robin_astro

You would need to watch out for flat field effects as the two measured spectra will be in different places in the field. (Best avoided with slitless spectra by placing the target and reference spectra in the same position) You could perhaps test for this by taking an additional spectrum with the 1st grating at a diagonal (adjusting the dispersion to match that of the combined spectrum) and comparing the shape with that of the horizontal spectrum

You first measure the horizontal spectrum without the second grating in place

The light in zero order is not the same as the original source. It is the undiffracted light so the spectral content is the inverse of the combined grating responses but that does not matter. All we are using the first grating for is to produce a horizontally dispersed light source which is then modified by the second grating. The light in the diagonal spectrum only comes from this dispersed light source. The ratio of the light at a given wavelength in the diagonal spectrum to that in the horizontal spectrum then gives the efficiency of the second grating at that wavelength. (It probably should not be used to give an absolute efficiency measurement but the variation in efficiency with wavelength should be correct. EDIT (change horizontal for vertical to match your diagram) Robin

An interesting question. Does a theoretical perfect unblazed transmission grating have a flat response? I dont know and have not yet been able to find the answer on line. One potential way of measuring the response of a transmission grating without a calibrated light source could be to use 2 transmission gratings at 90 deg. The first disperses the light into a horizontal spectrum and the second then disperses this spectrum vertically. The resulting diagonal spectrum divided by the horizontal spectrum should give the grating response. Robin

Another good one currently for the Star Analyser is Nova Cas 2021. It exploded back in March but unlike most novae it refuses to fade and has been bouncing around mag 7 ever since. This is the AAVSO light curve and an ALPY 600 spectrum I took a couple of weeks back, here in the BAA spectroscopy database https://britastro.org/specdb/data_graph.php?obs_id=10399 There is a long thread on it here Cheers Robin

Yes camera response is tough to measure as you need both a calibrated light source and spectrograph or a reference sensor to compare with. If this is scientific astronomical imaging though the atmosphere will also have an affect so you could use a similar process to that used in spectroscopy/photometry , matching the colour of a known star, typically making G2v stars in the image white Robin

It does need practise and for amateurs +-10% error in the shape of the continuum is about as close as you are likely to get (If you want to hone your skills try a few stars with known spectra to see how close you can get as I did in the examples on my website) At the end of the say though it is the features in the spectrum not the exact shape of the continuum which is most important as they contain most of the astrophysical information so don't get too hung up about response correcting, particularly with the Star Analyser which I developed to give a relatively gentle introduction to the subject and is mainly best for targets which show clear strong features. for a good example while it is bright enough I recommend looking at the recurrent nova RS Oph if it is high enough for you. It is in a rare outburst (every 15-20 years) and has and interesting spectrum for the star analyser. This is what it looked like back in 2006 with the Star Analyser http://www.threehillsobservatory.co.uk/astro/spectra_24.htm and this is what it looks like currently with my ALPY 600 spectrograph, from the thread covering this outburst https://stargazerslounge.com/topic/381251-eruption-of-the-recurrent-nova-rs-oph/?do=findComment&comment=4136409 Cheers Robin

A good quick check that your response is sensible is to apply it back to your reference star and make sure the result matches that from the library (again in my document) Robin

Still not good enough I am afraid as atmospheric conditions can be different from night to night even at the air mass. You still have to make a measurement of this on the night for best accuracy. If you are familiar with photometry it is the same as doing differential photometry Robin

It is difficult without specialise lab calibrated equipment but why would you want to ? Robin

You can get a rough response by this method. In fact this is essentially what happens with a slit spectrograph when you take a flat, all explained in my document. At the end of the day though it is not particularly useful as you still have to include the effect of the atmosphere which is done by taking a standard star. Professionals do tend to separate the effect of the instrument and of the atmosphere using a standard instrument response and then correct for the atmosphere separately by taking telluric standards on the night but they rely on much more stable atmospheric conditions. In general most amateurs take a nearby reference star on the night which includes all effects in one go. Cheers Robin

Steve, As I said earlier what you are measuring is not just the instrument response. (This is a common misunderstanding which is why I emphasise this in the document on my website). It also includes the effect of the atmosphere which is significant and changes from night to night and with height above the horizon (air mass). Although you can get a rough result by using a response from a previous night, provided the difference in air mass is not too great, for the most accurate results it is a good idea to take a response on the night at similar air mass, particularly for targets low in the sky where the air mass is high. See this example by Christian Buil of the effect of air mass on the spectrum http://www.astrosurf.com/buil/atmosphere/transmission1.png From his page explaining the effect of the atmosphere http://www.astrosurf.com/buil/atmosphere/transmission.htm Francois Teyssier has made a nice spreadsheet to find suitable bright stars with known spectra near your target. There is a link to it in my document on flux calibration Cheers Robin

Both are used for optical spectra in the professional community, Angstrom traditionally, particularly when identifying a particular line eg H alpha 6563 Angstrom. In publications though some Journals now insist on the SI unit nm Cheers Robin

Nice result. The pair are called 6 Lyr, 7 Lyr or Zet01 Lyr, Zet02 Lyr or HD173648,HD173649 https://simbad.u-strasbg.fr/simbad/sim-id?Ident=*zet01 Lyr https://simbad.u-strasbg.fr/simbad/sim-id?Ident=*zet02 Lyr How does the spectrum of 6 Lyr compare with that of 2 Lyr ? Can you see a difference ? Cheers Robin

