BlueMUSE (Blue Multi Unit Spectroscopic Explorer) is a blue-optimised, mediumspectral resolution, panoramic integral field spectrograph proposed for theVery Large Telescope (VLT) and based on the MUSE concept. BlueMUSE will open upa new range of galactic and extragalactic science cases allowed by its specificcapabilities in the 350 - 580 nm range: an optimised end-to-end transmissiondown to 350 nm, a larger FoV (up to $1.4 \times 1.4$ arcmin$^2$) sampled at 0.3arcsec, and a higher spectral resolution ($\lambda/\Delta \lambda \sim 3500$)compared to MUSE. To our knowledge, achieving such capabilities with acomparable mechanical footprint and an identical detector format ($4\text{k}\times 4\text{k}$, 15 $\mathrm{\mu m}$ CCD) would not be possible with aconventional spectrograph design. In this paper, we present the optomechanicalarchitecture and design of BlueMUSE at pre-phase A level, with a particularattention to some original aspects such as the use of curved detectors.

H$\alpha$ blobs are off-galaxy emission-line regions with weak or no opticalcounterparts. They are mostly visible in H$\alpha$ line, appearing asconcentrated blobs. Such unusual objects have been rarely observed and studied,and their physical origin is still unclear. We have identified 13 H$\alpha$blobs in the public data of MaNGA survey, by visually inspecting both the SDSSoptical images and the spatially resolved maps of H$\alpha$ line for $\sim4600$ galaxy systems. Among the 13 H$\alpha$ blobs, 2 were reported inpreviously MaNGA-based studies and 11 are newly discovered. This sample, thoughstill small in size, is by far the largest sample with both deep imaging andintegral field spectroscopy. Therefore, for the first time we are able toperform statistical studies to investigate the physical origin of H$\alpha$blobs. We examine the physical properties of these H$\alpha$ blobs and theirassociated galaxies, including their morphology, environments, gas-phasemetallicity, kinematics of ionized gas, and ionizing sources. We find that theH$\alpha$ blobs in our sample can be broadly divided into two groups. One isassociated with interacting/merging galaxy systems, of which the ionization isdominated by shocks or diffuse ionized gas. It is likely that these H$\alpha$blobs used to be part of their nearby galaxies, but were stripped away at somepoint due to tidal interactions. The other group is found in gas-rich systems,appearing as low-metallicity star-forming regions that are visually detachedfrom the main galaxy.