a question on the instrumental response

[Firefly]

If: in summer, the ambient temp. is high, usually CCD can get about -5~ -10 degree, then at this kind of situation, I make a IR [instrumental response].

then: if it is in winter or other seasons, the ambient temp. changes, i.e. in winter, CCD can go down to -20 or perhaps more colder. Should I make a new IR for current observing circumstance?

I individually think that it is necessary to do a new IR, for as the temp. changes, both mechanical and optical parts of spectroscope do begin to shrink or blend etc...and esp., CCD temperature changes largely. Am I correct?

I want to know if computer or ISIS conserves the IR.DAT permanently? If it is needed to delete it, how to do?

[Francois Teyssier]

Hi Li,

You have to take a reference star spectrum in order to compute the instrumental response EACH night and for EACH target, especially if exists a large range betwenn altitudes of your targets.

For instance, last saturday :

Target = M57 Alt 66° Ref Star = 69°
Target : NGC 6826 Alt = 76° Ref Star = HR 7420 Alt = 75°
Targets : P Cyg Alt = 66° and CI Cygni Alt = 65° : Ref star = HR 7736 Alt = 66°

Sometimes, for long exposures (i.e. 2 hours) the result can be improve by taking spectra of the reference star before and after acquisition of the target.

Best regards

François Teyssier

[Firefly]

Dear Francois,

Thank you for your detailed explanation. But I still have something I don't understand.

1. what kind of star is suitable to be selected as a reference star for different targets?

2.I begin to understand that it is best to find a reference star that is located at the same Alt as that of the target. Then if it has large effect on the result that 2 reference stars at the same Alt, but they are at east and west meridian, respectively.

3.Supposed that there are 2~3 targets at the same Alt., then can I just only take one spectrum of a reference star that is at the same Alt?? And for scientific work, is it permitted that how much the Alt difference between target and reference star is? +/- 5? +/-10?

4."...Sometimes, for long exposures (i.e. 2 hours) the result can be improve by taking spectra of the reference star before and after acquisition of the target" this means as following:?
step1: take a spectrum of a reference star 1h
step2: take the spectrum of target 1h
step3: take a spectrum of a reference star 1h

Total time is 3 hours. Then my question: how height [refer to the horizon] does a target arise at? then it's the most suitable positon to take spectrum. It is not necessary for a target to locate in zenith, is it? Although zenith is the best place to observe. 45degree? 60degree?

Again thank you!

Dong . Li

[Francois Teyssier]

1. what kind of star is suitable to be selected as a reference star for different targets?

Type AV

2.I begin to understand that it is best to find a reference star that is located at the same Alt as that of the target. Then if it has large effect on the result that 2 reference stars at the same Alt, but they are at east and west meridian, respectively.

Depends of the atmosphere homogeneity. In practice target and ref are in the same zone, so no problem

3.Supposed that there are 2~3 targets at the same Alt., then can I just only take one spectrum of a reference star that is at the same Alt?? And for scientific work, is it permitted that how much the Alt difference between target and reference star is? +/- 5? +/-10?

Yes, See P Cyg and CI Cyg
Atl difference of 5° is correct.
It depends of the altitude. Above 50° the flexibility is higher.
Near horizon even 1 or 2 ° difference produce significative effect



4."...Sometimes, for long exposures (i.e. 2 hours) the result can be improve by taking spectra of the reference star before and after acquisition of the target" this means as following:?
step1: take a spectrum of a reference star 1h
step2: take the spectrum of target 1h
step3: take a spectrum of a reference star 1h

Total time is 3 hours. Then my question: how height [refer to the horizon] does a target arise at? then it's the most suitable positon to take spectrum. It is not necessary for a target to locate in zenith, is it? Although zenith is the best place to observe. 45degree? 60degree?

Of course, try to get spectra around meridian, especilly for low declination targets. But sometimes, you can not choose.

Reference stars are bright enough that you acquire a spectrum un 10 minutes of less.
With Lisa you a get a correct spectrum of a 6th mag star with 4x1mn ou 8x30sec exposures

Best regards

François

[Firefly]

Thank you very much! Francois:

I am to learn much more and practise more according to your advice. Now my little observatory is completed well, with thermometer; barometer and hygromter. And I did the evaluation of gain & RON of CCD....the reason for doing all these is that I just want to standardize and normalize everything, as I think that science shall be treated carefully and severely enough.

In about one month, I am to test my LHIRES III (2400l) with Altair for its 1st spectrum, and I feel so happy to continue to get much more advice & help from you....

Best regards
Dong . Li

[Thierry Garrel]

Hi Li,
Instrumental response is very stable and you don't have to redo it each night or even with a great variation of temperature. It can be use for month, and depend only from your setup spectro/ccd, not even the scope. Delta T° make the calibration law becoming non linear during long exposures, specially in high resolution. It is sometime tricky, so be aware to take two neon one before and after exposure.
If you want to applied an atmospheric correction to your spectra, not needed in high resolution, you have to take a spectra of a standard star at the same elevation and same spectral type of your object.
Redoing flat if you change of wavelenght domain is mandotry. Flat are very important, first order corrections, i use to do it each night.
Cheers
T

[Firefly]

Thank you very much! Mr. Garrel

If you want to applied an atmospheric correction to your spectra, not needed in high resolution, you have to take a spectra of a standard star at the same elevation and same spectral type of your object.

This sentence means that ONLY low resolution needs atmospheric correction? Lines less than 2400 are thought as low resolution, aren't they?

Redoing flat if you change of wavelenght domain is mandotry. Flat are very important, first order corrections, i use to do it each night.
thank you for your helpful hints. I am so new to the spectral world and bad at computer technology that I am going to learn much more from you step by step. And I have visited your lovely specblog that is so great~~ so much interesting & useful knowledge....I will learn from that gradually and hope you can give me much more advice, suggestion & help....

Best regards,
Dong . Li