A small imaging refractor that eradicates misshapen stars
If there is one thing that causes angst among astrophotographers, it is the appearance of misshapen stars at the periphery of the field of view in deep-sky images. The Vixen VSD 100 f/3.8 astrograph tackles this issue head-on with a new optical design comprised of five individual lens elements.
The VSD 100 is supplied in a substantial flight case, the diminutive dimensions of which are a bit of a surprise when you first open the outer packaging, but this telescope is small for a very good reason. The clue is in the extremely low focal ratio of f/3.8 which, with an aperture of 4 inches, translates into a very short focal length of just 380mm.
Objects such as emission nebulae can extend across a surprising amount of sky: capturing these objects in their entirety requires a wide field of view, which dictates a short focal length. The VSD 100 has been specifically designed to cover a very wide yet flat field of view.
Vixen has not merely considered the optical requirements for this type of photography, as it has also included a very different type of focuser to normal refractors. The helical focuser supplied is beautifully engineered and held our imaging equipment very solidly indeed – there was absolutely no chance of any slippage while imaging near the zenith. The action is firm but smooth and produced no image shift when changing focus direction.
The focuser has a vernier scale rather than a simple, graduated one, and this allows the focus position to be read to an accuracy of 0.02mm. If you are not used to reading a vernier scale this can take a little getting used to, but for logging the start position for various camera and filter combinations, it can be invaluable.
Small but powerful
Supporting the optics and focuser is a white, high gloss tube and dew shield. Fit and finish are exemplary, with the micro-baffling and extremely matt black internal surfaces with absolutely no reflective areas showing great attention to detail. The various components have been engineered to very close tolerances and the main adaptor that slides into the focuser drawtube was such a perfect fit that it blew the dust cap off the rear of the telescope when inserting it.
Daytime imaging tests with a Canon EOS 450D DSLR camera indicated good colour correction. In fact, this telescope would be excellent for capturing high-quality wildlife images, especially as its handling has much in common with a camera lens. However, under the much more demanding spotlight of deep-sky imaging using a one shot colour CCD camera, we found it tricky to achieve best focus. Our Bahtinov mask and confirmation of correct focus using MaxIm DL’s FWHM tool resulted in images that were not quite as sharply focused as we would have hoped for.
Switching to a monochrome CCD camera, we again noted that luminance data was not as critically sharp as we had expected, with a little bloating to brighter stars. However, capturing narrowband data using our 7nm hydrogen-alpha (Ha) and oxygen III (OIII) filters produced extremely finely focused results, indicating that the optics are not entirely apochromatic.
We were very impressed with the flatness of the field of view. Capturing consistently well-formed stars right to the edges of the field of view with such a short focal length instrument is quite an achievement bearing in mind the actual sensor sizes involved. With our one shot colour CCD camera the field of view was 3° and 31 arcminutes by 2° and 21 arcminutes, while with our monochrome CCD camera it was 2° and 42 arcminutes by 2° and 2 arcminutes. There was some vignetting, but this was easily removed by applying flat frames.
Vixen has succeeded in producing a short focal length instrument with a truly excellent flat field ideal for many deep-sky imaging applications. Narrowband imagers will be very satisfied but one shot colour camera users and LRGB imagers may be a little disappointed with the sharpness of their images.
A five-fold flatness fix
All refractors suffer from field curvature to some extent. Although this can be tolerated when using eyepieces, the flat nature of a camera’s sensor highlights the issue. Images captured through a normal refractor show elongated stars at the edges of the field of view because a curved plane of focus projected onto a flat surface results in just the centre of the field achieving focus. These abnormal star shapes detract from the appearance of the view – which is why astrophotographers go to great lengths to eliminate them by installing external field flatteners.
Some manufacturers use a four-element optical configuration based on the original Petzval design in an attempt to combat field curvature, with some success. However, the single most important design feature of the Vixen VSD 100 is its five-element, five-group optical configuration. Vixen has resolved the flat field issue by using a pair of lenses at the front of the telescope to focus the incoming light and a set of three additional lenses at the rear of the telescope to correct the field curvature, resulting in a very flat field of view.
Internal baffles and blackening - Keeping light reflections to a minimum is important to maintain good contrast and the VSD 100 tackles this in two ways. First, the internal surfaces are coated in a very matt black finish; complementing this, there are a series of baffles within the tube to further absorb unwanted reflections.
Dew shield - The fixed dew shield extends 98mm past the front of the telescope and provides excellent protection from stray light and the effects of dewing. The push-in dust cap is made of aluminium and is retained in a rubberised holder, providing a very simple but extremely effective barrier.
58mm filter holder - The push-fit adaptor that couples directly with the focuser’s drawtube has an internal thread for holding a standard 58mm filter. This feature allows a light pollution filter to be installed for use with a one shot colour camera such as a DSLR.
Helical focuser and vernier scale - Unusually, the VSD 100 is supplied with a helical focuser, which operates in the same manner as a camera lens – adjustment is made by turning a focusing ring. The Vixen version is very substantial indeed, easily supporting our imaging gear with a superb focus action and a vernier scale for logging focus positions.
1.25-inch eyepiece holder - Although designed primarily for astrophotography, the VSD 100 is packaged with a basic 1.25-inch eyepiece holder. There isn’t sufficient backfocus to allow the use of a star diagonal, so observations must be made in a straight-through manner, which is quite typical of other Japanese telescopes.