Symbiotic ag Peg!

Hello Andy, Steve,

Thanks to both of you for posting your spectra – they are impressive and in my case certainly of great help because I can compare with my own low resolution spectra using SA200 & DSLR.  I have been following Chi Cygni & AG Pegasi [and also CH Cygni] since July/Aug, at low reolution and at other end of learning curve to you.  I have used several different references to correct for for instrument and atmospheric extinction effects, but am wondering if I should be using some recognised standard stars that are in common use – depending on target RA/DECL? So far I’ve tried to find a nearby A class star of roughly the same magnitude to use, but is there somewhere a preferred/recommended list?

Best Wishes

john s

Hi Steve and John,

That is a great spectrum John. These are exciting times as spectroscopy is becoming more easily available to amateurs, and there are plenty of fascinating targets.

My understanding of response correction is that to get a really good atmospheric correction you need to find an A or B star of similar airmass (altitude) to your target star, and with a professional spectrum available. There are ways to get a good compromise, but for the very best results this is the way to go.

There are a couple of very handy resources that make this much easier. First there is the Miles database of standard spectra. Second a spreadsheet that allows you to locate stars from the Miles database of similar airmass to your target at the time of observation. The ARAS forum post on this subject can be found here:

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=941

Best wishes,

Andy

As it happens – Francois Teyssier has just updated his reference star finder for low resolution spectroscopy

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1227

It finds the potential reference stars nearest in altitude to your target and now will even plot a trajectory so you can see how the relative altitude changes with time

It now includes all the MILES A and B stars (which have professionally measured spectra) as well as a long list of bright main sequence A and B stars with low interstellar extinction and published spectral classification. These should have spectra similar the  generic (eg Pickles) spectra for the particular spectral type but they do not have published spectra so you need to be a little careful using these, perhaps at some point comparing them with a MILES standard for example to confirm they are typical of their type.

I think measuring a few MILES stars of different spectral types is good practice if you are starting out or testing new equipment as any deviations between the measurement and  the database version can be seen and followed up to improve one’s technique. There are some examples here where I did this using the Star Analyser and ALPY  

http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm

I find getting the blue end below ~4000A perfect particularly difficult for a number of reasons:-

the sensitivity of the instrument drops off sharply 

any errors due to atmospheric extinction are greater

any problems from selective sampling of particular wavelengths by the slit due to for example atmospheric dispersion or chromatic aberrations in the optics are greater

 it is difficult getting enough signal from the (halogen) flat lamp to keep the noise level down

 the increased crowding of the Balmer lines towards the Balmer jump makes it more difficult to get a perfect match between measured and reference spectra.

Cheers

Robin