  Widefield quantum diamond microscopy is a powerful technique for imaging
magnetic fields with high sensitivity and spatial resolution. However, current
methods to approach the ultimate spatial resolution ($<500\,$nm) are
impractical for routine use as they require time-consuming fabrication or
transfer techniques to precisely interface the diamond sensor with the sample
to be imaged. To address this challenge, we have designed a co-axial sensor
holder that enables simple, repeatable sensor-sample interfacing while being
compatible with high numerical aperture (NA) optics. With our new design we
demonstrate low standoffs $<500\,$nm with a millimetre sized sensor. We also
explore the relationship between spatial resolution and NA spanning from 0.13
to 1.3. The spatial resolution shows good agreement with the optical
diffraction limit at low NA but deviates at high NA, which is shown to be due
to optical aberrations. Future improvements to our design are discussed, which
should enable magnetic imaging with $<500\,$nm resolution in an accessible,
easy-to-use instrument.
