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Planning Your OMI Observatory System
By Rich Williams
Overview
One of my responsibilities at Optical Mechanics is to advise prospective customers and existing customers on how to set up a complete observatory system. Our telescopes and control system are the core command and control for an automated/robotic observatory. Talon, OMI’s observatory control system, controls your telescope, instruments, and observatory dome directly and automatically. To make everything work together well the entire observatory system must be integrated properly. The basic components of an automated observatory system include:
- A custom OMI telescope
- CCD camera and filter wheel
- Monitoring and power recovery systems
- Dome or roll-off roof enclosure
The following sections describe each of these subsystems of an automated observatory.
OMI Telescopes
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You can read a detailed description and specification of our standard Nighthawk line of automated/robotic telescopes on our web site. The custom telescopes we design and manufacture use the same Talon software sometimes with added features and components for special requirements. The telescope system that you choose for your project depends on your needs and budget. Our Nighthawk telescopes fulfill the needs and requirments of most astronomy education and research programs. In addition to its capability to control the telescope, instruments and observatory enclosure directly and automatically, Talon offers many powerful data analysis tools for doing high precision photometry, astrometry, automated image calibration, and so on. And we include source code as part of our licensing agreement so you can customize and expand the software to suit your special needs in the future.
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CCD Cameras and Filter Wheels
CCD cameras
When planning your observatory project one of the most important decisions you'll make is which telescope and CCD camera combination best fit the goals of your project. Ultimately the best CCD camera for your project depends on the diameter and focal length of the telescope, the type of observing projects you plan, and of course your budget.
In the past few years many good articles have been published about matching CCD camera pixel sizes to the focal length of telescopes to get images that are not over-sampled or under-sampled. Some of the articles state that a pixel resolution of about two arc seconds fits well for "average seeing." The assumption in these articles is that the reader (typically an amateur astronomer) is using the telescope in a less than ideal location where the seeing is seldom better than two to four arc seconds or worse. Many of Torus' customers place their telescopes in observatories in locations where the seeing is often one arc second or less. Because a 2-arc second pixel resolution would under sample the seeing condition for these locations, you should choose a CCD camera that provides smaller pixel resolutions to take full advantage of the site.
Other factors to consider for your project are the field of view and quantum efficiency of available CCD cameras. If the main emphasis of your project is searching for asteroids, comets, or other objects without predetermined locations, you'll want to use a CCD camera that offers the greatest field of view to increase your chance of success. On the other hand, if you want to do photometry of fainter objects, choose a CCD camera with high quantum efficiency. Some CCD cameras offer very high quantum efficiency and relatively wide fields of view, but at a greater cost. Thus the telescope/camera system you ultimately choose will also depend on the budget of your project.
Real-World Examples
To give you a better idea about how to determine the best CCD camera and telescope system combination for your project, let's look at some examples using the OMI Nighthawk Series telescopes and several CCD chips found in popular cooled CCD cameras. The table below shows the relationship of telescope size and focal length to field of view and quantum efficiency for various combinations of telescopes and CCD cameras. The table includes the relevant specifications of the various telescopes and CCD cameras. The data in the last six columns contain information about the pixel resolution, the field of view, and the signal to noise ratio (SNR) qualities for each telescope/camera combination.
The pixel resolution and the field of view for each telescope/camera combination is a function of the size of the individual pixels and the total number of pixels of a camera's CCD chip relative to the overall focal length of the telescope. The SNR data provides a measure of the quality of the data you can expect to get with the telescope/camera combinations. To better understand the SNR data you need to know what it's based on. I used Bradley Scheafer's CCDLIMIT program that appeared in the May 1998 Sky and Telescope magazine to calculate the SNR data in the table. You can download CCDLIMIT from the Sky and Telescope web site. I modified the program to calculate the SNR data for the different quantum efficiencies of the Apogee cameras. The SNR data are based on the following constants:
- New moon and sun far below horizon.
- Zenith distance = 30 deg. (star is 30 deg. below the zenith).
- 1 arcsecond FWHM seeing in V (visual) at the zenith.
- Relative humidity = 40%.
- Air temperature = 15 deg. C.
- Latitude = 30 deg.
- Altitude above sea level = 1000 meters.
- Rural area with little light pollution.
- The color index (B-V) of the target stars are 0.7
- The SNR magnitudes are for V-band (visual)
The constants above are representative of a very good observatory site in the Southwest region of the United States. The data are used as a baseline to calculate the signal to noise ratios in the table. Observatory sites with less ideal conditions will not attain SNRs as good as those listed in the table. However, the relative ratios will be about the same.
I chose signal to noise ratios of 10 and 100 to show how faint of a magnitude of a star you can detect and the limits of photometry you can expect for each of the telescope/camera combinations. A SNR of 10 enables you to confidently detect a star or asteroid and determine the astrometric position of the object. While you might be able to marginally detect a star with a SNR as low as 3, the uncertainty is probably too great for the data to be reliable. The uncertainty of the magnitude for a star with a SNR of 10 is approximately 0.1 magnitudes, which is inadequate for photometry work. At a SNR of 100 a star's uncertainty of magnitude reduces to approximately 0.01 magnitudes. Thus SNRs of 100 and greater provide good data for photometry work.
