ULTRASPEC detector page

Vik Dhillon, Liam Hardy, version: 27 July 2022

This page collects together all relevant information on the ULTRASPEC EMCCD detector.

1. e2v datasheet for the CCD201-20 device used in ULTRASPEC
2. NORMAL output: bias level, readout noise, electronic gain, flatfield noise
3. AVALANCHE output: bias level, readout noise, electronic gain, flatfield noise, avalanche gain, clock-induced charge
4. Linearity
5. Dark current
6. Pumping the cryostat
7. Cooling the cryostat
8. Flat fields

NORMAL output

NOTE: The above measurements were obtained in CLEAR mode. However, tests show that similar results are obtained if NOCLEAR mode is used (see table below).

NOTE: Changing the binning factor makes little difference, as shown in the two tables below:

NOTE: The above measurements were obtained in NON-DRIFT mode. However, tests show that similar results are obtained if DRIFT mode is used (see tables below).

Below are links to example bias frames taken in the lab in Sheffield and at the TNT. Both are taken in normal slow readout mode and are provided in ucm format.

• Lab bias frame taken on ?.
• TNT bias frame taken on 7th November 2013.

AVALANCHE output

NOTE: The above measurements were obtained in CLEAR mode. However, tests show that similar results are obtained if NOCLEAR mode is used (see table below).

NOTE: Changing the binning factor makes little difference, as shown in the two tables below:

NOTE: The above measurements were obtained in NON-DRIFT mode. However, tests show that similar results are obtained if DRIFT mode is used (see tables below).

A note on gain in avalanche mode. We use the following definition:

• electronic gain (e^-/ADU). Measured from avalanche-output flats taken with no avalanche gain. The number of ADU that each electron measured at the output is converted into. This is the same as gain measured in a conventional CCD.
• avalanche gain (no units). The factor by which an electron entering the avalanche register is multiplied by when it gets to the output.
• system gain (e^-/ADU). Measured from bias frames with avalanche gain. The number of ADU that each photo-electron is converted into after passing through the avalanche register.

NOTE: Values in the final two tables come with significant uncertainties, due to the frames having small areas, thus fewer pixels. This means the histograms produced are less accurate, as can be seen in the plots.

Linearity

This was measured using the artificial star in the lab at Sheffield. The data were taken unbinned, using the normal output in clear mode with slow readout. The first plot below shows the total counts measured within the aperture centred on the star as a function of exposure time. Note that the peak counts per pixel at exposure times of 16, 17, 18, 19 and 20 seconds were 59733, 62133, 65535, 65535 and 65535, respectively. The second plot shows the residuals, where the data have been divided by the fit. It can be seen that the device is linear to better than 1% across its entire dynamic range, with the exception of the shortest exposure times, where the effect of the frame transfer is apparent. The data used to make this plot are available here.

Dark current

The above data were measured on 07/06/2013 with the chip at 160 K, using normal, clear, slow read-out mode and 600 s exposures. The data are shown graphically below:

The linearity of dark current with time was also measured, and the dark current per unit time was found to remain constant for varying lengths of exposure longer than 5 minutes.

The dependence of dark current on temperature was also investigated. As expected, the dark current increases with temperature, as follows:

Temperature (K)	DARK CURRENT (electrons/pix/hr)
155	3.2
160	13
165	38
170	106
175	288
180	659
185	1389
190	2682

The above data were measured on 08/06/2013 using multiple 15 minute exposures in normal, clear, slow read-out mode. A graph of the dark current dependence on temperature is shown below:

Below is an image of a typical dark frame taken in the lab in Sheffield on 3rd October 2012 with a chip temperature of approximately 160 K. The corresponding ucm file can be downloaded here.

Pumping the cryostat

Cooling the cryostat

Flat Fields

We include links here to example flat field frames taken at the NTT in June 2009 and at the TNT in November 2013. Both png and ucm files are available here. The flats have been bias-subtracted, and a suitable number of individual frames were combined to create each one.

• NTT g' flat .ucm file
• NTT g' flat .png file
• NTT r' flat .ucm file
• NTT r' flat .png file
• TNT g' flat .ucm file
• TNT g' flat .png file
• TNT r' flat .ucm file
• TNT r' flat .png file
• TNT u' flat .ucm file
• TNT u' flat .png file