robin_astro

Yep the ALPY guiding module is a must but the calibration module is a nice to have. The spectrograph is very stable so you can calibrate using known lines in a reference star, even the sun using the daylight sky and halogen lamp waved in front of the telescope is good enough for a spectroscopic flat. More on the development of the UVEX here. http://www.spectro-aras.com/forum/viewforum.php?f=45 Deigned by world leading amateur spectroscopist Christian Buil, it was principally designed to extend the range of amateurs spectroscopy into the UV but the the performance has proved to be impressive across the full visible spectrum. It is not quite fully developed yet (I think it is awaiting finalisation of the guider/calibration module, otherwise you need to use the one from the ALPY) and alignment is critical and tricky. I can see this being a commercially produced instrument at some stage though. The lowspec is a very conventional design and might be more straightforward for someone new to spectroscopy. https://www.thingiverse.com/thing:2455390 Cheers Robin

Sadly yes. Perhaps the downside of getting addicted through the Star Analyser is how it can lead to cravings for the harder stuff 😞

Yes but Gav asked what would be needed for detailed spectral classification. You can do a lot of other interesting stuff with a Star Analyser and plenty of real science with a cheaper ALPY 600 core plus guider module for example Cheers Robin

But if all you need is a general impression of how the spectrum changes with temperature then the Star Analyser can do this as Torsten Hansen showed here

If you are into self build then there are a couple of 3D printed designs (lowspec and Uvex) which potentially have enough resolution get you into this area UVEX in particular would make a very nice spectral classification instrument because of its good UV end capability http://www.astrosurf.com/buil/UVEX_project_us/ Robin

Something like a LISA spectrograph with a resolving power of 1000. To be honest though other than identifying cataclysmic variables (novae, supernovae etc) from their spectra precise spectral classification is a specialist area which needs quite a bit of skill and knowledge and is not an area amateur spectroscopists generally get into really. If you are looking for an up to date book on the subject a good reference is Gray and Corbally "Stellar Spectral Classification" Stars who's spectra vary are more interesting. For example there have been some recent posts on the AAVSO forum tracking the dwarf nova SS Cyg through its cycle every 2 months where the changes both in temperature and emission lines are clearly seen at Star Analyser resolution Cheers Robin

Hi Gav, It is not so much the brightness but the ability to resolve the key lines that determine the spectral type. This can be done approximately for types where the features are clear eg A and M but not in general for intermediate types F-K . The resolution of the Star Analyser is also insufficient to determine the luminosity class. The difficulties typically encountered can be seen in the post "A tale of two stars" where I discuss an attempt to classify two newly discovered variable stars using spectra at Star Analyser resolution. BTW if you are interested in a particular star's classification Brian Skiffs huge database of published classifications is a good resource http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk A quick browse of that will indicate in practise what an inexact science spectral classification is ! Cheers Robin

To answer a question raised in another post about the possibility of using the Star Analyser to do spectral classification, here is an example of a couple of stars I attempted to classify using spectra at the same resolution as a typical Star Analyser spectrum (~45A). It is rather long so I have put it in a pdf but the executive summary is "Spectral classification is based on what spectral lines are present and at what strength, not the shape of the spectrum continuum and this needs sufficient resolution to resolve these lines (Typically 10A or better). Spectra at typical Star Analyser resolution can contain clues as to the approximate spectral type but in general they are not good enoigh for precise spectral classification. The discovery in the early 20th century that the lines in the spectrum could be used to measure the temperature of a star and its size revolutionised stellar astrophysics" Cheers Robin A Tale of Two Stars.pdf

