WT5L Blog

In the past week, I have completed the latest round of major upgrades to the desktop computer that sits by and is connected to my amateur radio transceiver. Here are the details of this upgrade:

LCD Monitor

After years of working with nothing more than a 15″ CRT monitor at any of my desktop computers, I have now purchased a 24″ widescreen LCD monitor. The monitor is a Samsung Model 2494SW with a native resolution of 1920×1080 pixels. This unit was purchased from amazon.com for $201.45 with free shipping.

Video Graphics Card

When considering this monitor or any widescreen, high resolution monitor, I quickly realized the integrated video on the computer’s motherboard would not be up to the task of driving any such monitor at its native resolution. This realization led me to start looking for a suitable video graphics card. Specifically, I was looking for a AGP card since my motherboard had an unused AGP slot (Advanced Graphics Port). After some looking and comparing, I settled on a SPARKLE SF8855DT GeForce FX 5500 256MB 128-bit DDR AGP 4X/8X video card purchased from newegg.com for $34.99 plus $5.99 shipping. This graphics card features NVIDIA chipset and 256MB of video RAM. It also has has one D-SUB connector for analog video, one DVI connector for digital video, and S-Video connector for a TV connection. An image of this video card is shown below:

DVD-RW Drive

Now having access to a widescreen, high resolution LCD monitor, I then decided I needd to replace the very old CD-ROM drive that originally came with this computer. After a short search, I replaced this IDE drive with a Sony Optiarc Black 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 12X DVD-RAM 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM 2MB Cache SATA DVD/CD Rewritable serial ATA (SATA) drive. Since this is an OEM drive, I also had to purchase the SATA power adapter and data cable. These items were purchased from newegg.com for $27.99 (drive), $5.79 (cables), and $8.13 shipping. (Total order: $41.91)

Hardware Integration

The video card and DVD drive were installed in the computer case with no issues. The XP system automatically detected and loaded a driver for the DVD drive and I manually loaded the drivers for the video card and for the monitor from the supplied disks. Despite working the issue for a while, I was unable to get the graphics card to drive the monitor at its native resolution via the digital DVI cable but was able to get the native resolution via the analog interface. (I was pre-warned of this potential problem from user feedback on the newegg.com site for this graphics card.) For now, I will stick with driving the monitor through the analog cable.

Summary

I have now replaced every hardware item in this computer except the legacy floppy disk drive that I never use. In previous upgrades, the motherboard, motherboard memory, and power supply were replaced and the operating system was upgraded from Windows 95/98 to Windows XP Home. With a widescreen monitor in place, I now have enough room on the display to place windows side-by-side instead of having to stack them on top of each other. This capability will be particularly useful during radio contests where I was continually having to swap windows around due to a lack of room on the screen.

This photo was taken on the evening of 26 January 2010. At the time, the Moon was approximately 12-days past its new phase and was very high in the eastern sky. The image was taken with a Canon XSi DSLR (450D) attached to a Takahashi TSA-102S refracting telescope reduced to a focal length of 610mm. The image exposure was 1/100 second at ISO 100. The camera was focused on a 4th magnitude star using FocusMax software. Click here to view a larger representation of this image.

In any astronomical image there are three noise regimes that figure into the image’s signal-to-noise ratio (SNR). These regimes are the uncertainty associated with the signal itself (shot noise), the uncertainty associated with the readout electronics of the camera (read noise), and the uncertainty associated with the dark current that builds up within the sensor due to thermal effects (dark noise). All CCDs experience a certain amount of dark current during an exposure. Dark current is expressed as electrons per second per pixel and these electrons add to the electrons that build up in pixel wells as a result of exposure to the light from astronomical objects (stars, planets, galaxies, etc.). Dark current can be reduced by cooling the sensor and this is why astronomical CCD cameras usually have an ability to be cooled.

