Re: Everett and Howell Paper

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Sat, 03 Nov 2001 22:11:41 -0600, Tom Droege 
<tdroege@veriomail.com> wrote:

>Andrew,
>
>You really want to read the paper.  I will see if I can copy some of the 
>parts from it so you can see what they are doing.
Yes indeed - and thanks. I now have the whole paper.

They are doing a good deal better than any of us have
managed to do so far with the MK IV. I had quite a
struggle to get below 0.01 mags rms for putting together
randomly loacted images in the "TOM" data set. I and
others have done a little better with repeat sets that
put the stars in about the same place on each image of
the set: about 0.004 mags, maybe.

I think their better precision comes from two sources:

1) Putting the stars on the same pixel (+-1 pixel).
  a) Repeatable image area allows use of an unchanging set
     of reference stars.
  b) Repeatable star positions within the image eliminates
     the need for image-scale flat fielding.

2) They have superb pixel to pixel flat fielding to 5.10^-4.
  Even though they have systematic errors of at least 0.01
  mags across their 1 degree image, this does not degrade
  their analysis.

OK, Tom: To match their pointing accuracy you need something
like 2 to 3 orders of magnitude improvement in both DEC
drive repeatability and in polar axis alignment ...

Actually, it doesn't need to be anything like that good. To
use the same set of reference stars, one needs to do better
than 1/10 image width if one puts up with losing 20% round
the edges. Similarly, with flat field errors of order 0.1
mags across the field, one should be down at the 0.001 mag level
if one only has to correct inside 1/10 x 1/10 image: basically 
the sort of thing Arne has already done in the MK III post 
processing.

Better than 0.1 degrees would be nice.

As to the flat fielding, I'll just ride my hobby horse
around one more time:

Our flat fielding is entirely inadequate. We need some
sort of dome flat. The last time this was discussed, we 
turned up (among others) a design that produced circularly 
symmetrical illumination. IIRC this was good to around 
0.1% (a few digits in a 12-bit system) and could be built 
to fit the MK IV lens. So flat fielding is a solved problem 
for pixel to pixel calibration (but not across a 4 degree
image!) We should do it.

Andrew Bennett, Avondale Vineyard