robin_astro

The design there was just a bit of fun to demonstrate how spectrographs work It would not be commercially viable for Paton Hawklsely who are a grating manufacturer and the performance would not be as good as an alpy which is purpose designed with special opticsso they supply shelyak with gratings and let them make spectrographs . To be practical, slit spectrographs also need a guider which significantly adds to thew complexity Cheers Robin

If you have the zero order in the same field you can focus on that, then bring the spectrum to best focus either by eye or use a real time spectrum display tool like RSpec looking for the deepest lines. If you then note how much you have to move the focuser you can reproduce this on other targets. (Note the focus will vary along the spectrum, particularly with your achromatic refractor so it will be a compromise Cheers Robin

Hi Steve, To focus you chose a target with nice strong features and focus on these in the spectrum, not on the zero order. I suspect though you will have difficulties getting a sharp spectrum with this high dispersion grating and thick prism because the beam is converging through the grating and prism giving severe aberrations. (The Star Analyser deliberately uses a low dispersion grating to minimise this problem) You could adapt it to a parallel beam though using an afocal type setup, similar to this I developed using an SA200 (a sort of poor mans ALPY) http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm Cheers Robin

And finally... A brand new one to kick off the new year. AT2021K discovered yesterday at mag 15.7 in galaxy MCG +06-30-084 https://www.wis-tns.org/object/2021k "AT" signifies it has not yet been confirmed and classified as a supernova. It is another who's light dominates its parent galaxy as seen in the discovery image https://alerce.online/object/ZTF21aaabvjk My spectrum from last night is inconclusive so I cannot officially classify it. I suspect it may be a type Ia , (the thermonuclear detonation of a white dwarf in a binary system) though the usual identifying feature (Si II absorption at around 6300A) is not obvious Cheers Robin

Here's another one from last night. SN 2020adow discovered a week ago in a relatively nearby galaxy KUG 0830+278 (redshift 0.0075) https://www.wis-tns.org/object/2020adow Here overlaid on matching type Ic supernova SN1994I, and below compared with the professional spectrum used to classify it Type Ic are a relatively less common type of so called "Stripped Envelope" Core Collapse Supernova where the exploding massive star has already lost its Hydrogen and Helium so these do not appear in the spectrum. The narrow line at H alpha (at 6612A, red-shifted from 6563A) is from the host galaxy which is completely outshone by the exceptionally powerful supernova, here identified as a broad lined (BL) variant due to the higher than usual velocity of the explosion. The magnitude here was ~14.5. Note how my spectrum appears to be redshifted relative to the professional spectrum taken a few days earlier. This is because we are now seeing slower moving material in the spectrum (less blue shifted) compared with a few days earlier. Cheers Robin

An example of how tolerant spectroscopy can be of bright sky conditions. This supernova SN 2020uxz has now faded to mag 16.5. Despite the sky background being around 5x brighter than the spectrum (Taken using an ALPY modified for faint objects at lower resolution) there is still enough signal to identify it as a type Ia around 70 days past maximum, matching for example SN 2002bo from SNID (red) at a similar age. Cheers Robin

Doesn't this give a strong thermal signal? It does at 12 GHz http://www.threehillsobservatory.co.uk/astro/radio_astronomy/radio_astronomy_1.htm or is it insignificant compared with galactic hydrogen ? Cheers Robin

A "Dicke switch" With a low noise dummy load and a noise source you could correct both for noise floor/spurii and receiver gain/spectral response (rather analogous to darks and flats in imaging) Robin

As in any imaging, you just have to avoid being under sampled ie the size of the star image (FWHM) should be more than 2 pixels but in this case the size of the "pixels" is twice as big as the size of the actual pixels. (Since you will only be using the green channel it is not actually as bad as this as half the pixels are green)

Triangles can be a sign of pinched optics. I have a cheap 80mm f5 refractor which shows this beautifully if I tighten the objective lens retaining ring Cheers Robin

