The Kron photometer is a traditional-style aperture photometer using a photon-counting photomultiplier tube (PMT) system. However, unlike previous Lowell photometers, the Kron photometer was designed to be run from the control room and so has an acquisition CCD camera and motorized wheels for remote operations. As such, it can be used in conjunction with the autoguiders at the 72-inch and 42-inch telescopes; it can also operate at the 31-inch telescope. A scripting language permits the automation of observing sequences, while communication with the telescope MOVE computer allows the photometer to direct the telescope to new locations. Designed and constructed in-house at Lowell Observatory during 1998, primary funding for the new photometer was generously provided by Gerald and Katherine Kron, longtime supporters of Lowell Observatory.
The existing "backend" of the instrument consists of a thermo-electrically cooled detector usually operated at about -25 degrees C. The phototube is an EMI 6256, with a quartz window. This "blue" tube has good sensitivity from 3000 to 5200 A, then falls-off out to 6000-6500 A, and has no throughput redward of this. Dark current averages to 1-2 counts/s. Measured dead time correction is 60 ns.
An intensified CCD camera is used for acquiring and centering objects. There are four motorized wheels in the photometer: a filter wheel, an aperture wheel, an optics turret, and an ND filter wheel in front of the camera to protect it and extend its dynamic range to brighter objects. Light entering the photometer first encounters the aperture wheels, then the 4-position optics turret where the light is directed as follows: a wide-field mode where the camera can see a field about an inch across at the focal plane; a narrow-field mode so the camera can work with the smaller apertures; an eyepiece mode for viewing directly by eye instead of using the camera; and an open hole for light to reach the filter wheel and the PMT detector. The camera (and the eyepiece) are located behind the aperture wheel for centering an object in the aperture.
The filter wheel and the aperture wheel each have 15 positions, one of which is "dark" in each wheel. Aperture diameters range from 0.01" to 0.50", plus one "open" position (a 1" hole). Specific sizes and projected values on the sky at the 42" f/8 are as follows (projected aperture sizes are about 4x smaller at the 72"):
1 = 0.010 in diam 6.1"
2 = 0.020 in diam 12.1"
3 = 0.039 in diam 23.7"
4 = 0.062 in diam 37.6"
5 = 0.078 in diam 47.4"
6 = 0.100 in diam 60.7"
7 = 0.125 in diam 75.9"
8 = 0.156 in diam 94.7"
9 = 0.203 in diam 123.2"
10 = 0.250 in diam 151.8"
11 = 0.328 in diam 199.1"
12 = 0.400 in diam 242.8"
13 = 0.500 in diam 303.5"
14 = 1.000 in diam 607.1"
15 = DARK
A generic PC is used for instrument control and data acquisition. The PHOT program has commands to both control all basic acquisition operations as well as the unique Kron instrument operations. Operating commands can be scripted, permitting automation of numerous data acquisition sequences. Some basic telecope moves can also be controlled from PHOT. Data are stored to disk and also printed out on an old-fashioned impact printer with fanfold paper so one can look back at what happened earlier in the night. A manual for PHOT is available from Larry Wasserman.
Engineer Ralph Nye performed the overall design work, with assistance from astronomer Ted Dunham on optical components of the TV system. Metal work was done by machinist Jim Darwin, and he and Nye put together the mechanical components. Electronic elements were handled by Rich Oliver. Finally, astronomer Larry Wasserman implemented all software programing needed to remotely command the instrument from the control room, and created the scripting or macro capability to automate observing procedures.
For additional information, contact Dave Schleicher (general issues and PMT specs), Ralph Nye (mechanical issues), Ted Dunham (optics), Larry Wasserman (software), or Rich Oliver (electronics).