I’m doing something wrong

[Mr Allan Brittan]

hi all,

Not quite sure why but once I’ve put my spectra through Isis and tried to do comparison with miles or pickles star, hydrogen alpha lines up but the rest are all to the right of the cam parison spectra. I redid all my darks, bias, neon calibration and tried again, but got the same result.

I’m now confused where I’m going wrong. Could my star not be fully on the slit so I’m not getting enough light?

kate

[Steve Cuthbert]

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

[Paul Luckas]

Hi Kate,

It sounds suspiciously like your wavelength solution is the culprit. Can you you elaborate on how you’ve performed the calibration (this is an Alpy, right?). If there was a way to share your spectral and calibration images I’d be happy to take a look in ISIS.

In the mean time, I created a guide some time ago on using ISIS’ calibration assistant that may, or may not, be useful to you:

http://jazzistentialism.com/blog/wp-content/uploads/2016/10/alpy-600-mixed.pdf

Cheers,

Paul

[Mr Allan Brittan]

i am using an alpy without calibration module. Star is delta cas. I have now got things closer by changing my pixel value slightly.

Great guide Paul.

kate

[Andy Wilson]

Hi Kate,

It is one of the idiosyncrasies of ISIS calibration for the Alpy that you have to tweak the value of the pixel size. This goes against common sense as of course the pixel size is fixed.

I think this is because no 2 Alpys are quite identical, and is due to minor differences in focal lengths and chip to grism distances. François Cochard explained this at the BAA workshop far better than I can remember it now, but I remember that tweaking the pixel size fixes the problem.

Best wishes,

Andy

[Robin Leadbeater]

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

[Mr Allan Brittan]

hi Andy and Robin,

i did the calculations and came up with p=4.66.

I tried this and got an RMS of 22.118.

I repeated it with 4.69 and got an RMS of 8.86, so will stick with that figure for now.

Thanks  for the help

Kate

[Robin Leadbeater]

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

[Mr Allan Brittan]

Hi Robin,

These are my numbers from the last run through

Wavelength fit deviation
point #1  x = 706.025  lambda = 3839.683  dlambda = -4.293
point #2  x = 724.012  lambda = 3879.507  dlambda = 9.543
point #3  x = 767.819  lambda = 3979.946  dlambda = -9.866
point #4  x = 816.480  lambda = 4096.430  dlambda = 5.320
point #5  x = 913.897  lambda = 4341.149  dlambda = -0.669
point #6  x = 1111.911  lambda = 4861.383  dlambda = -0.043
point #7  x = 1782.002  lambda = 6562.789  dlambda = 0.021
point #8  x = 1898.594  lambda = 6872.013  dlambda = -0.013
————————————————————————–
Coefficient a4 : 7.007126E-10
Coefficient a3 : -3.752157E-06
Coefficient a2 : 7.268176E-03
Coefficient a1 : -3.45201
Coefficient a0 : 3798.122
————————————————————————–
RMS : 8.861652

Kate

[David Boyd]

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

[Robin Leadbeater]

Hi Kate,

What smile settings have you used?  If you do not have a calibration module, you can measure the smile using sky lines.  I tried your images assuming vertical lines with no smile (by setting the Y smile to the same as the spectrum position and the radius to a large number eg 999999) and got an RMS of 1.14 A using a pixel size of 4.68um

————————————————————————–

Wavelength fit deviation

point #1  x = 704.340  lambda = 3834.491  dlambda = 0.899

point #2  x = 725.984  lambda = 3889.299  dlambda = -0.249

point #3  x = 758.121  lambda = 3970.993  dlambda = -0.913

point #4  x = 809.533  lambda = 4102.375  dlambda = -0.625

point #5  x = 901.554  lambda = 4339.253  dlambda = 1.227

point #6  x = 1102.840  lambda = 4861.706  dlambda = -0.366

point #7  x = 1771.045  lambda = 6562.745  dlambda = 0.065

point #8  x = 1898.937  lambda = 6872.037  dlambda = -0.037

————————————————————————–

Coefficient a4 : 3.006774E-11

Coefficient a3 : -2.735230E-07

Coefficient a2 : 6.837095E-04

Coefficient a1 : 1.92972

Coefficient a0 : 2221.769

————————————————————————–

RMS : 1.116470

————————————————————————–

In his ALPY tutorial, Christian Buil suggests an RMS of 2-3 A maximum is acceptable for this calibration method

Cheers

Robin

[Mr Allan Brittan]

Hi David and Robin,

i set the smile using a neon light source.

kate

[Robin Leadbeater]

Hi Kate 

I am able to obtain an acceptable calibration (RMS 1.25 A) with your files using the neon file to measure the smile. I will email you a set of output files and screenshots which hopefully will allow you to reproduce it.

Cheers

Robin

[Mr Allan Brittan]

Thanks Robin

[Author]

Posts