I have a Sky-Watcher 200PDS with a DSLR and a 2x Barlow. However, my images of planets have coloured fringes on either side. Do you know how I can eliminate the effect?
Steve says: “Your photographs show an aberration known as atmospheric dispersion which is particularly noticeable in planetary images captured using larger aperture telescopes.
Dispersion is the displacement of the different colours of light received from celestial objects as it passes through Earth’s atmosphere.
The atmosphere acts in a similar way to a prism, separating white light into its component colours.
The colour dispersion we see here is caused by differential refraction, where shorter wavelengths of light (blues) are deflected to a greater extent than longer wavelengths (reds).
The lower the object appears in the sky, the more noticeable the effect as the light has to pass through more of the atmosphere than it does at higher altitudes.
If the planets are currently at very low altitudes, exacerbating the problem, you could simply wait until they’re higher in the sky.
However, optics can also come to the rescue. Atmospheric Dispersion Correctors (ADCs) use pairs of prisms to negate the ‘prism’ effect of the atmosphere.
Examples include the ZWO 1.25-inch ADC, the Pierro Astro ADC and the Altair Astro ADC.
However, you will need to check that you can still achieve focus with the extra length of the ADC inserted in the light path as Newtonian reflectors are notorious for not having sufficient inward travel of the focus tube when accessories are added.”
Some useful guides that might help:
- How to get rid of unwanted colour fringes on stars in astrophotos
- Optical aberrations: How to fix errors in your telescope
Email Steve your astronomy queries to firstname.lastname@example.org and they could be answered in a future issue of BBC Sky at Night Magazine.