Hi Andrea, 
If the pixel size is not defined in the header of the .mrc file if your EM map, chimera will use a default 1A/px.

You can set correct pixel size using volume command with voxelSize option. For instance, if the target EM map is volume #1 and pixel size 1.5A, use
volume #1 voxelSize 1.5

Best.
Oleksiy

On 21 Jul 2020, at 13:14, Andrea Dallapè <andrea.dallape@unitn.it> wrote:

Hi,

I have an EM map -from a cryoET experiment- and I would like to use ChimeraX in order to fit the protein -the ribosome, in this case- in such EM map.
I don't understand why the PDB of the ribosome I open in ChimeraX is far bigger than the EM map I have, and I don't really know how to resize them in order to have a fittable model.
Could you suggest me a solution?
Thank you for your help and attention,

Andrea
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Dr Oleksiy Kovtun
Research Fellow
Briggs group
MRC Laboratory of Molecular Biology
Francis Crick Avenue
Cambridge
CB2 0QH
UK

Tel: + 44 1223 267551
Email: okovtun@mrc-lmb.cam.ac.uk