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NATIONAL OPTICAL ASTRONOMICAL OBSERVATORIES

Cerro Tololo Inter-American Observatory

ACTR MEMO #28


DT:    Apr 23, 2001  (last update: Jun 8 status report)
TO:    ACTR, scistaff, ets
FM:   Roger Smith
RE:    Cloud Camera Proposal

C L O U D    C A M E R A

ACTR  PROJECT  432

History

While thermal IR sensors are the clearly excellent cloud detectors and have the advantage of operating similarly under starlight, moonlight or sunlight, they are still very expensive and must be mechanically scanned to cover the whole sky.  Apache Point Observatory has had such a scanner for some time, and another with lower resolution and presumably lower cost is under development at Steward Observatory (see sample movie). The observatories on Mauna Kea plan to purchases a microbolometer array on a panning head for $140K.

For several years, I have been exploring the feasibility of using a commercial CCD with a fisheye lens as a much cheaper alternative for detection of clouds in the moonless sky.  As well as being cheaper than the IR cameras, it offers higher resolution, no moving parts except the shutter and cooling fan, and no cryogens.   I supervised an engineering student, Mario Caceres, for several summers, who helped with this feasibility study and the presentation to ACTR in March 2000.  This clearly demonstrated the detection of clouds under starlight alone, and projected an extinction limit of a few percent with angular resolution of 0.18 degrees.   (Available in Powerpoint, on CD-ROM.)

On the basis of this presentation a backside illuminated 1Kx1K camera, the IMG1024S made by Finger Lakes Instruments, was approved for purchase.  It was discounted to $8.5K as a result of a clearance sale at Scientific Imaging Technologies (SITe).  A fast fisheye lens (Nikkor 8 mm, f/2.8) and data acquisition computer (Linux PC) were purchased with year end funds, at which time SOAR also elected to purchase a similar camera for Cerro Pachon as a site infrastructure contribution.  [To be more precise, CTIO bought the camera for SOAR with year end funds also, with a promise of reimbursement ...conditional on delivery.]

Various hemispherical windows were also ordered with year end funds, ranging from perspex domes from Edmund scientific to a custom optical quality MgF coated hemisphere.  The domes for large binnacle type compasses used in commercial shipping are one low cost source.  The one supplied by Ritchie Navigation for only $34, though polycarbonate and uncoated, is optically superior, and is often used for underwater cameras.   E&C Precision Optics have failed to produce an optical quality hemisphere, so the possibility of applying a hard AR coating to the polycarbonate window from Ritchie navigation will be investigated.   [2001 May 8: "Subal", an Austrian maker of underwater camera housings, claims to use optical quality glass domes which may be suiitable.]

The IMG1024S camera and fish eye lens have been operated in the lab and are about to be tested on the night sky.  I have not yet verified gain/noise/dark current performance.
 
 
 

Expenditures to date for Tololo
FLI  IMG1024S  CCD System 
$8254
Nikkor 8mm f2.8 fish eye lens
$2495
4 uncoated stock windows, various suppliers 
$  227
Linux Workstation + 50 Gb disk 17" monitor
$2500
TOTAL
$13476

 
Expenditures to date for SOAR
FLI  IMG1024S  CCD System 
$8919

Design Overview

The primary goal is to provide qualitative assessment of cloud patterns (detection, layout and motion).  Thick clouds on large spatial scales can be detected by almost any camera, but as they can also be readily seen by eye, this is not a very interesting case  ...except for remote users.  The primary goal of this project is to detect diffuse Cirrus which is more common and very difficult or impossible to see by eye in the moonless sky.    Side benefits (which I will discuss later) are aircraft detection (for laser safety), sky brightness monitoring (light pollution versus cloud cover and time of day), monitoring of OH emissions and auroras, and monitoring of clouds under moonlight or even daylight for remote observers.  Finally, the impact of high quality images of the skies over Tololo on the public, particularly the astronomical community, should not be overlooked, particularly if they are superior in quality to those offered in the North (e.g.  Mauna Kea, and Kitt Peak)

Since the dark sky is almost invariant from night to night it is possible to subtract a reference frame formed from the median of previous (dark) nights so that a nominally flat image can be displayed at sufficient contrast to allow extinction and scattering to be perceived down to the limit imposed by the photon shot noise.   This has been demonstrated crudely by de-rotating clear frames taken on the same night.  Much better results will be obtained when the camera remains in a fixed position from night to night so that no de-rotation is needed.

