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The Bahtinov mask
An accessory to help with focusing Photographing the night sky, like any other subject, requires precise focusing. As with a telescope, with a DSLR or a CCD camera, the focusing tolerance (Tf) does not exceed a few hundredths or thousandths of a millimeter and increases with the focal ratio (for example Tf = ±0.176 mm at 550 nm for a telescope at f/D = 20 and Tf = ±0.0034 mm for a f/2.8 lens, see this form). The slightest deviation therefore results in a blurred image. When the optical system does not allow automatic focusing because the subject is too pale or too small, like stars for example, an alternative method must be found to ensure focusing. This is where the Bathinov mask comes in, invented in 2005 by the Russian amateur astrophotographer Pavel Bahtinov (en translated into English), although similar solutions existed well before as the Hartmann mask displaying three round holes or preferably triangular that make one when focusing is perfect. That said, if you get sharp pictures or videos of the Milky Way for example by using the autofocus or by focusing manually, so much the better because in this case you do not need the Bathinov mask and you have saved between 25 and 200 € depending on the model. In all other cases, this mask will be useful to you and the following review may interest you. The Bahtinov mask is an accessory that is placed in front of the optics (refractor, telescope, astrograph or camera lens) in order to facilitate manual focusing. Most astronomy stores sell this accessory which is really cheap. In France, Pierre-Astro for example offers circular Bathinov masks for refractors and telescopes.from 65 to 340 mm in diameter (23.50 € for the 65-100 mm diameter). They are plastic but more durable than their metal version. He also sells high quality models. Astroshop and Optique Unterlinden among other companies also sell Bahtinov masks. But you can also make it yourself with a 3D printer (see end of page).
As illustrated here, the Bahtinov mask consists of a pattern of horizontal and inclined lines regularly spaced but intentionally forming a periodic asymmetrical pattern. Without a mask, the image of a bright star at the focal plane forms an Airy disk. But when the mask is placed in the path of the star beam, the Airy disk transforms into a Fraunhofer diffraction pattern with interference fringes in the shape of a large star whose branches or egrets become deformed and asymmetrical out of focus, displaying off-axis chromatic aberration. The narrower and tighter the parallel lines, the more discontinuous and iridescent the egrets are. Simply adjust the focus until the egrets are perfectly symmetrical and the thinnest (or on other models that the middle branch is perfectly centered between the other two branches). Of course, once the focus is obtained, the mask must be removed to take the picture (some people forget it). The Bathinov mask can also be used with a DSLR, whether it is fixed behind an astronomy instrument or used independently with its own lens. To read : Variation of Bahtinov mask, Cloudy Nights In French : Le masque de Bahtinov expliqué, SAHavre Mise au point et masque de Hartmann
Due to the darkness, usually the DSLR autofocus cannot focus on the stars; in active mode (it emits ultrasounds or an infrared beam) it does not find the surface of the subject and in passive mode either the rangefinder cannot correlate the phases or the collimation cells do not find a contrast area because the stars are too small and too faint (it only works for the Moon and on bright planets photographed with a telelens). In this case, you have to deactivate the autofocus and focus manually. When focusing manually at infinity, it is not enough to put the lens ring on infinity because mechanically the position of the ring is not very accurate. Thus placing the focus ring on infinity can work but not always. And a fraction of a millimeter next to the infinity sign, it is no longer infinity with a blurry image assured. Frustration guaranteed. In these conditions, it is necessary either to proceed by tests which is long with a lot of wasted time or to find a trick to achieve the focusing the first time. This is where the Bathinov mask comes in.
