Why I’m *still* not worried about ISON’s brightness

As you have probably heard, Comet ISON was recovered last week as it emerged from solar conjunction. This was big news because it was the first time ISON had been observed since June and there has been a lot of speculation about how bright it would be. So how bright is ISON now? About what we expected it to be!

I’ve added the new measurements to the plot of ISON’s brightness. It is true that they are under the curve I’ve plotted there, but then again so were most of the measurements throughout the late spring and before it went into solar conjunction. I discussed this in considerable detail a few weeks ago so I won’t belabor the point1, but I just want to reiterate that reasonable observers are well aware of this trend and have already taken this into account when planning observations.

If anything, the new measurements should be reassuring! ISON has most definitely NOT disappeared. It has NOT fallen apart. It continues to brighten (although admittedly not as quickly as we had hoped it might when it was first discovered). In other words, it is acting like a typical new comet from the Oort cloud.

As has been said time and again, comets are highly unpredictable and we still don’t know for sure what ISON will do between now and late November. But as of now, it seems to be behaving relatively normally. There is no need to panic!

1Okay, I will belabor the point for the initiated. One thing that I didn’t discuss in my last article about ISON’s brightness is that the last few weeks of observations prior to conjunction and the new observations have all been made in twilight at high airmasses (e.g., near the horizon). As Bruce Gary mentioned in his account of recovering ISON, this makes it very challenging to do reliable photometry. Some potential problems include:
1. Depending on the setup of the telescope, there may be occulting problems which block some of the light and cause the telescope to perform like a smaller telescope (this happened to Bruce).
2. Near the horizon, the atmosphere absorbs a lot more light (because the light has to pass through more air to get to the telescope than if the telescope is pointing nearly overhead) so the comet and nearby stars look a lot fainter.
3. What is more, the amount of light lost is wavelength dependent, with shorter wavelength (bluer) light being attenuated more than longer wavelength (red) light. This can lead to calibration problems when comparing to catalog stars if they have significantly different spectral types than the Sun (since most of the light we see from ISON is reflected solar continuum).
4. This increased path through the atmosphere also causes the images to be in much worse focus than if ISON was overhead, so the light is spread out over more pixels, again causing it to be fainter.
5. Finally, the twilight sky is much brighter than if ISON were observed in full darkness, thus requiring shorter observations (so the signal-to-noise is worse) and causing faint coma to be undetectable.
As discussed by Dr. Richard Miles of the British Astronomical Association, the result is that ISON’s brightness is likely being underestimated right now. Thus, I expect that as ISON’s solar elongation increases and it is observable in darker skies, the magnitude estimates will move closer to the trend line. In other words, it will appear to get “brighter” due to nothing other than better conditions for observing it. Hurray!