Description Title of Invention: HEAD -MOUNTED DISPLAY WITH TILT SENSOR FOR MEDICAL USE Technical Field [0001] The present technology relates to a head- mounted display that can be used for medical purposes. Background Art [0002] A head-mounted display (HMD) that the user puts on the head for, for example, viewing images is known. For example, as one of the HMDs, there is known an HMD including image display surfaces and display elements for right and left eyes (see, PTL 1). The HMD having such a configuration can display images having a parallax to the left and right eyes of the user through the left and right display surfaces, and hence can present three-dimensional (3D) images without crosstalk. [0003] Meanwhile, also in an endoscope apparatus and the like used for medical purposes, a practical use of a 3D endoscope apparatus that can present 3D images is in con sideration. An endoscopic surgery is less invasive for patients than a general surgical operation, and hence is popular in recent years. However, an affected part(s) is checked only by images during a surgery, and hence it is sometimes difficult to perceive a depth with traditional two-dimensional (2D) images. Therefore, it is expected that connecting and using the HMD capable of providing 3D images to a 3D endoscope apparatus can realize a more correct and rapid endoscopic surgery while viewing realistic images of the affected part. Citation List Patent Literature [0004] PTL 1: Japanese Patent Application Laid-open No. 201 1-145488 Summary of Invention [0005] For wearing the HMD having the above-mentioned configuration, it is typically necessary for the user to adjust an HMD mounting position to be such a position that a blurred image and an out-of-focus image are not generated, that is, a position in which the display surfaces and the left and right eyes are opposed to each other while viewing images displayed on the display surfaces. [0006] However, a doctor (user) is not allowed to touch the HMD from a hygiene viewpoint upon wearing of the HMD during a surgery, and hence the user cannot adjust the mounting position by himself or herself. Therefore, a person other than the user needs to help the user wear the HMD. However, in this case, it is difficult for such a person to check position relationships between the eyes of the user and the display surfaces, which makes adjustment of a relative position of the HMD to the user difficult. [0007] In view of the above-mentioned circumstances, there is a need for providing an HMD that allows adjustment of a relative position thereof to a user without checking displayed images. [0008] As described above, according to the embodiments of the present technology, it is possible to provide an HMD that allows adjustment of a relative position thereof to a user without checking displayed images. According to an illustrative embodiment, a head-mounted display includes a detector for detecting an angle of tilt of the head-mounted display; and an output unit for outputting an indication of the angle of tilt. Brief Description of Drawings [0009] [fig. 1]FIG. 1 is a view schematically showing a configuration example of an en doscopic system according to a first embodiment of the present technology. [fig.2]FIG. 2 is a schematic side view showing a head-mounted display according to the first embodiment of the present technology. [fig.3]FIG. 3 is a schematic front view showing the head-mounted display according to the first embodiment of the present technology. [fig.4]FIG. 4 is a schematic side view showing a state in which a user wears the headmounted display according to the first embodiment of the present technology. [fig.5]FIG. 5 is a schematic front view showing the state in which the user wears the head-mounted display according to the first embodiment of the present technology [fig.6]FIG. 6 is a block diagram showing an apparatus configuration of the headmounted display according to the first embodiment of the present technology. [fig.7]FIG. 7 is a flowchart showing an example of a mounting method for the headmounted display according to the first embodiment of the present technology. [fig.8]FIG. 8 is a schematic front view showing a state in which the user wears the head-mounted display according to the first embodiment of the present technology, specifically, a state in which the user wears the head-mounted display with a main body tilting with respect to a horizontal surface about a second axis. [fig.9]FIG. 9 is a block diagram showing an apparatus configuration of a headmounted display according to a second embodiment of the present technology [fig. 10]FIG. 10 is a block diagram showing an apparatus configuration of a headmounted display according to a third embodiment of the present technology. Description of Embodiments [0010] Hereinafter, embodiments according to the present technology will be described with reference to the drawings. [0011]