Astrodon

October, 2009

NEW!  Santa Barbara Instrument Group (SBIG) STX CCD Camera

SBIG recently announced its eagerly-awaited STX camera that houses the largest amateur imaging CCD detectors, such as the 16 megapixel 4096 x 4096 pixel Kodak full-frame KAF16803E that measures 36 x 36 mm with a 52 mm diagonal.  The STX boasts many new features, such as fast USB2.0, full-frame image buffer, continuous imaging and guiding while downloading, better cooling to -50C, and many other advancements.  Please see the link for more details.

SBIG continues its tradition of placing a guide detector inside the STX camera for their patented self-guiding. As a result, larger filters are required to cover both the imaging and guiding detectors. For this reason, SBIG will also introduce a filter wheel capable of housing 65 mm square filters that cover both STX detectors. 

Astrodon Introduces 65 mm Square Filters for the STX

Astrodon introduces its latest Generation 2 E-Series LRGB imaging filters and 5 nm H-a (hyrdogen alpha)  narrowband filter in 65 mm square size to support SBIG's introducton of the STX technology.

The E-Series is designed for the red-sensitive Kodak full-frame detectors with the goal of equalizing the fluxes, and hence, the signal-to-noise, for the red, green and blue filters.  This simplifies your imaging, because it allows you to take the same length exposure for each color and only one dark frame exposure time.  You will appreciate this since each 16803 FITS file is ~32MB in size.  Within ~10% your color combine weights based upon a G2V (sun-like) star white point should be 1:1:1, making processing much easier.

Spectral Scans

Spectral scans of the latest Astrodon E-Series filters in 65 mm square size are presented in the graph below.

Each RGB filter achieves an amazing 99% transmittance within itsbandpass and  has steep sides for minimal internal reflections (ghosting, that causes halos around bright stars). 

Generation 2 filters are well known for minimizing or eliminating halos around bright stars due to their construction.  The bandpass and blocking layers are all placed on one side of 1/4-wave fused silica glass substrates.  The other side has the highest quality anti-reflective coating.  As a result, there is virtually no "cross-talk" between the two sides, thereby eliminating halos. We do not need absorptive glass substrates that lower the transmission in order to minimize halos. We keep performance at a maximum.   All coatings are made with durable hard oxides using the latest sputtering deposition methods.

The E-Series filters are also designed to minimize the leading source of local light pollution from high-pressure sodium street lamps.  Their yellow color occurs in the 580 to 600 nm region.  This is the region in the E-Series BETWEEN the green and red filters that is not passed to the STX detector.

The high-performance luminance filter blocks both UV and Near-Infrared (NIR) light, and is supplied with the RGB filters as an LRGB set.  Astrodon also offers an optional Clear filter that is similarly UV-blocked, but allows NIR light to pass. 

Deep Red E-Series Filter Brings Out HII Regions in Galaxies

Lastly, Astrodon pioneered filter sets with two different formulations for red-sensitive Kodak full-frame detectors (Astrodon E-Series) and less red-sensitive interline detectors (I-Series).  The E-Series has a much deeper red filter (left) with a bandpass of about 60 nm, whereas the I-Series red is an orange filter (right) with a bandpass of about 110 nm. 

Other filter suppliers only offer LRGB filter sets that contain an orange filter. They were most likely designed for the popular SBIG STL11000 (Kodak Interline KAI 11000 detector).  As a result, HII emitting regions in galaxies tend to get "washed out" with the orange filter.  Conversely, most imagers who use the Astrodon E-Series filters with the deep red filter report HII regions in galaxies that "pop out".  They do not need to augment their galaxy images with H-alpha. 

This effect can been seen in the following sequence of images of the Crescent Nebula in Cygnus from an orange filter, to the E-Series red filter and then to various narrowband filters of different bandwidths.  The increase in detail is obvious between the orange and red filters.  You will therefore benefit from this deeper red filter in your images of both galaxies and emission nebula.

Highest Performance 5 nm H-a Filter

Lastly, Astrodon offers its popular 5 nm H-a filter in 65 mm square size.   It continues the goal of providing the highest performing narrowband filters available to amateur imagers.  The throughput or peak transmittance of the filter determines the signal in signal-to-noise.  Our typical throughput is 95% and we guarantee >90% transmittance at the 656.3 nm H-a emission line or you get a replacement filter or your money back.  The narrower the filter, the more non-emission signal is eliminated in the background.  This lowers the noise in signal-to-noise thereby increasing your contrast, especially for faint detail and bringing out background nebulosity, as shown in the above sequence. Your background signal will likely be 30 - 50% lower compared to a 7 or 8.5 nm narrowband filter. This will also help when imaging with the moon up.

Parfocal

Finally all Astrodon filters are coated on 3 +/-0.025 mm substrates and will likely be parfocal on reflectors (f/4 or slower).  Systems with refractive optics, such as refractors and SCTs, all have some degree of focus difference between red, green and blue.  This tight thickness tolerance in Astrodon filters will therefore not make the existing focal differences in the optical system any worse. 

Availability

Filter sets are now in stock. Astrodon will introduce the 65 mm square filters at the Advanced Imaging Conference (AIC) in San Jose, California at the end of October at a special introductory price.