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Baader AstroSolar Filter for Telescopes - 120mm Filter Aperture # ASTF-120

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Important Note: For safety reasons, we cannot accept returns of this item once the sealed filter box has been opened. Please measure your optical instrument's tube diameter accurately BEFORE ordering a filter to make sure that you order the correct size.

  • Baader # ASTF-120 AstroSolar solar filter with temperature-compensated cell for all telescopes (refractors, reflectors, & catadioptrics)
  • 120mm filter aperture
  • Suitable for high resolution, high magnification applications over a wide range of temperature conditions
  • Includes mounting accessories (centering bolts, sliding washers, fasteners, and safety straps)
  • Filter can be attached to items with an outside diameter (not aperture) of 140 - 180mm (5.51 - 7.08")
  • Sold individually

ASTF – Baader Solar Filters for All Telescopes (Refractor, Reflector, and Catadioptric)

Baader's ASTF line of solar filters permit safe visual observation and solar photography with all types of telescopes. These telescope filters feature a temperature-compensated cell design to provide high-resolution views or images and are recommended for use with high-end amateur telescopes.

This family of solar filters is made in the most elaborate way, and its construction and design is superior to that of the ASBF and ASSF line of Baader filters. Ideally, AstroSolar film must not be put under stress, either during mounting of the film or during any temperature change. For this reason the ASTF filter features a temperature compensated cell. The film itself is cemented onto an injection molded ring, and the substrate of that ring has the same thermal expansion properties as the AstroSolar Filter material itself. This free floating film-carrier ring is held onto the aluminum frame with the help of another holding ring made of fiber reinforced plastic to provide maximum security against breakage of the cell. This temperature compensated cell enables the AstroSolar filter material to retain its excellent optical quality at any temperature – be it -30°C or +50°C (-22°F or +122°F). 

As a result, this filter is suitable for high magnifications and detailed observation of individual sunspots. You will experience no astigmatism during winter observations or with temperature changes in the summer. The filter comes with two sets of 3 injection molded centering-bolts (length 38mm) with threaded brass inserts to center the filter-cell in front of the tube opening. These bolts come in two diameters, 10mm and 20mm.

Important Note: SCT Users must only mount the filter to the outside of the tube (the limited depth of the SCT front ID prevents adequate engagement of the centering bolts).

ASTF-120 Solar Filter Specifications

  • Item number: ASTF-120 or 2459312
  • AstroSolar film aperture: 120mm (4.72")
  • Outside diameter clamping range (with included 10mm bolts): 150 - 180mm (5.90 - 7.08")
  • Inside diameter clamping range (with included 10mm bolts): 170 - 200mm (6.69 - 7.87"). See additional info below.
  • Outside diameter clamping range (with included 20mm bolts): 140 - 170mm (5.51 - 6.69")
  • Inside diameter clamping range (with included 20mm bolts): 180 - 210mm (7.08 - 8.26"). See additional info below.
  • Suited for aperture range: 110 - 130mm (4.33 - 5.11")
  • Outside diameter of filter cell: 210mm (8.26")
  • Weight: 5.5oz / 155g

