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* Eyepiece solar filters. They crack when overheated and when an eye is behind the eyepiece this will result in blindness and no opthomalogist can help you. Furtunately they are not sold anymore - as new.
* Straight-through large binoculars on tripod. Only useful for terrestial viewing or astronomical objects low in the sky. Otherwise, you have to be in a very acrobatic position to look through the eyepieces and the tripod is in the way as well.
* Many finderscopes, as these easily become misaligned with the main scope, I have a simple 1/2" PVC pipe opticsless peephole mounted on my 16" Dobson which makes finding easier. For smaller scopes (my 110 and 80mm) I use no external finder at all: use the telescope itself as finderscope by popping in the eyepiece with the largest TFOV (2.5-3º).
* Sigma Octantis finding tools for polar alignment. I use my polar scope and put the less known +6.9 BQ Octantis (10' off the real Pole) in the center of the field. Even at 600mm (Fullframe) I can track up till four minutes per frame without noticeable trails.
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