As an example, the Kodak sensor in the SBIG ST-8 camera specifies a dark current of 1 electron per second per pixel at a sensor temperature of 0.0^o Celsius. This means that over the course of a ten-minute exposure, each pixel will accumulate approximately 600 extra electrons over and above that provided by the light from the object being imaged. The specifications for the ST-8 sensor also say that the dark current doubling temperature is 6.3^o Celsius. This means that if the sensor is cooled to -6.3^o Celsius the dark current will then be 0.5 electrons per second per pixel and for each additional cooling by 6.3^o Celsius, the dark current will be cut in half again.

To combat dark current, calibration dark frames are created by taking an exposure of the same length of time and at the same sensor temperature as the light frame but with the shutter closed. This dark frame records only the dark current the builds up over the course of the exposure for each pixel. The dark frame can then be subtracted pixel-by-pixel from the light frame to completely remove the dark current effects from the light frame. However, this subtraction operation is not without consequences because by performing the subtraction, extra noise is injected into the light frame. This noise can be mitigated by taking multiple dark frames and combining them into one “master dark”. The usual way to create the master is to find the average or median value for each pixel across all dark frames. As more dark frames are added, the noise contribution to the light image becomes less. The question becomes: How many dark frames is enough?

In an effort to figure out how many dark frames is enough, I captured 100 dark frames (600 second exposure @ -20^o Celsius) and combined different number of dark frames in to master dark frames. For each master dark frame, I measured the noise in the frame by finding the Standard Deviation of a small patch of the frame not affected by hot pixels. The table below summarizes the results of this effort:

¹ Method “Clip Min/Max Mean” first removes or “clips” the specified number of max values and min values then calculates the mean of all the remaining values for each pixel in the image.

The graph of the Standard Deviation versus the number of dark frames combined is shown below.

Summary: As is apparent from the table and the graph, the point of diminishing returns in reached somewhere around the area where 40 dark frames is reached. At this point, the master dark frame has a Standard Deviation of around 1.65 ADU or about 4.3 electrons (assuming a gain of 2.6 electrons per ADU). Since the dark current noise is very much less than the read noise (20 electrons), 40 dark frames seems sufficient.

^*All Dates and Times are given in Univeral Time (UT)

As discussed in a previous post (here), read or readout noise is the noise or uncertainty added to every pixel value read out of a CCD array. This noise is largely associated with the imperfect conversion of an analog signal into a digital value.

The method to determine a CCD’s readout noise contribution to the final signal is as follows¹:

1. Obtain two bias frames (0 length exposures).
2. Subtract one bias frame from the other bias frame.
3. Find the standard deviation for a region of the resulting frame.
4. Calculate Read Noise (e^- RMS) = StdDev * Gain / SQRT(2)

¹From “Handbook of CCD Astronomy Second Edition” by Steve B. Howell

The results of performing these operations on ten pairs of bias frames is summarized in the table below:

²Based on published gain = 2.3 e^-/ADU

³Based on measured gain = 2.6 e^-/ADU

⁴Published read noise for this device = 15 e^-

UPDATED 18 Jan 2010: All values in the table above were updated on this date to reflect the correct Standard Deviation calculation. In the first posting of this data, I was using Maxim DL to subtract one bias frame from another bias frame. I subsequently found out that when Maxim DL performs this subtraction, all pixel values that calculate to a value less than zero are assigned a value of zero since negative numbers are not allowed in 16-bit unsigned integer space. After finding this problem with Maxim DL, I switched to CCDStack for performing the subtraction since CCDStack converts all FIT files from 16-bit unsigned integers to 32-bit floating point values before subtraction. This change meant that instead of the read noise coming out to be slightly less than the published readout noise value (~ 12 electrons per read), the read noise turns out to be slightly more than the published value (~ 20 electrons per read). NOTE: This problem with Maxim DL can be easily avoided by adding an offset value (say 200 ADU) to all pixels in one of the frames then subtract the other frame from this offset frame. This will avoid the less than zero equals zero problem and not affect the calculation of Standard Deviation.