I’m doing something wrong

Hi Kate

Not quite sure whats going on there. I think the star not being fully on the slit would just reduce the intensiity I don`t think it would cause any line shifting,maybe someone else could chip in ?.

cheers

Steve

Hi Kate, The pixels size is usually not too critical whe using  the H alpha lines as there are no other lines nearby but it can be super critical with the calibration module where it needs to distinguish between some very closely spaced lines

I find Christian Buil’s tip here

http://www.astrosurf.com/buil/isis/guide_alpy/resume_calibration.htm

works well for calculating the pixel value to use:

“A tip for calculate the scaling factor p (or virtual pixel size). Use the formula:

p = 3123 / dx

where dx is the distance between the H alpha and H beta lines measured in pixels along the horizontal axis in a raw image. Try to find the distance to the nearest pixel (the reading of the mouse pointer is sufficient). In the example, the H alpha line is at x = 882, while the H beta line is at x = 535. So dx = 882-534 = 347, and thus the pixel size to adopt is p = 3123/347 = 9.00 pixels.”

Another trick is to keep an eye on the RMS value that ISIS generates in the running commentary on the “go” page when it is running the calibration fit. If you keep on repeatedly running varying the pixel size slightly, it should be obvious when the program locks onto the right lines as the RMS will suddenly tumble to a very low value. 

Finally double check you are picking the H alpha line and not the nearby Telluric band as the reference  point. (It has been known !)

Cheers

Robin

Hi Kate,

There is still something wrong I am afraid. The RMS fit should be better than 1 Angstrom. For example I have just run a Balmer line calibration on one of my recent reference stars and get the following results (ALPY 600 2xbin 4.54um pixels, pixel size set to 8.97A   (3123/348) )

————————————————————————–

Wavelength fit deviation

point #1  x = 143.059  lambda = 3835.148  dlambda = 0.242

point #2  x = 154.045  lambda = 3888.984  dlambda = 0.066

point #3  x = 170.621  lambda = 3970.476  dlambda = -0.396

point #4  x = 197.253  lambda = 4101.992  dlambda = -0.242

point #5  x = 245.161  lambda = 4340.028  dlambda = 0.452

point #6  x = 349.413  lambda = 4861.473  dlambda = -0.133

point #7  x = 696.362  lambda = 6562.787  dlambda = 0.023

point #8  x = 762.564  lambda = 6872.013  dlambda = -0.013

————————————————————————–

Coefficient a4 : 3.601046E-10

Coefficient a3 : -1.409022E-06

Coefficient a2 : 1.183699E-03

Coefficient a1 : 4.63613

Coefficient a0 : 3146.763

————————————————————————–

RMS : 0.408720

————————————————————————–

If you email me your spectrum image fits file, I can see if i can get a better result if you like

EDIT: typo in pixel size corrected (4.54 not 5.45)

Cheers

Robin

Hi Kate,

It looks like you are giving ISIS the wrong wavelengths for some of the Balmer lines so it is giving large errors on those lines.

David