CHAPTER 13: Digital Astrophotography — What’s New?Chapter_13__Digital_Astrophotography.htmlChapter_13__Digital_Astrophotography.htmlshapeimage_1_link_0
Filters for Piggyback and Prime Focus Work

While filters are required gear for shooting with advanced monochrome CCD cameras, for single-shot color CCDs and for 
DSLR cameras filters have more limited use for deep-sky imaging. But some can be of value. Here’s a summary.

For piggyback and prime-focus deep-sky shooting in color, a prime goal is cutting out light pollution and achieving a darker sky.

Unfortunately, there is a limited selection of useful filters. A 1A Skylight or a UV filter helps cut down sky fog a modest amount, although both serve best as lens protectors. We rarely use them as they can create ghost images caused by reflections from bright light sources.

Lumicon markets an interesting variation called the Minus Violet filter. Like UV and skylight filters, it also eliminates ultraviolet and violet light but has a deeper cutoff point that pushes into the visible part of the spectrum. It filters out the shorter wavelengths that contribute most to chromatic aberration. This yields sharper star images, especially with telephoto lenses. However, in fast telephotos this filter can exacerbate central hot spots from vignetting by producing an odd color gradient from center to corners that is almost impossible to eliminate, even with computer processing.

Most nebula and light-pollution filters are not suitable for color photography due to their very strong color cast. An exception is the IDAS Light Pollution Suppression filter available through Hutech, a broadband filter that is selective at notching out unwanted emission lines from light pollution. Our initial tests with this filter show it does help punch out nebulosity even under good skies without unduly affecting color balance or uniformity of illumination across the frame.

A useful filter for aesthetics is the Kenko Soften Filter (also sold by Hutech). This adds a soft glow around stars, an effect commonly used by astrophotographers to enhance the visibility of bright stars and constellations in wide-field shots (what we call the “Akira Fuji Effect,” after the photographer who pioneered this look).