robin_astro

Remember that you are not measuring a narrow well defined line 1 pixel wide. What you are trying to do with the Star Analyser is compare the distance between two poorly defined fuzzy blobs in the target and reference spectra (The zero order image and the image of the star at H alpha) They will be several pixels wide, dancing about due to the seeing and distorted by the change in focus and the aberrations caused by the converging beam configuration. See the images in the examples I gave above. Measuring this to better than 1% precision is a challenge. With a slit spectrograph the situation is much better as the slit (not the seeing, focus and other aberrations), define the spectral line which is reproduceable so you can measure the position of the (centroid) of the line to a fraction of a pixel. You can measure the wavelength to high precision by comparing with the spectrum of a calibration lamp with well defined lines at known wavelengths. Go for the SA100, not the SA200. It works better in the converging beam configuration, particularly with fast telescope like your f5 Newtonian (smaller dispersion angle, less field curvature, less chromatic coma.) The SA200 is for if you cannot mount the grating far enough away to get the dispersion you want eg if you have a close coupled filter wheel. See my website for more details http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm The Star Analyser is not designed for making high precision observations. It is for learning the basics by observing objects with bold features qualitatively to reveal the astrophysics going on before possibly investing in much more expensive equipment. Observations like these I posted near the top of the thread for example If you experiment with one you will learn first hand the fundamentals of astronomical spectroscopy and understand the issues I am describing much better than me trying to explain them here. Cheers Robin

because of the limitations of accurate wavelength calibration with the slitless Star Analyser I would estimate the smallest doppler shift you could reliably detect with the Star Analyser is probably around 1% or 3000km/s based on the measurements of redshifts I have made using it Robin

The recession velocity of NGC7331 (800km/s = 17A at H alpha) is too probably low to measure with a Star Analyser in any case. For high precision measurements you need a slit spectrograph where you have fixed wavelength reference points from a calibration lamp For slitless spectrographs like the Star Analyser you need a point source but there are active galaxies (Seyferts, QSO) which appear point like and have high enough redshifts to be measured eg http://www.threehillsobservatory.co.uk/astro/spectra_3.htm http://www.threehillsobservatory.co.uk/astro/spectra_21.htm Instead of the small achromatic refractor I recommend using the C8-N which is the perfect choice for the Star Analyser 100 (and ALPY). You will need a mono astro camera though for faint objects like these. The DSLR will not be sensitive enough. In general colour camera are not a good choice for spectroscopy for many reasons Cheers Robin

If you start with the Star Analyser though you don't need to guide, just expose for as long as your mount can track for and align and stack multiple exposures

To get the best out of any slit spectrograph you are going to need a mount which can guide sufficiently well to keep the star on the slit for long exposures using the spectrograph guide camera. The telescope has to be reasonably well matched to the spectrograph too, for example the ALPY works best at ~f5 but the DADOS and LHIRES need an f10 scope

it really depends where your interests lie. The LHIRES III cannot do everything. It is good for high/medium resolution on bright objects but does not work so well at low resolution. The ALPY works better at low resolution and can go much fainter (and the spectra with my even lower resolution modified ALPY 200 are some of of the faintest recorded by an amateur) Cheers Robin

Yes it is possible for amateurs to detect galaxy rotation but unless you have access to a large telescope, only on a few of the brightest galaxies. Some examples http://www.astrosurf.com/buil/forum/ngc7331_poster.png http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1682 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2232 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2420 http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2618 Robin

This is probably beyond the range of amateur equipment because of the trade off between resolution and limiting magnitude. ie you can either measure high velocities of faint objects like this high redshift QSO with my modified ALPY 200 (resolution ~45A) or low velocities of bright objects eg the pulsations of Deneb at ~0.3A resolution using a LHIRES III

This was done using a LHIRES slit spectrograph at ~0.5A resolution https://britastro.org/observations/observation.php?id=20200710_225300_18b30a0a785a7ab3 Cheers Robin

There are also some nice examples on here using the 3D printed Lowspec spectrograph at high resolution eg

