Bin x 2 - problem with ISIS

[David Boyd]

Hi Pete,
In answer to your final question, it seems to me that if you open the slit and expose for the same time, the light in each line will be spread over a wider area of the chip so the intensity per pixel will be lower so you will need a longer exposure to achieve the same ADU count per pixel as before.
David

[Robin Leadbeater]

Hi Pete,
In answer to your final question, it seems to me that if you open the slit and expose for the same time, the light in each line will be spread over a wider area of the chip so the intensity per pixel will be lower so you will need a longer exposure to achieve the same ADU count per pixel as before.
David

Hi David,

But the area of the approximately uniform field sampled by the slit increases doesn't it ? In which case I would expect the intensity per pixel column to remain constant and the total intensity in each line to increase approximately proportional to the slit width. See the ALPY lamp spectrum with the very wide photometric slit half way down the page for example
http://www.astrosurf.com/buil/alpy600/p ... c_slit.htm

slit_width_lamp.png (222.2 KiB) Viewed 5916 times

Cheers
Robin

[David Boyd]

Hi Robin,
Yes, of course you are right with a uniform source of illumination. The two slits should diffract into gaussian-like peaks whose area will scale with the slit width and should have the same height. I was thinking in terms of a point source diffracting into a wider but shallower peak. However that doesn't explain why Pete was seeing fainter lines with a wider slit. Possibly a focusing issue?
David

[Christian Buil]

Hi Pete,

I apologize for giving you such a late response, for various reasons (not critical!) I have not been able to be too present on this forum lately. We will solve your spectral calibration problem. I have no doubt!

First, you can publish a screenshot of the tabs:

1 - General
2 - Calibration
3 - Configuration

Second, transmit to my email adress (christian.buil - at - wanadoo.fr):

- one raw frame (2D) of NeAr calibration lamp
- one raw frame (2D) of a A0V like star (Vega for example).

(of course, in Bin 2x2 mode).

Do not hesitate, you must fix your difficulty (clearly ISIS cannot find the right calibration lines with your parameters)!

Christian Buil

[Peter Velez]

Thanks Christian

I have emailed you the files and screenshots as requested.

Now I have changed the LISA slit to the 35 micron alternative, I am finding calibration is better but not always. Initial calibration with either my .lst file or the Predefined mode presents a RMS at anything up to 22. Sometimes I can then refine this with the Calibration Assistant down to under 0.5. At other times it fails with an RMS of 5 - 8.

Looking carefully at what is happening, I can see what is causing the failure when I use the Calibration Assistant. I use 5944.82 as the reference line in the Ar Ne calibration image. With my rig this is at or around x-pixel 716. When I then run the Calibration Assistant using a reference star to match the H alpha absorption line, the x pixel position of the Ar Ne reference line changes to around pixel 801. Looking at the Ar Ne calibration image, there is no line there at all. I have tried to reset the reference emission in the Ar Ne frame to another line - but it keeps defaulting back to 5944.82 and the incorrect pixel position. Robin suggested that I look out for this.

My Ar Ne images look clean. I have tried to apply darks, bias and cosmetic correction to the Ar Ne frames and computed a mean of several images to try and stop this happening - with no luck.

Hopefully this helps with troubleshooting.

Pete

[Peter Velez]

Hi Robin,
Yes, of course you are right with a uniform source of illumination. The two slits should diffract into gaussian-like peaks whose area will scale with the slit width and should have the same height. I was thinking in terms of a point source diffracting into a wider but shallower peak. However that doesn't explain why Pete was seeing fainter lines with a wider slit. Possibly a focusing issue?
David

Thanks for your comments David and Robin

It may indeed be a focussing issue. However, what I find interesting is that I didn't change the focus of the LISA after rotating the slit mirror for the wider slit. The FWHM for the central line I used to check the focus was pretty much the same and was the best I could achieve. If anything, the focus with the 35 micron slit is better - I measure FWHM for the 5852 Ne line at 1.59 with ISIS. This is binned x2 so I assume that equates to just over 3 unbinned. Not perfect but I would have though this was acceptable. With the 23 micron slit I was at around 1.8 binned x2.

