UV FLOODING RECIPEES

[Olaf Iwert]

Dear CCD folks, 

Based on our recent experience with the characterisation of Mike Lesser's 
devices the following EMail was compiled. It is also triggered upon the 
occasion of some recent EMails by John Geary and Mike Lesser as well 
as the Danish CCD group. I would like to get your comments and point of view 
to better compare the UV flooding results and methods of the different 
organisations and to reach the best AND MOST REPRODUCIBLE device behaviour, 
also simplifying its maintenance in a REMOTE observatory.

(The flooding works, but seems to depend on at least some other issuses 
we did not compare in detail so far, so they are probably not applied similar 
and therefore not really reproducible between different sites.)


As a compilation of the different groups' results I therefore try to list 
first those points which seem to be CLEAR by now 

!!! IF ANY OBJECTIONS PLEASE LET ME KNOW !!! :

1.) Oxygen and air filling of the dewar to 1 atmosphere do NOT cause any 
    difference in the CCD QE behaviour
2.) The filling with oxygen can be done, but complicates the maintenance (also
    in terms of potential explosion danger, should the pump not be 100% oil 
    free). 
    (Even more complicated would be the UV flooding directly at the 
    telescope in terms of maintenance, should a routine or accidental 
    warm-up occur)
3.) The use of normal ambient air (humidity between 30 and 50%) does not seem 
    to result in very good reproducibility
4.) Therefore the best method seems to be the usage of dry air which can be 
    obtained in bottles (and is therefore better reproducible in terms of 
    its humidity)
5.) The lamp type used for flooding (Zn / Cd) results in a negligible 
    difference in the CCD QE 

NOW TO THE REAL QUESTIONS AND THINGS WE SUSPECT TO BE DIFFERENT AND GIVING AN 
EFFECT ON THE QE BEHAVIOUR :

A.) Baking out of the dewar seems to matter to prevent a contamination of 
    the dewar with gasses not necessarily contaminating the CCD surface 
    visible, but influencing the chemical UV flooding reaction ?
B.) What is the window material in use at the different sites using these 
    CCDs ? (It could be that the window material in use at site A caused by 
    its different transmission causes a different UV flooding time, compared 
    to the set-up at site B) 
C.) Which is the time needed to let the device sit in air or in oxygen BEFORE 
    applying the UV flooding and does it affect the QE data ?
D.) What is the time duration of the applied UV lamp  
      i.) heating up ?
      ii.) flooding of the CCD ?
E.) What is the temperature of the CCD during the flooding process ? 
F.) What is the temperature of the CCD from which on it is storing 
    (descending temperature) the UV flooding (0 degrees ?) ?
G.) What is the time needed for evacuating the dewar to a given vacuum 
    pressure range (e.g. 10exp-5 / 10 exp-6 ) after the UV flooding ? 
H.) What is the cool - down rate / profile of the CCD actually in effect ? 
I.) Is the UV flooding lamp left on during pumping / cool down of the device 
    and
      i.) is it necessary ?
      ii.) does it have an effect on higher dark current generation ?
J.) What is the physical/chemical process requiring the ambience of oxygen ?
K.) Flat fielding of the device is affected by :
      i.) the duration of UV flooding ? 
      ii.) the distance of the lamp ?
      iii.) the history of a flooding applied before ?
L.) To which extent did you measure the long term stability of the UV 
    flooding ?
M.) What is the physical / chemical reaction with hydrogen (as it erases the 
    UV flooding)  very rapidly and is there any coincidence with ambient 
    HUMID air ?

THE IDEAL CASE WOULD BE THAT WE WOULD HAVE A POOL OF CCD COOL DOWN PROFILES 
WITH MARKED UV FLOODING / PUMPING TIME POINTS / TIME DURATIONS TO MORE EASILY 
COMPARE....

Some points may also seem trivial and were listed already, nevertheless I 
think it is useful to compare.
Any information is appreciated but would be most helpful if directly 
referenced to the above given numbers / letters.


With best wishes 


Olaf Iwert

ESO 
Optical Detector Group
Karl Schwarzschildstr. 2 
85748 Garching near Munich
Germany

Telephone and voice Mail : +49 / 89 / 320 06 353
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