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This is one of the disadvantages of switch mode power supplies. They all have common mode AC voltage noise on their output. On some cheep ones it can be as high as 80VAC. As it is common to both outputs, you cannot measure it by placing multimeter on your positive and negative outputs. You can measure it by switching digital multimeter to AC volts and measuring voltage on one of the DC outputs against AC Earth. The current is less then 1uA therefore it doesn’t present any danger of electrocution. In fact if try to measure this voltage with old style analogue multimeter, you wouldn’t not get any reading on it because it got input impedance of around 10K Ohms against digital multimeters 10M Ohms.
I have received so many electrical shocks in my life that I’m kind of immune to little tingle but I know that many people are quite sensitive to it.
Linear power supplies don’t have this problem but are bulkier, heavier, get quite hot and are more expensive.
Also, when you attach the camera (the one with metal body) to your telescope it is in electrical contact (with most of telescope types) with the telescope, mount and tripod. But most of tripods have plastic or rubber inserts on their feet and therefore whole apparatus is insulated from the earth. This is another possibility for charge to build up on it and to discharge thru your body to the ground.
And check all your mains power points and your extension leads for incorrect wiring. So you don’t have the Active and the Neutral wires crossed and check continuity of the Earth
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