Re: Data reduction methodology for V-I colors

[Michael Koppelman <lolife@bitstream.net>]

But aren't you getting a different zeropoint solution for every frame? 
Why not use a single zeropoint for the entire night?

On Aug 31, 2004, at 12:20 PM, Thomas Droege wrote:

> Mike,
>
> Yep, this and a lot more.  I cannot identify a "bad" night.  I did a 
> lot of
> work on the assumption that there was such a thing as a bad night.  For
> example, go through all the data and look for stars with big deviations
> from the mean.  (This should work because most stars are not 
> variable.) Now
> sort all frames by fraction of stars with big deviations from the mean.
> Now eliminate these from the data set and look at the result. I did 
> this
> eliminating the noisiest 10%, 20%, 50% ...  No improvement at any cut
> level.  Conclusion:  Eliminating frames with lots of deviant 
> measurements
> does not improve the quality of the data as a whole. No doubt 
> eliminating
> noisy frames will improve the data when we solve other problems.  But 
> noisy
> frames are not even close to being the largest source of error or the 
> above
> experiment would have made some improvement.  It made no improvement.  
>  I
> was surprised.  But when you think about it, the star measurement 
> scheme
> that Michael uses really does a good job.  It is something else.  
> Something
> fundamental.  Something related to position in the frame.  If you 
> track a
> field, the result is much better.  But this just hides the problem.
>
> I think there are two likely suspects.  I am working on one, Michael is
> working on the other.
>
> 1) There is some gradient in the sky that produces errors in all 
> frames.
>
> Michael is working on this.
>
> 2) We don't have a good reference catalog.  The result is that 
> depending on
> where a star is in a frame, a different set of reference stars are 
> used.
> If the mean of the reference star set is different for the two frame
> positions, then this will produce an error.
>
> I am working on this.
>
> OK, I suspect that both have an effect and that there are still more
> problems to be found.  I know that I have made some small improvements
> using 2.  I am just waiting for more data and winter time with no
> observations to sit and compute on this for a couple of months.
>
> Tom Droege
>
>
>
>
>> [Original Message]
>> From: Michael Koppelman <lolife@bitstream.net>
>> To: Tass <tass@listserv.wwa.com>
>> Date: 8/31/2004 11:57:14 AM
>> Subject: Re: Data reduction methodology for V-I colors
>>
>> This got me thinking and I'm sure there are tech notes and such I 
>> could
>> find about it but nonetheless -- have you guys considered calibrating
>> the night rather than the frame? i.e. take the whole dataset, read in
>> all the stars from the entire night, throw out the ones with high
>> errors, match up the Landolt/Henden or other standards and get
>> transforms for the whole night?
>>
>> Cheers,
>> Michael
>>
>
>