What’s in a name?

Volunteer Observatory is the latest in a progression of observational facilities built to satisfy the growing needs of an amateur astronomer. While many observatories are named for famous astronomers or their benefactors I sought to provide some clue as to the location and mission of the observatory.

Being a student of history I also wanted to honor the volunteer tradition of the people from Tennessee and our state’s unique contributions to American history so the observatory name is derived from the “Volunteer State “ nickname of Tennessee. The name also provides a clue to its geographic location for anyone familiar with the Southeastern Conference of colleges and schools as it is located in the heart of “Volunteer Country” which for folks from parts out yonder may be more precisely described as Knoxville.

 The term "volunteer" may also be thought of as a reference to this being an amateur observatory albeit a very well equipped one. Observations are made without remuneration however generous grants are always welcomed!
 

Scientific investigations are the primary focus of the observatory with an emphasis on extra-solar planet photometry. To date the observatory has been used by the author in the co- discovery and/or characterization of three extra-solar planets. Periodically asteroids are also observed to determine their rotational periods and spin vectors. The observatory features state of the art equipment and operates autonomously under control of a computer automation sytem gathering data approximately 125 to 150 nights per year.
 

When Tennesse first became The Volunteer State
 

Design

The observatory consists of a 10 foot “Home Dome” by Technical Innovations. This is a DIY kit that I assembled and erected on a tall deck in my backyard. I have found this to be an excellent product and well worth the expense. I also recommend that prospective purchasers also pay the extra charge for “pre-drilling”. I did and credit this to quick and effortless installation. If you have ever drilled fiberglass then you know what a bear it can be.

The dome rests on a short octagonal structure that I constructed. Described down the page. The dome features a 36” wide dome slit that extends past the top of the dome to permit following the object across the meridian. Despite claims to the contrary by some upstart companies offering novel new dome designs this slit feature is not problematic and is a key reason why you want a domed observatory in the first place. Much of the misinformation about “dome-seeing”  is due to ignorance or feeble attempts to discredit the value of a domed amateur observatory.
 

Dome Seeing
So what about the dreaded “dome seeing” inherent to all domed observatories? The myth is that all domed observatories suffer from a chimney effect where the air outside cools faster than the air inside the dome never to reach thermal equilibrium and the resultant turbulence degrades seeing all night long.  After  reading a few papers on the subject and a few conversations with technicians at Kitt Peak National Observaotry I learned this really only becomes problematic in situations where the telescope is operating under extremely good seeing-like sub arc-sec fwhm’s. It’s also easy to remedy with active ventilation. While I often see 1 minute exposures in the 1.8-2 arc-sec range I never have to worry about stellar fwhm’s < 1 arc-sec!  If I did I already incorporate designs that would effectively mitigate this issue. Even if seeing at your site is well within the mediocre limits at most amateur observatories you will still want to incorporate the following features into your site design:

Ventilation.All observatories must breathe! This can be easily accomplished by proper design and selection of building materials along with the use of AC/ventilation. My observatory rests on a wooden deck. Fans inside pull air from the bottom of the deck and circulate it around the dome and out through air vents. Ventilation also runs during  the observing run to ensure thermal homogeneity of the airmass inside and outside the dome. Seeing issues are related to air masses of differing densities. Constantly circulating air currents also minimize the thermal lag time of the SCT with ambient and permits the Optec temperature compensating focuser to maintain accurate focus throughout the night. Tube currents are also very quickly elimnated. I suggest that most of the “dome-seeing” encountered in amateur observatories is either due to poor design with materials that soak up heat all day long only to radiate it back out into space at night or tube current issues with the telescope. I have worked with several institutions regarding this issue and find their issues largely related to concrete bldg’s that absorb heat during the day and slowly radiate it at night. These situations require more aggressive environmental controls. In these cases air conditioning should be employed as well as ventilation systems capable of a high rate of air turnover.

Elevation.Most of the “bad seeing” outside of tube currents that amateur telescopes encounter happens in the last meter or so of air above the observing site. Many serious amateur and nearly all professional observatories are built above grade. My telescope rests ~ 3meters above grade in my backyard.

Materials.In addition to incorporating the elements of elevation and ventilation into your design you should also consider building materials that will quickly come to thermal equilibrium during the night. The use of reflective materials will help in this regard. My octagonal bldg is covered with white vinyl siding. The white dome also is highly reflective. The result is that things do not heat up inside the bldg during the day above ambient. Thermostatically controlled blowers also move air around.
 

Pier Design

I’ve built several piers and find the best way to “model” an observatory pier is to think of it as an inverted pendulum. I developed a few spreadsheets to help with this. You want to minimize the mass of the of the pier meaning the trick of adding sand to a hollow small diameter metal pipe to dampen vibrations is actually counterproductive. I’ve used both and can tell you by experience that a very rigid steel pipe functions much better than one filled with sand. I hope to add a section later further explaining this and my spreadsheet calculations. My pier was constructed from 13” steel water pipe from flanges that I constructed from mild steel. This was welded together into two sections and the sections were later bolted together during installation. Gussets  were also added at the bottom section  to tie things together with a 20 inches square 1” thick steel base plate. The base plate rests atop the footer with ¾” ani-vibration padding material between the baseplate and footing. This is actually the same material used in machine shops to isolate heavy machinery from vibration,etc The baseplate is secured with four large bolts, nuts and washers. Leveling adjustments are incorporated at the top of the pier. To support this massive structure a large concrete footer ( ~1 cubic meter ) was excavated and formed using nearly 1.5 cubic yards of concrete. Your actual footing size will depend on many things such as frost line, soil stability, etc.  The result is that the massive pier does not move and polar alignment does not vary from season to season. Folks up North will probably need a much larger footing to deal with their much deeper frostlines.

