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With a guided telescope long exposures are possible which would collect much light. This would allow to use masks which would not work for unguided photo lenses on ordinary tripods. That much light also is not available for the live preview mode of DSLR cameras. Some masks that didn't work for me and my equipment might work for others and their equipment just perfectly.

Each optics-detector-system will have its own most suitable focusing mask. The "best" mask certainly would depend on focal length, aperture, exposure conditions, detector properties, etc. But also mechanical properties of the lens or telescope are important. For my 1000 mm lens I got the impression that I just could not take full advantage of some focusing masks just because I could not adust the focus fine enough. My focusing wheel is indeed no micrometer screw. In the picture above the focusing wheel is turned to about 85 m and the focusing bellows is extended accordingly (below to the left). There is need for a focusing unit on micrometer scale ...

Diffraction masks (Bahtinov, Carey, Oleshko) are more easy to use than imaging masks (Scheiner, Hartmann, Mrotzek) for a certain focal length range. Imaging masks work independent from the focal length and I would expect that they would be better to use for very big focal lenghts, because I expect that the imaged star also would be bigger at very high magnifications. Diffraction masks introduce dispersion to the incoming light and therefore would only work well for the focal length range for which they have been designed; for longer or shorter focal lengths a new diffraction mask with another dispersion would be more suitable. I think, diffraction masks perform quite well for focal lengths that are still too small for imaging masks. Combining both types does not work so far fo rme because the image and the central diffraction peak do overlay and have comparable intensities (best thing would be to suppress the central diffraction peak completely).

There is a big variety of possible layouts for diffraction masks, but at the end it is always about angles and straight lines that build the grating. It is not so important how the lines are actually grouped. But there are some guidelines to find a good layout. Lengthwise the grating bars should stop before reaching the next quadrant to ensure maximal spike movement while focusing. In dispersion direction the center of the grating should fall together with the center of the mask; otherwise there is an initial offset in the diffraction pattern.

I like well defined spectra more than diffuse spikes. Small grating constants and directing all intensity to only one order of the spectra produces nicely looking images in live preview mode and allows to use even fainter stars for focusing purposes. Disadvantage is that 0th and 1st order have a big separation. For small grating constants punching the masks can help in having longer and better defined spikes filled with spectras (the diffraction peaks move closer to the center and the spikes seem to be more continues).

What does not work well so far is using a shift along the dispersion direction of the spectra for alignment purposes. This is because such diffraction masks would have an unsymmetric grating pattern and because of this the spectra would already have an initial offset which would destroy the symmetry. Spectra could be useful in case neighbouring spikes can be better distinguished because of the color contrast in case different grating constants are used within the spikes. Beside this the spectra are only nicely looking features and the images could be taken as well in b/w mode. For alignment only the direction perpendicular to the dispersion is important.

The website astrojargon.net creates nice Bahtinov masks but can also be used to build own mask layouts that are based on diffraction gratings (for own masks: calculation of a Bahtinov masks with appropriate parameters, build of own mask by re-using parts of the converted bitmap of the mask). Use high resolution images for the templates to reduce rendering and anti-aliasing artefacts in the templates.

Random noise on the masks seems to be no problem. Only for extremely noisy masks the spikes are shorter and higher order spectra disapear. Already a little blurring of thin grating bars with high spatial frequency has a big effect: the spikes vanish. On single thick structural bars the blurring has no big effect. Therefore it is better the just do not use structural bars which is easily possible with transparent overhead sheets.

Print in black ink only rather than in color mode. But having dark black masks is not extremely important, they should be just dark "enough". For me even some grey experimental masks worked fine. Print on more transparent overhead sheets than on less glossy once.

Spectra are sensitive to vibrations and long exposure times (star trails). To which extend a spectrum is affected would depend on its r

The best of my images from a long session in average conditions during the transit. Io is just about to catch up with the Great Red Spot.

This stacked image was taken with the mono DMK21AU, 2.5x Televue Powermate, Astronomik 1.25" colour filters, manual filter wheel and 12" Meade LX200 f10 SCT. Final image is a stack of 600 red, 600 green and 550 blue in RRGB combination, processed twice, once for best Jupiter alignment and one for best Io alignment and then the best Io combined with the best Jupiter.

Peter