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Thread: Hartmann Mask - project and results (long and image heavy)

  1. #1
    Arch-Σigmoid Ausphotography Regular ameerat42's Avatar
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    Hartmann Mask - project and results (long and image heavy)

    (This was a recent project done between plumbing and building an awning.)

    A Hartmann Mask is a simple focusing aid used on telescopes and telephoto lenses to aid focusing.
    Its main use has been in astrophotography, but it can be used anywhere. It is particularly useful with
    cameras without live view, but also to properly focus celestial objects for optical viewing - when no other
    means are handy. There are many references to Hartmann Masks, and they can be made in many different
    patterns here is one:
    http://www.astropix.com/HTML/I_ASTRO...METHODS.HTM#HM

    The one shown here was recently made to help focus the borrowed MTO 1000/10, a Russian (actual)
    f=1100mm mirror lens operating at f/10.

    Construction
    The materials were simple - a washed out Bushells coffee tin, 500 gram size (made of tin ends and lid and
    cardboard cylinder). The tools used were almost as simple - a box-cutter knife; a small rigid wooden box
    (about 15 x 8 x 6 cm); a computer printer to make a precise template of the mask; a 30mm diameter metal
    hole-cutter saw and battery-powered drill; matt black and gloss black spray paint.

    The small wooden box was the rigid platform to hold the box-cutter level and in the same place, while the
    coffee tin was placed on a table and rotated by hand past the protruding blade. This made a progressively
    deepening cut around and through the cardboard cylinder. Two such cuts were made, one to get the
    container to the right height and the other to later fashion an inner sleeve. The problem was to use materials
    that nearly matched the diameter of the lens and of its extended sliding lens hood. The outside diameters of
    the lens hood and coffee tin were similar, as were the inside diameters of the lens hood and coffee tin. The
    inner sleeve - used to join the two items - was slit lengthwise to remove a small, measured width of
    cardboard. This was then slid firmly into the extended lens hood. The remaining coffee tin then slid firmly
    over this resulting sleeve.

    The coffee lid - made of tin - was used to form the actual mask, by cutting two circular apertures into it. To
    find the optimal design, an on-line formula was used. It can be found here:
    http://www.billyard-ink.com/Hartmann.shtml

    To maximise the amount of light entering the system, the two apertures of the mask required the maximum
    possible diameters. This came out to 30 mm per aperture. The lid was stamped with the Bushells logo and
    its surface was also stepped, so the apertures had to fit inside these features. Ideally - for maximum
    resolution - the outer edge of the apertures would have to match the outer edge of the objective mirror of the
    lens. In the end, the two measurements were pretty close.

    In the construction, the tricky part was using the hole cutter saw and drill, as the pitch of the teeth was
    rather coarse for the thin metal of the lid. Ideally, the lid should be firmly sandwiched between to pieces of
    plywood, but becuase of the size and different levels of the lid, only a firm baseboard - and much care -
    was used. The result was quite successful just the same, but care should be taken with the power tools in
    particular. Very low speeds and a very low clutch setting were used.

    Results
    In between rain periods the resulting mask pieces were painted - matt black on the inside surfaces, and
    gloss black on the outside. The finished result can be seen in the images below.

    Fig 1. Interior of Hartmann Mask.


    Fig 2. Mask mounted on MTO1000 lens


    During overcast weather the mask was assembled and attached to the mirror lens and tested on some distant
    trees. The results were successful. The out-of-focus (OOF), double image is typical of what is seen before
    focus is reached.

    Fig 3. Distant tree through mask: Triptych of OOF image, focused image, and image with mask removed.


    Today the setup was tested on the sun - USING A PROPER AND SAFE SOLAR FILTER. Again, the
    results from the mask were successful. The Baader film ND 5 (100,000X) solar filter is shown attached to
    the mirror lens in the following figure.

