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Hi Kevin,
I will reply to all posts here to avoid having to chase up and down the thread. (I have to say I don’t like this type of forum layout. I much prefer the more common layout like Stargazerslounge etc which are linear and you can quote)
Reply to #8
>(i) The spectrum gets wider (more height in Y-axis) as it goes down the sensor.
Yes to me this is unexpected but we need confirmation if it is normal. No clear skies here but I will dig out my artificial star or perhaps replace the slit for a hole and see what I get. I think we need to check this first before adjusting anything else.
>(ii) As well as this, the overall integrated flux gets bigger – an increase of around 50% from the two extremes explored here. I am assuming the final Isis image has removed the background and so the flux is signal, rather than background+signal. (You might expect total flux to increase just because its a bigger area…)No the total flux should stay the same. (The light from the star is just being spread over more rows). The flux may be changing as the mirror vignettes the return beam
>(iii) However, the spectrum is not so good in the larger-flux, lower-in-image version. In particular, it doest work well with the optimal binning optionOptimum binning selects particular rows to maximise signal/noise. For flux measurements, turn off optimal binning (and just set the binning zone to just include the full height of the spectrum (stretch the image to see the full extent) and subtract the background setting the background zones away from the spectrum above and below
Reply to #14
>By the way, do you know how Isis calculates R? It seems to do it from the image/spectrum and not from parameters as such.
I think it calculates it from the lamp spectrum. It can give variable results though. I prefer to measured the FWHM directly off the lamp spectrum. R = wavelength/lamp line FWHM
Reply to #23
>I have not tried loosening the screws on the grating holder – it sounded scary 🙂 But maybe I should try and re-tighten to get a feel for how this works
I recommend we clarify the widening with mirror position first before touching the grating.
Cheers
Robin
Attached are examples of before and after spectra when the grating was overtightened after a rebuild, though unfortunately I did not do the zero order test before I cured it which would have conclusively proved astigmatism. With the spectrum at its narrowest, the star was out of focus on the slit and the throughput was low. Adjusting the telescope so the star was in focus on the slit gave good throughput but a wide spectrum. Once the grating clamps were slackened off. Everything returned to normal (The star was in focus on the slit, the spectrum was narrow and the throughput was good)
Robin
>A possible test for this could be to check the zero order image of a star without the slit in place. In this configuration the grating is effectively a mirror so the image should be round. I have not tried this though.
Actually, checking back I see I did run this test. (though only after I had cured the astigmatism.) The test report is attached.
Cheers
Robin
Yes mine was one of the original kit versions. Having to assemble it from a box of bits taught me a lot about how it works. You can still see the assembly photos and drawings on Olivier Thizy’s website
http://astrosurf.com/thizy/lhires3/index-en.html
I have asked for confirmation about the main mirror adjustment on the ARAS forum.
>it does increase the height of the image. However I think this might not matter
The large change in width of the spectrum with the mirror position is strange. I am pretty sure I do not see this but I need a clear sky to check and the run of good weather has broken at this end of the country.
A wide spectrum, even if it does not affect the total flux is not ideal as you have to sum more rows, introducing more noise from the camera and sky background.
If the spectrum is wide when the star is focused on the slit (ie the flux is maximum), this can be due to astigmatism. ie the focus of the collimator lens in the dispersion direction (as set up by making the lamp lines sharp) is different to that in the vertical direction. (The spectrograph design has some inherent distortion of the image but this is small) . I had this issue once when I overtightened the grating in the holder, distorting it slightly into effectively a cylindrical lens. I am note sure how the gratings are supported in the latest design. A possible test for this could be to check the zero order image of a star without the slit in place. In this configuration the grating is effectively a mirror so the image should be round. I have not tried this though.
When the grating was not pinched I did not see any significant difference in focus between the guider and the spectrum image. (ie when the star was in focus on the slit the spectrum was narrow.) This is true for both my ALPY and LHIRES.
Cheers
Robin
>Do you mean the star disc spreads beyong the slit? For the stars I am looking at this is certainly the case. In this respect (star >diameter vs slit width) my guide images look liek the one Andy posted above.
Images of the star on the slit can be deceiving if the star is over exposed as the overspill looks worse than it is. Best to measure the typical FWHM of the point spread function of the star when correctly exposed and in focus. Matching the slit width to this is a good compromise as most of the light will pass through the slit and you will optimise the resolution. With your setup 23um corresponds to 2.4 arcsec and 35um is 3.6 arcsec. I would not go any wider as you lose too much resolution. You cannot reduce the star size using a focal reducer either as that produces more vignetting in the spectrograph. When you do these sorts of calculations you begin to see how the telescope, local seeing and spectrograph design are intimately linked. The LHIRES is optimised for an 8 inch f10 with 2.5arcsec seeing.
Robin
EDIT: Found the calculator I was looking for. For a gaussian shaped star profile, 76% of the light will pass through a FWHM wide slit.
https://spectroscopy.wordpress.com/2009/05/22/slitpinhole-flux-calculator/
So it does ! I need to recheck this then. (I was shown how to set it up this way at OHP workshop back in 2006.) The photos in the manual are rather confusing as they appear to show the spectra running vertically, displaced to the left. It also looks like the manual (on line at least) is well overdue for an update as it still shows the original kit built instrument with the adjustable slit and the old guider and calibration lamp setup.
