spectro-aras.com

One of the mysteries I've pondered regarding my LHiRes is why I continually see asymmetric spectral traces. A vertical cut through the spectrum will show extended wings in one direction, usually towards lower numbers in y. Tonight while again working on getting the focus to work I made a discovery. When I turn the doublet the Neon lines move along the x-axis (wavelength direction). Now, I'd always noticed the Neon lines move a bit when I focus the doublet each night - but those were always small turns, usually less than 1/12 turn. Tonight I rotated through three full turns and made notes on where a specific line fell along the x axis and found the location varies in a sinusoidal fashion with a period equal to one rotation of the doublet and an amplitude of around 80 pixels (9 micron pixels so about .72mm at the focal plane). I'm assuming that is not normal? It would seem the doublet is not set square in it's holder. I guess I should take it out and see if there is some way to adjust it, but I'll ask here first to see if anyone else has encountered the same thing and if so what you did to address the problem? I bought the LHiRes used so I don't know if it was a kit version or not but I think it was not. It has serial number 1K-307.


Mike Potter

Okay - so I took the doublet out of the spectrograph and sure enough, it had been badly assembled. The inner fiber spacer was bent way out of round and only part of it was set inside the inside retaining ring. Because of that one side of the lens had no support and so was skewed to the optical axis. I did the best I could to "unbend" the inner ring (the one closest to the camera) - and used the good ring (originally facing the grating) in it's place, basically swapping the two fiber spacers. Then slowly and evenly re-tightened the screws on the rear retaining ring and re-inserted the lens block. The repair is not perfect but now the peak-to-peak movement is around 8 pixels - so a factor of 10 improvement! It also appears that whoever assembled the lens housing only used three of the tiny screws - which should be fine (maybe better than using all six).

Mike

Well done Mike, I find there is much satisfaction in finding a problem analysing it and then fixing it. First class detective work. I am sure your spectra are all the better for it.

Andrew