Determining camera+lens distortions with Scamp

Hi

I am digitizing old astronomical photographic glass plates with a digital camera, and I want to determine the distortions caused by the camera+lens combination.
I have got the following recipe:

1) Plot an artificial star field on a paper (just small dots on paper)
2) Take images of the field at different orientations
3) Use Sextractor to find the stars and their positions
4) Use the obtained 'star' catalogues as input to Scamp, and setting the following parameters in Scamp:

MATCH N # Do pattern-matching (Y/N) ?
SOLVE_ASTROM Y # Compute astrometric solution (Y/N)
ASTREF_CATALOG NONE

Furthermore, I need to set
CROSSID_RADIUS 3000.0 # Cross-id initial radius
or otherwise I get
WARNING: Not enough matched detections in instrument A1

Scamp gives the following information about the catalogs:

Group 1: 5 fields at 4.37e+03 2.93e+03 with radius 5259
instruments epoch center coordinates radius scale
distort-1.cat A1 P1 +4.3675e+03 +2.9275e+03 5259. 1.000
distort-2.cat A1 P1 +4.3675e+03 +2.9275e+03 5259. 1.000

and the same for all the rest of the catalogs.
Obviously, all the values are in pixel units.

However, Scamp crashes in the following way:

> Making mosaic adjustments...
> Solving the global astrometry matrix...
> WARNING: Not a positive definite matrix in astrometry solver

----- Astrometric clipping:
> Computing astrometric stats for group 1
> out of memory, core-dump

Does anybody has a solution for this ?

cheers, Kimmo