Mirach's Ghost

NGC 404 near Mirach. Click to enlarge

Continuing my Black Friday imaging blitz, I came across this galaxy while recalibrating the telescope's setting circles on Mirach, a bright star in Andromeda. I was making sure they were as accurate as possible while heading to the quasar 3C 48. On such a bright star a CCD image will show beautiful diffraction spikes and a gradient of grays that to me are nearly the best thing out there. So while taking some images for that one of the stars looked a bit fuzzy, and sure enough NGC 404 popped out. Phil Plait has a nice post about it, and that post was the push to complete this round of processing.

Quasar 3C48

It was clear on the Friday after Thanksgiving, so I spent a long evening imaging some neat targets.

The first was the quasar 3C 48 in Triangulum, one of the first to be identified as such--as a "quasi-stellar object", a radio source coming from an apparently point source, and yet stronger than any radio emissions previously identified from stars.

In 1960-62, using two radio telescopes in Owens Valley, astronomers Thomas Matthews and Allan Sandage were able to zero in on the location of the radio source to within 10 arcseconds, and the only object in the field was a 16th magnitude "star". And for two years, they noted its optical variability, its lack of any motion across the sky, and a very faint nebulosity associated with it, with a non-descript spectra. And yet, the QSO remained to the astronomers studying it only a potentially Milky Way object, something obviously further than "close" in the galactic sense, but far enough away to be stellar in appearance and non-moveable.

After submitting a paper about three of these objects in 1963, Matthews and Sandage get word that Schmidt has discovered that another QSO has a significant redshift (which is published in Nature in March 1963), and this allows them the scientific freedom to reinterpret their own spectra as a high-redshift spectrum of z=0.367.


22 minutes total exposure, Click to enlarge. 10-inch f/6 reflector and Starlight Express SXV-H9 camera


Here's another CCD image from Anthony Ayiomamitis of the quasar.

I showed some RAS folks how to use the CCD camera last night. We couldn't get the drivers installed on the new laptop, despite tweaking the INF files to identify the USB device (at least in Windows, when you first connect a USB device, the device sends an ID number down, and Windows looks in the INFs to locate the driver). Unfortunately the manufacturer of the Starlight Xpress SXV-H9 camera changed the camera and the driver without changing the name or offering the old driver for download--the result is old camera owners can't get it working at all. Having to ask for the driver when it could just be on a website someone is really dumb.

Anyways, we got the camera working on the ancient soon to die laptop (94MB of RAM!) and took this 3 minute (12x 15sec) exposure of the Dumbbell Nebula. Enjoy.

When in doubt, blame the instrument: It wasn't the lightning.

The WGN Weather blog shows a video from Bucktown of the intense thunderstorm of August 6th here: http://blogs.trb.com/news/weather/weblog/wgnweather/2008/08/incredible_viewer_video.html, but they claim that the nearby lightning strike at the end of the video actually produced arcing close to the camera. That wasn't the case. The "arcing" is actually an artifact of the CCD sensor in the video camera. To understand what's going on, you'll have to deal a little bit with the physics of CCDs. In silicon, incoming photons will excite electrons out of a lower energy state and into the "conduction" band where it can then migrate through the material. You can call this liberating the electron. In a CCD control voltages create zones where these freed electrons are trapped in the silicon until they are moved out and measured. Those zones are best known as pixels. Depending on the type of CCD, when the exposure is over, the electrons are moved pixel by pixel in columns to be read.


Intensely bright sources of light will produce so many electrons that they will overwhelm the control voltage and flood out of the pixel and into the surrounding pixels and circuitry, producing spurious effects. You've probably seen these effects -- it starts showing up at 1:57 in the Yeah Yeah Yeahs video for Maps for example, or in the SOHO image above (it's Venus doing the blooming).
In this case the electrons flow out and down the columns that the electrons would normally be read. The Sun is a great source of column bleeding in a lot of videos online: see this one, for instance. Or bright stars--here is a weak version (it's the faint vertical column, not the diagonal streaks):


Since it's difficult to control all the sources of light in any possible photo scene, the CCD manufacturers have ways of trying to mitigate the overflowing electrons. One technique is to put drainage canals around the pixels and dump the electrons. This is good, but the extra space for drainage costs you some light sensitivity and light measuring accuracy.


Another problem can develop while you are moving the electrons off the CCD to be measured--if you have a shutterless camera, then light is still hitting all the pixels and can still cause overflow problems. One technique (used a lot for video cameras, at least in the old days) is to make the CCD twice as big with half of the chip covered up. At the end of the exposure you quickly move the electrons in the lit part over to the dark part and then leisurely read them out. This helps, but you can still have those overflowing electrons come down into your dark area.

So, in the lightning video, you can see that the extremely bright strike produces too many electrons in the CCD of the camera, and they flow 1. into the dark frame-transfer area and 2. down the columns (the vertical bleeds).
You can see at least one of these effects in some of the other strikes in the video.