Hereford Arizona Observatory (HAO)

HAO#2, illuminated by a full moon, with Venus setting above mountains with snow-covered peaks at 9500 feet.
Links Internal to this Web Page
    Tour of HAO
    Hardware Specifications
    Filter Passbands
    Extinction (broadband spectrum)
    System Throughput for All Bands
    Flat Field Performance
    Horizon
    Measurement Capability with Meade 14" HyperStar (magnitude & wavelength)
    Clear Sky Statistics
    Directions to HAO

Tour of HAO

The HAO consists of two telescopes in two separate observatory structures: HAO#1 and HAO#2. But let's start the tour with this south-looking view of my house from near the driveway entrance.

 
Notice HAO#1 behind and to the left of my house. Looking south-south-west.


This view, looking west, shows both dome observatories. HAO#1 (foreground) houses the Celestron 11-inch (CPC-1100) telescope and HAO#2 houses the Meade 14-inch (LX-200GPS). Both are controlled using buried cables. The observing site is at 4670 feet (1423 meters) above sea level. HAO#2 (the most-used) is located at east longitude -110.23785 and latitude +31.445208.


Both domes are 8-foot diameter ExploraDomes. They are motorized for azimuth movement only (no shutter motors). This picture was taken before completion of HAO#2, and shows one of the two trenches open with conduit in position. One conduit is for AC power and another is for control cables. The left end of the house is my office (control room).



Looking inside HAO#2 which houses a Meade 14-inch LX200GPS on an equatorial wedge. Note the sophisticated RA balance tool, secured by duct tape.

 
This more recent configuration hows an off-axis aautoguider system, consisting of an Orion 80 mm, f/5 telescope and Lodestar CCD.
   
 
The optical backend consists of a wireless focuser, focal reducer lens, a 10-position filter wheel and a ST-10XME CCD. The dome material is polyethelene, which is transparent to the wireless focuser frequency. More information is given in the "Hardware Specifications" section.


The HAO#2 dome is moved in azimuth by a motor/gear that I modified to achieve 4 times better azimuth resolution. The original design used a shaft painted white and black (total of 2 sectors) that were viewed by an optical sensor; I moved the optical senor for a down-looking view of an 8 sector black and white pattern. The quantization interval is 4 degrees, and this provides reliable slaving of the dome to the telescope azimuth with no occasions of dome slit obstruction.

HAO#1 houses a Celestron 11-inch CPC-1100 telescope. It employs a wireless focuser, a 10-slot filter wheel and SBIG ST-10XME CCD. Note the sophisticated Dec balance weight, held in place by bungie cord.


The control room has dedicated computers for the M14 (right) and C11 (left). Each computer commands the telescope, focuser, CCD, etc using serial and USB cables or wireless units. Communication with the CCD cameras employs 100-foot ethernet cables and pairs of USB boosters (Icron). Each computer has a dual DVI video card for display on two LCD monitors. In this view both telescopes are observing the same object, as seen by their respective left panel displays. The right displays are usually used for display of TheSky/Six which assists in positioning the main chip FOV so that the autoguider chip has a sufficiently bright guide star. The M14 station includes a wireless audio/video camera display; this provides audible and visual feedback of M14 movements.

Hardware Specifications

M14 (in HAO#2)
Meade LX200 GPS, a 14-inch Schmidt-Cassegrain telescope on an equatorial mount (Meade Super Wedge), driven by SciTech (after market) controller.
Dome: 8-ft ExploraDome, controlled using MaxImDL 6.11 via MaxDome II and ASCOM Dome Control. Only azimuth is computer controlled (the shutter is opened and closed manually).
CCD: SBIG ST-10XME (KAF 3200E main chip, TC-237 autoguide chip next to main chip). The main chip has a 2184 x 1472 array of 6.8-micron pixels (physical size is 14.9 x 10.0 mm). Gain is 1.3 electrons/ADU (unbinned). Read noise = 8.8 photoelectrons. Full well capacity is 77,000 electrons.
External 10-position filter wheel for 1.25-inch threaded filters. The CFW contains the following filters: B (Astrodon), V (Schuler), Rc (Schuler), g' (Astrodon), r' (Astrodon), i' (Astrodon), z' (Astrodon), NIR (Astrodon) and CBB (clear with blue-blocking, Astrodon).
FOV without focal reducer (current configuration): 15.3 x 10.3 'arc. Image scale = 0.42 "arc/pixel (unbinned).
Autoguiding is usually performed with the CCD's autoguider chip, but sometimes is done using a Lodestar CCD mounted on an off-axis Orion 80 mm, f/5 telescope.
Focusing is performed using a Starizona MicroTouch focuser.

