Doubled calibration lines and grating clamp pressure?

[Mike Potter]

Every year about this time the focus of my LHiRes deteriorates rather significantly. It is, of course, temperature related. During the summer I get nice sharp focus, skinny and symmetric emission lines from the internal Neon-Argon source. But as the temperatures fall going into autumn, the focus slowly gets worse, with the lines showing a skewed shape (plotting a slice perpendicular to the line). By the time temperatures get into the low single-digits (Celsius) the lines are very out of focus, and no amount of doublet twisting can solve it. Eventually the lines begin to show a doubled shape, starting at the "bottom" of the image, but spreading upwards as the temperatures fall. A long time ago I figured that I'd try to loosen the clamps that hold the grating to its mounting plate, but despite that the problem remained. Until last night. It was nice and clear and cold so, of course, focus was terrible. In desperation I decided to revisit the grating clamps to see if they were still too tight. I was amazed. Basically I decided to loosen the machine screws so that the grating was only barely being held against its holder, just tight enough that I could shake it gently and it would not move. After replacing the grating I could not believe the difference. I'm not certain that was the only thing going on as, during the process, I may have moved the grating a bit in its holder - maybe it was skewed a bit. Anyway, I'd seen others with the same problem and read that loosening the grating clamps was a possible solution, but did not appreciate just how gently they need to be tightened - only enough to keep the grating from moving. Here are before and after pictures, taken maybe 20 minutes apart.