to be directly usable.

Fig. 9 The mask printed on a laser printer does not suffer from a lack of contrast.


Alexandre