Pluto Is Certainly Possible

I made a faint stab at imaging Pluto a while ago with no result. I was obsessing over whether it was even possible to capture something this faint – magnitude 15. But, it occurs to me now, that many of the galaxies I’ve captured are much fainter than mag. 15 and they suffer more because that’s spread over a significant area, M31 for example has a magnitude of 16.6 and surface brightness of 22.

I may be overstating my case though because this article from Sky and Telescope refers to a 10 minute exposure with a modest telephoto capturing magnitude 16. Anyway, I no longer think it’s hopeless.

Table 1 on this page gives specific magnitudes for lenses at 30 second exposures. It shows a 50mm f/1.8 lens yielding mag 13 and a 200mm f/2.8 yielding mag 15.2. He gives an intermediate calculation of a “Clark Exposure Factor” of pi/4*aperture-in-cm^2*exposure-time-in-minutes. If I apply them I get a figure of magnitude 14.5 for my 200mm f/4 camera lens and 15.5 for my on order Z61 telescope. Also, woohoo, I just pasted an image into my wordpress blog – that’s a big advance!

So if i’m right a several minute exposure with my 200mm lens should be easily able to show Pluto.

Unfortunately, tonight is about the last night before the moon casts its blight on the landscape and it’s not supposed to be great.

This post from reddit says they imaged pluto with a DSLR and an Orion ST80. I think the Scope is 400mm f/5 and he was shooting 60 second subs but doesn’t mention the ISO.


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