Mars has approximately half of the earth’s diameter. When it is closer to the earth it is at about ½ AU from the earth. Calculate at what size it will disappear when seen through the same telescope.

Also,

So,

At 1AU, the sun’s diameter is = (1/2) degree

Therefore, diameter of mars will be = (1/400) degree

At 1/2 AU, mars diameter = (1/400)(2) = (1/200) degree

With 100 magnification, mars diameter = (1/2) degree = 30’

As a result, because of atmospheric variations, the value is larger than the resolution limit, and hence seems magnified.