MDM 4K Imager

Contents:

Summary

The 4K is a wide field CCD imager designed for the 1.3-m telescope as a facility instrument. Its 4064 pixel, 21.3' square field of view is read out by four amplifiers, and can be binned up to 16x16, and/or pre-defined centered region of interest can be selected. It attaches to the Buckeye filter wheel, which holds up to twelve 4" square filters and is currently equipped with Johnson UBVRI filters. The four amplifiers have a gain of 2.2-2.4 electrons per ADU and a read noise of 5 electrons.

4K Pictures: On 1.3-m | Horsehead | M15 | M33 | M42 | M77 | Moon | NGC3628 | M51

CCD Characteristics & Parameters

Read Time

The read time consists 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.

GO n:

  • Full TCS query before each image
  • Full pre-image erase cycle
  • Full pre-image instrument status query
  • Does additional pre- and post-image housekeeping queries of all instrument subsystems between images.

MGO n: Does GO for first image, then on subsequent images same as GO except

  • No pre-image instrument status query => don't change filters during a sequence
  • No housekeeping queries between images => next file counter won't update

TGO n: Same as MGO except during subsequent images

  • Time-only TCS query => computes own time dependent quantities (SECZ, HA, etc.)
  • No pre-image erase cycle => extra cosmic rays collected during previous read
bin\mode
go
mgo*
tgo*
1x1
133.875
132.5
129.375
2x2
40.25
38.875
35.875
4x4
15.875
14.5
11.5
8x8
9.25
8
4.875
16x16
7.5
6.125
3

*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.

Guider

Because of the large field of view of the 4K, 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.

Focus

The image quality is extremely sensitive to focus. The focus must be right to ~ 2 focus steps on the 1.3 m. An easy way to focus the telescope is to use the newly implemented prospero command "focus", which takes an exposure, closes the shutter, shifts the charge a user specified number of pixels, and keeps exposing and shifting until the user wants to read it out. Since the pixel scale of the binned images is somewhat course, it is recommended to do a focus sequence unbinned. To use it,

set the first focus position
pr> focus n first
adjust focus
pr> focus n
adjust focus
...
pr> focus n
set focus to last position
pr> focus last

where n is the number of *unbinned* pixels to shift between exposures. 30 is a good default value. In the upper quadrants, the top-most image will be the first focus position. In the lower quadrants, the bottom-most image will be the first focus position.

Shutter

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.

Humidity

Because of the large, cold dewar window, the 4K 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 4K.

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.

Performance and Throughput

TBD...

Image Quality

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.

Known Problems

Reductions

An IDL program to do the basic reductions of the night's data can be found here. To run it, simply type

IDL> proc4k, 'filelist'

The steps are outlined below. Further details and more explicit operating directions it are in the 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 ( ) .

Operations Manuals

Additional Documentation & Information

Last revised: April 3, 2008 [jde/osu]