Sunday, June 3, 2018

Cause of triangular stars in a short tube 80 telescope (ST80)

I bought a Meade short tube 80 achromatic refractor as an inexpensive way to do some wider field astrophotography. The first night out, I was surprised to see that the images of brighter stars were triangular! Here's a closeup.

I did some reading online, and found some forum pages mentioning that this can be caused by pinched optics, which are the result of the front objective being overtightened. I didn't find any sample images though - the purpose of this post is to get some out there.

The dew cap is easily removed, which gives access to the front objective retaining ring. Mine was significantly overtightened - it took a fair amount of force to loosen it. The ring should be snugged down lightly, just enough to prevent it from moving.

This resolved the problem immediately. I'm getting round stars now.

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