Electricity and Data

Underground conduits carry electric power and data lines to the observatory. Inside a UPS bank filters power for the observatory’s vital loads ( Telescope, camera, computers, security system, etc.) as well as provides backup power during momentary outages during operation.  I decided against wireless computer networking and have two cat 5e lines to handle data and control signals from any computer inside my home via Windows Remote Desktop. Data are saved to a dedicated network drive inside as soon as it is acquired.
 

Observatory Control

I regard automation a must for anyone involved in serious scientific projects who also has normal responsibilities.The notion of staying outside all night at the telescope while gathering data is out of touch with the reality that most of us as amateur astronomers face. Many solutions exist for those wanting to automate their observatory. Volunteer Observatory  can accurately be described as a robotic facility with the only human intervention required during startup and shutdown before leaving for work the following morning. This has been designed and implemented using commercial and self built hardware coupled with ASCOM compliant software and drivers.

The dome is driven by a pair of AC gear motors that supply power to roller drives. These drives can be manually controlled by a handpad or computer operated by means of a freely distributed interface and control driver that I found on the internet. ( More info on my automation system can be found on this page ). I adapted the LESVE dome interface to work with AC motors and am using the standard drivers developed by the inventor. This system is ASCOM compliant and functions under control of the Astronomer’s Control Panel ( ACP ) software for full robotic observatory control.
 

A typical night starts with me opening the dome and starting the hardware and computers. Ventilation fans are started as is water assisted cooling for the CCD during the summer months. A diffuser is placed over the telescope ota and dewshiled and then the scope is slewed to a ready position for acquiring sky flats for calibration at dusk. My goal is to be able to achieve 1 mmag rms precision with 5 minute averages while conducting millimag photometry runs of extra-solar planet candidates. This is not trivial and I have examined my processes extensively. I now have a system that works brilliantly and allows me to achieve this goal when observing conditions permit. Some may find these practices questionable while other may see them as inordinately complex. While I generally have good results with the practice of maintaining a library of dark frames ( remade at least on a monthly basis ) for calibration I make it standard practice of acquiring fresh flats and flat darks each night for each filter to be used. Since the aluminum SC ota has a linear temp expansion coefficient I also guestimate where the correct focus position is based on the previous position and temp at shutdown from the previous night and anticipated low temperatures for the night of the observing run. Temp info is acquired from the Optec TCF-s temp probe mounted on the ota. I usually average the two positions to arrive at the focus position to be used for flat field acquisition. I also have developed a method for accurate dome flats but prefer sky flats as they are speedier. I have measured my flat field errors and find that I achieve calibration to better than 0.5% across the ST10’s KAF 3200ME CCD array.

I also have polar alignment tweaked to better than 30 arc-secs of the pole. This is necessary to prevent field rotation around the guidestar during an imaging run. To minimize systematics it is necessary to not only create good flats but also to maintain the (x,y) centroid positions of the stellar psf’s constant throughout the observing run. The Astrophysics 1200 GTO mounting is a superb piece of machinery that tracks exquisitely. Uncorrected periodic error is ~ 3 arc-secs peak to peak and pointing is better than an arc-minute across the sky with the built in pointing corrector feature with ACP. Polar alignment permits relative motion across the chip during a night to be held to values << stellar psf fwhm’s. ( before meridian flips of course ) I typically see a drift of < 2 pixels. This should compensate for any flatfielding errors and indeed my differential photometry now routinely approaches 1mmag rms.

 Target data and script details are entered into a plan using the ACP Planner utility. Trial exposures are first verified with the target as well as the availability of suitable guide stars. Once the script is executed by ACP the computer has full control of the observatory until the end of the run unless it is aborted by the operator.

ACP controls the observatory during the run and automatically calibrates each image immediately after acquisition and if west of the meridian flips it to align with those from the east side. The data is then stored in a directory located in a networked HDD inside the house. Additionally each image can be automatically plate solved using the built in Pinpoint astrometric engine and the fits file updated with ra/dec and plate constants.  At the end of the run ACP automatically parks the scope and shuts down the CCD. The dome is also parked and if it a shutter motor were installed, would be automatically closed. In the mornings I have a folder ready to be burned to DVD and I spend less than three minutes outside shutting down the observatory and disconnecting everything from the grid. This is the only way to be sure  your equipment is protected from lightning damage. The security system and video surveillance cameras remain online via a surge arrestor circuit. These are also tied in to a network to permit monitoring from any location via the internet.
 

I have no formal plans for the observatory. The deck design is unique to my backyard and the support structure is a simple octagonal bldg. that stands four feet tall. The radius is 120”. I figured the details out on paper then discovered this excellent site online that figures out the cuts. The frame is constructed of 2x 6’s and made for a fun project with a 12” compound miter saw.

Construction Photos
 
 

Dome and Support bldg
Octagon details
Dome and Side walls ( made from Dome shipping crate ! )
Alot more progress
 

Machining Pier Flanges ( playing with my mill )
Welding ( This is Brent Holt- a friend and metal working extraordinairre)
Rigging!
Pier installed
 

Coming together now

FINIS! ( almost )

Automation in progress!

This is what I'm talkin' bout- It's ALIVE!
 
 

More Volunteer Observatory Images