I might be able to refine this with more tuning - but as my gear is 6 hours drive away, it may need to wait. Provided I can overcome my calibration issue, taking longer calibration images is not a concern

Pete

[Peter Velez]

So - focussing. One for the brains trust...

A reminder - I have a LISA with an Atik 460EX attached. Images are currently binned x2. Calibration with ISIS is very hit and miss. It has improved a bit since changing the slit mirror to the 35 micron option - possibly because I am no longer undersampling.

Yesterday, while trying to manually determine the dispersion using the Dispersion tool in Profile (ie manually selecting lines from a .dat file taken from a calibration image and allocating the appropriate wavelength, I realised how poorly defined were the lines shorter than 5852. The emission lines with shorter wavelengths were no more than dimples in the profile. I expect there is scaling at work but it did lead me to look at my Ar Ne frames.

I noticed that there is a clear difference in how sharp the emission lines are across the whole spectrum. Lines with wavelengths between about 5700 and 6700 are quite sharp. The lines around 4000 are very indistinct and smeared out.

Testing this with some FWHM data (measured in ISIS for lines at 4158 and 5944, I found a big difference - FWHM of 2.85 compared to 1.43. This may explain why ISIS is struggling.

To check this, I looked at some calibration frames taken in mid May. Same exposure length (but with a narrower slit) - the FWHM for the same lines were 1.78 and 1.88 respectively.

What is causing this? I have 2 possible explanations:

1. Resolution - with a wider slit, resolution is lower. So there is uniformity in the lines produced by the LISA.

2. Focus and temperature - the current frames were taken during the day in a hot(ish) shed - at around 16 degrees C. The earlier calibration frames were taken at around 8 degrees C at night. I understand that temperature variations impact on resolution of the LISA.

A further interesting issue - I have been focusing the LISA during the day when it is warmer. If I am imaging at night, I may have lost focus before even starting imaging. This might explain why a number of my recent spectra of dim objects are noisier than those taken last month.

I'd welcome any thoughts on this.

Pete

[Benjamin Mauclaire]

Hi,
Could you join a FITS file of the 2D spectrum of your calibration lamp in bin 2x2?
Be aware not to over expose the red lines.
Is your calibartion lamp a Relco?

Benji

[David Boyd]

OK, I will offer my thoughts.

The LISA is certainly temperature sensitive. Focusing during the day and then taking spectra at night without refocusing is likely to lead to relatively poor results. That is my experience. I leave my LISA on the scope outside at night for around half and hour to let it acclimatise before focusing it. Temperature changes during the night are then relatively small and don't affect focus much. I usually focus on the 5852 line in the middle of the spectrum and aim for a FWHM of about 2.5 pixels. This is with the 23 micron slit and binning x1. I have the same CCD in my camera.

The lines at the blue end of the spectrum are very weak relative to the red lines in the LISA calibration lamp. Once you know which lines ISIS is using in its fit, you can expose your calibration frame to get an ADU count in the brightest line being used around 30-40K. It might even be worth taking a very bright line out of the list in order to be able to increase the calibration exposure for longer to bring up the blue end.

You can investigate how your resolution varies across the spectrum by measuring the FWHM of the calibration lines in pixels using the FWHM function in ISIS. Multiply these FWHM values by the dispersion in A/pixel from the A1 value of the polynomial fit to get the FWHM in A. Then divide the wavelength of each line by its FWHM to get the resolution you are achieving at each line. This is a very educational graph! You can then experiment with the focus to see how the shape of that graph changes as you vary the focus. What you find is that there is a compromise between balancing the resolutions at the blue and red ends of the spectrum. I can get a resolution of 1200 or better in the centre of the spectrum but it drops to around 700 at the ends. This reflects the compromises necessarily made in designing the LISA at an affordable price.

Hope this helps.

David

[Benjamin Mauclaire]

Hi David,
The method you described to compute RP (resolution power) will not describe LISA caracterictics, ie non linear dispersion and focus along wavelength band width covered.
RP depends on wavelength as you said, as RP=Lambda/Delta_lambda where Delta_lambda=FWHM(of line at Lambda), it is needed to use the FWHM associated to the calibration line.
Therefore RP of blue calibration lines will be lower and then describes the unfocussed of this wavelength zone.
Cheers,
Benji