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R4K User's Manual



CCD Properties
Readout Modes and Times
OSMOS at the 2.4m
Direct Imaging at the 2.4m
Direct Imaging at the 1.3m


The R4K is a 250um thick, fully depleted LBNL CCD with 4096x4096, 15um pixels. This detector is particularly notable for its extremely high red sensitivity. For example, it is approximately a factor of three more sensitive than the MDM4K between 900nm and 1 micron. See this QE comparison figure for the expected performance of the MDM4K and the R4K CCDs (these two QE curves are based on manufacturer data). The R4K also exhibits substantially reduced fringing compared to thin CCDs. Its two main disadvantages are a higher rate of radiation events (a.k.a. cosmic rays) and lower response in the blue region.

The R4K is interchangeable with the MDM4K device and may be used in direct mode on both the 1.3m and 2.4m telescopes, as well as with OSMOS on the 2.4m telescope. Observers should specify their preference for detector at the time proposals are submitted. Should a conflict arise between observers for the R4K (or MDM4K), the 2.4m observer has traditionally received precedence in similar situations with other instruments.

The R4K detector system was commissioned at the MDM 2.4m Hiltner telescope in September 2011.

CCD Properties

The R4K detector is LBNL CCD #127298-20-4. The detector electronics were built by Bruce Atwood at Ohio State and read out array through 8 amplifiers (2 per quadrant). The readout of the columns in each quadrant are alternated between the 2 amplifies such that there are effectively separate amplifiers for the odd and even columns. As the bias (and gain) levels for each of these amplifiers are somewhat different, the raw data will usually exhibit a striped pattern.

There are three ways to remove this even/odd pattern in the quadrants (and the differences between quadrants). The first is with a measurement of the 8 different bias levels from the overscan region. This is described further below in the software section below. The second method is to simply bin the data 2x2 after they are obtained. Finally, the bias level is fairly constant and consequently just subtraction of a bias frame appears to produce good results. Note that the gain level also varies slightly between amplifiers. The gain variations may be removed with flat field data. Once the gain has been measured, those data will be posted here. The gain is expected to be about 2 electrons per count.

Technical details are available from the LBNL Manual for the device.

Readout Modes and Times

Various region of interest (ROI) readout modes are available, similar to those available for the MDM4K. These modes may be used to readout a subset of the full array, or to return to full frame readout. These may be set in the same manner as for the MDM4K, namely with the following Prospero commands:

PR> call rroi1k.pro
PR> call rroi4x1k.pro
PR> call rroi2k.pro
PR> call rroi4k.pro

These change the ROI to 1024x1024, 4096x1024, 2048x2048, and 4096x4096 pixels. In all cases the ROI is centered on the array. Note that these scripts start with rroi and not roi to differentiate them from the scripts for the MDM4K (the scripts are slightly different because the arrays are slightly different sizes). Readout times for a representative subset of these modes are listed in the table below.

The binning may also be changed. For example, to change the binning to 2x2 type:

PR> ccdbin xbin=2 ybin=2

The current binning is shown in the Prospero Status Window. Readout times for 2x2 binning with various ROIs are also listed in the table. Note that binning 2x2 will NOT remove the different even/odd bias levels, as the array is still read through all 8 amplifiers.

Readout Times
ROIBinningTime [s]


Standard overscan subtraction codes, such as IRAF's colbias task, do not have an option to calculate the bias level separately for even and odd columns. Paul Martini has written a python code called proc4k.py that calculates and subtracts a constant bias level for each of the eight quadrants. This code works reasonably well, although it has not been exhaustively tested. Users are encouraged to report problems and requests for additional features to Paul. One future addition is likely to be to remove the gain variations between the amplifiers (once they have been precisely measured).

Obviously, use of this code requires that there is an overscan region. Good results have been obtained with 32 columns of overscan. To set the overscan region in Prospero type:

PR> overscan overx=32

OSMOS at the 2.4m

Operation of the R4K and MDM4K are effectively identical. Should any differences be identified, they will be described here.

Direct Imaging at the 2.4m

The R4K may be used as a direct imager at the 2.4m telescope with the 12-position filter wheel. The plate scale is 0.17 arcseconds per pixel and the field of view is about 11.5 x 11.5 arcminutes. The image quality at the edge of the field should only be marginally degraded in the best seeing. Specifically, the images will be elongated by approximately 0.5 arcseconds in the corners of the field. The main advantage to direct imaging with the R4K compared to OSMOS is somewhat higher throughput. This has not been measured, but is expected to be on order 30 percent. The main disadvantages are smaller field of view and poorer sampling. Note that while it is possible to bin 2x2 to approximately obtain the same plate scale as achieved unbinned with OSMOS, the radiation event hit rate per binned pixel will be larger than for unbinned data.

Direct Imaging at the 1.3m

The R4K has only experienced limited use at the 1.3m telescope to date, and not yet by the commissioning team. The operation should be identical to the MDM4K. The scale is 0.315 arcseconds per pixel and the field of view is about 21.3 x 21.3 arcminutes. Users are encouraged to refer to the detailed MDM4K Manual for further information. Note in particular the sections on image quality and the shutter correction.


There are saturated pixels shaped like 'icicles' at the top and bottom of the array

This is due to accumulated charge in the horizontal shift registers after either the saturation of the device or the device sitting idle for a long time. The Prospero script 'snapclean' will aggressively attempt to clean up the residual charge:

PR> call snapclean
If you encounter this error, please report it in the Trouble Report and mention if snapclean resolved the problem. Note that snapclean will leave the readout mode as full frame and 1x1 binning. If you were using a different mode, you will need to reset it, as well as make sure you are in object (not snap) mode.


LBNL 4K Manual
MDM4K Manual
MDM Observatory
MDM Page at Ohio State


The Red 4K has been generously funded by the National Science Foundation, The Ohio State University, Dartmouth College, the University of Michigan and Ohio University.

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Updated: 2013 Nov 3 [pm]