Astronomical Site Survey for Pikes Peak

INTRODUCTION

Pikes Peak remains an untapped resource for a wide variety of astronomy-related activities. In particular, Pikes Peak is potentially the finest ground based site in the continentel United States for studying the thermal infrared universe. High altitude, cold climate, and inland location, combine to produce extremely dry conditions atop Pikes Peak. This in turn produces highly transmitting and low emitting skies throughout the infrared portion of the electromagnetic spectrum. Geophysical isolation also favors laminar air flow and relatively low atmospheric turbulence over the summit, thus promoting what astronomers refer to as good "seeing". It is during good "seeing" conditions that one is able to obtain high angular resolution imagery.

It is presently believed that excellent conditions exist frequently on Pikes Peak. However, site survey data (particularily data obtained at the summit), is too limited to provide a statistically meaningful assessment of the astronomical quality of the site. A detailed site survey is needed to confirm the feasibility of placing an observatory on Pikes Peak.  Given the multi-purpose use of the facility for scientific research, education, and public outreach, we are confident that results from a detailed site survey will result in a decision to proceed as planned with the observatory.  The principal reasons for carrying out a thorough astronomical site survey are to:

• Determining some of the better locations for siting the future observatory.  This is primarily a "seeing" issue where one must determine the near-ground atmospheric turbulance at various locations on the peak.
  
• Establish the strength of the proposed observatory project on scientific grounds and gain greater interest and support from the astronomical research community.
  
• Provide information for future planning and scheduling of the many scientific research, education, and public outreach activities that will take place at the Pikes Peak Observatory.

Pikes Peak Observatory (PPO) associates have defined a variety of scientific measurements and data analysis projects that will determine the quality of Pikes Peak as a site for conducting astronomical research.  Many of these activities are waiting until sufficient funds have been raised to proceed. Item number one listed above (determining the prime locations for the observatory), is of primary importance so that we may begin negotiations with the US Forest Service and the National Parks Service to obtain permission to build the observatory on the peak.

This document discusses in detail our current plans to measure and assess the 1) astronomical "seeing" quality, 2) cloud cover, and 3) precipitable water vapor (PWV) expected at and above the site. The survey will bring together satellite, radiosonde, and pertinent historical data bases, along with actual measurements taken from the peak.

WATER VAPOR

Atmospheric Modeling

Water vapor  is a dominant source of opacity in the thermal infrared and is therefore a major factor in determining the quality of Pikes Peak as an astronomical site for conducting research in this part of the spectrum. To illustrate the effects of water vapor, the PPO presents the results of atmospheric model calculations wherein the atmospheric transmission throughout the thermal infrared is estimated for varying amounts of water above the summit. The atmospheric models and the paramaters used, as well as the individuals responsible for conducting this research are listed below:

Model: Line By Line Radiative Transfer Model (LBLRTM)

Code written by: Shepard (Tony) Clough at ARM (Atmospheric Radiation Measurement Program)

Code set up by: Ron Blatherwick, University of Denver

Code run by: Aaron Brachfeld, Eagle Crest High School

Spectral coverage: 210 cm^-1 to 2990 cm^-1 (50 microns to 3.3 microns)

Spectral Resolution: 0.05 cm^-1 FWHM

Atmospheric constituents used: H₂0, CO₂, O₃, N₂O, CO, CH₄, O₂, NO (No aerosols used)

Atmospheric conditions: Mid-latitude winter profile, elevation 4000 meters

Relative humidity: 10%, 20%, 40%, and 70%, corresponding to Precipitable Water Vapor (PWV) amounts of 0.25 mm, 0.5 mm, 1.0 mm, and 2.0 mm respectively.

Computed parameter: Atmospheric Transmission

The relative humidity and therefore the PWV values were chosen to be representative of the conditions atop Pikes Peak during the Fall, Winter, and Spring months. Relative humidity values of 10% to 20% and therefore PWV values in the range of 0.25 to 0.5 mm of water are believed to be quite common during these months.  This is a factor of two or better lower than the PWV amounts experienced on Mauna Kea, the premier infrared site in the world.

Aerosols which result in broad band continuum absorption were not incorporated in these model calculations. Though aerosol absorption is a strong function of altitude, it does not vary significantly from site to site for a given altitude. Therefore the conclusions drawn from these model calculations in establishing the credibility of Pikes Peak as a site, are not greatly affected by this omission.

The code was run at 0.05 cm^-1 resolution, however, the effect of thermal and pressure broadening on the profiles of atmospheric lines, tends to produce a limiting resolution around 0.1 cm^-1. This was the reason for not going to higher resolution in the model calculations as they would have resulted in more data points but no new information.

As an example of the results of these model calculations, we present the following graph wherein the atmospheric transmission profiles from 400 cm^-1 to 450 cm^-1 (25.0 microns to 22.2 microns) for varying amounts of water (0.25, 0.5, 1.0, and 2.0 millimeters) are plotted.

