Equipment and Method

SkyTech CLS

Skytech CLS filter

Doing astrophotography from the outskirts of a city is frustrating at times. Light pollution can cause colour casts or gradients across the image and skyglow will reduce contrast in DSOs to the point of drowning out faint details which makes them impossible to recover.

In the ideal case we could take trips to dark sky locations every night, which would produce the best results. However even if this were possible, in the best case scenario it still takes up time which could be used for imaging, every second with the shutter closed means wasted photons which could be put to work integrating the final image.


That's where a light pollution filter comes in, in my case the SkyTech City Light Suppression (CLS) filter. This is designed to clip in front of a DSLR sensor behind the lens. When light from the sky reaches the filter it reflects or absorbs the wavelengths of light associated with light pollution such as the 589 nm line emitted by sodium-vapour lamps. Wavelengths of light associated with deep sky objects such as 656 Hα are transmitted through the filter at close to 100% intensity. This allows for longer exposures before overexposure and reveals faint details which would otherwise be undetectable behind skyglow.

Skytech CLS spectral curve

While the filter does dramatically improve image quality in terms of contrast and gradients, it can't do this without compromise.
Some filters can cause decreases in sharpness, though I have not noticed this with the SkyTech version. More importantly is that while the transmission spectra is very good, it is not perfect, and so some light is being lost which could otherwise be gathered. This also causes issues with colour balance as cutting out a large part of the spectrum throws off the white balance of the camera requiring more processing time to correct. Although this would also be necessary in areas of heavy light pollution due to the cast caused. 


Skywatcher 130PDS

130 PDS

My primary OTA for planetary or lunar work is the Skywatcher 130PDS, a specialised version of the Explorer 130 designed for astrophotography. With a proper mount it would also be suitable for use with DSO's. Relative to other telescopes it has a fast aperture of f/5 due to its 650mm focal length and 130mm aperture which makes it more suitable for imaging though also more susceptible to coma.

Unlike other Newtonian designs it does not have issues with backfocus as the primary mirror is closer to the draw tube than normal meaning I can achieve focus even with a DSLR and no barlow which is ideal for DSO's. For lunar photography its 650mm focal length can fit the disk of the moon in or a 2x barlow can be used for detailed or mosaic shots


Spherical vs Parabolic mirror

The primary advantage of this scope over other cheaper designs is its use of  a parabolic primary mirror. This allows it to bring all light to focus at the single point, improving the sharpness of the image. However this benefit is sometimes negated as its fast f ratio means very precise collimation of the mirrors is required to properly align the optics and bring objects into focus. As well as this using a telescope is much more difficult than a telephoto lens due to the weight and the bulk. Generally I use this for lunar photography but for DSOs I use a standard camera lens