The MITLL2 Dark Current Problem

Jessica Chapman, AAO, February 9, 1999.

1. The Problem

Dark currents measured with both the TEK and the MITLL2 CCDs during UCLES and UHRF observations have been consistently higher than the dark currents previously published on the web, based on measurements made in the Epping lab. Here I do not discuss the MITLL3 which has not so far been used for coude observing (as far as I know).

The raised dark levels are a major problem for observations of faint sources where the dark current becomes the limiting factor in  determining the signal/noise ratios.  This problem is most severe for UHRF observations as a very large number of pixels (up to 1200) are  integrated (spatially) to form the final spectrum.

As an example, in his recent Jan 99 run using UHRF@1E6 + MITLL2, Ian Crawford measured dark currents with the MITLL2 which ranged from 2.5  e-/pix/hr, approximately 12 hours after powering the CCD up, to 0.72 e-/pixel/hr after 3 days. These are a factor of 9.3 and 2.7 times the published values. These higher levels effectively washed out his observations of 8th magnitude stars. Indeed it appears that a dark current of around 1 e-/pix/hr on the MITLL2 would restrict UHRF observations at 1E6 to sources brighter than approximately V=6. Ian's data appear to be  consistent with a dark current measurement I obtained last year  giving a dark level of 1 e-/pixel/hr 1.5 days after the MITLL2 was installed.

High dark currents have also been a serious limitation to UCLES observations of faint sources and this has raised several complaints  from users over the last couple of years. Here the spectra are extracted over a much smaller number of pixels and the dark  current levels have limited observations of sources fainter than around 15-16th magnitude (Brian may have more precise information on  this).
 
 

2. Dark Current Sources

The raised dark currents are caused by:
 
  1.  the room flourescence - this has been known about for many years.  The coude room walls appear to 'glow' for 2-3 days after the room lights have been turned off.
  2. The long clean-out time of both the TEK and the MITLL2 CCDs.
The dark current on the TEK CCD initially drops fairly rapidly but takes at least one day to reach its lowest value. Previous data I have show a dark level of 1.5 photons/pix/hr after 25 hours. This compares with an expected best-level of 1.0 photons/pixel/hr.

The clean-out time has not been systematically measured on the MITLL2. However, tests by John Barton with the MITLL3 show a clean-out time of 2-3 days. For Ian Crawford's run, the dark current (measured with the camera shutter closed to avoid room glow) had dropped to 0.72 photons/pixel/hr after three days. This is still more than a factor of two higher than the level of 0.3 e-/pixel/4000s measured by John. Its not clear to me why the level was still so high or whether the best measured value of 0.3 e-/pixel/4000s could in practise be reached.
 
 

3. Impact on Operations

 

3.1 Long Term Solution

It seems to me that the only long term solutions are to:
 
(i)  Paint the coude walls (I gather that paints are being investigated).
 
(ii) In future acquire a CCD for coude observations that has a much shorter clean-out time - so that the nominal dark level can be reached during  a single day. Do any CCDs have this characteristic?

 

3.2 Short Term Solution

For the present I offer the following advice:
 
(i) It would be really helpful to do some systematic tests of the MITLL2,
to properly measure the dark level with and without the camera shutter
closed, over an interval of several days or more. This would give a better
understanding of both the full clean-out time and the level of room
glow. John Barton also advises that tests be done of the dark current
linearity with time. At present we have very limited data for the MITLL2. 

(ii) At present users are advised that  BEST dark level they are likely to reach with the
MITTL2 CCD is 0.7 e-/pixel/hr, and that the dark current will be much
higher than this if the CCD is installed on the day of observations. 


(ii) For UCLES observations of faint sources (> 14th magnitude?)  the
TEK should be installed at least one day in advance of the run and the
MITLL2 should be installed several days in advance.


(iii) For all UHRF observations, install the TEK at least 1 day in
advance and the MITLL2 several days in advance of the run. This is
absolutely critical for faint sources (>~ 6th magnitude) but I'd
advise this is done routinely for all UHRF observations. 


(iv) Once installed the CCDs should be run on a no-break supply and
should not be switched off! If the CCD power is turned off for any
length of time, the dark level is reset back to the starting value
again! The CCDs must also be kept at a stable temperature.

For both the flourescence and clean-out problems it's clear that the longer
the CCD has to 'settle down', the better the outcome will be.
 

Report by Jessica Chapman (jmc@aaoepp.aao.gov.au)
9/2/99
Converted to HTML by Chris Tinney