.
Kepler Star KIC 8462852 Amateur Photometry Monitoring Project

B. L. Gary, Last updated 2017.05.23, 16 UT

This web page records my observations of KIC 8462852 (hereafter KIC846) in an attempt to measure small amplitude variations on timescales of hours, days and weeks. A secondary goal is to begin a multi-year measurement of a suggested gradual decline of 0.34%/year (Montet & Simon, 2016). In support of both goals 25 nearby stars have been calibrated using all-sky photometry.
 
Latest Status Report


This "final result" for May 23 observing session.


Detail of above, showing the fade event with symbols for hourly average measurements. Solid traces are based on measurements (with brave interpolation), dashed traces are speculation. (The future trace is wild speculation!) When everyone's data is available for combining this graph will "fill out" with abundant detail.

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All-sky photometry for B- and V-band was completed on 2016 Sep 25, involving 25 nearby stars. I've begun the monitoring phase of observations. If consideration is restricted to observations with my standard hardware configuration (Meade 14" and SBIG ST-10XME CCD) using either a clear or V-band filter, there's "a hint" of a 30-day sinusoidal variation with amplitude 2.6 mmag; I don't believe it yet. There's also fade evidence during the last 1.7 years of ~ 0.8 %/year. Here is the latest day-timescale light curve using a V-band filter, showing a ~ 2% fade in May, 2017:


In addition to a slow fade, of ~1.0 %/year during the past year, there appears to be an abrupt drop of ~ 1.7 % starting between May 14 and 19.

My first observations were in 2015 October, when I used a clear filter (bad choice). After 6 observing sessions I switched to using a V filter. In order to evaluate fade rate over this long 1.6 year interval, in 2017 May I resumed observing with a clear filter using the same calibration stars, and same processing procedure. Here is a first version of "clear" filter V-mag measurements over the 1.6-year interval:


These clear filter observations, processed to yield approximate V-mags, spans 1.6 years. It shows a fade rate similar to the briefer interval of V-band observations of 2017.

Let's combine the above two graphs.


This data is combines clear and V filter observations made with an identical hardware configuration (Meade LX200 GPS 14", SBIG ST-10XME CCD). The clear filter mag's, which are uncertain due to their strong "star color sensitivity," have been adjusted by applying a -0.038 mag correction in order to achieve agreement with well-calibrated V filter observations. Both data sets exhibit the same fade rate.

Links on this Web Page

    KIC846 basic info
    2016 V Filter and Clear Filter Observations  
    2015 Clear Filter Observations 
    Finder chart with calibrated star B-mag and V-mag table
    References
    All-sky photometry B- and V-band calibration of KIC846 star field (linked to another web page) 
    Related links 

Observing Project Goals & Plan 

My original goal was to detect the suggested fade rate of 0.34%/year during the 2016 observing season. The suggested fade rate corresponds to 3.4 mmag/year, or 1.0 mmag every 3.5 months, which is the length of my observing season (due to the summer monsoon ending in mid-September). However, I have determined that systematic errors are present at the level of ~ 3 mmag per observing session, and I do not know what systematic errors may exist during monthly timescales, so it is unreasonable to expect success in measuring a 1.0 mmag fade during a 3.5-month interval. Instead, I will attempt to measure the slow secular fade by comparing results during two observing seasons. 

My original secondary goal was to search for short timescale variability, and this has become a primary goal for the current observing season. A few observing sessions will be long enough to search for hourly variations, but most observing sessions will be limited to an hour. This should allow for a search of day-to-day changes, as well as weekly timescale changes. If none are found then the observations will be viewed as providing an observational upper-limit to such variability.


The observational strategy is to improve the calibration of nearby stars so that each observing session produces a better-calibrated V-mag for that date. The tighter the star color sensitivity scatter plot, which I use for establishing an observing session's calibration, the smaller are the systematic errors for that session's target V-mag. The APASS magnitudes for this star field appear to be flawed, in both average value and internal consistency (star-to-star ratios). I will use all-sky photometry, involving many Landolt stars, to accomplish this calibration.

In order to know how long each weekly observing session will have to be for the average target V-mag to have a stochastic SE smaller than the estimated systematic SE I will conduct a couple long observing sessions (> 6 hrs) and simulate having just limited time chunks of the data, with a sampling chunk lengths. KIC846 is bluer than all but one nearby star to be used for calibration. This means I have to worry about air mass effects that differ between the target (KIC846) and the set of nearby calibrator stars. In order to assess the usability of high air mass data I will determine a best value for my "air mass curvature correction," AMC', and demonstrate the stability of this parameter, and use simulations of long observing sessions to determine how safe it is to use only high air mass observations (made late in the observing season) for monitoring secular fade rate.