C11 (in HAO#1)
Celestron CPC 1100, a 11-inch Schmidt-Cassegrain telescope on an equatorial mount (wedge).
Dome: 8-ft ExploraDome, controlled using MaxImDL 5.2 via MaxDome II and ASCOM Dome Control. Only azimuth is computer controlled (the shutter is opened and closed manually).
CCD and CFW  are the same as for M14 (above).
FOV without focal reducer (current configuration): 18.2 x 12.3 'arc. Image scale = 0.50 "arc/pixel (unbinned).
Autogiding is always performed with the CCD's autoguider chip.
Focusing is performed using a Starizona MicroTouch focuser.

Observing Strategy

All observatory control is performed using MaxIm DL (MDL). Calibration images (bias, dark and flat) are updated at appropriate intervals. A master bias image is obtained from 2 or 3 dozen images about once a month. A master dark image is produced by median combining 1 or 2 dozen images made at the same temperature and exposure time as used for most target imaging. A master flat is obtained by median combining (using MDL's linear option) about a dozen images of the sky shortly after sunset made with a home-made "double T-shirt diffuser" placed over the telescope aperture. Every such flat image has its own dark calibration. Exposure times are set manually to obtain peak counts of ~ 40,000 (the 16-bit A/D readings have a maximum output of 65,535 counts). A new master flat is obtained typically once every two weeks (and also whenever any configuration change is made that could affect the flat).

Approximately 55 minutes after sunset (at my latitude) it is possible to start observing a target with minimal systematic errors present for precision photometry. The CCD coolers are started, with a set point that can be expected to be reached before target observations begin. The set-point temperature might be -15 C during the summer and -25 C during the winter. The dome azimuth is set using the "home" feature. is Approximately 1/2 hour after sunset it is possible to refine star alignment (pointing), using a bright star if necessary as a first step. Focus is performed just before the start of target observations (by which time the telescope temperature should be stable) and equilibrated with ambient. The focuser is always adjusted starting from a setting that is "in" with respect to expected final setting; in other words, because of backlash focusing adjustments are always made with incremental outward adjustments and measurements.

When the observing objective is to perform good day-to-day repeatability of brightness (e.g., 1 or 2 mmag precision) the target is carefully positioned at a pre-adopted x/y location in the CCD FOV (to within ~ 3 pixels). For such an observing session the off-axis autoguider cannot be used because "mirror flop" and tube flexure cause the star field to move during a long observing session (e.g., 8 hours) by many pixels (possibly 1 or 2 'arc). No flat field is good enough to correct for such movement at the 1 or 2 mmag level. Such precision observing can only be done using the CD's second (autoguider) chip. If the objective is merely to measure an exoplanet transit, then the same precautions must be taken, but without concern for placement of the star field with respect to the CCD pixel field. If the objective is to monitor large changes in brightness (e.g., 0.1 mag), then none of these precautions are needed. Since M14 is controlled using MDL 6.11 I am able to specify automatic observatory shut-down when any of various conditions occur: elevation < 15 deg, twilight, etc. Since C11 is controlled by MDL 5.2 shut down has to be performed manually. 
 
Image Processing


Image processing is always performed using MaxIm DL 5.2 (MDL 5.2). MDL 6.x lost a feature of easily specifying a large set of "check stars" and since I use ensemble photometry with 10 to 30 stars, typically, it's too inconvenient to use MDL 6.x for image analysis.

See http://brucegary.net/asteroids/#Image_Reduction_Procedure for the rest of a description of how I process images to produce a light curve.
 
Filter Passband Shapes

Here are some useful filter passband shape plots.






SBIG's "pretty picture" B, G and R filter passbands.

Extinction

My site altitude is 4670 feet, which determines the average Rayleigh scattering spectrum. The other extinction components, which are weakly or not correlated with altitude, are aerosols, water vapor and (stratospheric) ozone. Here are some atmospheric extinction measurements with model fits for a sampling of seasons, showing the range of variability of zenith extinction at the various filter bands.

 

 

System Throughput for All Bands

The "counts" produced by a star (also called ADU intensity) depends upon more than just exposure time and telescope aperture; it is also affected by the star's spectral energy distribution (SED), CCD response (relaive Quantum Efficiency), the filter transmission function, atmospheric extinction (and air mass) and telescope optics (corrector plate and focal reducer transmissions). In planning an observing session it is helpful to know the expected SNR of a particular star for each of the filter bands. The following figure shows expected SNR versus filter band for my Meade 14-inch telescope system (M14) for the following assumptions:  exposure time = 15 seconds, air mass = 1.3 (typical atmospheric extinction), V-mag = 12.0, B-V = 0.2 (blue star) and 1.4 (red star).



If the target is a red star the highest SNR is acheved by using a r'-band filter. If it's blue, the g' filter is best. Notice the low throughout for the u'-band filter; for a red and blue stars the u'-band SNRs, relative to the V-band SNR, are 1/550 and 1/26. In other words, observing with a u'-filter incurs a penalty that ranges from 3.5 to 6.9 magnitudes (for blue and red stars).

