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Pluto/Triton occultations
Young et al.
Pluto and Triton: observing their evolving
atmospheres with stellar occultations:
Facilities and Equipment
Laboratory
Southwest Research Institute in Boulder has an optics lab suitable for maintaining the Portable
High-speed Occultation Telescope (PHOT) systems we propose to use.
Clinical:
Not applicable
Animal:
Not applicable
Computer:
Southwest Research Institute provides all of the computer equipment and support needed to
conduct the intended research.
Office
Southwest Research Institute provides all of the library facilities and staff support needed to
conduct the intended research, and a clean, safe area in which to store the two PHOT systems in
use at SwRI Boulder.
Other
Not applicable
MAJOR EQUIPMENT
1.
Summary
The Boulder office in Southwest Research Institute and with Lowell Observatory have been
funded by NSF Major Research Instrumentation (MRI) to build four lightweight, portable 14"
occultation systems that can be easily deployed to sites around the world. The Project
Description is self-contained, and includes a summary of these systems. We repeat and elaborate
on that description here. (Note that without the duplicated tables, the sum of the Project
Description and this description of Major Instrumentation is still within 15 pages.)
Our occultation systems will use high-speed, low-noise CCDs suitable for high-frame rate
occultation observations. Our acronym for these systems is PHOT, for Portable High-speed
Occultation Telescope. Development is underway; we received shipment of our cameras on
November 11, 2003, on schedule for a useable system by summer of 2004. We expect the PHOT
systems to have a useful lifespan of at least ten years.
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Pluto/Triton occultations
Young et al.
2.
Performance
The PHOT systems are being developed by Eliot Young (SwRI), Leslie Young (SwRI), and Ted
Dunham (Lowell), each of who has helped design and operate stellar occultation systems for
more than a decade, and understand the unique requirements that stellar occultations place on
telescopes and cameras. The requirements on a stellar occultation system are summarized in
Table 1.
Table 1: System performance of the Portable High-speed Occultation Telescopes (PHOT)
Component
Frame rate
Dead time
Full field of view
Pixel scale
Subframe size
Timing knowledge
Storage capacity
Deployability
Ease of setup
Sensitivity
Requirements
1-20 Hz (5-1000 ms/frame)
< 10% of frame rate
1-20 arcmin
0.5-1.0 arcsec/pixel
30-200 pix
1 ms
> 10 Gb
Worldwide, <3 weeks lead time
Observe < 1 h after 1st star
acquisition
Pluto: SNR=16 at 5 Hz
KBO: SNR=10 at 1 Hz
Centaur: SNR = 10 at 20 Hz
Designed performance of PHOT
0.4-200 Hz
<0.01% of frame rate
6 arcmin
0.75 arcsec/pixel
1-512 pix
< 0.001 ms
20 Gb
2 week lead time typical
20 min setup typical
Pluto at V< 14.3
KBO at V<17.4
Centaur atV<14.2
High frame rates are needed in order to derive vertical profiles of temperature and pressure
from an atmospheric occultation or to resolve narrow features like jets. We determined the frame
rate achievable on the Roper Instruments 512BFT camera with the existing WinView software is
7 Hz for a 64 x 64 subframe. Our planned and budgeted custom software will require the
minimum number of parallel and serial shifts and will be limited by the digitization rate. We will
achieve frame rates of 25 Hz for a 64 x 64 box at the slow digitization rate of 100 kHz (with 3.1
electron read noise), and over 200 Hz frame rate at the fast digitization rate of 1 MHz (11.8
electron read noise). The pixel smear caused by image rotation with our alt-az mount is less than
1/10 of a pixel if we maintain frame rates faster than 0.4 Hz.
Dead time needs to be minimized for photometric fidelity. Atmospheric occultations are
often characterized by "spikes," or local refocusing of the stellar flux by small density
fluctuations in the occulting atmosphere. If the dead time is larger than 10%, then these spikes
may not get recorded, degrading the photometric interpretation of the occultation lightcurve. The
dead time per frame is dominated by the serial transfer rate, and is only 600 ns to shift all 512
rows to the serial register.
The full field of view should be relatively large to help identify star fields. Pixel scales of 0.5
-1.0 arcsec are a good compromise: much larger pixels lead to object confusion and excessive
background contamination, while much smaller pixels give unnecessarily large read noise and
data volumes. Occultations are typically run with small sub frames sizes to increase frame
rates. Typical subframes range from 30x30 pixels to 200x200 pixels, and are sized to allow good
subtraction of sky background. The full field of view for the 6.7-mm area of the 512BFT array is
6.4 arcmin for the 3.556 meter focal length of the Meade 14" LX200-GPS telescope, and the
pixel scale for the 512BFT's 13-micron pixels is 0.75 arcsec. Both the existing WinView and
planned PVCam software will allow the specification of multiple subframes of any size from a
single pixel to the full array, surpassing our requirement of subframes ranging from 30 to 200
pixels.
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To know the timing of a given frame to 20% at the fastest frame rate (5 ms/frame) requires a
timing knowledge of 1 ms. The timing knowledge of the 1 PPS - 1 million PPS (pulse per
second) output of the Trak 8821A GPS is specified to be within one microsecond of Coordinated
Universal Time (UTC). The computer needs enough storage capacity for an event's data, to
ensure data is archived properly. A 30-minute occultation at 20 Hz will produce 36000 frames.
