Optical device for microscopic observation

An optical device for microscopic observation which makes light from an object incident on an imaging element, comprising:

• a cold stop which has a first opening corresponding to an optical system on the object side having a first magnification and which is arranged in a vacuum vessel to let the light from the object pass to the imaging element;
• a warm stop which has a second opening corresponding to an optical system on the object side having a second magnification and which is a stop member arranged outside the vacuum vessel to let the light from the object pass toward the cold stop;
• and
• a support member which supports the warm stop so that the warm stop can be inserted to or removed from on the optical axis of the light from the object,
• wherein the warm stop has a reflective surface on the imaging element side, and
• wherein the second opening is smaller than the first opening.