Question

Let us pretend that you want to re-discover the dwarf planet Quaoar.

a).  Given that Quaoar’s semi-major axis is 43.6 AU and you need Quaoar move at least 3 arcseconds to observe that it has moved, what is the highest frequency you should visit the same patch of sky to “discover” Quaoar? Every night? Every three nights? Once a month? Show your math and make sure to factor in that there is only about 8 hours of dark time to observe every night… For reference, 1 arcsecond is 1/60th of an arcminute, and 1 arcminute is 1/60th of a degree. (Hint: for very small angular moves, you can treat Quaoar’s motion as a straight line.)

b).  Given how far Quaoar would have moved between nights, why did Mike Brown need to use the 48-inch Schmidt telescope with a 36 square arcminute field of view to find dwarf planets like Quaoar in the Kuiper Belt?

Answer 1

(b) Though Quaoar moves significantly every night in the sky, and henchence one would think that a moderate telescope would suffice for its observation, one has to note that it is a dwarf planet and we could detect it by receiving the sun-light reflected by it towards us.

In other words, being very faint source, one need a telescope with large enough objective lens so that it actually sees the source as faint as Quaoar. That is the reason Mike Brown needed a 48 inch Schmidt telescope to find the dwarf planet.