I posted an image of the M81 area a few days ago. It is based on images obtained Mar 21, 2020 with my 100 mm (4 inch) APO and my Sony a7s (unmodified) mirror-less camera. After I made this image, out of curiosity, I downloaded a professional image of M81 to compare. It is taken with the Advanced Camera for Surveys of the Hubble Space Telescope. Would it at all even be possible to compare? See for yourself. This animation is going back and forth between my image and the HST image.

There are quite a few simularities, the dust bands, some larger structures, the location of the stars, but mostly my image is of course greatly inferior. HST's bright stars are nice pin points while mine looks like big baloons. Most likely it is due to the extreme difference between the telescopes (and/or my processing or ability to focus). If the bright stars that appear overexposed in both images grow to the width of the lowest part of their profiles, I guess something like this will happen.

The ratio of aperture sizes between HST and my APO is 2.4m/0.1m or 24:1, and the ratio of focal lengths is 57.6m/0.58m or 100:1. The pixels are about the same same, however, 15 um and 8.4 um, thus a ratio of about 2:1. As a matter of fact with these big pixels I am undersampling should my telescope be diffraction limited. The diffraction disk (Airy disk) should be 1.4" in my telescope, but my pixels are 3". In HST the Airy disk is 0.06" and the pixels 0.05". I'm not sure if the atmosphere is limiting my telescope since my pixels are so large. Color channels are also different; I have RGB, HST use B,V,IR in this image.

Maybe this is a stupid comparison, but it is still fun to do and reflect over how much you actually can see in your back yard with a small telescope, a turbulent atmosphere, and a miniature budget compared to HST. There are a lot of small interesting details to look for in the animation. I have for example in my image a strange structure close to the core that seems to be missing in the HST image. Unclear how I got that. In the lower spiral arm to the right there is a pair of stars that seems to change relative brightnesses. One is brighter in my image and the ofter is brighter in the HST image. Again, unclear why. I hope you also find it fun to compare. Please comment if you find other interesting differences or if you can explain the phenomena.

The HST image is the "Hubble photographs: Grand Design Spiral Galaxy M81", NASA, ESA and the Hubble Heritage Team (STSC/AURA)