When the mask is attached to the lens, you can display the picture on the Live View monitor of the DSLR and observe the diffraction egrets as illustrated on the right. While the classic Bathinov mask shown above works with optics with a large focal ratio, including small telescopes and telelenses, it is difficult to see the diffraction egrets in Live View mode with wide-angle lenses, even at maximum zoom (6 or 12x depending on the model), not to mention the possible electronic noise of some old LCD monitors compared to the new generation of OLED. The only option is to display the picture on the DSLR monitor in playback mode and to zoom in hoping to spot a bright star to check the focus. To do so, after the shoot, you need to activate on the DSLR the function that allows to enlarge the picture (often by pressing the Set or button and a dial wheel placed on the back of the camera) to zoom in a bright star in the field up to 31x on some models. Some people also use a 10x photographer's loupe (see the video below). Image magnification is no longer necessary if you use a telelens of at least 135 mm of focal length. But this method does not work with wide angle lenses (< 50 mm of focal length) because the pattern of the classic Bathinov mask is too large and too much light passes through. But there is a solution. Currently on the market there are four formats of Bathinov mask for APNs and they are made of two different materials which are reviewed here. Bathinov mask made of glass or plastic In order to be able to use the Bahtinov mask with APN lenses, some manufacturers have found a solution by developing a Bathinov mask which pattern is laser-etched in a transparent glass (Focus on Stars) or in an acrylic plate (2filter, KASE, Lonelyspeck), sometimes called organic glass or optical resin. Depending on its quality, this plastic transmits 84 to 90% of light and is relatively shock resistant but these masks are not treated anti-reflective or hydrophobic and remain sensitive to scratches. They must therefore be handled with care. KASE pompously calls this plastic "Wolverine glass" and 2filter calls it "KW glass", a way to deceive customers. Note that several KASE branches including in their Amazon store (uk, de, ca) and even KASE Europe offer "Wolverine filters" made of mineral tempered glass. But they are only offered for photographic filters such as the ND8, but not for the Bathinov mask. Confusion is therefore possible. Indeed, KASE branches do not all support the same items. In addition, in the glass industry there is a mineral glass called "Wolverine" used in particular to manufacture windows and French windows. A Bathinov mask made of mineral glass is of course more transparent and more efficient than a plastic model, but this latter works too although its resolution is lower and it is much less resistant (as it is also true for eyeglass lenses, filters, including polarizers, and diffraction ratings made from glass vs. unbreakable plastic models). A
voir : How to focus the Milky Way under the night sky | Milky Way Photography,
Aaron King, 2018 How to achieve perfect Focus in 90 seconds using a cheap Bahtinov Mask, A&D. Grelin, 2019 Kase Filters Bahtinov Mask Review, Peter Zelinka, 2021 Square Bathinov mask with filter holder For DSLRs, this mask is generally available as a square "filter" of 100 or 150 mm on each side to be placed in a filter holder that screws onto the lens. This model is easier and quicker to remove when shooting than a screw-on filter. But we will see that there is an alternative, the magnetic attachment. KASE Benelux in Belgium offers a square Bathinov mask K100x100 of 100 mm a side (69 €) made of Wolverine glass, an organic glass, in other words in plastic, for filter holders. But according to some users, it should be not very effective with very bright wide-angle lenses, to check. Lonelyspeck in the USA offers a square plastic Bathinov mask "SharpStar2" of 85, 100 and 150 mm on each side ($69 or 62 € for the 100 mm) for filter holders. It works well on wide-angle lenses at a small aperture (f/1.2- f/3.5). It is compatible with Cokin, Lee and other Formatt-Hitech filter holders. Focus on Stars in Hungary, outside the Euro zone, offers a 100 or 150 mm square Bathinov mask for filter holders for wide-angle lenses (WIDE model for $125 or 112 € for the 100 mm) and telelenses (TELE model for $110 or 99 € for the 100 mm). This mask is laser-etched in a tempered glass and treated anti-reflective, scratch-resistant and shock-resistant. Currently it is the sole model made of mineral glass. According to its manager Gábor Takács, with video support, the egrets are visible on the DSLR monitor even with a wide-angle lens. He confirmed this to me again by email in 2024: "The WIDE model is the only one in the world designed for wide-angle and ultra-wide-angle lenses!" Some amateurs doubt that the egrets are visible with a wide-angle lens but they have not tested it. A video presented on the Focus on Stars website proves that the egrets are clearly visible through a 14 mm f/2.8 wide-angle lens around the star Arcturus of apparent magnitude -0.04 but on condition of enlarging the image on the DSLR monitor. Let us add that the Focus on Stars model received a favorable advertisement in the Japanese magazine "Tenmonguide" in May 2023 (a double page) and 80% of sales are in the United States (which is why prices are indicated in dollars and not in euros). This mask is ~38% more expensive than "competing" plastic models. But according to Takács, "it cannot be compared to ordinary Bathinov masks" (implying 2filter, KASE and Lonelyspeck), but this remains to be proven. K&F Concept (via Amazon.com, also on Amazon.fr) offers an excellent 100 mm aluminum filter holder (48 €) that Takács uses. Unlike other brands such as Cokin, it comes with several adapter rings (from 49 to 82 mm in diameter) to screw onto the lens. This filter holder is also versatile because it can also accept any other filter (neutral, graduated, anti-light pollution, etc.) and even several filters if necessary.
