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SA Notes On PFIP Setup

This page describes the setup tasks that need to be carried out on the first day and night that PFIP is mounted on the telescope. The general task of changing filters is also described.
  1. Changing Filters
  2. Afternoon Checks
  3. Night Checks
  4. Light Leak

1. Changing filters

Changing or installing filters in the PFIP filter wheel implies taking out the filter wheel, physically changing filters, putting the wheel back and introducing these changes into the filter database.

The task of changing filters can take anywhere from 15 minutes to more than an hour. For this reason, filters should not be changed during the night. It is strongly recommended that you ask for the help of the duty engineer to extract the filter wheel. Note that the filter wheel is very heavy, so care is required when handling.

In case you are installing narrowband filters, make sure that the observer is aware of the effects of the f/2.8 on the spectral characteristics of the filter. Also, check in advance that there are adaptors available and that the filters fit well in them (the dimensions of the filters are not very accurately measured and the adaptors may need small mechanical modifications).

Steps for changing filters:

  1. With the help of the duty engineer, bring the telescope down to the access park position (AP2), with the rotator in a position in which the filter wheel cover is to the side. The cover is recognisable since it is held in place by 6 large brass screws (3 on each side). It is possible that liquid nitrogen starts to spill from the cryostat, in which case adjust the rotator position as appropriate.

  2. Remove the filter wheel cover: First take out the six large brass screws that hold it in and then slide it down. Be careful not to knock any of the cables.

  3. Disconnect the two black cables: These cables are connected on the right hand side of the wheel and they will have become that visible after removing the filter wheel cover. Unscrew the metallic connectors and pull the cables out. Also disconnect the thin pneumatic tube by pushing in the blue ring at the base of the connector and pulling on the tube at the same time. Be careful to hold the pneumatic tube because the air pressure can make the tube whip back.

  4. Take out the filter wheel:

    • Release the two red clamps located on either side of the filter wheel rails. Now the locking pin is the only thing that holds the wheel in place (visible on the left hand side of the filter wheel rails, gold coloured).
    • Hold the filter wheel with your right hand (you will have to support all the weight of the wheel with that hand) and with the left one, pull and hold the locking pin to the left. Now you can slide down the filter wheel (be careful, it weighs a few kilograms). As soon as you can, use your left hand to hold the handle on the front of the filter wheel for extra support.
    • Place the wheel in a safe and clean place (maybe one of the big trolleys available in the dome, covered by some white tissue; there is normally a roll of tissue in the prime cabinet), with the wheel handle pointing up.

  5. Change filters: Normal PFIP filters are 125mm in diameter. Putting these filters in the wheel is quite straightforward. TAURUS or JKT filters can also be installed in the wheel, but they need special adaptors (see the list of adaptors currently available). Filters and their adaptors are kept in the WHT Prime Cabinet. When mounting filters in the PFIP filter wheel, it is important to take into account the following considerations:

    • Balance: Try to keep the filter wheel properly balanced: if installing 50mm square filters try to place them in opposite positions (since the 50mm filters and their corresponding adaptors are much heavier than the 125mm filters).
    • Vignetting: The 50mm square filters must be orientated parallel to the CCD mosaic in order to minimize the vignetting; i.e. with two opposite edges of the filter perpendicular to the radius of the filter wheel.
    • Screws:
      • Make sure that the screws are properly tightened so that they cannot fall into the instrument, which could be very dangerous.
      • Look at the back of the adaptor and make sure that the screws do not stick out by more than ~1mm (you can use extra plastic washers under the screws if no shorter screws are available).
      • Do not over tighten the screws that hold the filters in place since the filters are easily damaged and expensive to replace.
    • Mounting: Make sure that the filters are mounted such that their top surface (including any holding ring or screws) lies at or below the top level of the mounting petals, otherwise the filter will clash with the internal surfaces of the instrument when the filter wheel turns, possibly causing damage. Attention to this issue is required for the thicker (usually narrowband) filters.
    • Orientation: The coloured side of the interference filters must be placed facing up (i.e. the red side up for Halpha filters), so that when the wheel is back in the instrument, the reflective side looks at the primary mirror of the telescope and the coloured side the at detector.

  6. Keep a record of the current positions of the filters in the filter wheel, and note down which filters were removed from the filter wheel.

  7. Place the filter wheel back in the instrument again: Slide it up, push the locking pin to the right (make sure that it is holding the wheel), and lock the two red clamps. Connect the two black cables, the pnuematic tube and reinstall the black metal cover with the six brass screws.

  8. Update the ING filter database with the changes made (see the manual here). Note that if a PFIP filter position has been left empty, then one should reflect this in the filter database by recording the movement of the PFIP_EMPTY filter to the empty filter position. If this is not done, then the PFIP control GUI will not allow you to move the filter wheel to the empty position, since the system cannot read a set of "filter" properties from the filter database.

