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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
I see the author of AIP4WIN is jointly presenting a poster paper at the BAA/AAVSO meeting this weekend. Not sure if he is attending in person though
Robin
I see the VSS are already investigating possibilities, There are two papers on potential pro-amateur cooperation concerning exoplanets in the variable star meeting on the Sunday
11.00 – 11.50 Dr Guillem Anglada Escude (Queen Mary, University of London) Red Dots Initiative: science and opportunities in finding planets around the nearest red-dwarfs
11.50 – 12.10 Lukasz Wyrzykowski (University of Warsaw) How to find planets and black holes with microlensing events
Hello Jack
What limitations are you trying to overcome compared with the Lodestar ? If you are looking to guide on the faintest stars, the 314 will probably go a bit fainter with a few seconds exposure. (I tried a Lodestar with my ALPY 200 but returned it as my old cooled 16ic-s would guide on fainter stars) On bright stars though you might have to use the cropped frame feature to get a decent update speed. Also what main camera are you using ? I dont think for example you can fit two 314 sized cameras side by side on the LHIRES. Anyway since you have both why not try them and let us know which works best?
Cheers
Robin
Peter Somogyi has published a spectrum taken with an ALPY. It confirms reports of an almost featureless blue spectrum but does not shed any more light on this puzzling object
Reports are it is now showing spectral features of a 1c-BL supernova. There is an HCT spectrum (rectified to remove the blue continuum) linked from David Bishop’s page
http://www.rochesterastronomy.org/sn2018/sn2018cow.html
but the features are not as obvious yet compared with this one for example I called as a 1c-BL last year
https://wis-tns.weizmann.ac.il/object/2017ixv
Robin
Yes the ATK 414 is one of the best for low dispersion ripples but sadly one of the worst for high dispersion ripples seen with the LHIRES at the highest resolution (unlike its predecessor the 314 which is good at both low and high dispersion.) Unfortunately none of this is known until one tries to use them for spectroscopy. The low dispersion ripples are all but eliminated by using a flat but the high dispersion ripples can be really tough to remove.
Cheers
Robin
I have been keeping an eye on development with this one too. The reported spectra is blue and featureless which would be consistent with it being a CV. I think the speculation about it possibly being an SN comes from the fact that it is coincident in position with the galaxy and is within the brightness range expected for an SN at this distance (though the latest measurement at mag 13.8 would put it towards the top end for an SN at this distance I think.) The problem at the moment is, because the spectrum is currently featureless and there are confusing spectral features from the galaxy there is no definitive redshift measurement yet which would conclusively prove if it is a foreground object.
Robin
