15 exposures 600sec. each are summed up in order to enhance the signal to noise ratio. This results in an equivalent overall exposure time of 2.5 h.
With the naked eye only the two brightest stars in the image are visible and you need at least a 10 inch telescope to get only a glimpse of the horsehead nebula.
This is not the case for photographic observation. A 3 inch Pentax 75 SDHF apochromatic refractor with a focal length of 500 mm was used to shoot this picture.