Why your astrophotos look wrong

• Stars trail in the same direction across the whole frame — Tracking. Either the mount is not tracking, the polar alignment is out, or the exposure is longer than the mount can hold. Shorten the exposure until they are round, then fix the alignment and lengthen again — that order tells you which of the three it was.
• Stars are round in the centre and stretched at the corners — Optics rather than tracking. Field curvature or tilt, usually solved by a field flattener matched to the scope, or by correcting the spacing between the corrector and the sensor. Tracking faults affect the whole frame equally; optical ones do not.
• Stars are round but bloated and soft — Focus, or seeing. Refocus using a Bahtinov mask rather than by eye, and remember that focus drifts as the temperature falls through the night. If refocusing does not fix it, the atmosphere is doing it and no equipment change will.
• Occasional frames trailed, most fine — Wind, a cable snag, or somebody walking near the tripod. Discard the bad frames rather than chasing a fault; this is what stacking exists to absorb.
• Stars have coloured fringes — Chromatic aberration in an achromatic refractor, and it is a property of the optics rather than a mistake. It can be reduced in processing and removed only by different glass.

• One side of the image is brighter — A gradient, almost always light pollution or the Moon, occasionally dawn. Normal, and largely removable in processing — but shoot the target when it is high rather than low, because low targets look through much more atmosphere and much more town.
• Diagonal streaky noise that survives stacking — Walking noise: the same sensor pixels landing on the same sky pixels frame after frame, so the noise stacks instead of averaging away. Dithering between frames — nudging the mount slightly — is the fix, and it is the single most effective thing most beginners are not doing.
• Dark rings or dust shadows — Dust on the sensor or filter. Flat frames correct it, and taking flats is worth learning before buying anything else.
• The background is grey rather than dark — Usually over-stretching in processing rather than a capture fault. It can also be genuine sky glow. Either way, more exposure time helps and aggressive stretching does not.
• Amp glow in a corner — Sensor heat, common on some cameras. Dark frames matched to the same temperature and exposure remove it.

The general principle behind most of this: total integration time beats any single frame. A beginner rig with two hours of stacked sub-exposures will beat the same rig with twenty minutes, and it will beat a much better rig used for five. Before buying anything to fix a noisy image, try collecting three times as much of it.