[Aavso-photometry] IL Aqr Sequence

[Arne Henden]

Michael Koppelman wrote:
> On Oct 11, 2004, at 10:46 PM, Arne Henden wrote:
> 
>> Why is the astrometry "not particularly great"?  Tycho2 is certainly
>> of the same astrometric quality as is UCAC; a few arcsec is completely
>> out of range for either catalog.
> 
> 
> I think it is because it only finds about 10 stars in the image. If I 
> was using UCAC it would find 100 or more.
> 
10 stars is more than enough for astrometry; 3 is generally sufficient
to get you pretty darn close to the final answer.  I think you have
another bug to trace down.

>>   I'm a little surprised that I don't see the exponential error
>> increase towards the faint end.  Are you deleting stars with high
>> error?
> 
> 
> See the attached JPEG. There is a vaguely exponential rise. I am not 
> deleting stars with high errors. When I run daofind in IRAF I set the 
> threshold pretty high so I don't find all the faint stars. I do see an 
> exponential rise more so on the individual sequences than the combined 
> one. I think the way I calculate the errors in the combination loses 
> some of the error information. I should properly propagate the errors 
> rather than just taking the standard deviation of the average.
> 
I'll wait until you get a 3rd night, then, so that the standard devation
is more reliable.  It might be good to make your threshold about a
magnitude fainter just so that you can see the rising error.
Arne