Pixels can be binned together to improve the noise statistics, but only until the typical spatial scale of the clouds is reached.   Unfortunately a comparison of the angular scale of daytime Cirrus with that of the the moon or an outstretched thumb will quickly confirm that  the proposed resolution of 0.18 degrees (960 pixels across the sky) is not excessive, and that only slight binning can be used if at all.  This fine angular scale and the high winds found at altitude combine to require exposure times shorter than ~3 seconds to maintain acceptable contrast for Cirrus.

The unsurprising consequence is that camera sensitivity is crucial to detection of thin clouds.  To be specific, it was found that with fast optics (f/2.8), broad bandpass (no filter) and high quantum efficiency (backside illuminated CCD), one can detect extinction at about the 3% level.  Enough photons are collected so that moderate read noise can be tolerated.  Exposures short are enough that standard thermo-electric cooling is sufficient to keep dark current well below sky counts.

The smallest image circle for a fast fisheye lens (Nikkor 8mm, f/2.8, or Coastal Optics, 7.5 mm f2.8) is 22 mm requiring a moderately large CCD.  This is the primary cost driver since CCD prices scale roughly by area rather than by pixel count.   Fortunately the SITe 1024x1024 backside illuminated CCD with 24 um pixels was available at a bargain price ($8500 including camera electronics and software).  The 23 mm image circle for the Nikkor 8 mm maps into 960 pixels which can be displayed at full resolution with only very minor vertical truncation by most graphics cards.

Faster and shorter focal length lenses (e.g. GM23514C, 3.5 mm, f1.4), designed specifically for 1/2" CCDs, are available from Rodenstock Optics (Linos Photonics), but these are only quoted as providing 103.6 x 76.5 coverage.  These would be very attractive for a cheaper camera (smaller CCD) where horizon coverage was not required.  The faster lens would offset the loss of sensitivity due to the small CCD area.

Images will be acquired at 25 second intervals, flat fielded archived and scaled to 8 bits ready for distribution and display.  As a minimum, the most recent frames will be served up on the web, possibly binned 2x2.  An archive of previous images at full resolution could also be provided relatively easily.

A significant enhancement would be to provide animation the most recent hour or at 15 Hz frame rate.  Such smooth animation is needed to harnesses the eye's formidable pattern recognition capabilities which can extract correlated motions from the noise.  This considerably enhances the system's sensitivity to clouds and provides an intuitive way of showing weather patterns.  With a little practice, the eye can easily separate cloud and air glow .

Here are some sample Cloud Camera movies (*.avi, Windows Media Player), binned heavily to compress the movies to about 5 MB:

Comparison with ConCam

After I had purchased the above hardware, the installation of the "Continuous Camera" on Kitt Peak was brought to my attention by Ed Olszewski at Steward Observatory.  The ACTR chairman has asked whether the installation of a ConCam should be funded instead of continuing our own Cloud Camera effort.

ConCam is a project at Michigan Technological University funded by NSF & NASA to monitor the whole sky continuously.  See www.concam.net  for  complete description.  They state that...

"Scientific objectives of the CONCAM project include the tracking of bright stars and highly variable phenomena such as novae, supernovae, optical counterparts to gamma-ray bursts. Astronomers might be interested in CONCAM images, however, for information they provide about weather and seeing conditions. CONCAM images will be uploaded soon after being acquired to a publicly accessible web page which can be inspected by astronomers in neighboring domes, astronomers attempting to observe from a remote location, or others generally interested in observing site conditions on any particular date and time."
In private correspondence last year the PI, Robert Nemiroff, described the capital costs...
We would be happy to build a CONCAM for the VLT.  We would ask you for about $10K to cover the cost of materials and ask that we not be charged internet transfer costs.  We ask in addition that all CONCAM data be made public domain as soon as it is taken.  We would then plan to transfer a copy of CONCAM.VLT data back to http://concam.net for display there under the link http://vlt.concam.net .
and the customer's responsibility for installation....
If you agree, we can ship you a CONCAM.  We will talk you though installation and set up http://vlt.concam.net from which VLT observers anywhere in the world can see the VLT sky.  You can mirror our software to make a local VLT mirror so that observers in the south can see the images with shorter download times.
Concam is optimized to achieve its scientific objectives with lowest possible capital investment....about 2/3 that of the Cloud Camera.  As shown in the table below, its sensitivity would be expected about 15 times lower ...a factor 5 for CCD area and 3 for integrated QE.   Exposure time is increased to compensate, which is acceptable for their stationary science targets but causes severe smearing of clouds.  Clouds are detected of course, but what is the sensitivity limit?   Users at Kitt Peak who I have consulted (Olszewski, Massey) have been unable to make a judgment since they have not been doing photoelectric photometry.  One of the Telescope Operators, Hillary Mathis, has supplied comments on the usefulness of ConCam indicating Cirrus are seen as a general increase in scattered light, causing a brightening in the sky background.   An examination of the ConCam archive and nightly movies shows that the lights of Tucson increase the visibility of clouds, particularly in the East, so it is not clear how applicable these reports would be to Cerro Tololo.  A ConCam moving GIF from April 17 has been stored locally to demonstrate performance when running from the intranet: he update rate is nothing like that planned for our CloudCam.  It shows a variety of cloud conditions before and after moonrise: the fact that new clouds suddenly become visible when the moon rises is a bad sign.  Other movies can be accessed easily from the ConCam archive.
 
