MDM Blue 4Kx4K CCD
The OSU Blue 4K CCD is a wide field CCD imager designed for the 1.3-m telescope as a facility instrument for direct imaging. The detector is a Silicon Technology Associates STA-0500 CCD with 4064x4064 15-micron pixels. This delivers a 21.3-arcminute square field of view on the MDM 1.3m telescope. The CCD is read out using four amplifiers, it can be binned up to 8x8 pixels, and readout in a number pre-defined centered subframe regions of interest. It is mounted to the "Buckeye" 12-position filter wheel on the MIS unit. this filter wheel can hold up to twelve (12) 4-inch square filters and is currently equipped with Johnson UBVRI filters and SDSS ugriz filters.
The four readout amplifiers have a gain of 2.2-2.4 electrons per ADU and a read noise of 5 electrons (rms). It has superb blue sensitivity out to the U band, and because it is a thinned, backside-illuminated CCD, it fringes in the far red (I, i, and z).
MDM4K Pictures: On 1.3-m | Horsehead | M15 | M33 | M42 | M77 | Moon | NGC3628 | M51
The readout time estimates include of the overhead to prep the chip, query the telescope, and update the status of the instruments, in addition to the time it actually takes to read the detector. The time it takes to read the chip scales with the number of pixels, and the amount of overhead depends on the image-taking mode. See the table on the right for read times, in seconds. The three readout modes and their consequences are explained below.
MGO n: Does GO for first image, then on subsequent images same as GO except
TGO n: Same as MGO except during subsequent images
*Applies only to images after the first in a sequence. The first image in every sequence will take the same time a "go" takes for a given binning. All times are accurate to 1/8 of a second.
When stars are strongly saturated on one quadrant, there is significant crosstalk into pixels in the other three quadrants. The effect is strongest in side-by-side quadrants, and weaker in top-to-bottom quadrants, consistent with the expected crosstalk signal pathway through these devices.
An algorithm for removing the cross-talk artifacts is included in the proc4k IDL reduction pipeline.
|Because of the large field of view of the MDM4K, the guider space is limited and it is easy to vignette the science field with the guider probe. You cannot exceed x=2500, y=3500 at the same time without vignetting the field. The cartoon to the right is not to scale.|
In order to ignore the shutter correction, a minimum exposure of 10 seconds is recommended. For a one second exposure, the central area of the chip is ~5% high.
A 10 second exposure has a ~1% error in the center of the field and a 60 second exposure has a ~0.1% error.
Because of the large, cold dewar window, the MDM4K is particularly susceptible to condensation on the dewar window (see figure on the right). We therefore revise the safe observing limit to 80% humidity for the MDM4K.
A dry nitrogen line will be installed to recover from this. If this happens before a line is installed, run the nitrogen line (by the pier) into the filter wheel, turn it on, and take a 15 minute exposure (to open the shutter and vent it out). Repeat if necessary.
The image quality is poor at the edges of the field and out of focus. The corrector was designed using an incorrectly given prescription of the 1.3-m.
To the right is a contour plot of the brightest, non-saturated star in the corresponding 1/25th region of the image. North is up, East is left. The image was taken on 2007/09/16, which was the night with the best seen during 6 weeks of observing. Each box is 10 unbinned pixels or 3.15 arcseconds square. The number is at the bottom of each plot is the FWHM in arcseconds derived from a 2d gaussian fit of the PSF.
Click on the image to download the postscript version or right click view image to see a higher resolution jpeg.
An IDL program to do the basic reductions (crosstalk, overscan, flat, [fringe,] wcs) of the night's data can be found here. To run it, you'll need the IDL astronomy library in your path (available for download here) then simply type
IDL> proc4k, '*.fits'
where '*.fits' is a wildcard to select all the images you want to reduce, including appropriate calibration images. The steps are outlined below. Further details and more explicit operating directions are in the program file. Everything but the coordinate solution takes ~30 minutes for a night. The coordinate solution, depending on how accurate the pointing was, can take several hours.
Note: Biases are never used. The overscan does a much better job and what little large scale structure there is isn't stable enough to reliably subtract.
If you have questions about, have additions to, or find bugs in this program, please contact Jason Eastman ( ) .