Scope Doctor
Our equipment specialist cures your optical ailments and technical maladies
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? RICHARD WHITE I’m thinking of getting either a Meade NG 90 or ETX 90. Are these suitable for a beginner and how upgradable are they? JOHN PLACE
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.
The planets are currently at very low altitudes, exacerbating the problem, so 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. The two telescopes you mention are suitable for beginners but some of your cash will be paying for the electronics in the Go-To mount at the expense of the optics. Although these two are reasonably good telescopes, they are rather limited by their relatively small apertures and long focal lengths of around 1,250mm. As such, they are more suitable for lunar and planetary observations than for more general use.
Larger apertures, which give a much more flexible instrument suitable for a wider range of observations, can be found in the Dobsonian telescope design. You have to point them manually, but they are very simple and intuitive to operate. However, the big advantage here is that all your money will go into the optics which, at the end of the day, is what allows you to observe celestial objects at their best.