As one can see from figure 1, the atmospheric transmission over this spectral range is quite varied depending on the water vapor overburden.  Several atmospheric windows are seen throughout this portion of the spectrum and they are substantially more transmissive during drier conditions. Increased transmission results not only in increased light from the source of interest, but it also reduces unwanted background emission from the atmosphere. It is this background emission that dominates the signal throughout the thermal infrared and is the fundamentally limiting source of noise. In addition colder sky temperatures like those seen above Pikes Peak, further reduces this background thermal emission. High altitude also reduces the effects of pressure broadening of atmospheric absorption lines which in turn produces broader atmospheric windows.

Results of this analysis are available for the 210 cm^-1 to 2990 cm^-1 (50 microns to 3.3 microns) spectral range. Please contact dklebe@du.edu if you are interested in obtaining more results from this study. Some of the highlights of this analysis are listed in the following summary:

• Good transmission from the 17 to 27 microns (see Figure 1 above) is dependent on particularly low PWV values (0.25 mm to 0.5 mm).  These may be routine conditions for Pikes Peak.
  
• With the exception of molecular absorption due to molecules other than H₂0, the 8 -13 micron spectral region remains quite transparent for PWV values less than 2 mm.
  
• Several atmospheric windows open up in dry conditions (0.25 mm to 0.5 mm) in the 5 - 8 micron spectral interval. This spectral range remains untapped for ground based astronomical research.
  
• The 3 - 4 micron is largely unaffected by varying amounts of water.

Results of this study form the basis of the spectral analysis of data acquired by the instrument LWIR discussed in the following section.

LWIR Measurements

The PPO has acquired the use of a 17 to 27 micron grating spectrometer dubbed LWIR (Long Wavelength Infrared Radiometer) from the University of Denver that is capable of accurately measuring the background emission from the sky. With relatively few assumptions, the spectral emission data from this instrument can be used to determine the transmission spectrum across the 17 to 27 micron wavelength interval. From the atmospheric models discussed above, LWIR data can be further used to determine the PWV above Pikes Peak.

The instrument incorporates a single element HgGe detector cooled to 4 degrees Kelvin (liquid Helium cooled) and has a resolving power (wavelength / wavelength resolution) of 200. The basic procedure for determining the atmospheric transmission from LWIR data is discussed in the following:

• Spectral emission data is collected for a 10 degree square patch of sky for two different airmasses (i.e. two different altitude angles above the horizon). One measurement is taken towards the zenith (i.e. 1 airmass) and the other is taken at an angle close to 30 degrees above the horizon (i.e. 2 airmasses).
  
• The instrument is then occulted by a blackbody source whose temperature is identical to that of the ground. This temperature is recorded in the observing log.

• The two sky measurements are then used to determine the spectral emissivity of the instrument by extrapolation of the data to zero airmass. This analysis has revealed a nearly constant instrumental emissivity of 4 percent across the 17 to 27 micron wavelength interval.
  
• The emissivity, along with the blackbody spectral data is used to subtract the flux contribution of the window from the total flux, leaving only the spectral flux due to the sky.
  
• The sky emission data can then be used to determine the atmospheric transmission. The only assumption that goes into making this calculation is determining the mean radiometric temperature of the atmosphere. Based on radiosonde data which show the atmospheric lapse rates, this temperature is assumed to be 10 degrees Kelvin cooler than that measured on the ground.
  
• Finally the transmission profile at one airmass is fitted with data derived from the atmospheric model calculations discussed above to determine the PWV value for the measurement.

A sample of a reduced data set from measurements taken atop Pikes Peak on February 8, 1997 is shown in the following figure.

Series 1 and Series 2 represent the atmospheric transmission extrapolated to one airmass from the low and higher airmass data sets respectively.  The good agreement between the two curves, illustrates the quality of the measurement as well as the constancy of the window emmisivity. When compared with atmospheric models from the work discussed above, this data illustrates a day in which there was approximately 0.5 mm PWV above Pikes Peak. Though it has been some time since this particular data point was taken, we do expect to use LWIR more extensively to measure PWV. Site survey results will be posted in the following table:

Radiosonde Data Analysis

PWV can also be obtained from radiosonde data launced out of Denver, CO two times a day.  Pikes Peak Observatory associate Tom Damon has done some comparative analysis utilizing this data.  The results which are very promising will be posted at this location shortly.  Stay tuned.

Hand Held Water Vapor Monitor

One possible hand held water vapor monitor is the Microtops II produced by Solar Industries. The instrument is designed to look at the sun both in and out of a water band situated near 1 micon. The instrument has the advantage of being very portable and easy to use. The manufactures claim that it will have an accuracy of 10% for low PWV values which is more than adequate for the PPO's site survey needs. The instrument carries with it a price tag of $5,000 and the PPO is considering purchasing it if moneys allow. The PPO is also considering building a similar instrument utilizing student participation. This possibility is under investigation.