For 128x128 pixel subframes (32 kilobytes per frame), this requires 1.1 gigabytes of storage.
Test runs will be made for each event, which increases the storage requirements by a factor of
roughly 10. The storage capacity of the Latitude C840 computer is 20 Gb, providing room for
programs and system files, and 10 Gb of data storage.
The new system will be designed easy deployability, since frequent use provides a large
scientific return on the investment of hardware. Since salaries associated with travel and
preparation are a major factor in the cost of observing occultations with current occultation
systems, decreasing the preparation time allows more occultations to be observed on the same
budget. The lead time for preparing and shipping the PHOT systems should be typically less than
two weeks, based on our recent experiences and our plans to include a local expert on
import/export regulations. Ease of setup will increase the number of occultations observed.
Many occultations occur at difficult observing circumstances: shortly after sunset, shortly after
the object has risen above the horizon, or during breaks in cloudy weather. In these cases, the
ability to set up the telescope and acquire the field soon after stars are detectable can make the
difference between an excellent data set and a complete loss of the event. The setup time for the
Meade LX200-GPS systems is notoriously short, and we conservatively estimate 20 minutes
between acquiring the first bright star and taking data on the occultation objects.
The sensitivity requirements depend on the scientific goal. The required SNR and frame
rates for each type of occultation were discussed above in the context of Pluto, KBO or centaur
occultations. We recap these SNR requirements here for the three cases, and discuss the expected
number of events per year by sufficiently bright stars. Compared with the PCCD system. The
PHOT system allows our science goals to be met with stars that are 1.4 mags fainter (3.6 times
fainter) for observing Pluto's changing atmosphere and 1.1 mags fainter (2.7 times fainter) for
measuring KBO shapes and sizes. The importance of low read noise is most evident in the highspeed observations of centaurs, where the PHOT systems can go 2.2 mags fainter (7.6 times
fainter) than the PCCD systems. This dramatically increases the number of events observable.
3.
Design
Table 2 summarizes the planned PHOT system. The telescope will be a 14" with a computercontrolled Alt-Az mount. The detector is a high-speed, low-noise frame transfer array with an
active area of 512 x 512 pixels. This CCD is run from a laptop, and probably will include a dock
to house a full-size PCI card to control the CCD. A GPS-referenced clock will generate an
accurate external time sync to trigger the CCD. This GPS unit may be a PCMCIA card in the
laptop, although our default choice is a stand-alone unit from Trak that produces accurate 1 Hz
and 1 MHz pulses.
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Table 2. Baseline design of PHOT
Manufacturer
Meade
Roper Scientific
Dell Computer
Trak
National Instruments
Clear Night Products
Various
Atlas Cases
Component
Telescope
Camera
Laptop
GPS clock
Timing card
Wind screen
Field kit
Shipping cases
Items(s)
14" LX200GPS
MicroMAX-512BFT
Latitude C840
8821A-12
NI PXI-6602
Teledome
Various
3 custom cases
Newer telescopes have higher throughput than the Celestron 14 inch telescopes used for
PCCD, especially with high transmission coatings. Also, most observers have stories about close
calls or missed occultations that could have been averted with help from a GPS-assisted,
computer-controlled mount. Of the half-dozen CCD vendors we contacted, only Roper Scientific
has frame-transfer detectors that had low read noise, thermo-electric cooling of the CCDs, and
PCI-card connections to the host laptop computer that can handle the necessary frame rates. We
have purchased the "512BFT" CCD from Roper Scientific, where B means "back illuminated"
and FT means "frame transfer." The array has 1024 rows and 512 columns, but only a 512 x 512
area is used for imaging; the other 512 x 512 region is used for reading out the previous image
while an exposure is taking place. Tests at SwRI Boulder show this camera has a readnoise of
3.1 electrons at 100 KHz, much lower than the read noise of around 12 to 15 electrons per pixel
for the decade-old PCCD occultation camera. The Roper Scientific CCDs can be triggered from
an external TTL signal generated by the 1-MHz output of a GPS receiver, processed by counters
on a timing card.
For use in the field, we will use a windscreen to decrease windshake (a problem, for
example, with the Chillagoe observations of T176). A field kit will include tools and materials
needed for assembly and minor repairs. We have asked Atlas Case Corporation
(http://www.atlascases.com) for quotes on foam-filled cases made of 1/4" ply/fiberglass
laminate cases for the Optical Tube Assembly (OTA), the fork mount, and the CCD camera and
electronics (3 separate cases). Not only will the cases facilitate safe air transit of equipment, they
will permit two observers to load each case into rental cars or vans.
OTHER RESOURCES:
For this program to be feasible, we must be able to get the equipment to the desired location.
Since the events of Sept. 11th, shipping or traveling with certain technologies has been restricted,
and is prohibited to some destinations. Advance planning is necessary to get the correct licenses
and documentation, in order to prevent transportation delays. To address these concerns, we
have an SwRI consultant available to evaluate and classify our equipment, and then to review the
import/export restrictions on the equipment prior to making travel plans. For each occultation
we will review the planned destination against the most current government regulations (since
the restrictions are constantly changing with world events).
Facility and Equipment-4