Additional Filter Details and Features

  • Made of original Baader AstroSolar Safety Visual Film (ND 5.0), cell mounted, and quality inspected by skilled members of Baader's staff.
  • The AstroSolar filter material is mounted onto a plastic ring where the plastic ring itself works as stabilizing element for the filter cell. This still plastic ring is mounted onto the aluminum front frame. The thermal expansion coefficient of the plastic and the film are similar.
  • Clear white IR-rejecting front ring with reinforcing raised edges along the outer and inner circumference, designed to protect the solar filter material from contact with any sharp metal edge.
  • Includes two sets of 3 injection molded centering-bolts with threaded brass inserts to center the filter-cell in front of the tube opening (10mm and 20mm diameter; 38mm length). This fastening system keeps the filter snugly in place and centers the filter. Each bolt features injection molded rubber grip-faces, shaped to easily slide onto any telescope tube but hard to come off.
  • No tool mounting and adjustment of the centering bolts to fit a wide variety of mechanical tube diameters. Bolts are made to contact the outside of the optical instrument tube for most applications.
  • For some optical items (such as refractor and Newtonian reflector telescopes), you may be able to attach the bolts on the inside of the instrument tube. However, this requires a clear depth of about 20mm on the inside for adequate engagement of the centering bolts which many instruments (like binoculars, camera lenses, and SCT/Mak-Cass telescopes) do not have.
  • Includes non-rotating sliding fasteners made from aluminum that hold the centering bolts.
  • Additional Phillips screws are integrated into the sliding fasteners so that the centering bolts can be firmly fastened with a screw driver if preferred by the user.
  • Index scale provided beside each adjustment slot - adjacent to each sliding fastener - for fast and repeatable adjustment of the filter cell onto different telescope tubes.
  • Black sliding washers supplied to securely cover the full length of the adjustment slots in the aluminum frame – for blocking sunlight from passing trough the filter cell beside the filter area.
  • ASTF filters come with rubber plugs to be used as a cover for closing unused adjustment slots in the aluminum frame.
  • Three security straps included to secure the position of the filter cell with Velcro-adhesive pads. A safety feature to absolutely ensure a secure fit in front of the telescope under virtually all circumstances.
  • Securely packed in a cardboard box with stiff cardboard inlay for safely shipping the filter; Instruction booklet included.

About Baader AstroSolar Film and Solar Filters

The world's finest objective solar filter material - Baader Planetarium's AstroSolar Safety Film - is now available in a complete range of high-quality mounted Baader Solar Filters. Their innovative design and construction enables each filter to fit a wide range of instruments - from high end telescopes, to spotting scopes, camera lenses, and even special versions to fit binoculars. Each filter is precisely mounted at Baader onto special cells, guaranteeing a stress-free mounting that preserves the full optical quality of the AstroSolar film.

Baader Solar Filters provide a professional and secure solution to observe and image the Sun using AstroSolar Safety Film. Many ready-made filters stretch the filter material like a drumhead in order to make it look like a piece of glass. However, this absolutely destroys the image quality, rendering the film useless for high magnification work. AstroSolar is mounted entirely stress free onto Baader's filters in order to perform like a high precision plane-optical window. Even though it may seem counterintuitive, Baader AstroSolar film must show slight ripples! When the film is put under stress by an improper cell mount, its optical quality is degraded and it performs just like a common coated glass-filter made of untreated float glass.

In its September 2000 issue, Sky & Telescope magazine proclaimed Baader Planetarium's AstroSolar Safety Film to be "the new standard in solar filters." AstroSolar Safety Film delivers white light views of the Sun that are unmatched by any other objective solar filter or material. The views through AstroSolar display fine solar details, including fine penumbral detail, solar granulation, and lighter colored faculae. The disk is set against a dark background, free of the haze and scatter that is common in typical 'mylar' films or inexpensive glass filters. The performance of Astro Solar Filter Film is even more remarkable when considering its very low cost.

What is even more important - with AstroSolar Film the Sun appears in its real color - neutral white (with only a slight bluish tinge). Other films and glass filters produce a blurry bluish or reddish/orange/yellow Solar image, thereby cutting part of the spectrum. Especially with an orange sun, it is very hard to see faculae regions which are visible predominantly in the blue portion of the spectrum.

Due to its neutral color balance, AstroSolar Film enables the use of various color (or interference) filters which allows the observer to concentrate on certain spectral passbands for investigations of different layers within the solar "atmosphere". It is also possible to further improve the stability and sharpness of the solar image through the use of the Baader Solar Continuum Filter (sold separately). Views of granulation and faculae are particularly enhanced through the use of this continuum filter, in combination with AstroSolar film.

Please note that you cannot see solar prominences with white light solar filters such as this - that requires the use of sophisticated and significantly more expensive narrowband solar filters or telescopes. Solar filters made with AstroSolar Film with a neutral density (ND) of 5.0 can be used for visual and photographic, imaging and video applications.


Note: Occasionally, the product you receive might have a slightly different cosmetic appearance or labeling than what is shown. This can often change from batch to batch according to the whims of the manufacturer. However, you will always receive a brand new, original product as described above.

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