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Luna-tic

Starquest XV at Green Bank Observatory

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This isn't so much to advertise an upcoming party as to share my experience at a recent one. If needed, mods please move to a more appropriate section.

I realize much of the readership on SGL is European and UK, I thoroughly enjoy reading what like minds are doing across the big pond. I thought I'd share a bit about the star party I attended this past week on my side of the Atlantic.  Starquest XV is held yearly at the Green Bank Observatory in West Virginia, one of the most beautiful areas in the eastern US. GBO is a radio astronomy observatory, and is home to the largest steerable radio telescope in the world. At full tilt, it is over 135 meters tall, and the dish measures 100 meters across and encompasses 2.3 acres in area. It took ten years to build and weighs 17 million pounds. Located at the edge of a large national forest, it is also within the national Radio Quiet Zone, a square roughly 50 miles on a side, where radio emissions are highly regulated in order to minimize RFI at the observatory. There is no cell phone service within 20 miles of the telescope, and once on the observatory grounds, anything that emits energy is highly shielded or forbidden. You can't have a digital camera within a mile of the telescope, and the control room and the computer room in the visitor center are built within huge Faraday cages to trap the RFI. Microwave ovens are also caged in the cafeteria.

I was amazed to learn that a cellphone, at a range of 5 kilometers, emits RFI in the 2.5 million Janksy range. The telescope routinely picks up cosmic energy in the 0.002 Jansky range, so it's easy to understand why RFI from local sources can impede reception of desired signals. As part of the star party, attendees can sign up for a tour of the telescope;  staff members take groups of four or five up on the telescope as far as the array platform, which is 100 meters up with the telescope in its "maintenance park" position. This position puts all the platforms and catwalks in a horizontal position, and the two elevators in vertical positions. The star party is held during a maintenance week, and there are numerous workers in climbing gear all over the scope, painting and repairing various parts on a specific schedule, in order to keep the telescope operating reliably.

Riding up the first elevator takes you to the level of the altitude pivot (the scope uses an alt-az type mount). You walk along a catwalk to a second elevator which takes you up the reflector array arm as far as the array turntable and control room (unmanned during use). You get off and are standing on an open platform over 100 meters off the ground. All the platforms are open grating, so a glance down at your feet has you looking straight down at the ground and the structure between it and you. The only portion of the telescope above you at this level is the secondary reflector, another 15 meters up. Climbing gear is required to get there. The telescope uses an off-axis design, the secondary reflector and receiver array do not hang over the primary dish, which is a modified parabola.  Signals from space reflect from the primary up to the secondary, which reflects them down to the receiver array. This array is on a turntable about 3 meters in diameter, which can be rotated to place the desired receiver in the beam path.

As we work our way back down the upper elevator, we stop at selected platforms to allow us to view different portions of the structure and mechanism. All the drive motors on the telescope are hydraulic, so they emit no RF while moving the telescope. We stopped at the altitude drive, where we saw the four motors (each about a meter in diameter) that tilt the dish. The staff member told us that the telescope was so finely balanced that a single person could move the dish if the motors were not engaged to the geared track. There is a massive counterweight built into the gear arch used to move the dish. Back on the ground, we saw the four sets of drive motors (four motors to each set, and twice the size of the altitude drive motors) that are used to point the telescope in azimuth. The telescope rides on a circular track and can rotate in azimuth thirty degrees per minute, and can pivot ten degrees per minute in altitude. There is also a central pivot point that shares the weight of the telescope with the azimuth track. Both of these are concrete structures that extend almost 20 meters into the ground to bedrock. The tour took about 1-1/2 hours, and much as I enjoyed it, I was happy to get back on solid ground. The scale of this machine is hard to describe, and everywhere you are on the structure is highly exposed. The view from the top is amazing. Since digital cameras are not allowed within almost a mile of the telescope, pictures can only be made with film cameras. I couldn't find a battery for my old Canon AE-1P, so I bought a cheap disposable 35mm camera to make pictures while on the telescope. I'll post some up when I get them developed. The pictures shown below were made just under a mile away, at our observing field, where I was camped for two nights.

The star party itself is fantastic. The Observatory is a tourist destination and strongly promotes astronomy and astrophysical sciences. There are scheduled speakers on topics from observing Mars during the opposition, to techniques for deep space imaging and planetary imaging, to new discoveries and technologies in radio astronomy. Attendees can sign up to learn how to control and operate the 40-foot radio dish, and can get work time on the scope at night. Our observing field is about three acres with fantastic horizons, even though we're in the heart of the Appalachian Mountains. Thursday at dusk looked like a washout for observing, about 70% clouds at dusk around 8:30 pm, but by 10:30 the skies were almost totally clear and stayed clear until about 2:30 am. There is no sky glow except for one small area where a ski resort lies about 25 miles away. The sky would classify as a Bortle 2 or at worst a Bortle 3. The Milky Way is clearly visible in its full arc, and seeing was very good. I had two telescopes set up, a WO GT81 on an AVX mount to view wide field, and an Edge HD 8" on an EQ6-R Pro to get higher magnification views. The telescopes on the field were varied, the largest was a 24" Dobsonian, and there were several 10" and 11" SCT's and a few 5" refractors. One gentleman was imaging with an Edge 11 using Hyperstar. I saw a few of his early constructs, he was imaging the Lagoon and Trifid Nebulae, and they were awesome. I stayed two nights; Friday night was even better than Thursday, by 8:30 the skies were clear and remained so until around 4 am. I spent most of my time in Scorpio and Sagittarius, but wandered around the zenith quite a bit. The site is at 38*N, Vega is directly overhead around midnight.

I had a wonderful time at this event, and will probably make it a yearly thing; it's a five hour drive for me, but the people, event and the dark skies are worth it. Now for a few pictures; first few are the telescope itself. Remember these are made almost a mile away. The first picture is in the "maintenance park" position, you can see the lower elevator track and the horizontal catwalk that goes toward the left. The small box at the intersection of the catwalk and elevator track is the elevator car, which is just slightly taller than a person, so you can get some idea of scale.  2nd picture is the dish at full tilt, facing toward me, and the third picture is the telescope  facing away. Fourth and fifth pictures are my camp spot and the gent with the Hyperstar/Edge 11. Last picture is the beautiful 24" f/5 Dob.

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Edited by Luna-tic
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As promised, a few pictures while on the telescope during the tour. 1st picture is a worker out on the dish, you can see how large it is. 2nd picture is at the dish level, looking up at the underside of the array platform, which is 325 feet up. 3rd and 4th pictures are from the array platform. It's still another 45 feet to the secondary reflector, but it can only be accessed by climbing the structure.  The telescope in the distance is the 140 foot telescope. Last picture is the base of the telescope, not the size of the trucks below it.

A couple of interesting notes about the 140 foot radio telescope. It is on an equatorial mount, one of the largest ever built. The RA bearing is the largest ball bearing ever manufactured.

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Edited by Luna-tic
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It looks a fantastic event, thanks for posting the pictures. I often fly from Manchester airport here in the UK and really like it when I get views from the plane of the Lovell dish at Jodrell Bank.

Edited by melsmore

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Referring to the picture of the worker out on the dish; you can see the dish is made up of many individual panels. There are, in fact,  2004 of them. The dish is an "active" surface, meaning it can change its shape to compensate for expansion, contraction, and sagging in order to maintain its highly tuned parabolic shape. There are 2209 actuators under the panels that move the panels to keep them aligned. The panels have a surface accuracy of 50 micrometres; the scope can operate at frequencies between 290 MHz and 100 GHz.

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