PF light leak

A 2-night narrow-band PF imaging run in Aug 1999 was seriously compromised by light leaks. The effects were particularly noticeable on the newly-commissioned 2-EEV mosaic detector, being strongest near one edge of the array, but clearly affecting the whole area. The leak was traced to light bouncing off the edge of the top surface of the poorly-baffled corrector, and to a number of lesser leaks. New baffling has been installed (1/2000) by Servando Rodriguez, who also took the opportunity to improve cabling and to lengthen the track along which the filter wheel slides, to make removal of the filter wheel safer for both the instrument and the observer's toes. Some technical details about the light leak, which will have affected all PF imaging prior to Jan 2000, are given below.


Below are a few notes on the symptoms and origin of the WHT PF-camera 
light leak, and a suggested cure (basically 3 baffles, indicated by the
sheets of card currently in the camera).  Where I give orientations, they
are as would be viewed by someone standing at the filter-wheel access
door, with the camera pointing down at the floor.


Symptoms
--------
The leak is visible as a V-shaped patch of light on exposures made 
through narrow-band filters with the 2-EEV mosaic.  The apex of the V
is on the join between the CCDs (at the bottom as usually displayed,
= opposite B capstan), with the arms reaching out to about halfway up
the left and right edges of the displayed frame.  The intensity in
dark of moon peaks at about 0.1 counts/sec, which is about 4% that
expected for sky as seen through a V filter.  Hence for narrow-band
imaging (dlambda ~ 30 A), the magnitudes of the leak and the sky
background are similar.  The data taken by this month's narrow-band
imager (Steve Smartt) were seriously compromised by the leak.
The leak is faintly visible on broad-band images.
The leak does not change position when the camera is rotated.
It is unaffected when wrapping the entire camera in black cloth,
so is due to light coming through the main optics.  
It is unaffected if all the filters are blacked out, so must be light 
scattering round the filter wheel.

It happens that when only one EEV CCD is used at PF (EEV12), it is oriented
with the long axis perpendicular to the join in the 2-EEV, so the light
leak is not then prominent.


Origin
------
I replaced the 2-EEV cryostat with a translucent screen at the approximate
position of the CCD, and covered all except the entrance aperture with
black cloth.  With a near-opaque filter in position,
an arc-shaped leak was immediately visible on the screen, with its
apex in about the right place.
With the front aperture covered, the leak is not present.
The leak is sky light passing through the main optics and bouncing
off the edge of the top of the corrector, skimming between filter wheel and
autoguider head, and landing on the mosaic opposite the capstan B end of the 
CCD join:


   ---- 2EEV
       .         *
         .      **
-F-wheel---.   *** autoguider head
             .****    
               .    
----------------- top of corrector (illuminated by sky)  


I was able to reduce the leak by a factor of 200 (as measured on the
CCD) by blocking the edge of the top of the corrector with card.

There is only about 1 cm horizontal clearance between the filter wheel
and the autoguider head at the limit of its travel.  Any baffling
has to fit in this space.

There is a large vertical gap between the top of the corrector and
the filter, and the diameter of the corrector is much greater than that
of the filter, so the complete absence of internal baffling means that there
are many other routes for light to get onto the CCD.
When the main leak is blocked, many other distinctive leaks are visible
on a short exposure e.g. light passing through an unprotected cable
entry port on one corner of the camera appears on the CCD as a diagonal
streak.  The obvious leaks need blocking up, and internal baffling
is needed.


Suggested cure
--------------
Servando and I looked at the camera yesterday, and concluded that
the following baffles are needed (I've taped pieces of card in the approx
positions required):

(1) A baffle running left-right between filter wheel and autoguider head,
from side to side of the box, supported on the box.  In cross section
as viewed from left or right of the camera:

___F.wheel__   -----
              / **
             / ***
            / ****

where * = the autoguider head, ___ = filter wheel and ///--- = baffle.
This baffle blocks the main leak.
The shutter bar projecting below the base plate which sits on top
of the camera may get in the way of part of this baffle.  In this
case, a similar shaped baffle should also be stuck to the autoguider head,
so that it moves left and right with the head.  The length of this
extended hood will be determined by the left-right travel of the
autoguider.
The front-back position of the main baffle needs checking with the 
autoguider driven to its limit in that direction to check no fouling.

(2) A circular baffle blotting out all of the top of the optics 
except the area used by the autoguider and a central circular area
below the filter X mm in diameter, where:
    X = 78 mm (corner-to-corner diameter of CCD) + H/2.8 (to allow 
        for spreading of the beam)
    H = distance between top of optics and front of CCD
I think H is about 100 mm, which gives X = 114 mm, but it would be a good
idea to measure H (to an accuracy of a few mm), and then add about 10 mm
to the calculated value of X, as a safety margin.
This should be just above the optics, and if mounted on them should 
be removable when the PF optics are used with AF2, 
This baffle blocks most of the unwanted light emerging from the optics.

(3) Two baffles just underneath the filter wheel, i.e. about 10 cm vertically
above (2), symmetrically left and right, between them blocking the whole 
area except under the filter in use.  These baffles are bounded by the
box, by baffle (1) and by the central circular structure of the
filter-wheel holder.  They need to be mounted on the box (not the
filter wheel, which is where I've taped the pieces of card).
These baffles prevent light from the aperture left in (2) passing 
through one of the filters not in use (e.g. a much more transparent one) 
and being scattered about inside the box until it lands on the CCD.

In addition, the following modifications are desirable:
- Light leak at unprotected cable entry point top left needs blocking.
- Small light leak at cable entry point about 30 cm below filter wheel
  needs blocking.
- There are 6 bolt holes to fix the top-plate on the camera, but only
  4 bolts.  Light leaks through the remaining holes.
- The flaps between the petals of the filter holder are loose, and I'm worried
  that one of them will bend, and foul against something.  Can't think
  of an easy solution though.
- The inside of the main box should be black everywhere (e.g. I noticed
  there's a white sticky label on the underside of the shutter plate).

A few other things which have nothing to do with light leaks:
- Is there any possibility of extending the slide that the filter wheel
  goes into?  On the telescope, this is normally withdrawn vertically,
  by someone working alone.  The handle isn't accessible until the 
  holder has fallen out of the slide, so there is no safe way to 
  take it out.  One day the entire filter wheel is going to leak
  through someone's fingers and make a large dent in the floor.
- The cables passing through the filter-wheel door have to be disconnected
  every time a filter is changed.  This seems to me a recipe for broken
  connectors, but I can't think of any easy solution.
- There are some flecks of black paint visible on one of the optical
  surfaces, looking down through the camera.  Can they be removed,
  or is it sealed?

CRB

Chris Benn (crb@ing.iac.es)

1999 Oct 26 updated 2000 Mar 3

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