Comparison stars are used to calculate the brightness of the target (and check) stars. Several comparisons can be combined to form an "ensemble" a sort of super comparison star which can improve accuracy and precision. Check stars are normally just to check the precision and accuracy of the measurement, useful for checking stability eg during a long time series, where the check star graphs should be flat (multiple comparison stars can also be used as check stars, looking for agreement between them) Cheers Robin

Was going to do a back of envelope calculation but decided the answer must be out there already https://www.youtube.com/watch?v=oauf6W3Uz04 The barycentre is currently outside the sun Cheers Robin

Mine is at a crazy angle to fit them in the frame. (the aim was to get them in line vertically but ran out of room!)

Funny you should mention that. The idea for this field originally came from Christian Buil when he borrowed my Star Analyser in the early days of it back in 2007 and came back with a version taken with a DSLR. I often use it with permission in presentations on spectroscopy but finally got round to looking at it myself. Here is Christian's colour version on his website. It does look good in colour but his W Per is rather over exposed. (I did a non linear stretch on my image to avoid that so don't try to measure the flux 😉 ) http://www.astrosurf.com/buil/staranalyser/wr5_2.jpg

A conventional image of this star field in Perseus (~20x15 arcmin) would look rather ordinary but a diffraction grating (100 lines/mm Star Analyser) placed in front of the camera to spread some of the star light into spectra reveals some interesting astrophysics going on in these stars thanks to the power of spectroscopy. (34x10 sec, Celestron CII, ATIK 314L) Top of the frame is W Per, a Red Supergiant star and like most other supergiants (Betelgeuse for example) is variable in brightness. Here it is currently magnitude 9.5 measured at visual wavelengths (from the AAVSO database), but much brighter in the Infra-Red where most of the light from this star is produced. (The spectrum here extends out to wavelengths beyond 1 micron, the limit of the CCD sensor sensitivity.) We can tell it is a cool star (spectral type M5i) from the sawtooth shape absorption bands in the spectrum produced by spinning and vibrating molecules of Titanium Oxide which are able to form in the relatively cool (~3000K) atmosphere of this star. Centre is BD+56 727, a hot star (effective temperature ~15000K) of spectral type B5 with no obvious strong absorption lines in the spectrum, typical of very hot stars. The star should look very blue with the spectrum much brighter at the shorter wavelengths but in this case the spectrum is almost even brightness across the visible range due to the large amount of interstellar dust between us and the star which dims and reddens its appearance. (It is V magnitude 10.6 but the dust absorbs over 90% of the visible light so would look much brighter and bluer without the dust). There is one clear feature in the spectrum though, the bright spot in the red at the Hydrogen alpha wavelength (6563A). This emission line comes from a rotating disc of gas around the star, the hydrogen atoms being excited by the UV light from the star and glowing in H alpha. (It is a Be star, one of many followed by amateur spectroscopists in support of professional astronomers) Bottom is WR5 a Wolf Rayet star with a spectacular spectrum showing many emission lines. (It is visual magnitude 10.4 but most of the light is concentrated in the emission lines). Wolf Rayet stars are extremely hot (tens of thousands of degrees K) massive stars nearing the end of their life (probably destined to become supernovae.) They are shedding their outer layers in powerful high velocity stellar winds. It is the material in these winds (made up of elements formed by nuclear fusion within the star) which produces the emission lines, variously of ionised Helium, Carbon, Nitrogen and Oxygen, excited by the UV radiation from the star. Cheers Robin

Novae can be unpredictable in their decline For example nova Cas 2020 has been rebrightening repeatedly for 4 months now but nova Del 2013 faded over 2 months (AAVSO data dates month/day) Cheers Robin

This is how it looked through the Star Analyser (20sec exposure, 2020-12-05). Nice bright Hydrogen Balmer emission lines Cheers Robin

