Pinched optics

I’ve been having an ongoing problem since buying my telescope a couple of months ago. Stars appear triangular, as you can see in this picture of Pleiades.

M45 Pleiades cluster
M45 Pleiades cluster

I’ve done some research and it seems the most common cause of triangular stars is a defect called “pinched optics”. The primary mirror in is held in place with several clips. If there are three clips, and they are overtightened, they have the effect of distorting the mirror into a slight triangular shape, which will cause triangular stars.

Usually, the mirror clips are clamped down too hard on the front surface of the mirror. This was the first thing I checked. They were indeed clamped down tightly, so I reduced the tightness so the clips hover a fraction of a millimetres above the mirror. The clips are not supposed to hold the mirror in place; only to prevent the mirror from falling out if the telescope is turned upside down.

But even after loosening the clips, I am still seeing triangular stars. I looked harder, and this time I noticed that isn’t just the rubber L-shaped clip that was touching, but the metal base peg. The mirror seems to be wedged pretty tightly between the three pegs. So tightly, that it isn’t even sitting neatly on its cork pad which is just about visible at the bottom.

Primary mirror clip
Primary mirror clip

This is clearly a manufacturing defect and I surely have grounds to return the telescope, or at least the mirror cell. I’m going to contact the retailer to ask if they can send a replacement, but I don’t want to be without my mirror cell for any length of time. I’m tempted not to bother with the returns procedure, but to remove the mirror from its cell and file off the insides of the metal support pegs. We’ll see how I get on.

More on this as the highly-charged drama unfolds 😛

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