You can use the SNR data in the table to help determine what telescope/camera combination best fits the needs of your project. The SNR = 10 columns show how faint of a star with a SNR of 10 you can detect with 1-minute and 10-minute exposures for each telescope/camera combination. For example, A TORUS CC04 equipped with a Finger Lakes Instruments CCD camera with a SITe 512 x 512 back-illuminated chip with 24-micron pixels can detect a 19.4-magnitude star (or asteroid) for a one-minute exposure and you can attain a SNR of 100 for a 15th magnitude star in about 35 seconds. The field of view with this combination is only 10.3 x 10.3 arc-minutes. The same telescope with an Thomson 2048 x 2048 front-illuminated chip with 14-micron pixels provides a 24.1 x 21.4 arc-minute field of view providing 5.47 times more imaging area; a real advantage for search programs. However, the Thomson chip has a lower quantum efficiency (front illuminated) and smaller pixels and only detects stars to magnitude 18.3 for a one-minute exposure and requires 75 seconds to reach a SNR of 100 for a 15th magnitude star. The Nighthawk 400 with Finger Lakes Instruments CCD camera with a SITe 1024 x 1024 back-illuminated chip with 24-micron pixels gives you the quantum efficiency and larger pixels of the 512 x 512 pixel chip with four times the imaging area of the smaller chip.
When deciding which CCD camera to use with your telescope system, there are many factors to consider. The final decision will be based on how you intend to use the telescope/CCD camera combination for your particular project and your budget. If the emphasis of your project is searching for objects such as asteroids or comets, then you'll want a system that gives you a wide field of view to decrease the time and the number of images required to cover as great of an area as possible each observing session. If your project is primarily concerned with acquiring the best photometric data, then you'll want to use a camera with a back-illuminated chip and the greatest quantum efficiency.
We currently have device drivers to adapt Finger Lakes Instrumentation and Apogee CCD cameras to our Talon observatory control system. These companies offer comprehensive lines of CCD cameras that should provide you with the quantum efficiencies and fields of view to meet your needs and requirements. We can contract with you to develop device drivers for other cameras or you can write the software yourself.
Although this article uses the Nighthawk Series telescopes as examples, the same principles apply for all our custom telescope projects. New CCD cameras are continually being developed opening up new possibilities for integrated telescope/CCD camera combinations.
| Nighthawk 400 |
Primary Diam. (cm) 40.6 |
| Focal Length (mm) 4064 |
| CCD Camera |
Kodak
KAF-1001E |
SITe
SI-502AB |
SITe
SI-003AB |
Thomson
THX7899M |
| Pixel Size in Microns |
24 |
24 |
24 |
14 |
| Pixels in Rows and Columns |
1024 |
512 |
1024 |
2048 |
| Arc Seconds/ Pixel |
1.21 |
1.21 |
1.21 |
0.71 |
| Field of View in Arc Minutes |
20.66 |
10.33 |
20.66 |
24.10 |
Magnitude Attained for
60-second exposure
@ SNR = 10 |
18.7 |
19.4 |
19.4 |
18.3 |
Magnitude Attained for
600-second exposure
@ SNR = 10 |
20.79 |
21.4 |
21.4 |
20.6 |
Seconds to Attain SNR = 100
for 15th Magnitude Star |
70 |
35 |
35 |
75 |
Seconds to Attain SNR = 100
for 18th Magnitude Star |
1230 |
620 |
620 |
1310 |
| Nighthawk 500 |
Primary Diam. (cm) 50.8 |
| Focal Length (mm) 5080 |
| CCD Camera |
Kodak
KAF-1001E |
SITe
SI-502AB |
SITe
SI-003AB |
Thomson
THX7899M |
| Pixel Size in Microns |
24 |
24 |
24 |
14 |
| Pixels in Rows and Columns |
1024 |
512 |
1024 |
2048 |
| Arc Seconds/ Pixel |
.97 |
.97 |
.97 |
.56 |
| Field of View in Arc Minutes |
16.53 |
8.26 |
16.53 |
19.28 |
Magnitude Attained for
60-second exposure
@ SNR = 10 |
19.1 |
19.7 |
19.7 |
18.6 |
Magnitude Attained for
600-second exposure
@ SNR = 10 |
21.2 |
21.7 |
21.7 |
20.9 |
Seconds to Attain SNR = 100
for 15th Magnitude Star |
44 |
22 |
22 |
49 |
Seconds to Attain SNR = 100
for 18th Magnitude Star |
770 |
385 |
385 |
840 |
| Nighthawk 600 |
Primary Diam. (cm) 61.0 |
| Focal Length (mm) 6100 |
| CCD Camera |
Kodak
KAF-1001E |
SITe
SI-502AB |
SITe
SI-003AB |
Thomson
THX7899M |
| Pixel Size in Microns |
24 |
24 |
24 |
14 |
| Pixels in Rows and Columns |
1024 |
512 |
1024 |
2048 |
| Arc Seconds/ Pixel |
.81 |
.81 |
.81 |
.47 |
| Field of View in Arc Minutes |
13.77 |
6.88 |
13.77 |
16.06 |
Magnitude Attained for
60-second exposure
@ SNR = 10 |
19.3 |
20 |
20 |
18.8 |
Magnitude Attained for
600-second exposure
@ SNR = 10 |
21.4 |
22 |
22 |
21.1 |
Seconds to Attain SNR = 100
for 15th Magnitude Star |
31 |
16 |
16 |
35 |
Seconds to Attain SNR = 100
for 18th Magnitude Star |
535 |
265 |
265 |
600 |
Filter Wheels
We offer a 9-position filter wheel that accepts 50mm filters designed specifically to integrate with our telescopes and software for automated operation. Most serious scientific photometry is performed using standard U, B, V, R, and I (ultraviolet, blue, visual, red, and infrared) narrow bandwidth filters. Also many amateur and professional astronomers use R, G, and B (red, green, and blue) or C, Y, and M (cyan, yellow, and magenta) filters to produce visually appealing ‘true color’ images. Nine position openings for filters enables you to have all five UBVRI filters, as set of RGB (or CYM) filters and one other filter such as a neural density, clear, H-alpha, and so on.
