[vsnet-chat 5967] (fwd) Re: [AAVSO-DIS] CCD-V Vs visual observations = U ORIONIS

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

From: BailyHill@aol.com
Date: Sun, 9 Feb 2003 15:29:45 EST
Subject: Re: [AAVSO-DIS] CCD-V Vs visual observations = U ORIONIS

Hello Sebastian;

Your email highlights some important points that I think are worth 
understanding.  First let me say that I think there are many forms of 
Variable Star Observing, and there is enough room for everyone to enjoy 
whatever form of Observing that brings them satisfaction.  I do favor any 
form over another form for others, but for me the thing that I am interested 
in is CCD Photometery, and I hope that not everyone wants to do the same 
thing.

Moral:  Visual, PEP, CCD, Automated, All Sky and Other forms all have 
strengths and limitations and each can contribute to our understanding of 
Variable Stars.


Your first observation about the U ORI data is that there are two sets of 
observations at JD 2452638.7993 at different Magnitudes, V = 7.31 and 7.26!  
This should not surprise anyone, there is going to be scatter in the data.  
We are dealing with a statistical process here and if one makes ten 
observations, scatter of this order is what I have seen in my observations 
and others.  Perhaps this observer should report different JD's for the two 
observations, but on the other hand, perhaps this  is data taken at the same 
JD and reduced using two different techniques?  We do not know.  I believe 
there is nothing wrong with these observations.

Moral:  CCD Data has Scatter in it.  While Visual data has a scatter of 1 
magnitude peak to peak, when one looks as most any well observed AAVSO field, 
there seems to be the expectation that if this was done with a CCD, that the 
scatter should go to less than .01 magnitude, but it just does not seem to be 
the case.  The reasons may have to do with the non photometric nights, the 
non-photometric sites and the capabilities of the telescopes, drives, cameras 
and software that we all use.  


The next observation in your email was about an observation done 9 days later 
by (WGR), which is myself, at V = 6.97 and B-V of 3.69---actually I reported 
B of 10.66.  This does seem unusual.  I went back and looked at the 
photometry sheets for these two.  I did find that my chart showed comp star 5 
with a magnitude of 12.991, while the latest chart on the web shows 13.991.  
I also had a note on my chart that Ron Zissell had found this error, but I 
used the old value.  I have corrected my chart.  This would change my value 
to V= 7.31.  I gave the B observation two different days of scrutiny, and 
could find nothing obvious.  The comps are in the right places and are the 
same ones used in the V observation.  The 10.66 is the average of 3 
observations which gave 10.18, 10.73, and 11.09 for comps 5, 4 and 3.  This 
gives a Std Dev of .37 mag (I know that with 3 samples the Std Dev is not 
appropriate, but I use it to measure the scatter of my observations on every 
one).  Good numbers may be as los as .03 mag, and rarely better.  So No 
Closure on the B observation.

Moral:  Report your Comps.  Since I know the number of comps, and the values 
that I used, My observatoin was really saying, " If the Comps 5, 4 & 3 are 
12.991, 12.250, & 9.060, then my estimate of U Ori was 6.97".   Since we know 
that Comp 5 was actually 13.991,  we can fix the observation of 6.97 and it 
becomes 7.31.  Thank you very much for pointing this out to me, I have been 
using the old value in many of my observations on U Ori.  It is fixed now.


The next observations in your email, "A Day later, MTK recorded it in V at  
7.38 and B = 9.17 (B-V = 1.79).  Another 3 days and WGR found it at V=7.13 
and B=8.66  (B-V=1.53).  I rechecked and found nothing unusual.  

So lets summarize at this point, with the correction, we have from the 
beginning V = 7.31, 7.26, 7.31, 7.38 and 7.13.  This is for 3 different 
observers, over 14 days, on 4 different nites.  The max peak to peak is .25 
mags.  I wish it were better, but this is about typical, I beleive.  As for 
the B, B = 8.83, 9.01, 10.66, 9.17, & 8.66 from the same observers at the 
same time.  Not as  good as the V, but B is tougher with a CCD.  

Moral:  Expect some Scatter, how much is too much, where does it come from?


I also checked the observations at JD 2452652, 2654, 2655, 2661, 2665 and 
2667.  Once again, the comps seemed correct--here I did not use comp 5 for 
any of the V measurements.  I did find an error in the B.  On the JD 2452655 
and 2452667,  the Photometry sheets show that the B data was identical to the 
V data.  This means that the Measurements Results used were not updated.  I 
observe in the V, B, R, I order.  This update is a manual operation, and it 
must have been omitted.  So the B observation 7.91 on JD 2452655 and 8.90 on 
2452667 should be discared.  (Aaron I hope you are reading this).  I am 
looking into including this in the automated tasks so that it cannot happen 
again.  Thanks for pointing this out to me.  

Moral:  Be Careful.  Do things the same way each time.  Develop a routine.  
Do not deviate from it.  Keep good records, so that you can go back and check.

For a final Point, I want to address the observations from JD 2452651 to 
2452667 from the above.  These V observations were 7.13, 8.01, 8.11, 8.17 and 
7.26.  By ploting this on the Light Curve Generator, I can see a dip in my 
observations, which might at first seem like an error.  There is however, 
only one other obervation on 2452655 which is a Visual one that lists 7.7.  
This is about a half a mag lower that the other Visual observations the 
preceed and follow this observed dip.  If anyone has additional observations, 
it would be interesting to see if this dip is real.  This is not the only 
time that short term variations have been observed in the CCD data.  

Moral:  Do not look back at your previous observations when making tonights 
observations.  Do not try to force your observations to meet a pattern.  The 
change one observes might just be real.  I also observe blind, I do not look 
at previous data, which is probably why I did not pick up the things that 
Sebastian did.

Moral:  This is the world of UNEVALUATED DATA.  This is why the discalmer at 
the top reads "Do not use for Publication".  EVALUATED data scrubs this out 
of the data base.  We need to work towards eliminating these errors.  The 
electronic submission has helped greatly.  We need to do more.

Thanks again for pointing out these errors Sebastian.  I hope this gives the 
readers a better expectation for data.  I would be open to a quick project on 
U ORI, to observe every clear nite for the next month or so, by a dozen CCD 
Observers, just to see if we can understand more about this variation.  Any 
Takers?

Gary Walker
(WGR)
Chair CCD Committee

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