Flat Field Performance

Since my observing involves bands that extend to the short and long wavelength extremes, u' and z', it is important to use a focal reducer (FR) that has an anti-reflection coating that is wide-band. The Optec 0.5x FR has multi-layers to achieve this purpose, and it also is designed to correct for optical imperfections in the model of Meade telescope that I use (LX200GPS, sold in 2004). The following image shows flat fields for three configurations: top row is without a FR, middle row is with a Celestron f/0.63 FR (showing presence of reflections that are worst at u' and z'), bottom row is with the Optec 0.5x FR (showing minimal to no reflections):

Flat fields with no focal reducer (top row), a standard focal reducer (middle row) and a higher quality Optec focal reducer designed for use with this model telescope (and having multi-layer anti-reflection lens coatings).

Why does it matter to minimize reflections in flat fields? Because reflections add a component to the flat field that does not relate to the losses of starlight at the CCD pixel location. During a long observing session small pointing drifts require correction for each star's transmission loss corresponding to each pixel location, and if an incorrect flat field is used the calibrated star flux will be in error, causing ratios of star fluxes to exhibit errors that vary during the observing session. If absolutely no drift occurred, and no image rotation was present, even though star flux ratios would have errors they would remain fixed during the observing session and would therefore not contribute to a target's light curve shape. However, for an all-sky observing session no errors in star flux ratios can be tolerated.

Horizon

Looking south at mountains that are at 4.0 deg elevation on the meridian (vertical red line). The star Canopus (declination -52.7 deg) is transiting. Stars at declination -53 deg can be observed for more than an hour either side of transit, although the air mass would be ~ 10 for most of that time. Observations with the NIR filter would be subject to an extinction of 0.75 magnitude at this air mass. Mexico is 7 miles in this direction, so this sky is quite dark.

Measurement Capability

Under average conditions my Meade 14-inch telescope in prime focus configuration with a HyperStar and SBIG ST-10XME/CFW10 (31% blockage) can be used to obtain useful measurements described by the following figure.

 
 Magnitude/wavelength observation capabilities using the Meade 14" telescope in prime focus configuration (HyperStar, SBIG ST-10XME/CFW10).

The SA-100 is a transmission grating with ~ 55% of light in the first-order spectrum (25% in the zero-order straight-through path). It has an advertized resolution of 1% (hence the "100" in the product name). Anything brighter than the SA-100 curve can be "measured;" thus, bright asteroids (e.g., Ceres and Vesta) can produce spectra extending from ~ 380 nm to 1050 nm, whereas fainter asteroids will have a more limited wavelength coverage. The 20.7 V-mag point is for 2 or 3 hours of median combined images, unfiltered (effective wavelength ~ 650 nm). The "LC (unfiltered)" symbol corresponds to seeing the asteroid image in each 30-second exposure; this allows easy "moving target" photometry using MaxIm DL. An extra ~0.3 magnitude is achievable if the asteroid is moving slow enough to permit combining several images in order to "see" the asteroid at the beginning and end of the observing session for specifying "moving target" photometry.

Clear Sky Statistics

There's a "clear sky band" in the USA extending from Yuma, AZ toward northern California. Although Yuma is the "sunshine city" it is also a low altitude and hot place. The small black cross east of Yuma is Hereford, at an altitude of 4670 feet, which accounts for the almost ideal weather.  Summer afternoon's are ~ 7 F cooler compared with Tucson, and the winter nights are below freezing less than half the time. We get snow once or twice each winter and it usually melts in a few hours.



The following graph summarizes my first year living in Hereford, AZ. Note that we have a monsoon that typically starts July 7 and lasts until mid-September. We have a second "rainy season" (very mild) in February. May and June are the clearest months.


"Possible starshine" is defined as nightime conditions that are either clear or scattered clouds (<10% coverage).

Directions to HAO

The HAO is located at West Longitude 110:14:16 and North Latitude +31:27:08, at an altitude of 4670 feet. As the following map shows the HAO is ~80 miles southeast of Tucson, 7 miles from the Mexican border.

 


 
Note that a street name has been changed from Janice to "Edward V St." 

  
Google Earth image of my property, showing two domes. HAO#2 is the main telescope (14-inch Meade); the other dome (Celestron 11-inch) is fully operational, and is sometimes used at the same time on a different target. It's also my "backup" observatory for when the Meade is out for repair and I'm desperate for my "photon fix."

Related External Links
 
wikipedia description of HAO  
 
All-Sky Photometry (simplest possible to more advanced procedures described)
  Amateur Exoplanet Archive (AXA)  a place where 640 amateur light curves were archived (no longer supported)
  Exoplanet Observing for Amateurs, book by Bruce L. Gary
  Overview of Bruce Gary web pages  
  Resume

WebMaster: B. Gary. This web page is in the public domain; nothing on it is copyrighted.
This site opened: 
2009.06.14 Last Update:  2017.09.25.