Spectrograms of UV Cut Filters Suitable for Astrophotography
Unmodified Canon cameras have spectral sensitivity roughly between 400 and 665nm. Whereas true APOs and highest quality telephoto lenses will produce images free of chromatic aberration (CA), most conventional achromats, semi-APOs, and standard (affordable) telephoto lenses will exhibit CA in the shape of blue or red halos around brighter stars. One way to improve CA is to reduce aperture by 1-2 stops. Another one is through the use of appropriate UV filters to reduce the spectral range of the light reaching the sensor.
Full-spectrum modified Canon cameras manifest a much wider range, between 370 and >750nm. No refracting system, including the best APOs, is designed to correct such a wide range without prominent red chromatic aberration. Use of UV/IR CUT filters which block light in the violet and in the far-red region is mandatory.
With unmodified cameras, conventional, "photographic" UV filters generally improve star images to a degree by reducing atmospheric light scatter (Rayleigh scatter) which is most prominent in the violet region. They also reduce CA by blocking some unfocused violet light, however the effect is relatively small since most UV filters block below 400nm where unmodified camera sensitivity is very low anyway. As a typical example, look at the spectrogram of the Hoya Pro 1 Digital UV(0) filter which seems to have only minimal effect on the violet end. The most effective "photographic" UV filter is Tiffen Haze 2A. It begins to cut violet light below 415nm, and actually produces very good results in reducing CA and haze. Formatt Haze 2A has an identical bandpass at a substantially higher price. Unfortunately, neither one of these filters is coated, and will generate internal reflection artifacts on very bright stars. In spite of this, Tiffen Haze 2A is so affordable, it should be the first choice for a beginning telephoto lens astrophotographer.
Specialized astrophotography filters are much more expensive, but they are anti-reflection coated, more restrictive in bandpass, and more effective in reducing haze and chromatic aberration. Also, they can all be used with unmodified and full spectrum modified cameras because they block light in the violet and in the far-red region. All of them are available in the 2 inch threaded version which can be screwed into 2 inch telescope focusing tubes, or easily attached to telephoto lenses with step-down filter thread adapter rings.
My personal favorite is the Astronomik UV/IR Cut L3 filter because it is also manufactured in the Canon EOS clip-in version. Clip-in filters are close to the camera sensor, and tend to produce less aberrations than more distant 2 inch filters in telescope focusing tubes or in front of telephoto lenses. Its bandpass is between 421 and 687nm, which yields excellent images with reasonably good optics.
The most restrictive UV/IR cut filter is the Baader Fringe Killer. With bandpass between 420 and 675nm on modified cameras, and 420 - 665nm on unmodified cameras, it is probably the best choice for marginal optics.
Another filter I have found very useful with full spectrum modified cameras is the Astronomik CLS-CCD light pollution filter. Its overall range between 450 and 690nm is even narrower than the Baader's, but it excludes light between 530 and 640nm, which requires doubling the exposure time. The rewards are very high contrast, marked reduction in atmospheric scatter, no chromatic aberration with decent optics, and exclusion of sodium and mercury light pollution.
Edited by rekokich, 20 December 2018 - 11:56 AM.