You need the right kit for the magnitude of the velocity you are measuring. The Star Analyser can measure the high velocities (redshifts) of some galaxies eg http://www.threehillsobservatory.co.uk/astro/spectra_21.htm and the expansion of supernovae (bottom of page) http://www.threehillsobservatory.co.uk/astro/spectra_6.htm but to measure orbital velocities you need a slit spectrograph with higher resolution (with good technique and a stable slit spectrograph a velocity precision is ~1/10 of the resolution is relatively straightforward eg at a resolution of 5A ~20km/s and at 0.5A resolution 2km/s) examples are:- David Boyd "observing with a Lisa spectrograph" measuring binary star radial velocities to ~6km/s, slide 43 on https://www.britastro.org/downloads/15701 My measurement of velocities to 1km/s precision in the dusty eclipsing disc during the eclipse of epsilon Aurigae using a LHIRES spectrograph https://britastro.org/node/19640 and velocities due to pulsations in Deneb to 0.5km/s in my talk "pushing the limits of commercial spectrographs" using a LHIRES spectrograph https://britastro.org/node/19378 and measuring exoplanets to a few metres/second precision using a stable high resolution fibre fed echelle spectrograph as here by Christian Buil http://www.astrosurf.com/buil/exoplanet2/51peg.htm Cheers Robin

Unfortunately most of these gratings tend to be very inefficient. (Spectroscopy spreads out the light very thinly so we need all the efficiency we can for astro spectroscopy.) A simple test is to look through the grating. You want as much light as possible to be in one of the first order spectra. Any light in the zero order (the light that goes straight through) or any other order is wasted. This is what the SA100 looks like (from the user manual)

If you do decide to go this way, I would not worry about the wedge prism, at least to start with. It does not make a big difference to the resolution with the SA100 but makes wavelength calibration more difficult. You can see an example of the difference in tip #3 of Christian Buil's useful tips using the Star Analyser here http://www.astrosurf.com/buil/staranalyser3/userguide.htm Cheers Robin

This is only part of the problem (The focal plane is curved so you can only be in focus at one wavelength. You will also find with achromatic refractors that you will not get perfect focus at all wavelengths, particularly at the blue end because of chromatism in the telescope optics. This appears as a fishtail like shape at the blue end). The main problem is the converging beam. Because the beam converges leaving the telescope you get what is called chromatic coma which means you can never get a perfectly focused spectrum image even using a wedge prism which as you have seen can actually make the problem worse. This gets worse at higher diffraction angles (more lines/mm) and at lower telescope focal ratios (The beam converges at a steeper angle.) This is the reason the Star Analyser has 100l/mm and is not recommended for low focal ratio telescopes. For a full analysis of the converging beam setup, see Christian Buil's website here http://www.astrosurf.com/buil/us/spe1/spectro1.htm Cheers Robin

If you want to try using your 500l/mm grating I can suggest mounting it in front of a DSLR lens and using it to produce spectra of bright stars using the method on my website. (The high dispersion grating works well in this case giving a sharp spectrum as the beam is very parallel, having come from the distant stars) http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm About 20-40mm focal length lens would be about right, not the 200mm shown there as that is for the 100l/mm Star Analyser. Note though that most cheap gratings are not very efficient so the spectrum may be too faint. (Unlike more expensive blazed gratings most of the light ends up in the zero and other orders) Cheers Robin

Hi Steve, As I said in my earlier post, high dispersion gratings with thick wedge prisms like this just don't work at all well with converging beam setups. The aberrations are just too bad so the lines are never in focus. To get a converging beam setup to work you need small dispersion angles ie a low lines/mm grating Cheers Robin

A spectrum of SN 2021K from last night gives a good match to a type Ia near maximum (The characteristic broad Si II absorption is now clear at ~6200A) Cheers Robin

K spectral class stars like alpha Cas are not the best choice for a first try as the spectrum has a myriad of faint fine lines which are unresolved resolved at low resolution. Delta Cas would be better as it is a main sequence A star with clear Hydrogen Balmer absorption lines, or if your camera has sufficient sensitivity at H alpha, Gamma Cas which is a Be star so shows He alpha in emission. See here for examples of both using an early prototype of the Star Analyser (In Patrick Moore's back garden!) http://www.threehillsobservatory.co.uk/astro/spectra_12.htm Cheers Robin

Unfortunately the weather prevented me from getting another shot at this and it has now been classified (unusually from a spectrum at the UV/blue end of the spectrum). My suspicion that it was a type Ia turned out to be correct though https://www.wis-tns.org/object/2021K