OK time for reality check here I am afraid. The key point to understand is that spectral classification uses the presence or absence of particular lines, not the shape of the spectrum continuum. The reason is that using the shape of the spectrum continuum can be totally misleading due to interstellar extinction. (If this could be used it would be easier just to use the colour (UBVRI) photmetry. I will post an extreme example of this for a star I am currently studying, later today when I have reduced it. Detailed spectral classification is not the Star Analyser's strength I am afraid because the resolution is not high enough (You really need a resolution of 5-10A or better to resolve the lines sufficiently) You can get a rough idea (ie hot stars show H and He lines, cool stars show molecular bands but the spectra in between tend to be a mush of overlapping lines which are difficult to distinguish.) For me the Star Analyser strength is looking at the more unusual objects with bold features in their spectra. These are much more interesting astrophysically in any case. (eg exploding stars, stars with hot discs around them or with intense stellar winds, planetary nebulae, methane in planet atmospheres, distant quasars and active galaxies Cheers Robin

Taking spectra is like any other astro imaging. The key is to get the best stacked image. ie the sharpest spectral features (without software sharpening), lowest noise, full bit depth (ie stack the avi to give a 16bits mono image) without any saturation in the spectrum, that you can to pass to your spectral analysis program. With bright targets planetary imaging techniques work well. Spectroscopy of faint targets is more like deep sky imaging with longer exposures (Your ASI1600mm will be perfect for that when you are ready) Cheers Robin

Hi Nigella, Yep, that's exactly why I developed the Star Analyser. (Well actually, mainly I just wanted one for myself ! But I convinced Paton Hawksley that they might sell a few and the rest is history as they say) Have fun ! Robin

You can indeed do spectroscopy in bright skies but with slitless spectroscopy (eg the Star Analyser) the target does need to be brighter than with dark skies. With a typical Star Analyser setup the spectrum brightness is about 6 magnitudes fainter than the star because the light is spread out so if you can image say a mag 12 star in bright moonlight the faintest star you could record with the Star Analyser on that night would be ~mag 6. With dark skies if you can get to mag 20 in an image you would be able to record spectra to ~mag 14. A slit keeps most of the sky background out which means it is possible to record faint objects even in full moonlight or bad light pollution. I have a couple of little demonstrations of the effect of a slit on the sky background here on my website http://www.threehillsobservatory.co.uk/astro/spectroscopy_4.htm http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm

I think it is easy to get lost in all the complexity of processing. Most of the key features are immediately visible in the spectrum image without any processing. I sometimes forget how remarkable it is what we are doing. In 1836 the philosopher Auguste Compte famously suggested the make up of stars as an example of something we can never know. 25 years later Kirchoff and Bundsen used spectroscopy to detect several elements in the sun and 160 years on we are doing this and much more from our back garden for any object visible in our telescope. Cheers Robin

The main thing is not to give up though. Your Castor spectrum resolution is very good. Resolving 5 Balmer lines with the Star Analyser is typical. I can see 7 in your spectrum which is excellent Cheers Robin

If you like to orientate the spectrum so the dispersion direction is in the Dec direction(for example if your tracking is not so good), you can still orientate the grating dispersion to be horizontal relative to the camera and then rotate the grating plus camera so the dispersion direction is in the Dec or RA. The dispersion is only dependent on the distance of the grating from the sensor so moving to a longer focal length will not change the dispersion. I would stick with the short focal length for now as this will potentially give you better resolution (smaller star image). The pixelation noise is not a big issue, for example you could try 2x binning the image after rotation and processing that in Rspec. They will probably disappear without significantly affecting the resolution. Cheers Robin

Hi Louise, Rotating a pixelated image will always introduce some artifacts so the long term solution is to align the grating so the spectrum is horizontal. The Star Analyser cell has a mark on the side to get you somewhere near but a good way to do it off the telescope is to look through the grating at the camera sensor. If you get the lighting right you should see multiple images of the sensor from the various spectrum orders. You just rotate the grating so they line up. Reflection from laser pointer also works well giving a line of dots but take care not to aim it directly in your eye of course. The Star Analyser comes with a externally threaded locking ring to keep it in the right orientation on camera nosepieces but to be honest I find is easier to put a bit of plumber's ptfe tape round the thread which is enough to hold it in the right orientation. Cheers Robin

