Calibration Issue

[Keith Graham]

Hi,

I am having an issue with my calibration errors and linearity. My equipment is an SBIG ST8xme/lhires 2400/35 u slit/LX200 12" classic slope. Guide camera is the Lodestar.

Up until about a week ago, my neon calibrated spectra generally required a .06 telluric correction to bring them into BeSS specs. Most, if not all, of the BeSS check lines averaged around +/-.05 A when submitted. The last time I got such results were on Sept. 18, but I have been noticing an increasing amount of error since then. On Sept. 19, there was a .17 A correction, Sept.20 a .2 correction, Sept. 29 a .33 correction. These corrections were not the same all across the spectral range as there are some tellurics I cannot get to match at all. The issue appears to get worse towards longer wavelengths.

I have checked the following:

1. I re-processed all of these spectra to see if there was a setting issue in ISIS. I get the same values mentioned above, so I have eliminated that possibility.

2. I checked the focus of the neon lines and of the 2D target spectra as well as the fwhm of the neon lines. The spectra are thin and sharp with visible telluric absorption lines. With a 35u slit and 9u pixels, my neon lines should have a fwhm of about 4, which they are.

3. On one evening, the neon lines were shifted between the first and last spectra taken that night. So I tried calibrating with each spectrum. The results were the same.

4. I have noticed the guide star jumping back and forth across the slit causing some large guide corrections. But I have noticed this for quite some time and it has never caused this issue in the past.

5. I suspected something might be changing within my equipment, but I have changed nothing there. The camera and Lhires have not been removed or changed in any way during this period. I did rotate the collimating lens to get the optimal neon fwhm, but that is the only change made, and this made no difference.

So right now I am at a loss to explain this gradual increase in calibration errors. Can anyone please offer some help here.

Thank You,

Keith Graham

[Robin Leadbeater]

Hi Keith,

Has the neon moved sideways on the shaft ? An offset neon can give big shifts. I regularly look a the slit in the guide camera with the neon on to check the illumination is still symmetric over the slit. (Not sure the error would be wavelength dependent across the field though)

Robin

[Keith Graham]

Hi Robin,

You brought up a point that I had considered but dismissed because I had no idea the neon could move within the Lhires. Also, I am not sure what to look for, so I took an image with the neon on and was surprised to see the right part in the dark and a column of bright just to the right of the slit. I do not even know if this is normal, but I would guess it is not. I have attached the image to this message and ask for your opinion.

Now, if the neon lamp did move, can you please tell me how I would fix that? Is the lamp accessed through the guidescope port? And how would it be tightened down.

I must say that since this issue emerged. I did notice the knob was quite difficult to rotate. But now it appears to have loosened up and rotates normally. I am wondering if that difficult rotation may have caused a shift.

Cheers,

Keith

[Keith Graham]

Hi Again, Robin,

Actually, the bright area is to the LEFT of the slit, not the right. In fact, none of that bright zone falls directly on the slit, so I will bet you are spot on with your assessment. Also, I did not see the attachment on the previous post so I will try again. If it does not appear, let me know and I can send it to you privately.

Cheers,

Keith

[Keith Graham]

OK - back again. I was able to see that by removing the slit I could see the neon lamp and shaft. It appears that the lamp is secured to the shaft by a screw that actually runs through the shaft, so it does not appear that that lamp can actually slide along the shaft. And I do not see any markings on the shaft that shows the lamp had shifted. I guess that leaves the possibility that the shaft itself might be able to shift laterally. Is this possible?

Since I did not see my image as an attachment, I took the liberty of sending it to your email. Hope that is OK.

Cheers,

Keith

[Keith Graham]

My calibration puzzle has been solved. I knew it must be something simple I was missing and indeed it was. It turns out that I neglected to double click the X pixel location of the 6532.88 line in the ISSI Calibration tab. So I was using a value of 461 for that pixel location when in actuality it was over 500. Yep-that'll do it. Now that I have rectified that issue, we are back in business.

Cheers,

Keith