Messier 93 is an open cluster located in constellation Puppis and it is about 3400 light-year away from Earth. The cluster is one of those deep space objects, which I cannot see from my home, therefore I had to pack the gear and enjoy starry night outside. Compared to my previous attempt, I took the laser collimator with me and waited sufficiently long the temper the telescope. Well I was surprised that the stars are slightly oval and there is blue halo on one side and red one on the other side of nearly each star. I was speculating if there is something wrong with the optics, but then I realized that this object has very low declination (it’s not far from horizon), therefore it’s exposed to atmosphere refraction/dispersion. This means that the light must pass through the thick layer of an air. In this case the atmosphere works as a lens and bends red and blue light differently. The effect is described here or here.

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