[Robin Leadbeater]

Hi Tony,

Except for a mismatch in the strength of the lines, probably due to a mismatch in resolution, the disparity at the blue end does not look that obvious to me from here. What does the fractional error look like if you divide one by the other and smooth the result?

I (and others) have also found getting a good flux calibration below 4000A tough. Most of the sources of error were probably been covered in this thread.

https://britastro.org/node/9700

and Chrisitan Buil’s site covers chromatic aberration, atmospheric absorption and the effect of position on the slit

http://www.astrosurf.com/buil/dispersion/atmo.htm

I suspect the best (though time consuming) solution when after highest accuracy in the continuum shape at the blue end is to use a spectrophotometric technique using a very wide photometric slit to determine the shape of the continuum and combine that with the resolution of a narrow slit spectrum eg

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

David found using a wider slit gave better results, though the reason for the improvement was not clear in this case

https://britastro.org/node/9199

Cheers

Robin

[Robin Leadbeater]

Hi Tony,

These look really good, particularly below 4000A where i had trouble getting a perfect fit.  What reference star did you use to calculate the instrument response ? Did you use the same one for all the targets?

Cheers

Robin

[Robin Leadbeater]

Hi Peter,

You are getting there. The Hydrogen Balmer lines in Denebola in particular are nice and clear.

There’s no Methane in Sirius but the Hydrogen Balmer lines are visible.  The H2O telluric line marked should be at an absorption line. The most obvious telluric line is the broad O2 band at ~7620, off the edge of your plot but very clear in the image you posted on “Cloudy Nights” and “StargazersLounge” forums. The other humps and bumps are from the camera’s 3 colour filters. (Monochrome cameras are better in this respect for spectroscopy)

Cheers

Robin

[Robin Leadbeater]

Hi Paul,

Yes, The way I understand it is if you are using the AOD function in ISIS to calculate and correct for atmospheric extinction then the resulting instrument response  just takes into account the instrument  (ie not the “IR+extinction” commonly calculated using a nearby reference star.) You are therefore correcting your spectrum in two stages, instrument response and atmospheric extinction. (I understand this is the usual technique used by professionals who know their instrument response and therefore just correct for the atmosphere for that particular observation) You can see an example of the procedure here.

http://www.astrosurf.com/buil/atmosphere/annexe.htm

EDIT: linked from

http://www.astrosurf.com/buil/atmosphere/transmission.htm

Cheers

Robin

[Robin Leadbeater]

Hi Kevin,

I think the “type wrong” is Brian Skiffs comment against the reference. G8III and K0III are very close but  your K0III reference (and your own confirming spectrum of course !) is missing from Brian’s catalogue so it could be worth letting him know so he can add it.

Here’s another oddity currently under investigation (BD-1 2458 referenced  as G0v in the literature but clearly much hotter from a preliminary spectrum). Even visually the colour looks obviously wrong for the catalogued classification so it is not clear what is going on here.

https://stargazerslounge.com/topic/289163-s617-triple-star-work-to-do/

Robin

[Robin Leadbeater]

Hi Andrew,

email sent

Robin

[Robin Leadbeater]

It sounds like the prnu map which ISIS uses. For ISIS this is generated separately, illuminating the sensor disconnected from the instrument

http://www.astrosurf.com/buil/isis/eshel/reduction/echelle.htm

Cheers

Robin

[Robin Leadbeater]

Hi Andrew,

Ok it would need to be observations going forward then. I can take two sets of flats in future for anything that might be suitable. Probably best not to depend on me short term though as I am back to the ALPY 200 supernova hunting setup currently which is not the most typical of applications.

Cheers

Robin

[Robin Leadbeater]

Hi Andrew,

Are the low and high ADU flats conventional imaging or spectroscopic flats? The reason I ask is my individual ALPY spectroscopic flats for example can typically range from a few hundred to a few tens of thousand ADU even within the flat due to the instrument response and lamp spectrum. If they are conventional imaging flats I could perhaps take a few of these to supplement my test data set of MILES stars posted on here for you to test.

Cheers

Robin

[Robin Leadbeater]

I have had IRAF sitting installed on a spare laptop running Linux Ubuntu for some time, waiting for me to pluck up enough courage to dive in and start using it. This could be the spur I need. I must admit I find the prospect rather daunting though. It is a long time since I used command line programs!

[Robin Leadbeater]

Thinking about it though, I guess in planetary imaging where focal ratios are invariably very high it is not so much of a problem

Robin

[Robin Leadbeater]

This look pretty good value as a couple of decent wedge prism eg from Edmund Optics alone would be close to this figure. This arrangement though would generate astigmatism placed in a converging beam wouldn’t it?  Shouldn’t there ideally be some form of collimator ?

Robin

[Robin Leadbeater]

Hi Alun,

The ALPY600 is an impressive tool for measuring active galaxy redshifts. For example Etienne Bertrand has found it is possible to get down to mag 15-16 on active galaxy and QSO with a C8 scope. You might be interested in his growing collection that he has been reporting on the ARAS forum here.

http://www.spectro-aras.com/forum/viewforum.php?f=6

His latest ( QSO 1247+267 at z=2 and V mag 16) is here

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1663&p=7726

Cheers

Robin

[Robin Leadbeater]

Also some additional comments here which might be useful where a change to 256 colours was needed

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1660

Robin

[Robin Leadbeater]

Hi Peter,

The shape of your spectrum looks different from the one in the library because your spectrum is not corrected for the response of your instrument or the absorption of the atmosphere. You can see an example of this for the Star Analyser in the VSpec tutorial lesson 5 here

http://www.astrosurf.com/vdesnoux/tutorial5.html

You can also see the same process in my BAA Workshop presentations

http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm

Cheers

Robin

[Robin Leadbeater]

You should try keeping a flip top dry!  The moisture condenses on the inside of the open roof and can even rain down on the gear when you close it up.  I run a dehumidifier 24/7 (with a tubular heater when it is below freezing), though apart from after I close up, it never seems to be running when I go back to it. The observatory volume is very small though!

http://www.threehillsobservatory.co.uk/astro/observatory/observatory_C11_LHIRES_feb_2010.jpg

[Robin Leadbeater]

Hi Andrew,

Yes the effect of atmospheric dispersion (particularly if combined with CA) is well known in amateur spectra too eg  

http://www.astrosurf.com/buil/dispersion/atmo.htm

but that does not seem to explain this case where the results were ok when the seeing was better, unless the target was also higher in the sky or the location was such that the parallactic angle fortuitously aligned better with the slit

Cheers

Robin

[Robin Leadbeater]

Hi David,

I am not sure I follow this. I can understand how the wavelength dependency of the Airy disc size due to diffraction could potentially cause this effect in a diffraction limited situation but in your setup I would expect this to small compared with the slit width and the seeing.  Also the atmospheric effects which affect seeing and atmospheric dispersion are due to refraction, so wouldn’t they reduce with increasing wavelength?  Comparing the star FWHM from photometric B and I band measurements could be interesting. Do you know if that this affect on seeing is seen there ?

Robin

[Robin Leadbeater]

Hi Tony,

I forgot about that. In fact it was me who asked for it !

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=516#p1829

It was added in v5.1.3

http://www.astrosurf.com/buil/isis/new/release.html

Cheers

Robin

[Robin Leadbeater]

I am wondering if using the AOD technique to do a second order extinction correction coupled with our standard reference star technique as a first order correction might be useful, reducing the need to chose the reference star precisely at the same elevation as the target. ie The MILES star need not then be at the same elevation, as the effect of the difference in air mass is calculated separately using a typical AOD. 

Robin

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