Hi Lousie, I flipped it and then rotated it using Visual Spec. Some rotating algorithms introduce less artifacts than others but it may be the result of starting with the jpeg rather than the original fits file which would be sharper to start with. The noise is much smaller scale than the resolution of the star analyser though so you could smooth (filter) your spectrum to reduce the noise without losing any spectrum features. A similar effect can sometimes be seen with colour cameras too, even when perfectly aligned horizontally, because of the "missing" pixels in the red and blue. You can see the effect here for example (bottom of the page) http://www.threehillsobservatory.co.uk/astro/spectroscopy_11a.htm Cheers Robin

and an example of students new to astronomy and spectroscopy using the Star Analyser https://www.cloudynights.com/topic/643276-thanks-to-celestron-starsense-and-michael-swanson

Yes the spectra of most "normal" stars look pretty boring with the Star Analyser but the more unusual stars make the best targets for the Star Analyser and are the most interesting astrophysically. Those with strong features, particularly emission lines . This one of a recurrent nova for example https://www.cloudynights.com/topic/674213-recurrent-nova-v3890-sgr-erupts-again/?p=9624439 or planetary nebulae, Wolf Rayet stars and Luminous blue variables like P Cygni, recorded here with a Star Analyser with a very small 55 mm aperture https://www.cloudynights.com/topic/677560-grab-and-go-spectroscopy/ Robin

Hi Louise, Here is a quick result using Visual Spec from your jpg image. It is quite a bit less noisy so I suspect the noise in your spectrum may well be an artifact caused by rotating the spectrum in software which the jpg has smoothed out. Cheers Robin

Hi Louise Nice result. Rather noisy though, particularly for a 60x2 sec stack on such a bright target. The noise looks like it may be pixel artifacts which are sometimes seen with short focal length systems where the star size is very small. (a pixel to pixel sawtooth) Did you have to rotate the spectrum to get it horizontal? Rotation can introduce this sort of effect so taking care to get the spectrum horizontal helps The difference between A0v and A2v is very small but if you want to be spot on you could average the two Pickles references to produce an A1v (Not worth it though!). Vega is usually considered to be A0v but precise spectral classification is an inexact science. You can see the various classifications in the literature for many stars in Brian Skiff's huge catalogue here. http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk The position of the maximum in the raw profile depends on the spectral response of grating plus the particular camera so is not consistent between different setups even for the same star. Cheers Robin

Focus on the zero order star first then tweak the focus a bit to best see the spectrum features. As an alternative to main sequence A stars (Like Delta Cas for example) stars with a bright H alpha emission line can be easier to focus on, like Gamma Cas or P Cygni. Look for a bright dot in the spectrum towards the red end. As here for Gamma Cas for example http://www.threehillsobservatory.co.uk/astro/spectra_12.htm Cheers Robin

Also binning could give an under sampled image with such a short focal length which can give processing problems with spectra (artifacts) so I would recommend running unbinned

You can get RSpec to do a lot in real time but personally for best results I would first use the real time feature of RSpec using an avi to get the best focus on a brightish main sequence A star which would also be your reference calibration star and then capture a series of subs of both reference and target using Sharp Cap which you then preprocess off line before passing back to your favourite spectrum processing program which could be RSpec or another program. The subs can be an avi for bright targets, which are then dark subtracted, aligned, selected for sharpness and stacked like planetary imaging to produce a 16 bit fits image or longer exposure full bit depth subs up to how long you can track for with faint targets, like in deep sky imaging, The objective is to produce the best (well, not over, exposed, low noise, sharp) image of the spectrum you can to pass to the spectrum processing program. Cheers Robin

From my website here http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm The annoying design of the Canon lenses means the grating rotates as you focus but an empty rotating filter cell from a cheap set of polarising filters https://uk.telescope.com/catalog/product.jsp?productId=109834 completes the setup, making it easy to rotate the SA (glue the rotating filter cell into the hole in the lens cap and screw the SA into that) Clip on lens caps work best but it can be difficult to find ones where the clip mechanism does not prevent a large enough hole for the SA Robin