RS Oph is entering an interesting new phase not seen in typical novae as the explosion crashes into the material surrounding the red giant, producing a shock wave which heats the material to very high temperatures. This produces highly ionised "coronal lines" in the spectrum, for example [Fe X] (Iron with 9 electron knocked out of it) shown here. (Note the Y scale has been stretched massively to show these weaker lines. The peak intensity of the alpha line is well over 100 !). I saw the same line appear in spectra taken around the same time during the previous outburst back in March 2006 Using a Star Analyser grating http://www.threehillsobservatory.co.uk/astro/spectra_24.htm

Note than when you are correcting for the response (also known as flux calibration) you are not only correcting for the sensor response but also everything else between the sensor and the starlight arriving at the top of our atmosphere. This includes the effect of the atmosphere, the telescope optics and the transmission of the grating which is wavelength dependent and as significant as the camera sensor. By measuring the response by taking a known star we include all these effects in one go, all explained in the document above Cheers Robin

A very nice spectrum with clear Hydrogen and Helium emission lines in the powerful stellar wind of this blue supergiant. It looks like you have rectified the spectrum though (flattened it by removing the shape of the continuum) rather than correcting for the response. That is fine if that is what you intended but a response corrected spectrum of this hot star should slope upwards from red to blue as in this example in the BAA database by Kevin Gurney https://britastro.org/specdb/data_graph.php?obs_id=1658 The full details of how to do this are in this document on my website here http://www.threehillsobservatory.co.uk/astro/Relative_flux_calibration_20201007.pdf but the steps in outline are 1. Take a spectrum of a star with a known published spectrum (eg you could use your 2 Lyr which is type A2v a version of which from the Pickles library can be found in your spectrum processing program eg Visual spec, ISIS, RSpec, BASS) 2. Produce a raw digitised spectrum of it, doing dark subtraction and subtracting the sky background and then wavelength calibrate it 3. Divide this spectrum by the published spectrum of the star to produce the response 4. Produce a raw spectrum of the target star ie P Cygni 5. Divide this raw spectrum by the response produced in 3 to produce the response corrected spectrum of the target star Cheers Robin

Something's gone wrong there I am afraid. The absorption lines have change to emission lines and the spectrum continuum should rise from red to blue, not be flat if you have corrected for the response. Take a look at my page here for how to correct for response of the instrument (and atmosphere) http://www.threehillsobservatory.co.uk/astro/spectroscopy_21.htm also my presentation " Low Resolution Slitless Spectroscopy - Robin Leadbeater on the BAA website here https://www.britastro.org/downloads/15701 Cheers Robin

Excellent! You are on your way. Lots of nice Balmer absorption lines in this A star. The shape of the continuum perhaps looks a bit flat topped so double check that there are no saturated pixels in that region. The small peaks will likely be the zero orders of faint background stars. Cheers Robin

I was fascinated by a recent BAA webinar by Dr Amy Bonsor entitled "Planet Eating White Dwarfs" where she describes how by studying the spectrum of the white dwarf we can determine the composition of the planets which were thrown into the star by the gravitational imbalance created after the parent sun lost its outer layers. https://www.youtube.com/watch?v=4-JOL5NVZ4I I tracked down a couple of them which are bright enough for amateurs to take a spectrum showing clear evidence of the planetary material https://britastro.org/observations/observation.php?id=20210831_232116_04a034fbfd0ae832 Perhaps the same fate awaits us, the only trace of our existence just a few lines in the spectrum of the remnant white dwarf which was once our Sun.... Always look on the bright side of life ! Robin

Also have you tried imaging through the piggyback scope without the star Analyser? If you cannot get good well exposed sharp images without it, you will not get good spectra with it Cheers Robin

There are some examples on my BAA page here which give you an idea what in focus spectra should look like https://britastro.org/observations/observation.php?id=20201216_234948_8cabda965bfe692f https://britastro.org/observations/observation.php?id=20210406_144443_9e1c6a4cf219d14d note though at that the low resolution of the Star Analyser as used in its standard configuration, many star spectra look almost featureless. It works best on stars with strong features Cheers Robin

Hi Steve, You have a good setup which will work well but I would suggest taking a step back and start with a star with known clear features to focus on. You can then move to stars with less obvious features keeping the same focus. Main sequence A stars are often used as they show clear Balmer absorption lines but an even better star at this time of year to cut your teeth on is P Cygni which shows very obvious Hydrogen and Helium emission lines. The image you have posted is way too saturated to be able to see anything. It is important that none of the pixels in the spectrum is saturated. To see the features clearly err on the under exposed side. Have you downloaded the Star Analyser manual ? I have tried to include some useful tips in there too Cheers Robin

Interestingly the AAVSO data shows the visual observers are doing just fine

The flux calibrated spectrum from 2021-08-25. Red shows the full height of the H alpha line and blue at 20x scale shows the detail in the weaker lines. See this annotated high resolution spectrum in the ARAS forum for identification of the lines https://www.spectro-aras.com/forum/viewtopic.php?f=36&t=2804&start=60#p15994 though a few other high ionisation lines have appeared since that spectrum was taken The peak intensity of the H alpha line is currently ~120x compared with the continuum at 5500 A in the green. Approximately 50% of the total light in the visible range 4000-7000 Angstrom comes from the H alpha line. Cheers Robin