    Fig 4. Baader film solar filter attached to MTO1000 and camera
    [imghttps://public.bay.livefilestore.com/y2psoUPQLKIOY45yGMyMwBPjkhFbKIgImxcWUC99ii4tDNsGAYd3n12ardC906_wRvG3y5__uj8YJKrdxv7ss8quDCQdxR2JrZEEGMSoNwSgeo/SDIM0475c.jpg?psid=1[/img]

    Three phases of the focusing procedure through the Baader filter are shown in Fig 5, along with
    AN UNEXPECTED PROBLEM that occurred when the mask was removed for a normal, full aperture shot of
    the sun through the solar filter. A BRIGHT STRAY PATTERN OF LIGHT marred the image.

    Fig 5.

    This was later found to be due to a PINHOLE size break in the filter material, rendering the filter useless in
    that state. The filter will be made useful again by blocking the pinhole with a small amount of opaque
    material, as this would have negligible effect on the image. It does, however, serve to show the tremendous
    filtration required for safe viewing and photography of the sun. The following figure shows the small hole
    that caused this effect.

    Fig 6. Pinhole break in Baader film filter.


    For comparison, an earlier image of the sun using the same equipment is shown below.

    Fig 7. May 2013 Partial Solar Eclipse seen from Sydney


    An image of the sun through the repaired filter will be posted as soon as possible.

    Am.
    CC, Image editing OK.

  2. #2
    Former Username : Wetpixels Dazz1's Avatar
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    Fascinating! Sounds like an experiment worth trying myself. Thanks for posting this.
    80D, 600D, Sigma 150-600mm F5-6.3 DG OS HSM Lens - Contemporary, Sigma 18-250mm 1:3.5-6.3 DC Macro OS HSM lens, EF-S 18-55mm f3.5-5.6 IS STM lens, EF-S 18-55mm f3.5-5.6 IS II lens, EF-S 55-250mm f/4-5.6 IS II lens, Yongnuo YN500EX flash, Velbon Sherpa 5370D tripod, PH-157Q head, Klika W1003 monopod, AF Macro Extension tubes, LED Ringflash Software: DPP4, Gimp, UFRaw, Rawtherapee, DigiKam, Hugin

  3. #3
    Arch-Σigmoid Ausphotography Regular
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    Quote Originally Posted by wetpixels View Post
    Fascinating! Sounds like an experiment worth trying myself. Thanks for posting this.
    You're welcome for that. Hope it works.

    And...
    My photos always look best after a few Islay single malts - preferably Lagavulin
    now I know how you arrived at your user-name. I'm partial to a wee dram of Chivas Regal betimes, and
    I don't mind the peaty reek of Laphroiag s1ngle malt. At its price, demanding a less frequent sampling.
    (Dr)Am.

  4. #4
    Former Username : Wetpixels Dazz1's Avatar
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    Quote Originally Posted by ameerat42 View Post
    now I know how you arrived at your user-name. I'm partial to a wee dram of Chivas Regal betimes, and
    I don't mind the peaty reek of Laphroiag s1ngle malt. At its price, demanding a less frequent sampling.
    (Dr)Am.
    Lagavulin is from the same island as Laphroiag, and has a similar taste. That and Ardbeg make up the three I choose most often. When pressed for cash, Bailie Nicol Jarvie is a nice blend that has that lovely peaty taste as well.

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    Ausphotography Regular
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    Oh yeah - those Islay single malts....
    David

    Nikon D810
    Nikkor AF-S 24-120VR, Nikkor AF-S 16-35VR, Nikkor AF-S 70-300VR, Nikkor AF 50 f1.8
    Tamron 90mm Macro

  6. #6
    Arch-Σigmoid Ausphotography Regular
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    Quote Originally Posted by davidd View Post
    Oh yeah - those Islay single malts....
    You could say that again, and then use the Hartmann mask to help you focus!!!

  7. #7
    Arch-Σigmoid Ausphotography Regular
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    Well, here is a pic of the sun with the above lens after repairing the solar filter (see pic above). It HAS NOT affected the performance
    of the lens.
    (I had forgotten about it after processing because of plumbing problems and many trips under the house)
    Am.

    NOTE: it is at cropped full size.

    Sun through "repaired" solar filter.

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