I will sue the same hardware configuration for all observations: Made 14" LX200 GPS telescope, in a dome, with a x2 focal reducer (designed for this Meade model, A CFW-10 with an Astrodon V-band filter and an SBIG ST-10XME CCD. All hardware control is performed using MaxIm DL, via 100-foot buried conduit cabling between my residence office and the dome observatory. The unbinned image scale for this system is 0.725 "arc/pixel, and the FOV is 28 x 18 'arc. I will use 2x2 binning for all observations, placing the target at the center of the image, and autoguiding with the 2nd chip of the ST-10XME CCD. This will remove drift of the star field with respect to the pixel field during an observing session, and if I place the target accurately at the same center location for each observing session this should reduce the effects of imperfect flat field calibration.    


Basic Info for KIC846  

RA/DE = 20:06:15.5, +44:27:25
All-sky photometry: B = 12.493 0.025, V = 11.912 0.025 (B-V = +0.581 0.035)
APASS Mag's: B = 12.360, V = 11.852 (B-V = +0.51), g' = 12.046, r' = 11.697, i' = 11.554
There's a 0.11 mag discrepancy between BV mag's in Boyajian et al (Table 3) and APASS (article is brighter).
There's a 0.23 & 0.21 mag discrepancy between BV mag's in Boyajian et al (Table 3) and my all-sky V-mag (article is brighter).  


Observing Sessions (Most Recent at Top)  Note: Only data above elevation 30 degrees (air mass < 2.0) should be considered for scientific use.


2017.05.23 V-band, 6.7 hrs







2017.05.22 V-band, 6.7 hrs







2017.05.21 V-band, 6.7 hrs

Just a "hint" of a decrease and recovery during the observing session. We're talking about a 0.2 % variation, so it may be just "systematics" that appear when data is averaged too much.







2017.05.20 V-band, 6.1 hrs


Normalized flux version of 11-point average data.






2017.05.19 V-band, 7.1 hrs.







2017.05.14 V-band, 4.1 hrs.







2017.05.13 C-band, 6.2 hrs.


Will use only data above 30 deg elevation (air mass < 2.0).



2017.05.05: C-band, 5.7 hrs. 






2017.05.03:  V-band, 3.55 hrs.


Revised LC showing only data for air mass < 2.





2017.05.02: V-band, 3.1 hrs.





2017.01.08: C-band, 1.3 hrs.





2017.01.06: C-band, 0.9 hrs. 





2017.01.05: C-band, 1.0 hrs. 





2017.01.02





2016.12.26


Revised LC showing only data for air mass < 2.





2016.12.24







2016.12.20  





2016.12.18 


Revised LC showing only data for air mass < 2





2016.12.15





2016.12.12 







2016.12.09 







2016.12.05







2016.12.04







2016.12.02





2016.12.01





2016.11.30





2016.11.25  





2016.11.18





2016.11.15





2016.11.14





2016.11.12





2016.11.10






2016.11.07 





2016.11.06  






2016.11.02  

All-sky session, for V-band. Same V-mag as previous ones: V-mag = 11.895.






2016.10.31







2016.10.30 

Using Meade 14" with ST-10XME CCD, V-band. Finally, a reliable system! Data quality is better due to large FOV with smooth flat field and V filter (small Star Color Sensitivity slope).






2016.10.27





2016.10.24

Switched back to no filter (bec V filter had bubbled surface, causing bad flat field).





2016.10.20

I began using a V-band filter with the Lodestar.


The variations are undoubtedly due to flat field imperfections!




2016.10.19





2016.10.18





2016.10.17


The wiggles near the end must be due to flat field imperfections.



2016.10.16


The wiggles near the end must be due to flat field imperfections.



2016.10.15





2016.10.12


CCD failure occurred at 3:30 UT!



2016.10.11





2016.10.10





2016.10.09






2016.10.08






2016.10.06

Fifth C11 observing session.





2016.10.05

Fourth C11 observing session.





2016.10.04

Third C11 observing session.






2016.10.03

Second C11 observation; a slight increase in brightness.





2016.10.02

First observation with the Celestron 11" telescope (C11).






Note: On 2016.10.01 the Meade 14"failed to turn on, so I moved the SBIG ST-10XME to my Celestron 11" telescope (also in a dome), and have continued observations with this "back-up" telescope system.