Cheers
Robin
Here is a post on spectro-l from Francois Cochard back in 2010 which helped me when setting up my guider.
https://groups.yahoo.com/neo/groups/spectro-l/conversations/messages/7119
and here is my comment in the same thread on the sweet spot position
https://groups.yahoo.com/neo/groups/spectro-l/conversations/messages/7079
Robin
Here I am talking about the light beam after the slit. It has to fit through the collimator in both directions. The collimator is f8 but there is an angle between the beam in each direction and the return beam is also dispersed by the grating so is wider. This means an f8 beam will overspill the collimator hence the f10 design specification for the LHIRES. Running at f8 does not affect the spectrum quality appreciably though, you just lose a bit of light. In spectrograph design there is a trade off between slit width, focal ratio and resolution. The upshot is though for a given resolution, the size of spectrograph optics scales with telescope aperture.
Robin
Hi Kevin,
OK as you have the latest version, you get the updated optics which should have improved the off axis coma. (I have not really used my LHIRES since I installed the upgrades but am just reinstalling it on the scope so should be able to report back in the next few days. The new design has removed most of the adjustment degrees of freedom of the guiding mirror (you can just push it in and out) so you are dependent on it being aligned correctly at the factory (with the old system you had full adjustability but was tricky to get spot on. I have kept the old adjustment system on mine) The sweet spot with round stars should line up with the slit but in this case being so far from the centre suggests an alignment problem somewhere. In the back of my mind I remember a similar comment from someone ese. You could check your guide camera is concentric with the guide port and square but beyond that I would be tempted to bounce that back to Francois at Shelyak and ask him to comment.
Cheers
Robin
For a circular star image, the amount of light passing through does not increase proportionally to the slit width so the penalty is not as large as expected. Having said that,I would expect your scope optics to be seeing limited on axis so for good throughput I would aim for a slit width equivalent to around 3 arcsec for typical uk seeing. (I use a 35um slit with my C11 at f10). Is your scope f8? in that case 23um might be better for you and give higher resolution. You will get a bit of vignetting in the spectrograph at f8 though which will lose you a bit of light compared with my setup at f10. (The spectrograph optics are f8 but you have to allow extra for the angle between the undispersed and dispersed beams so an f8 beam will not quite fit through the optics)
Robin
If you have the latest design, Shelyak supply an alternative calibration lamp which can be used in place of the neon which includes Ar and other elements.(The same as used in the ALPY and LISA)
https://www.shelyak.com/produit/se0148-argon-neon-spare-bulbs/?lang=en
It has the disadvantage that there are lots of lines which can be difficult to identify so I suggest sticking to the neon and H alpha initially while we get the spectrograph up and running.
Cheers
Robin
If you look into the spectrograph with the imaging camera removed, you can see the diagonal mirror which sends the light towards the grating through the lens. It is important that the return beam misses this mirror when it returns to the camera. This is achieved by rotating the mirror. The mirror should be adjusted so the spectrum is approximately 1/3 the way up from the bottom of the camera field when the spectrum is horizontal and orientated with red on the right.
The fact that the focus of the spectrum (The width of the spectrum line) is varying so much as you adjust the mirror. (80 pixels means the spectrum is way our of focus). I do not recall seeing this before This suggests to me something may be out of alignment. We may have to revisit this.
Cheers
Robin
Hi Kevin,
I have seen all these issues with the LHIRES and hopefully can help work through them. Firstly can you confirm it is the latest model (ie it is a new LHIRES, not just new to you ?)
Cheers
Robin
Thanks Nick,
I was familiar with colour measurement (CIE colour space,tristimulus values etc using standard light sources C, D65 etc) from my life in the paper industry but it was the definition and determination of the correlated colour temperature of the light source which I was particularly interested in. Following your lead I found this specificwikipedia page
https://en.wikipedia.org/wiki/Color_temperature
I wondered about the usefulness of the definition of CCT for light sources like LEDs which have spectra which deviate significantly from a smooth Planck curve. (Rather like trying to measure Teff of a star from the continuum shape in the visible region, a source of much confusion to beginners when they fail to get the “right answer”) The wiki page does caution about the range in which it can be used but the second plot I posted does at least suggest a good correlation between CCT and “blue light hazard efficacy” for a range of light sources regardless of their spectrum shape
Cheers
Robin
Here is a plot comparing different light sources wrt “blue light hazard”
The source for the plots is these interesting documents published by the US Department of Energy
https://www1.eere.energy.gov/buildings/publications/pdfs/ssl/opticalsafety_fact-sheet.pdf
https://www1.eere.energy.gov/buildings/ssl/pdfs/true-colors.pdf
Cheers
Robin
Hi Nick,
Yes I understand the concept of colour temperature. I was looking for a quantitative measurement method. I found these spectra of various LED lamps with their rated colour temperature.
It looks like a measurement of the ratio of flux at 450nm and 600nm would give a good indication. I might knock up a simple portable spectrograph and take a few measurements
You should see Filey where we were on holiday for a few days last week. They have fitted all the lamps and strings of lights along the prom seen here
http://www.geograph.org.uk/photo/833144
with the bluest LED light bulbs I have ever seen. (Hundreds of them. They looked almost purple, like mercury discharge lamps. A shame as just off the front they have a few warm white LEDs ) We stayed in a hotel on the front and the rooms were lit with this eerie light and the seagulls seemed to be awake all night.
Robin
Is there an established method of determining the rated effective temperature of these LED lights? (They are a long way from being black bodies or even the spectral energy distribution of the sun)
Robin
Hi Tony,
The CAOS group site is a good source for design ideas
https://spectroscopy.wordpress.com/fibres/
but the tough bit is sourcing/making a mirror with the end of the (typically 50um) fibre embedded in it. I think most amateur built fibre fed spectrographs have ended up using the Shelyak guide head
https://www.shelyak.com/produit/pf0008-f-6-50%C2%B5m-injection-unit/?lang=en
Cheers
Robin
