1. Long-term Photometric Monitoring of a Single Stellar Field
Jeff Wilkerson
Luther College
Iowa Academy of Science
April 27, 2019
Support:
Roy J. Carver Charitable Trust (Grant #00-50)
Luther College
R.J. McElroy Trust/Iowa College Foundation
American Astronomical Society
Luther College HHMI
Luther College Office of the Dean

2. Approximately 1625 stars regularly identifiable in the data set
OUR DATA SETS
Cluster
Dur. (s)
# Nights
Total Images
Date Range
NGC 6514 (M23)
3.5 25
45,000
19 June 2003 – 8 Sep. 2003
2.5 20
23 June 2005 – 30 Aug. 2005
5.0 37
49,000
28 Mar – 25 Sep. 2006
2.8 49
91,000
9 Mar – 27 Sep. 2007 53
82,000
3 Mar – 16 Sep. 2008 45
50,000
11 Mar – 17 Sep. 2009 63
59,000
27 Feb – 8 Oct. 2010 57
46,000
1 Mar – 11 Oct. 2011
7.0
41,000
11 Feb – 3 Oct. 2011 41
31,000
1 Mar – 1 Oct. 2013 40
28,000
18 Feb – 27 Sep. 2014 43
30,000
1 Mar – 15 Oct. 2015 35
22,000
19 Feb – 2 Oct. 2016
NGC 6514 (M23)
7.0 28
17,000
9 Mar – 12 Sep. 2017 12
14,000
3 June 2018 – 31 July 2018 / ?
21 Mar – present
Approximately 1625 stars regularly identifiable in the data set

3. Student Participation:
Zebadiah Howes
Buena Vista
Univ.
Travis DeJong
Dordt College
Forrest Bishop
Decorah High
School
Ujjwal Joshi
Nathan Rengstorf
Andrea Schiefelbein
Todd Brown
Brajesh Lacoul
Kari Frank
Alex Nugent
Drew Doescher
Alex Sperry
Jennifer Schulz
Clara Olson
Ben Anderson
Joe Novak
Elizabeth Larget
Jonathon Goldstein
Robyn Siedschlag
Siri Thompson
Matt Fitzgerald
Heather Lehmann
Amalia Anderson
Hilary Teslow
Steve Dignan
Kirsten Strandjord
Donald Lee-Brown
Andrew Becklin
David Pfotenhauer
Daniel Herman
Kevin Honz
Erik Floden
Torger Jystad
Madilyn Heinke
Isaac Stivers
Alex Pigarelli
Linnea Lee-Brown
Aiden Berdahl
Colin Weber

4. We have discovered unambiguous variability in about 80 stars

5. Aperture Photometry

6. Data Normalization Normalization All Images Unfiltered!
Identify four reference images from throughout the night
Calculate average signal for each star in all four frames – this is the reference signal
Determine the signal of each star in the frame to be normalized – this is the sample signal
Calculate (ref. signal/sample signal) for each star
Normalization factor = median of all ratios in (4)
Inter-night normalization factor = median of all star signals: ref. night/data night
All Images Unfiltered!

7. Short Term Photometric Resolution (STPR)
Long Term Photometric Resolution (LTPR)
At large signal values, STPR approaches a constant (plateau) value determined by our frame normalization, itself limited by scintillation. For faint stars STPR increases as signal-1. In between STPR increases as signal-1/2. Counting statistics of the stellar signal measurement dominate STPR in this region.
Functional fits shown of form: STPR=
[(C1)² + (C2signal-1/2)² + (C3signal)2]1/2

8. Color Normalization for 06 - 25 -14
Reference Night / Data Night
Mean R-I

12. What About STPR?

13. Conclusion/Where We Are Headed
To improve photometry:
Annular photometry
To better understand our photometric limits (and, perhaps help photometry?):
Correlate normalization factor, STPR and LTPR with observing conditions such as atmospheric water column, aerosols aloft and winds at various atmospheric heights.