I still like to use the Star Analyser from time to time, though mainly for fun rather than for Pro-Am science. See here for an example and see here on my BAA personal page for more examples of spectra taken by all three instruments https://britastro.org/observations/user.php?user=146 Cheers Robin

Hi Steve, I developed the Star Analyser in 2004 and convinced Paton Hawksley to manufacture it after experimenting with similar setups as the one you are trying. It is a great simple low cost way to try spectroscopy and you can discover a lot about how to do spectroscopy and what it can tell you before spending too much money. (There are at the last count around 6000 of them worldwide). If you are interested you can listen to me talk about the development of the Star Analyser (and how I modified my ALPY to measure some of the faintest objects ever recorded by an amateur) in this BAA talk. https://britastro.org/video/11250/12234 You will also find more useful information on the BAA spectroscopy resources page https://britastro.org/node/19378 The other goto website for everything about spectroscopy for amateurs is Christian Buil's http://www.astrosurf.com/buil/index.html I would not say most of my spectra are taken with the ALPY. (I do not put most of my spectra on my website these days. ) I use them both fairly equally. (eg the LHIRES on bright objects at high resolution on moonlit nights and the ALPY on faint objects at low resolution on dark nights) You can see my current spectra (400 of them of 203 different targets currently) under R Leadbeater in the BAA database. (click the all headers button to see what equipment was used) https://britastro.org/specdb/ I bought the LHIRES as a kit when it first came out in 2006 and I was a beta tester for the ALPY when it came out in 2013. The LHIRES and ALPY are quite different instruments which fulfil different functions so they complement each other. With spectroscopy the more you spread the light out the fainter the spectrum so you can either measure bright objects at high resolution or faint objects at low resolution. (High and low resolution spectra tell you different things about the astrophysics) The LHIRES is optimised for the former while the ALPY is a low resolution instrument which can measure much fainter objects. The ALPY is a more straightforward next step from the Star Analyser and easier to use than the LHIRES with no adjustments once it is set up, though the processing is somewhat more complex. Cheers Robin

There are alternative solutions to calibration so you can get away without the ALPY calibration module, to begin with at least. Baader do not supply one for the DADOS (well a crude neon lamp which is not sufficient) I would definitely not buy an ALPY (or any spectrograph) without a guider module though. There is no simple answer to which spectrograph is best though. It depends on what you want to do with it. Each can do things another cannot. I have yet to see any serious results from the SX spectrograph. Cheers Robin

Slit spectrographs are very difficult to use except on a few bright targets without a built in (mirror slit) guider The mirror slit guider in a slit spectrograph does more than just keep the star on the slit. First it allows you to see the spectrograph slit and the surrounding field so you can actually place the star on the slit (which is typically only ~20 microns wide) Without this, until the star lands on the slit you see nothing in the spectrograph so you are completely blind It then allows you to focus the star on the slit so you get the maximum amount of light into the spectrograph Finally it allows you to keep the star on the slit in long exposures which might be an hour or more in total on faint on objects without any issues like flexure or field rotation etc I would say in the past this was one of the most overlooked item in amateur spectroscopy resulting in amateur built spectrographs collecting dust in cupboards. The introduction of the mirror slit guider into spectrographs for the amateur was, along with the CCD camera probably the most significant development contributing to the advancement of amateur spectroscopy Cheers Robin

The prism gives only a marginal improvement for an SA100, more for an SA200 but brings its own issues (eg non liner calibration) and most users don't use one. It reduces the aberrations a bit giving a sharper spectrum and less change along the spectrum but the focus in the spectrum is still different compared with the zero order. Generally you can see the best focus by eye, helped perhaps by deliberately trailing the spectrum. See Christian Buil's pages for example for tips on how to get best results from the Star Analyser. http://www.astrosurf.com/buil/staranalyser/obs.htm http://www.astrosurf.com/buil/staranalyser3/userguide.htm At the end of the day though these simple setups are about good fun to learn on without spending too much before moving on to a "proper" spectrograph of good design either commercial like the ALPY or a good home build design like the lowspec. (It is easy to design a spectrograph on paper but there are lots of important subtleties and to produce one that actually works well in practise is much more difficult) Cheers Robin

the simple SA configuration varies in focus because of the angle of dispersion which means the distance from grating to sensor varies. The Alpy in focus along the spectrum because of the optics. My junk box design is also in focus within the limits of the camera lens performance. (field flatness, chromatic aberration) Cheers Robin