2016.09.25

This observing session had 4 goals, or questions to answer: 1) Does the "star color sensitivity" diagram exhibit the same small MRS scatter about a slope predicted by the all-sky measurements of the previous week?, 2) What's the "air mass curvature" coefficient of the blue target star when calibrated by mostly red stars?, 3) Is there evidence for smal amplitude, short-period variations during a long observing session? and 4) How accurately can a 1-hour observing session measure V_mag?


Supercedes the next graph.


Based on cal using V mags adjusted on 2016.11.15.


Supercedes the next few graphs.


Uses the adjusted V-mags of 2016.11.15 (based on 2016 Nov 10, 12, 14 & 15).



After minor adjustments of 11 stars for persistent departures from Star Color Sensitivity plots (based on 3 observing sessions, Sep 25, Oct 02, Oct 03).


Star color sensitivity relationship for all-sky calibrated nearby stars, with an offset solution used for setting the calibration for this date's observing session.

The star color sensitivity relationship has a small slope, as expected (same as before), and the scatter of 13 mmag about a slope fit is what I expected, based on the all-sky measurements. This scatter is ~ 1/2 of what was present when APASS mag's were used.


This 6.6-hr LC employs an AMC of only 3.0 mmag/airmass. Even using zero produces a LC that is almost indistinguishable to the eye. When airmass < 2 it doesn't matter whether AMC is zero or 3.5.

Averaging mag's from groups of 5-images yields a RMS scatter of 1.3 mmag. The formal sinusoid solution is amplitude = 1.3 +/- 0.4 mmag, P = 1.0 hr. This is below my empirical threshold for being real, so I conclude that during this 6.6-hr observing session there were no detectable variations (above ~ 2 mmag). For future reference, a 1-hour data segment (above EL 30 deg) should be sufficient to determine V-mag with a precision (and repeatability?) of ~ 3 mmag. This SE is dominated by the use of 23 cal stars and their "star color sensitivity" solution. The best AMC' = +0.003 mag/airmass. The effect of AMC' departing from zero is not apparent in the LC, visually, until airmass > 2.

To investigate how long a data chunk must be in order for its stochastic SE to be smaller than estimated systematic error (~ 3 mmag), I divided the data into 1-hour chunks of data. The SE per hourly median is 0.9 mmag, so hour-long observations provide an average V-mag with a stochastic SE that is several times smaller than estimated systematic uncertainty. Here's a plot of the hourly medians and 5-image averages.


LC for 5-image averages (blue diamonds) and hourly averages (red circles).

I conclude that 20-minute observations should be adequate for monitoring KIC846 on a weekly basis in search of a secular fade.

2016.09.15

B-band all-sky observations for calibrating the target star field, involving 8 Landolt star field (25 stars). The observing sequence was L1-t-L2-t-L3 ... t-L8, where Ln = Landolt star field #n and t = target (KIC846).









2016.09.05

V-band all-sky observations for calibrating the target star field, involving 7 star fields. 








 


Clear filter observations from 2015 October/November

    2015.11.12, B. Gary, C filter LC, V-mag = 11.929, r'-mag = 11.703 0.005
    2015.10.24, B. Gary, C filter LC, V-mag = 11.929, r'-mag = 11.689 0.003

    2015.10.23, B. Gary, C filter LC, V-mag = 11.932, r'-mag = ??.??? ?.??? (dew problem)
    2015.10.20, B. Gary, C filter LC, V-mag = 11.933, r'-mag = 11.691
0.005
    2015.10.16, B. Gary, C filter LC, V-mag = 11.930, r'-mag = 11.697 0.005
    2015.10.15, B. Gary, C filter LC, V-mag = ??.???, r'-mag = 11.689
0.010 (too short for use)



2015.11.12: Clear filter, 1.3 hrs.

The following two graphs are calibrated using my revised B & V mags for 18 nearby reference stars, and my processing procedure yielding approximate V-mags (with later-determined bias of ~ 0.038 mag).





The following graph, calibrated using APASS r'-mags, may be ignored because the preceding two graphs replace it.




2015.10.24
: Clear filter,  5.8 hr.

The following two graphs are calibrated using my revised B & V mags for 18 nearby reference stars, and my processing procedure yielding approximate V-mags (with later-determined bias of ~ 0.039 mag).





The following graph, calibrated using APASS r'-mags, may be ignored because the preceding two graphs replace it.


Faint gray trace is a predicted variation based on previous observing sessions (which doesn't allow for air mass curvature).


2015.10.23: Clear filter observations, 3.0 hr. Dew on corrector plate created a growing systematic error.