Hi John, Nice Balmer emission. What date was this? (The lines other than H alpha can be quite difficult to see at the moment at the resolution of the Star Analyser compared with a few days ago but they should become more prominent again over the coming days as the spectrum evolves.) This series of amateur spectra taken for Nova Del 2013 shows an example of how the spectrum evolves but every nova is subtly different http://www.astrosurf.com/aras/novae/Nova2013Del.html There is a nice collection of amateur spectra building for this nova too http://www.astrosurf.com/aras/Aras_DataBase/Novae/2020_NovaPer2020.htm Cheers Robin

Great ! The lack of H alpha in the flare is interesting (to me at least, not really knowing anything about flares 😁) Cheers Robin

I don't know about the origin of the wow signal but there appears to be a repeating signal in this forum https://stargazerslounge.com/topic/366374-identified-possible-origin-of-the-wow-signal/

I look forward to the peer reviewed version of this paper. Apart from the implicit unsupported assumption that intelligent life would only be found on planets orbiting "sun-like stars", the search area is so wide that it is pretty much inevitable that you would find stars of every type including "sun-like stars" in the Gaia catalogue regardless of the direction you looked in. They might as well have said "sun-like stars found throughout the galaxy and stars of every type found as possible targets for the wow! signal" Robin

A couple of spectra, one about 6 days before maximum and one about 13 days after https://britastro.org/specdb/data_graph.php?obs_id=8176%2C8020&multi=yes&legend_pos=ne The spectrum has evolved quite a bit in that time but continues to give a good match to typical type Ia supernovae of the same age (using SNID the supernova identification program, black measured, red match) Robin

No. I think you are confusing the apparent movement of an object across the eyepiece due to the earth's rotation with change in the apparent position of nearby and distant objects with a change in observing location (parallax). You can demonstrate the difference with your finger held out in front of your face. If you focus on your finger and rotate your head so your finger moves across the field you will see the finger stays in the same position relative to objects in the distance. If you move your head from side to side though the finger moves relative to the distant object. The observed change in position of a fixed object with time from one side of the eyepiece is almost entirely due to a change in the viewing angle, not due to the effect of a change in viewing position (parallax). eg , if a planet is at the same position as a distant star at one side of the eyepiece, they will still be at the same position relative to each other when they get to the other side of the eyepiece field to a degree of precision you could possibly expect to measure, (ignoring any effect of the relative orbital motion of the earth and planet). In parallax measurements you change your viewing location significantly relative to the nearer object (eg from one side of earths orbit to the other) and then the relative position of nearby and distant objects does change. Robin

Hi Adam, I am the Robin that Andrew referred to. I am also the person who developed the Star Analyser. Option 2 with the grating between the lens and camera sensor will not work as the focal ratio is too low (I recommend using the calculator on the RSpec website which is based on my recommendations to the Star Analyser manufacturer, Paton Hawkley Education Ltd. https://www.rspec-astro.com/calculator/ If you use the calculator you will see that the minimum recommended focal ratio for the SA100 is f4 and for the SA200 f4.5 and the calculator gives a warning for lower focal ratios) You could use the grating with the lens in an objective grating configuration though with the grating on the front of the lens. here are some examples of this on my website http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm http://www.threehillsobservatory.co.uk/astro/spectroscopy_17.htm Use the same calculator to calculate the focal length range which will allow the spectrum to fit in the field by putting the focal length of the lens in place of the grating to sensor distance (Ignore the other warnings as they apply to telescope applications) Option 1 might work ok but it depends on the distance between the grating and the camera sensor. Use the RSpec calculator to test this. It will tell you if there are problems (The SA200 is more commonly used in filter wheel applications as it can be used closer to the sensor and has a lower profile so fits in most filter wheels) If you need more information you are welcome to contact me direct via the email address on my website Cheers Robin

Possibly but to alter a post without any obvious sign of editing and then attempt to gaslight someone who called it out is not acceptable in my view. I have added a comment to my original post to clarify what happened Robin