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[ccp4bb]: unmeasured data in map-calculations

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Dear all,
with many apologizes (because it's mostly a a cns-related question) I
like to submit a question to the ccp4bb. But it's soon x-mass and you
might forgive this.

My data set is quite incomplete in the highest resolution shell. When I
calculate maps with the cns script model_map the sigmaA-weighted
fofc-map (mFoDFc) has many "unreasonable" peaks at 3sigma-level. When I
make cns to take Fc for the unmeasured Fo's the sigmaA-weighted fofc-map
shows much fewer and much more reasonable peaks, simply this map becomes
significanly better.

This seems to make sense, but what is the physical/mathematical reason
for this? 
How treats cns unmeasured hkl's in the "normal" mode in difference maps?
Is there a related ccp4-option in refmac/fft?

Thanks for any reply, kind regards and merry christmas

Jan Abendroth
Institut fuer Biochemie
Universitaet Koeln
Zuelpicher Strasse 47
D-50674 Koeln

Tel: +49-(0)221 470 6455 (Labor)
                    6445 (Buero)
Fax: +49-(0)221 470 5092