The following two graphs are calibrated using my revised B & V mags for 18 nearby reference stars, and my processing procedure yielding approximate V-mags (with later-determined bias of ~ 0.039 mag).  The last 40 minutes was ruined by the autoguider not keeping the star field fixed to the pixel field (that data was ignored in the analysis).


Worsening dew on corrector plate means later data can be ignored.



The following LC was calibrated using APASS r'-mags to yield r'-mags. You may disregard this graph, which is superseded by the previous two.




2015.10.20: Clear filter,  0.7 hr.

The following two graphs are calibrated using my revised B & V mags for 18 nearby reference stars, and my processing procedure yielding approximate V-mags (with later-determined bias of ~ 0.039 mag).





The following LC was calibrated using APASS r'-mags to yield r'-mags. You may disregard this graph, which is superseded by the previous two.


The small-amplitude sinusoidal variation, with P = 0.8 hr, appears to be statistically significant (same P as in previous LC).


2015.10.16: Clear filter, 4.2 hrs.

The following two graphs are calibrated using my revised B & V mags for 18 nearby reference stars, and my processing procedure yielding approximate V-mags (with later-determined bias of ~ 0.039 mag).





The above graphs replace the following two (calibrated using APASS r'-mags).


Best 5 ref stars chosen for color similarity to target & other behavior. The small-amplitude sinusoidal variation, with P = 0.8 hr, appears to be statistically significant.




Square root of "Power Spectrum" showing weird shape.


Finder Image and Calibrated Star B-mag and V-mag Table 

This is a finder chart.


FOV = 26 x 18'arc, north up, east left. KIC846 is in the square.

Here's a smaller FOV finder chart showing 25 stars that have been calibrated using all-sky photometry.


FOV 14.0 x 9.4 'arc showing KIC846 (red square) and 25 calibrated reference stars. North up, east left.

The table below is a listing of my all-sky photometry B- and V- magnitudes.

  
All-sky photometry of KIC846 and 25 nearby stars. Left panel is based solely on all-sky calibrations; right panel includes small adjustments to V-mag based on 5 observing sessions and persistent departures from a Star Color Sensitivity fit. I recommend use of the right panel magnitudes.

Two stars are "dangerously" red for use with a slightly blue target.

Observing Start/Stop Local Times  

If an observation of at least 1/2 hour is needed for a usable measurement, and if measurements should be made when elevation (EL) is above 20 or 30 degrees, then the following graphs can serve as a guide for the season that KIC 846 can be monitored from the ground under dark sky conditions (for my HAO observing site). 


Local time for observing start and end, for the HAO observing site (Latitude +31 deg, Longitude 110 E) for the requirement that EL > 30 deg. Times for sunset + 1 hour and sunrise - 1 hour are shown as dashed traces. Local midnight is 12:21 AM due to the observing site's longitude being -110 deg (vs the standard -105 deg for the HAO time zone).

The number of hours that are potentially available for observing is plotted in the next graph.


Number of hours when KIC846 is above specified elevation (EL), and when the sky is dark (more than 1 hour past sunset and 1 hour before sunrise) versus month number, for an observing site at latitude +31 deg (Hereford Arizona Observatory).

From this graph it can be seen that January is the only month when KIC846 can't be observed from ground observing sites for monitoring purposes. December is marginal, since KIC846 is above EL = 30 deg during darkness for only 0.6 hour (Dec 15). For observing sites at more northerly latitudes the observing times will be longer.

References

    Boyajian et al, 2015, MNRAS, "Planet Hunters X. KIC 8462852 - Where's the flux?"
    Landolt, A., 2009, Astron. J., 137, #5
    Montet, Benjamin T. and Joshua D. Simon, 2016, "KIC 8462852 Faded Throughout the Kepler Mission," arXiv 1608.01316
    Smith et al, 2002, Astron. J., 123, 2121-2144.

Related Links

    Boyajian et al, 2015, MNRAS, "Planet Hunters X. KIC 8462852 - Where's the flux?" link 
    Washington Post article, 2015.10.15: link 
    AAVSO Campaign Notice requesting KIC646 observations 
    AAVSO LC Generator https://www.aavso.org/data/lcg (enter KIC 8462852)
    Web page tutorial: Tips for amateurs observating faint asteroids (useful for any photometry observing)
    Book: Exoplanet Observing for Amateurs, Gary (2014): link (useful for any photometry observing)
    My web pages master list, resume

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WebMaster: B. GaryNothing on this web page is copyrighted. This site opened:  Oct 15, 2015