Deep sky objects are the part of the hobby that photographs beautifully and looks least like the photographs. What the eye gets is grey, subtle and, once you have learned to see it, more affecting than the pictures, which is a claim worth explaining rather than asserting.
Andromeda, which is enormous and faint
The Andromeda galaxy is six times the width of the full moon and almost all of that is too faint to see. What a small telescope shows is the bright core as an oval glow; what a larger one under a dark sky adds is the extent of the disc and, at 200mm and up, the two dust lanes across it. It is best in binoculars or a wide field eyepiece, because at high magnification you are looking at one part of something far larger than the field.
The Orion nebula, which is the exception
Orion is the one deep sky object that rewards everything: visible to the naked eye as a fuzzy star, obvious in binoculars, and in a telescope a genuinely structured cloud with four bright stars, the Trapezium, embedded in it. At 130mm the wings of the nebula are clear and the Trapezium splits cleanly. It is the object most likely to convince somebody the instrument was worth buying.
Averted vision, which is the skill nobody mentions
The centre of the human retina is poor in low light. Looking slightly to one side of a faint object puts its light onto the more sensitive part of the retina, and objects that are invisible when stared at appear when you look away. Every experienced observer does this automatically, and it is the single largest improvement available for free.
Questions people ask about telescope
What does the Andromeda galaxy look like through a telescope?
An elongated grey oval, brightest in the middle and fading outwards. The spiral arms visible in photographs are far too faint for the eye at any aperture.
What is the best deep sky object for a beginner?
The Orion nebula in winter and the Andromeda galaxy in autumn from the northern hemisphere, plus the brighter open clusters, which look magnificent in almost any instrument.
Why do deep sky objects look grey?
Because they are too faint to activate the colour receptors in the eye. Cameras accumulate light over minutes and the eye cannot, which is the whole difference between the view and the photograph.