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Because you were using low power eyepiece, it could have been an issue with too large exit pupil - if larger than your own eye iris, you could perceive effects like described.
Also it could be the eye relief issue.
Basically, what you saw was the diagonal shadow over image of Venus...
Next time try to move further or closer to eyepiece - the telescope exit pupil should coincide with the iris of your eye, both in size and distance from eyepiece
What is the f number of your mirror?
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