Measurement of PWV from Global Positioning Satellite Receivers

Water vapor induces measurable temporal delays in signals broadcast by Global Positioning Satellites (GPS's). Considerable research has been done as of late to explore whether or not one could use signal delays to accurately determine the water column. This research has met with some success though it is questionable whether or not this technique will produce accurate results for low water columns like those found from the top of Pikes Peak. The PPO is negotiating to see if one of these GPS water vapor measuring devices can be installed on Pikes Peak for proof of concept purposes. If and when the device is installed, results of the data will be published at this web location. Stay tuned.

ASTI Measurements

The PPO is also hoping to utilize the University of Denver's Atmospheric Spectral Transmission Interferometer to make water vapor measurements atop Pikes Peak. This instrument is a single element high resolution (1 wavenumber) interferometer designed to look at the sun over the 1 micron to 5 micron spectral interval. The instrument is capable of determining the water vapor column to an accuracy of 5%. This instrument, though difficult to set up and operate, will provide a source of calibration for the many other water vapor measuring techniques being utilized by the PPO.

CLOUD COVER

Line of Site Observations

Line of site observations of the cloud cover above Pikes Peak were taken from September 22, 1996 to December 31, 1997. Observations were made primarily from Manitou lake located 8 miles north of Woodland Park. This location affords a clear view of the conditions atop Pikes Peak without the complication of fog which occasionally covers the city of Colorado Springs. The measurements were taken in the early morning hours since it is believed that morning conditions are a very good proxy to the conditions of the previous evening. The purpose of these observations was to get a rough idea of the clear sky conditions expected above Pikes Peak. The following graph shows the frequency of clear sky above Pikes Peak over this period of time based on these measurements. This figure shows that photometric conditions (i..e. 100% clear) are experienced nearly 40% of the time during the early morning hours.

Cloud Monitor

Line of site measurements are very qualitative and somewhat subjective. To more accurately measure the cloud cover, the PPO has plans to build and operate a cloud monitor on Pikes Peak. The monitor which will utilize an uncooled microbolometer infrared array detector, will image a large portion of the sky in the thermal infrared. This instrument will allow continuous measurements (both day and night) of the cloud cover and may also have sufficient sensitivity to measure water vapor as well. The monitor will be of great utility for efficient operations after the observatory is built  The PPO is making steps to acquire the necessary component to build this instrument.

Satellite Data

Another possibility for measuring cloud cover would be to utilize meteorological satellite data. Images of North America in several infrared wavebands are taken on an hourly basis by the GOES8 satellite. This data which is archived and is available real-time over the net, has the advantage of allowing a direct comparison of Pikes Peak with several other well established observatory locations within the continental United States. This data can also be used to determine the PWV with useful precision. The PPO is currently investigating the feasibility of doing this study either in house or having an outside consultant do the analysis. A three year comparitive study would be ideal. The main issue in going ahead with either option, is time and money.

SEEING MEASUREMENTS

Hartmann Differential Image Motion Monitor

The PPO has acquired the use of a 14 inch Celestron telescope and mount from the University of Colorado, Boulder to quantitatively measure the astronomical seeing conditions on Pikes Peak. The telescope will be configured as a Hartmann Differential Image Motion Monitor (H-DIMM). The basic technique is to place a mask with a series of holes (small individual apertures) at the front of the telescope and to take an imaging device capable of short exposures (1/50th of a second or less) slightly out of focus. The result when viewing a star, is to produce several images of the same star at the focal plane of the camera. This is made possible because of the larger depth of focus for an individual sub-aperture as compared to that of the entire primary mirror. Since each sub-aperture samples a slightly different portion of the atmosphere when imaging the star, a measurement of the variance in the relative displacement between separate images in the focal plane, results in a characterization of the atmospheric turbulence. This in turn produces a reliable estimate of the seeing. This technique has the great advantage of being insensitive to drive errors and wind shake at the telescope. For further information on this technique, the reader is referred to Bally et al in Journal ?, vol ? page?. As these measurements are made and analyzed, they will become available at this web location. Stay tuned.

Double Star Measurements

To provide a seeing measurement that is somewhat more gratifying but less quantitative than the technique described above, the PPO also plans to use the 14 inch telescope to image double stars of known angular separation without the Hartmann mask in place. This technique allows the viewer to witness the image quality first hand with the image plate scale given directly from the separation distance of the stellar pair. Samples of these images as they are acquired, will be made available at this web site.

METEORLOGICAL STUDIES

The Pikes Peak Weather Station

The Colorado Springs Utilities Department finished installation of the Pikes Peak Weather Station in the fall of 1997. The data will be archived for twenty years and the PPO stands to benifit greatly from this information. Analysis of this data has begun and results will be available at this web site when ready. To see the current conditions atop Pikes Peak, please click here.

Initial Site Survey Results

The first site surveys in the 17 to 27 micron part of the spectrum were made on February 8th, 1997, in order to get an idea of the water vapor content of the atmosphere above the summit.

Conditions were average-good (for the time of year) and a graph of the data suggests a very dry site (transmission above 80%) which would be ideal for work in this part of the infrared (IR) spectrum.

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