spectro-aras.com

For the second time this month the calibration lines have shift on the chip between sessions overnight.
I installed a roll off roof last fall and never touched the Lhires tuned to Ha until late this spring when I would do an occasional HeI at 6678. I noticed that the movement of chips across the camera chip did not track in direct relation to amount of movement with the micrometer. In fact, in the beginning it sometimes moved in the wrong direction. I could not rely on the micrometer reading to return to Ha. I now use the 6598 line to move between Ha and HeI. I try to end the night on an Ha target so I don't forget change the micrometer for the next session. In both instances I was able to confirm that the 6506 was where I set it at the end of the previous night.
Now that the daytime temperatures are ranging between 37C to 43C I am starting to see the calibration lines shift left about 145-150 pixels (Atik 460 binned 2x)
only on the session following turning to HeI. It's like something is slipping.
I checked on the thumb screws on the back of the grating and they are tight. The micrometer does not have any noticeable play. I have not removed the grating. I assume at some point I reluctantly must, but I'm hoping someone can provide advise on probable causes.
Thanks, Al Stiewing

Hi Al,

Shifts of a few pixels during a night because of flexure and temperature are normal for the LHIRES. (It is therefore important to take lamp spectra as a minimum at the beginning and end of each observation) but a 150 pixel shift without moving the micrometer is not normal. I suggest checking that the grating is rotating freely in its holder and the spring is correctly in place (it can foul the rotation movement if displaced) and it is firmly holding the bar against the micrometer foot. See 3/4 down the "assembly" page here for how it is constructed (the original version, the latest version may be slightly different)
http://www.astrosurf.com/thizy/lhires3/index-en.html

The grating holder is easy to remove and is designed to do so to change gratings. To remove the holder just wind the micrometer off fully, undo the 4 thumb screws and pull it out. To minimise flexure, it is also worthwhile going round all the screws on the body making sure they are all tight. Check also that both nylon screws that hold the doublet lens in place are tight.

Cheers
Robin

Worth also checking the imaging camera coupling and the slit holder thumb screws are tight, though if movement of the slit is the problem you would also see a large shift in the position of the slit in the guide camera.

Cheers
Robin