 
CloudCam
ConCam
 
CCD
 SITe 1024x1024
Kodak 1530x1020
pixels
Camera Model
FLI - IMG1024S
SBIG - ST8
 
Pixel Size
24 
9
um
Active Area 
26.6 x 24.6
 13.8 x 9.2
mm
Image quality
Good
 Marginal
 
Peak QE
 80%
 40%
%
 Wavelength range
(half max QE points)
 360 - 940
 500 - 930
nm
 Lens
 Nikkor 8mm 
fisheye
Nikon Coolpix FC-E8 
Fisheye Adapter
+
Computar M8513
8.5mm  f1.3
 
Lens Price
$2500 (2nd hand)
$200 + $49.95
 
Effective  f - ratio
 2.8
  > 2.8 
 
SENSITIVITY RATIO
(not including smearing)
> 15
1
ratio of photons detected per square degree
Exposure time
 2-4
 60
seconds
 Exposure rate
 2.2
 0.5
frames/minute
 Reference frame subtraction
 yes
 no
 
 Replay rate
 10-15 
 network limited frames/second
TOTAL CAPITAL
$15K
$10K
 

By Products

P R O P O S A L

STAGE  1  :  Basic deployment

If ACTR funds the completion of this project, it is my intention to seek the services of a recently graduated programmer, or final year student to provide write scripts for the various stages of this project.  I see my role as developing specs, supervising and testing, while dedicating most of my time to other higher priority projects.   Once a programmer is contracted Stage One could be completed quickly.
 
Resource Estimates for Stage 1
Roger Smith
2 mw
Mechanical Draftsman
0.6 mw
Contract Programmer
4 mw
Carpenter
0.8 mw
Concrete constructor
1 mw
Capital (concrete, rebar, fiberglass enclosure)
$500

 

STAGE  2  :  Enhanced visualization

There are two enhancements which are equally important and are quite independent of one another....

 
 
O P T I O N A L      E X T R A S

( Not supplied with ConCam either )

The remaining items are not required to meet the primary goals of the project but are included to show the merits of several upgrade options and the work involved.

Costs are not given since approval
is not being sought at this time.

 

Occulting Ring :

Operation in moonlight will be needed by remote observers and for aircraft detection (laser safety for adaptive optics).  It will be convenient but not essential for observers on site.  The moon and its halo saturate the CCD even at the shortest exposure times, and at large angles from the moon performance is degraded significantly, since moonlight scatters off the window and entrance optics.  Hillary Mathis (Telescope Operator on Kitt Peak) describes ConCam as being "completely useless" under such conditions.  The solution is to shade the window with an obstruction slightly larger than the window, placed at a distance where the angle subtended is just great enough to occult the halo around the moon.  The entrance window has a 6" diameter requiring that the occulting mask be about 1.5 m away.

Rather than attempting to track the moon, a simple and much more robust approach is to place a 3 meter diameter ring around the entrance window, inclined at 30 degrees from zenith so that it lies in the plane of the equator when the moon is at zero declination.  It is translated along the polar axis to accommodate the declination change of the moon (or sun).  It can either be made wider and be left in a fixed position all night or be driven during the night.  Automation is preferable since the same motion can be used to park the occulting ring below the horizon when the moon is down.    Click here to see the how the dimensions are calculated.

The rails and supporting structure could be built by a competent contract welder such as Benjamin Leiton.  Since the declination motion is moderate, the occulting ring can have considerable depth without changing the projected profile appreciably.   This allows it to be stiffened against high winds.  Fiberglass-foam sandwich (like a surfboard) will provide a good stiffness to weight ratio and can be out contracted.  The wheels from in-line skates will carry the weight easily while accommodating imperfections in the track built from welded angle iron.  Given that positioning requirements are very coarse, one could think of solutions requiring less capital outlay, but our favorite RS485 controlled servo motors (QuickSilver) would require least labor and is therefore preferred.

