Focusing LHiRes III

[Mike Potter]

I'm trying to get the focus for the LHiRes III as close as I can. I find that getting best focus on Neon lines using the internal calibration source, vs getting best focus by removing the grating and imaging the slit using a long-focal length (300mm) telephoto set to infinity, (again using the internal Neon lamp) is just about one full turn counter-clockwise of the doublet in it's holder as viewed looking towards the grating. I understand that moves the doublet 1 mm closer to the camera. That would mean a 2mm error in the camera placement - but in which direction? I'm guessing it means the imaging chip is 2mm too far back from the front of the LHiRes III. Do I have that correct?

Thanks in advance for your help!

Mike Potter
Beverly Hills Observatory

[Mike Potter]

Okay - so I think I've answered my own question. Assuming parallel rays returning from the grating it would mean my camera/chip is about 2 mm too far back (since I had to move the doublet lens back by 1mm). I've measured the distance from the front plate of the LHiRes and the front plate of the camera and, when added to the distance from the camera front plate to the imaging chip surface, I come up with a distance that is about 1 mm too far back, so at least the direction of the needed correction is the same.

Mike

[Andrew Smith]

Mike, I would not trust the infinity focus of the camera/lens unless I had focused it on the stars. The change in focus can be very sensitive in my experience.

Regards Andrew