Monitoring and Power Recovery Systems
An automated/robotic observatory system needs to have a way to monitor the local and external environment to react in an appropriate manner to protect your equipment and data. We supply a weather station with each telescope that continually monitors weather parameters such as temperature, wind speed, and humidity. You can set up limits to any of these parameters that will trigger Talon to do specific actions in response. For example, you might set the observatory shutter or roof to close if the wind speed is greater than 40 kilometers per hour, or the humidity is greater than 90 percent, or the temperature is less than -20 degrees Celsius. You can also set up Talon to reopen the shutter or roof and resume operation of the observatory when the chosen parameters are back within a safe range.Power protection and recovery are crucial to the safe and continual operation of an automated/robotic observatory. We supply an appropriate uninterruptible power supply (UPS) with each telescope. The UPS works with Talon to protect and filter the incoming power to protect the sensitive electronics from power spikes and variable power voltage levels. Also, in the event of a power failure, Talon and the UPS work together to gracefully shutdown the observatory closing the dome shutter or roof, saving current data, and shutting down the control computer.
Dome and Roof Enclosure
There is a continual debate in the astronomy community on the benefits of enclosing telescopes in rotating domes or roll-off roof buildings. Regardless of which type of observatory building you chose, opening, closing, and positioning of the shutter (for a dome) must be controlled by Talon to enable automated operation of the observatory. Whichever type of observatory enclosure you choose, it’s important to place the building where the thermal effects in the local environment in and around the building will not degrade your local seeing. There is a lot of information on this subject in books and on the Web.
We ship your telescope to your observatory site and install it on a pier that you or your contractor constructed to our specifications. The best pier is made of steel-reinforced concrete sitting on top of a footing several feet below the frost line. Six to eight feet below the grade of the building is a good rule of thumb for the depth of the footing. The pier and its footing should be completely isolated from the floor and any other part of the building to avoid the transfer of vibrations from the building to the telescope.
There are many dome manufacturers around the world and many ways to design and construct a roll-off roof observatory. There is no single readily available solution for interfacing domes or roll-off roofs with Talon for automatic control. We have worked with customers to adapt Technical Innovations, Ash Dome, Baader Planetarium, and other domes and custom roofs to work with Talon. For roll-off roofs, you need to have switches that tell Talon when the roof is fully opened and when it is fully closed. This is a straightforward system to set up. Domes are more complicated to control; you need to control and coordinate the azimuth position of the shutter opening with the position that the telescope points to and control the opening and closing of the shutter. Controlling the azimuth typically requires an encoded feedback loop to slave the azimuth position of the shutter with the telescope position. Like the roll-off roof the shutter can be controlled using switches that determine when the shutter is fully opened and closed. Because each dome or roll-off roof design is different, we will work closely with you from the early planning stages of your observatory project to ensure that your dome or roof with fully integrate with the Talon observatory control system.
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| Telescope |
Minimum Dome Diameter |
| Nighthawk 400 |
10 feet / 3.5 meters |
| Nighthawk 500 |
12 feet / 4 meters |
| Nighthawk 600 |
14 feet / 4.5 meters |
| Nighthawk 800 |
18 feet / 5.5 meters |
| Nighthawk 1000 |
20 feet / 6.5 meters |
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What size dome should you buy for your telescope? The answer depends on how you plan to use your telescope and observatory. If you plan to frequently have many people inside the dome (for education, outreach, or fun), then you should buy a dome that is bigger than the minimum required for a more automated observatory that is mostly unmanned. As a guideline below are recommended minimum dome sizes for our Nighthawk line of telescopes. If you plan to have many people in the dome at once frequently, then increase the size of the dome to the next size or bigger.
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