For operation in daylight an occulting ring is essential, and even if the camera is not used in the daytime the ring would protect the camera from heat and UV damage.
 

Switchable Filters :

For operation in daylight, one needs either to stop down the lens or to apply a neutral density filter.  The ND filter has the attraction that the same filter wheel can be used to look at the sky in various colors by night.  e.g. It could be used to block air glow.

The 8 mm Nikkor comes with a built in filter wheel.  Replacing the filter disk with one that has a toothed rim would allow a QuickSilver motor to be clamped to the lens so that remote filter changes could be made via the same RS485 port as the occulting ring.
 
 

Software refinements :

A graphical overlay could be added at display time showing current RA & dec and/or current telescope pointing.
 
 

Roger Smith,   2001 April 20



 

STATUS UPDATES

2001 May 7:

Approval was given at the ACTR meeting at the end of April to commence the basic implementation, reference frame subtraction and high speed animation immediately, with funding being assigned for contract programmer(s).

Tests with the camera on a dark photometric night have demonstrated that the camera forms sharp images and operates reliably in continuous aquisition mode all night.  Images are sharp with minimal wings on bright stellar profiles.    Dark current and read noise were not aparent in images on the sky, and there are only a few small cosmetic defects.   The OH emission and/or scattering from aerosols at low angles brighten the sky near the horizon sufficiently that some long wavelength blocking may be desirable.  Images under a wider varaiety of weather conditions will be required to determine whether a filter changing motor will be desirable.

It was verified that direct sunlight on the lens will not produce anything like enough heat to damage the shutter.

2001 May 12 - Jun 8:

David Walker was contracted to work on the camera control software on May 12.  (The other candidate, Cristian Gutierrez, was not available on the necessary timescale.)  David has succeeded in running the camera under Linux, using a software developers kit (SDK) provided by the the camera manufacturer.   The code to take an image had to be reorganized so that the camera initialization could be placed before the main acquistion loop. 

David has deciphered Skycalc and extracted the portions necessary to compute sun/moon rise & set times to allow the camera to startup and run autonomously.  This a general solution which can be applied wihtout modication at any site.

The workstation clock has been slaved to the GPS Time Server and exposures are now automatically taken at the same sidereal time each night to allow reference frame subtraction in future.  The frame rate need not produce an integer number of images per day.  We have devised an automatic image naming scheme which allows for easy file searches by either Date, UT or ST.

The SDK provided examples to read images from the camera into memory only.  David has incorporated the code to write the image buffer to a FITS file on disk.   Camera parameters, aquisition timing, and ephhemeris information have been added to the headers.

Roger has made a sketch of the enclosure concept.  Patricio Schurter is producing a 3D scale drawing as an exercise to learn his new CAD program.

Remaining Tasks:

Enclosure:

 Design
 Construct inner camera support (AOSS:carpentry)
 Construct molds and concentric fiberglass shells (external contractor)
 AR coat hemispherical window (outcontracted)
 Mount window on inner fiberglass shell
 Construct concrete pier on Tololo AOSS:OPSTOL)
 Install and test enclosure for mechanical fit, leakage, condensation, dust, ventillation, & image registration from night to night.

Finish data acquisition software:

 Schedule exposures at fixed times since start of each sidereal day. [done]
 Re-organize some library routines to eliminate delays due to redundant operations inside loops. [done]
 Log camera and timing parameters and weather stats in header [in progress]
 Get parameters from configuration file instead of hard coding them.
 Log messages to file.
 Automate exposure mode switching (Day, twilight, dark, moonlight)
 Schedule calibration frames; generate special names for these
 Automatic start on boot.

Web interfaces:

 Web page for local access only
 Web page with reduced BW requirements for off-site access
 Archive search and retrieval
 Camera documentation

Data processing:

 Subtraction and scaling to 8 bits as images acquired.
 Automatic median frame generation (daytime)
 Autocompress images no longer used in medians
 Email reminder to tape data, triggered by remaining disk space.
 When space needed, automatically delete % of images per night depending on age.
 Testing/contingency

Animation tool for local replay high resolution & frame rate:

 Initialize and continuously update image cache on local disk
 Find movie player or develop front end for ximtool
 Replay controls: rate control, play/pause, rewind, single step.



List of ACTR Documents

actr@ctio.noao.edu