Alpy Instrument Response

[Keith Graham]

Hi,

I have a few questions regarding instrument response using the Alpy600. My practice has been to create an IR curve each time I take a spectrum. I will use the same reference star and the same Miles star when creating the IR curve for a given target each time I take a spectrum for that target. What I found was there are times when the IR curve may be slightly different for each spectrum. Recently I discovered that two IR curves were quite different on two different nights, even though I used the same reference star and Miles star each time. I am guessing that I probably erred when I smoothed out the continuum in ISIS. This, of course, caused a difference in the two spectral profiles.

As I thought more about it, I began wondering if I could create an IR curve for a given target once and apply that curve to all subsequent spectra for a given target. I would think this would keep a greater consistency for all profiles. Of course I would need to be certain the IR curve I use was accurate. So my questions:

1. Would one IR curve work for a given target each time I took a spectrum for that target (i.e. reuse one IR curve for all spectra of a given target)?
2. Would one IR curve give accurate results for any target (i.e. reuse one IR curve for all targets)?
3. Or is it always best to create an IR curve each time I take a spectrum?

Thanks,

Keith Graham

[Miguel Rodriguez]

Keith,
Maybe I cannot be of help, but, I would like to know what are those IR curves and what is its purpose.

I've never used any of that doing spectra with Alpy.

Regards
Miguel

Edited: I have mistaken the meaning of IR, from Instrumental Response to Infrared

[Keith Graham]

Hi Miguel.

The IR curves are the instrumental response curves. In ISIS, you create them using the Response button in the Calibration tab. When doing low resolution spectroscopy with the Alpy, instrumental response is essential to develop a correct spectral profile. When the IR curve is applied to the spectrum, you will see a definite change in the profile from before and after its application.

Cheers,

Keith

[Robin Leadbeater]

Hi Keith,

Don't forget that the "Instrument Response" also includes various effects not directly due to the instrument which can change from observation to observation, so variations in IR curve from night to night are quite possible and are needed to correct the spectra for best accuracy. Examples include:-

1. Difference in atmospheric extinction due to altitude of target or variations in atmospheric transparency from night to night

2. Changes in the relative proportions of different wavelengths passed by the slit due to chromatism (either caused by the telescope optics or atmospheric dispersion), combined with differences in focus and guiding.

3. Changes in spectral content of the flat lamp eg long term drift during the life of the lamp or shorter term variations due to voltage changes.

This means an IR curve measurement made under as close as possible conditions as the target measurement should give the best results.

If you are finding unexpected variations in your results though, you can troubleshoot the problem first by checking if the IR applied to the reference star gives good agreement with the library version of the star spectrum. This should always be the case. If it is not this implies a problem with the IR curve calculation.

Once you have confirmed that the IR calculation is ok then any differences in IR curve from night to night are likely to be real and provided conditions do not change between the reference and target measurements, the IR correction to the target spectrum on a given night should work ok.


Cheers
Robin

[Keith Graham]

Hi Robin,

Many thanks for this reply. Your comments make perfect sense and nicely explain why IR curves are necessary for each observation. They also give me something to check out when IR curves re in question.

Cheers,

Keith

[Keith Graham]

Hi Again, Miguel,

I would like to expand a little on my previous answer to you.

You equipment most probably will have different sensitivities to different wavelengths compared with those of 6he equipment of another person. For example, your camera may be more sensitive to 500nm than another camera, so your spectrum will show a greater amplitude for 500 nm than the other camera. So if you and another person take a spectrum of a given target on the same night, the amplitudes of wavelengths on each of your profiles will be different depending on how sensitive your camera is to the various wavelengths within your spectrum compared with the equipment of the other person. By taking an image of a reference star close to the target and applying a reference spectrum of the same spectral class from a catalogue such as Miles, an instrumental response curve for your camera is created. The other person will do the same with his reference star. You both will now have an instrumental curve for your equipment which will probably look different because the wavelength responses of your equipment are different. But when your IR curve is applied to each of your target spectra, the target profiles will be very close. Without applying the IR curve, your target spectra profiles would be very different.

I use ISIS for my spectral processing. You can find the ISIS guide for the ALPY here:

http://www.astrosurf.com/buil/isis/guid ... uto_en.htm

Here you will also see how to determine the IR curve using ISIS.

Accurate calibration and instrument response are important for an accurate target profile that is suitable for submitting to the various data bases. Also, see Robin Leadbeater’s more detailed explanation in this thread.

Cheers,

Keith

[Miguel Rodriguez]

Hi Keith, thanks very much.
I didn't supposed you referred to instrumental response, I thought you were talking about Infrared

Yes, I do instr. response curves. I usually do one for every object I make an spectrum. Alpy seems to be very stable, and the obtained response curves are very similar in shape through the night, and from one night to the next. If the curve has a different shape than typical I repeat the spectra taking of the reference star, or I choose another different.

Anyway, I would never use a response curve from a former session.

[Keith Graham]

Hi Miguel,

That is funny. Yes, I do see why you might have interpreted IR to be infrared. Please forgive my rather long explanation to someone who obviously knows his way around spectroscopy.

My IR profiles from night to night,like yours, are usually very similar. That is why I was puzzled as to why two of mine were so different from what I normally get. And even more puzzling was that they were different using the same reference star. Actually, I always use the same reference star for a given target, and I do take reference star images for each session. Although I have never used IR profiles from previous sessions, I was curious if this was an option. Robin's message explained nicely why this should not be done I do agree that the Alpy is very stable, and this was the first time I experienced this IR profile differential. My lesson here is that when I see an atypical IR profile, it would be best to simply do a retake of that reference star or scrap the results entirely.

Cheers,

Keith