Re: More from ASAS

[Tom Droege <tdroege2@earthlink.net>]

Andrew Bennett writes:

>So - bite the bullet: the MK V has to put the same star on the
>same pixels night after night!

Well, this brings up one of the points that I am trying to make.  If you 
measure the same star on the same pixel night after night, the result looks 
nice.  It even has a small rms error.  But the real error in absolute
photometry is hidden by the process.  The error is still there, it is just 
harder to see.  If we move the star around on the ccd, then the error 
becomes obvious.  We still may not have all of it, but at least we are 
fooling ourselves less.

OK, I have parts on order that I hope will allow accurate pointing in 
declination.  Then I will be able to point to the same field night after 
night.  But do I want to????

Tom Droege


At 12:25 AM 10/17/02 +0100, you wrote:
>On Wed, 16 Oct 2002 18:44:38 -0500, you wrote:
>
> >Experts please comment!
>
>I'm no expert but you sent it to me so I'll comment anyway.
>
> >...
> >I keep wondering why I am not doing as will.  One difference is
> >that most of the other surveys always point to specific fields.  This
> >results in a star always being measured by the same CCD pixels.
>
>Yes! Then you get the results and don't have to fight
>with all the peculiar effects that we keep discovering!
>
> >  I note
> >that the errors I see are much smaller if I follow a star and take
> >successive images.  Now the star is in roughly the same position in each
> >image.  OK, it could still be flat fielding, but I think not.
>
>Or at least, not all.
>
> >One way to demonstrate this one way or the other would be to use a specific
> >set of comparison stars for each star measured.  Then as the star moves
> >across successive images it would still use the same set of comparison
> >stars instead of a different (large) set for each position as it does
> >now.  At the moment, I do not have the ability to get into the code to
> >accomplish this.
>
>I tried very hard to do this! I failed. I did it several
>ways and none worked. I quit and went away to grow grapes.
>
>I kept all the stars that had the same minimum set of comparison
>stars on every image. There weren't any.
>
>I cut the minimum size of the set to the bone and kept all
>the images of a star that had the minimum set. There weren't
>very many ... a lot of apparently good measurements got thrown
>out for lack of one of the comparison set. And with the small
>set of comparison stars, most of which were inadequate, the
>errors went up instead of down. Unlike you, I got the impression
>that a lot of the extra error was being inserted by the details
>of the "flat fielding."
>
>You are right that a lot of the error comes from the reference
>catalog via the use of different stars for each image [and
>diferent positions for the same stars on each image]. I found
>huge errors for Tycho2 Vmag and even worse for I, even when the
>positions were too close for there to be any possibility of
>misidentification. This suggests large numbers of variables not
>flagged as such. Clipping the large residuals is a pretty
>arbitrary process!
>
>So - bite the bullet: the MK V has to put the same star on the
>same pixels night after night!
>
>Or retire from astronomy and grow grapes. Another huge truck
>load went down the driveway today. One load more to go.
>
>Andrew Bennett, Avondale Vineyard