The solution with filter holder has two drawbacks : first the price of the Bathinov mask for a DSLR which can vary from simple to double if not more depending on the brand and the quality and which is therefore on average very expensive, and then the risk of vignetting with the shortest focal lengths of wide-angles, although the K&F Concept filter holder is only 5 mm thick. If vignetting occurs, the solution is to remove the filter holder at the time of shooting. This vignetting does not appear with magnetic circular filters (see below). Read also the review of Bathinov masks for square filter holders published by Royce Blair in 2020. Magnetic Bathinov mask Apart from holding the Bathinov mask freehand in front of the lens (some do it but it is amateurism), the only alternative is to use a mask fixed magnetically (with magnets) on the lens. Two models exist : the square model for magnetic filter holder and the circular magnetic model with its adapter ring, the two adapters screwing onto the lens. Magnetic square filter holder KASE Benelux in Belgium offers a K100-X K9 magnetic square filter holder with thread up to 86 mm in diameter. It is suitable for standard square filters with a width of 100 mm and a thickness of 2 mm and 1.1 mm such as the afore mentioned KASE K100x100. It also accepts circular magnetic filters and comes with 4 adapter rings from 67 to 82 mm in diameter. Disadvantage, its price (€169) without counting the Bathinov mask. Circular ring and mask K&F Concept (via Amazon.fr), KASE Benelux in Belgium and KASE Europe in Germany offer a circular magnetic ring in aluminum that screws onto the lens. It has a thin profile and is available in different diameters to support the different magnetic filters (20 € for the 77 mm model). In this configuration, it is imperative to use a magnetic and circular Bathinov mask. Currently there are only two manufacturers and both sell masks made of plastic. KASE Europe in Germany offers a circular magnetic Bathinov mask made of Wolverine glass, plastic, of 77 mm (59.90 €) or 82 mm in diameter. Called "ROUND Magnetic Precision Focus Tool Astro Bathinov Mask", it comes without magnetic adapter sold separately (12.90 €), for a total of 71.90 € plus delivery. It is the cheapest solution. 2filter (aka KASE USA) in the United States offers a circular magnetic Bathinov mask made of KW glass, in optical resin and thus in plastic, with its adapter to screw onto the 77 mm diameter lens ($95). Called "Bright Star Precision Focusing Tool", it is mounted in an aluminum alloy frame. The adapter ring accepts all magnetic filters. Shipping costs to Europe by USPS Priority Mail amount to $47.95, for a total of $142.95 or ~128 € (the value excluding fees being ≤ 150 €, VAT is not due, otherwise it amounts to between 12-17% depending on the country). To
see : How to Focus on the Stars and
Milky Way (6 Methods), Alyn Wallace To read : Kase Filters Bahtinov Mask Review, Peter Zelinka, 2021 A Bahnitov Mask Solution for DSLR Users, Cloudy Night
For DIY enthusiasts, circular magnetic filters cannot be removed from their frame (as can circular acrylic filters that are secured by a ring in their filter holder). Bahtinov mask to screw-on Although a screw-on model is far to be convenient and ideal to check the focusing, KASE Europe in Germany offers a double-threated 77 mm KASE Bahtinov mask also called "Bright Star Precision Focusing Tool" in Wolverine glass, in plastic, set in an aluminum frame (59 € or $63 on ebay). It is treated against UV and water resistant. The same model is available in KASE store on Amazon.com ($67.50). The only review posted on Amazon with pictures indicates that it gives good results on telelenses of at least 100 mm of focal length. Note that KASE Belgium also offers a threated 77 mm Bahtinov mask in Wolverine glass, plastic, but it is combined with a blue-magneta filter to neutralize the yellow-orangeous color of artificial lighting (119 €). It is called "Kase Wolverine circular neutral night Kit 77mm screw filter". But this colored filter has not the same effect as a LPR filter. Bathinov mask made with a 3D printer It is possible to make a coarse Bathinov mask yourself for transtandard optics and teleobjectives (because below 50 mm of focal its interest remains to be proven) with a plastic plate (like polystyrene or matte black ABS) for less than 20 €. The ideal is to make the mask from a black and white image with contrast using a 3D printer or a machine tool as explained by a user on the Cloudy Night forum. You can also buy a Bathinov mask from Azur3DPrint in France. His manager, Vincent Duchčne who is also a skilled astrophotographer, offers a model specially adapted to the Samyang 135 mm f/2 ED telelens. This lens measures 77 mm in diameter. The mask is made with a 3D printer and offered at the price of 17.50 €. The mask protected on the edges by felt is simply placed on the lens hood in tulip shape. Its overall size without felt is 97 mm in diameter. Azur3DPrint can also print custom masks suited to your lens. Due to its design, this coarse mask is not suited to wide-angle lenses. The reason is always the same : stars are too small to be visible on the DSLR monitor (a picture must be taken and checked with the playback zoom at high magnification). |
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