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Due to turbulence in the atmosphere during the day, it's not uncommon for an image to be somewhat blurry over 40X.
A 25mm eyepiece yields 50X in that scope, so, for daytime use, you only have a 50/50 chance of getting a decently sharp image with it.
Higher powers? or Barlow? Forget it. You will see fuzz, basically.
At night, when the atmospheric turbulence settles down, you'll get reasonable sharp images up to about 125X and, occasionally, above that.
That means if the eyepiece focal length is shorter than 10mm, you'll need calm, steady, skies.
It also, as was mentioned, depends where in the sky you view. When you look straight up, you look through about 10 miles (16km) of air. At 30 degrees above the horizon, it's twice that (20miles/32km). At the horizon? 100 miles(160km) of air. Needless to say, all the extra air adds extra turbulence, dust, smog, haze, water vapor, etc.
So try to look at things as high in the sky as you can. Instead of looking at an object when it's just above the eastern horizon, wait 3-4 hours and view it higher in the sky. The image will be sharper and your scope will be cooled down, too. This type of scope cools for a couple hours as it's outside and yields better images when cooled to ambient temperature.
One last thing: dew can lightly coat the front corrector plate and end your observing quickly. When using the scope at night, use a plastic or metal dewshield to extend the tube forward past the corrector plate by about 15-18cm. [I used firm foam to make one] It will help prevent dew for a few hours and give you a chance to view more objects.
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