  9. Initialize the PFIP main filter wheel on the PFIP control GUI, click on the Refresh Filter List button, and then close the PFIP control GUI (File => Quit). Restart the PFIP control GUI again by typing:

    SYS> 4MSControl -panels PFIP &

    Check that the list of filters and the focus offsets are correctly displayed on both pages of the control GUI.

2. Afternoon Checks

  • Check that the operations team has set the capstans of the CCD mosaic to their previously recorded values (have a look at the CCD Capstan Settings folder in the WHT control room). If this has been properly done, then at most a small adjustment of the tilt of the CCD mosaic is required during the night, which can save a lot of time and hassle.

  • Check that the mechanisms are working properly: Exercise the filter wheels, the autoguider probe and focus, and the ADC.

  • Check that the various automatic compensations are enabled: If this is not the case, then enable them from the PFIP control GUI, or use the following commands:

    SYS> pfip_set -m filtfocus enable

    This enables the automatic compensation of the telescope focus for different filter thicknesses.

    SYS> pfip_set -m gdfoc enable

    This enables the automatic adjustment of the autoguider focus for different filter thicknesses.

    SYS> pfip_set -m adc enable

    This switches the atmospheric dispersion corrector on.

3. Night Checks

Determination Of Rotator Centre And Pointing Test

The rotator center (RC) of PFIP lies within (or close to) the gap between the two CCDs. On the first night after PFIP is mounted on the telescope, it is necessary to determine exactly where the RC is and to perform a telescope pointing calibrate with 7 stars. Due to the field distortion at the autoguider chip position, the pointing test cannot be done with the autoguider and needs to be done with the science detector.

The following instructions describe how to determine the RC and do the 7-star calibrate:

  1. Determination of the rotator center:

    • Ask the TO to move to a bright star (~11 mag), and set the sky position angle (PA) to 180 degrees.
    • Translate the pointing of the telescope to CCD1 (to avoid the gap):

      SYS> user "ENTER APERTURE 0 250 0"
      SYS> user "AP 0"

    • For speed, set the CCD readout speed to fast.
    • Take an exposure and measure the position of the star (x1,y1) on the chip (should be close to the centre of CCD1).
    • Ask the TO to move the sky position angle to zero degrees and measure again the position of the star in the chip (x2,y2)
    • Compute the position of the RC: (xc,yc) = ((x1 + x2)/2, (y1 + y2)/2)

  2. Pointing calibrate with 7 stars:

    • The sky PA should already be at zero degrees, but check that this is the case. Also keep the same aperture as that defined when determining the rotator centre.
    • Make a window of 301x301 pix around the rotator center using the following command:

      SYS> window pfip 1 "[xc-150:xc+150,yc-150:yc+150]"

      Remember that the coordinates of the RC in this new window will change (for the example above, the RC will be at pixel (151,151) of the windowed frame).

    • Now tell the TO that you are ready to run the pointing calibrate with seven stars and that you would like to use the faint point grid. Remind the TO that the sky PA should be maintained at zero. Sometimes these stars are too bright for PFIP, so use a narrow band filter (or the U filter with short integration times) to avoid saturation. If all else fails, then you may close the primary mirror petals by a small amount.
    • For each star, take an image and calculate the telescope movement needed to put the star at the RC. Normally one, or a maximum of two, small offsets (<15'') for each star are needed.
    • Once finished, disable the window and reset the aperture:

      SYS> user "ENTER APERTURE 0 0 0"
      SYS> user "AP 0"

      Also remember to reopen the mirror petals fully if you closed them.

Removal Of Mosaic Tilt

On the first night of PFIP after an instrument change, it is necessary to check that the CCD mosaic lies flat within the focal plane. The following steps will take you through this complicated procedure:

  1. Point the telescope to an open cluster or any star field with stars well distributed across the 16'x16' area. You will need to have at least 10 (ideally 30 or even more) stars with some 3 to 30 kADU in the peak of their profile.

  2. Ensure that the CCD mosaic is not windowed, and for speed, set the CCD readout speed to fast.

  3. Put a PFIP broad band filter in the beam (e.g. R or V).

  4. Set the previous PFIP run focus or the past night focus as a start point to focus the telescope.

  5. Take a series of 7 (or 9) images with different telescope focus, centred at the start focus and stepping the focus by 0.05mm using:

    focusrun pfip <no. exposures> <exptime> <focus start> <focus step>

  6. Use the IRAF task mpf_head from the pfip_ql package to create new images (from the focusrun frames) with some new necessary header info in order to analyze the mosaic tilt (this can be done in the /obsdata/whta/YYYYMMDD directory):

    pf> mpf_head input=r??????.fit
    pf> mpf_head input=r??????.fit

    and so on... Note that mpf_head does not work with image lists.

  7. Run the IRAF task mpf_tilt:

    pf> mpf_tilt images=a1,a2,a3,a4,a5,a6,a7,a8,a9

    The task will prompt you to select an image to display and you should choose, which is the image closest to the best focus. Now you should select at least 10 (and up to 30) uniformly distributed stars from the whole displayed image mosaic using the letter "m", and you may use the usual imexam options of "e" and "r" to select "good" stars, i.e. PSF objects with no saturated pixels.

    Note that mpf_tilt will crash if it finds even a single saturated pixel in one of your chosen star profiles on ANY of the images to be analyzed. For this reason it is a good idea to pick stars with peak counts of less than 10 kADU, which are unlikely to saturate in any other image. Especially in case of bad weather (sky changes within few minutes during the setup procedure), it is advisable to choose faint stars, so no star from any image could saturate and crash the script! Stars may be removed from the list of stars to be analyzed by removing the corresponding entries in the file /obsdata/whta/YYYYMMDD/tmp_coord1. When you have finished your selection, you may press "q" to finish.

    IMPORTANT IN CASE OF CRASH: in case that mpf_tilt crashed or if you need to force it to stop it because of mal-functioning, then you must search for any remaining temporary files (pfip_ql> ls *tmp*) in the current folder (observing date) and delete these files manually (pfip_ql> del tmp*) before launching mpf_tilt again; otherwise a bug will occur and will prevent the script to work properly.

  8. A few seconds after pressing "q", a window will appear with a plot of FWHM versus focus. The options of the mpf_tilt task may be accessed by pressing "?". Use the option "b" to bring up a plot of the best focus for each star as a function of position in the CCD mosaic. Red and green circles show stars whose focus is below or above the mean focus, respectively.

    From the distribution of stars in this plot it is easy to see whether or not the CCD mosaic is tilted with respect to the focal plane (here is an example of a clearly tilted mosaic and a non-tilted mosaic). Another useful option, "a", displays the FWHM and ellipticity of the stars as a function of position across the mosaic for the mean focus. Note that a star can be deleted by pressing "d" with the cursor on top of it.

    On pressing "q", the task will report a recommended movement for capstans A, B and C:

    • If the recommended capstan adjustments are around one quarter of a turn or less, then it may not be necessary to apply them. In this case, check that there is no clear trend of FWHM across the field of view, and that the best focus value for each star is within ~0.05mm of any other value, indicating that the best focus is independent of position in the CCD mosaic.
    • If the recommended capstan adjustments are larger than one quarter of a turn, and you can see a clear gradient in the best focus estimate for each star across the field of view, then you will need to apply the adjustments and check the mosaic tilt again:
      1. Ask the TO to move the telescope to the access park position.
      2. Adjust the capstans in the order A, B and C. If adjusting only B, leave A and C alone, but if adjusting only A and C, release and lock B (but without turning the capstan) between adjusting A and C.
      3. Once the capstans are adjusted and locked again, make sure the cryostat is tight again on its mount.
      4. Return to the same star field and repeat the process from step 4 (NB: after adjusting the capstans the telescope focus will be different. The mpf_tilt script gives a prediction of this change).
  9. When the mosaic tilt has been properly removed, then communicate to the operations team that the capstan settings should be remeasured during the next day and their positions noted in the CCD Capstan Settings folder.
TIP TO SPEED UP STAR SELECTION: You can save a temporary file (tmp_coord1) which allows you to skip star selection process during subsequent mpf_tilt runs. Do this like this:
  1. TO BE DONE ONCE IN THE BEGINNING: after you have selected the stars (min 10 and up to cca 30) within mpf_tilt, in DS9 finish normally the process by pressing "q" which prompts you the irafterm plot. Right here, open another terminal and drive to the observing folder (ex: /obsdata/whta/20121108) then make a copy of the selected stars temporary file,

    > cp tmp_coord1 copy-tmp_coord1

    Come back to the irafterm plot and finish normally the mpf_tilt process, then correct the recommended tilt at the telescope.

  2. TO BE DONE ANY NEXT STEPS: Next time(s) point at exactly the same position (could be accurate up to a few pixels) and before running mpf_tilt, in the same terminal copy back the saved temporary file to be used by the new mpf_tilt run

    > cp copy-tmp_coord1 tmp_coord1

    then run mpf_tilt normally, which will prompt you in DS9 with the same marked stars and avoid re-selection.

Filter Focus Offsets

A number of filters for use in PFIP have not had focus offsets measured, and hence, for these filters, the telescope focus must be determined independently. To obtain the focus offset for such a filter, simply use the following formula:

Filter Focus Offset (mm) = F1 - F2

where F1 is the best telescope focus (mm) for the filter without a filter focus offset, and F2 is the best telescope focus (mm) for any filter with a filter focus offset.

The newly measured filter focus offset must now be entered into the ING filter database, and in order for the PFIP control GUI to pick up the change, then the GUI should be closed down (File => Quit), and restarted by typing:

SYS> 4MSControl -panels PFIP &

Check that the list of filters and the focus offsets are correctly displayed on both pages of the control GUI.

4. 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.

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.

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
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?


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Contact:  (PFIP Instrument Specialist)
Last modified: 08 November 2012