For any question, contact Olivier (thizy@free.fr).
Lhires III is a high resolution spectrograph designed for amateur telescope (200mm typically). Typical object magnitude for an acceptable exposure time is much lower than the magnitude of quasars and supernovae. Those objects are not reacheable with Lhires III (well, until we get our galactic supernova!).
Comets are also very faint and diffuse. With 150lines/mm optional grating, very bright comets may be reachable but we haven't tested it yet. Even a 300lines/mm grating should give good results - but it has not been tested yet!
Lhires III has a guiding system thanks to a mirror slit. You can guide visually but it's not very practical. We recommend a webcam (it sometimes works on the same PC than the acquiring PC). A video sysem is usually even better. Watec 120N with long exposures is a great choice that we successfully tested with the Lhires III.
Yes as this spectrograph has been designed and industrialized with the support of AUDE association. Kit distribution is done through this non profit ("loi 1901") association. Subscribing can be done at the same time as ordering the kit (one bank transfer being cheaper than two). Check out the AUDE web site: http://www.ccdaude.com/
Lhires III has been optimized for 200mm f/10 telescope with KAF400 type of CCD chip. Slit should then be 18µm. This means a 1.8 arcsec angle for a C8 telescope. This is an average seeing for an amateur. with a bigger telescope (ex: C14), this means 1 arsec which is small. Gain in flux is not proportional with the surface of the telescope, except if you reduce the resolution by widening the slit. It depends on your project & target.
Loss of flux (turbulence, slit too small compared to diffracting image) and diffracting phenomena when slit is too smal...
No atall. A mount with normal tracking is enough. It's not difficult to point to a bright star target and to manually guide. Slit has to be oriented on RA axis so Periodic Errors just move the star along the slit.
Can this kit be mounted on a f/8 FS102 for exemple?
[Christian Buil] Yes, it works at f/8. Not a too lower f/ ratio as you will loose flux through vignetting of the doublet. But be careful with a FS102 for the limiting magnitude. Check the ETC (ETC = Exposure Time Calculator) and make a simulation.
[Christian Buil & François Cochad] Short answer is that Lhires III will work at f/6 with around 0.3 magnitude loss through vignetting. Resolution will be slighly lower due to optical aberration. But guiding will be slightly easier as star will brrighter on the guiding chip...
Useful diameter of the grating is 25mm; with 200mm doublet, this means f/8. Using a f/6 instrumen, you will only use (6/8)^2=56% of the flux... so this means a 2.5*log(.56)=0.63 loss in magnitude. It will be less for detectivity because of the noise and the fact that the star image is thiner at f/6 than at f/10... I recommend you play with the Exposure Time Calculator and try to multiply grating efficiency by 0.56 (so 0.2*0.57=0.11). For a S/N ratio of 100 & 1h exposure & constant seeing:
Vignetting loss is partially compensated by seeing impact. A 30mm grating would only improve it by 0.1 magnitude. Real geometry is a little bit more complex, but you will loose around 0.3 magnitude. We didn't try on the sky but on the first Lhires I, Christian used it at Newtonian focusof a CN-212 telescope which was more open and it worked! :-)
It's not mandatory as Lhires III kit comes with a standard 2400lines/mm grating. But with a reasonable additional price, you can transform your Lhires III in a medium resolution spectrograph (R=1000) with a 150lines grating. It's not ideal as focus is not perfect on the side but it works (we tested it!) and it can be very useful in some projects.
Also, some lines are very large (profile's base of P Cygni, novae H-Alpha lines...) and it doesn't fit on a KAF400 chip. You can buy a KAF1600 camera or much cheaper sacrifice resolution and put the 1200lines grating.
Those options allows to adapt your resolution to your target.
Lhires III has been designed for 8" telescope but works well up to 24". Slit size of 20µm will only take a portion of the star on a large telescope. So you have to make a choice between resolution and flux and adapt slit width to your project. Slit access has been simplified but it still requires some time to adjust it.
Not necessarely as you can change the grating with a screwdriver. But it's a delicate operation (grating surface is highly fragile!). If you have to change configuration often, and specially in collectivity, it is recommended to have additional support that can be changed more easily.
[François cochard] Barèges has been designed for lower resolution (R=1000-2000) while Lhires III is first designed for high resolution (R=15000-20000) but can be adapted to lower resolution. Lower grating degrade "luminosity*resolution" factor compared to a Barèges.
Barèges does a better job at focusing through the spectra while Lhires III is limited by the doublet; you won't be able to focus in a very large spectral domain. Our test showed that with a 150lines grating and a KAF400, result are still acceptable but lines at the border will be thiner with a Barèges.
Barèges has an adjustable slit which is used to isolate the object not for the resolution. Lhires III has a very thin slit which contributes to the high resolution.
Barèges works with f/5 telescope while Lhires III requires f/8 or higher.
Lhires III has a guiding mechanism that Barège doesn't have. LISA is a low resolution spectrograph that use the same principle as Lhires III for autoguiding but LISA is not available commercially.
[Steve Dearden] The DSS-7 is a spectrograph intended for low resolution work, covering most or the whole of the visible spectrum. Lhires III is a high resoloution (R> 15,000 with a 2400 lpm grating) instrument intended mainly for high resolution studies of stars of Mv = 6 to 8. Having said that, it is perfectly well adapted to lower resolution work so it is being supplied with other gratings for lower resolution studies.
The slit width can be slightly increased for low-res work so that some increase in spectral throughput (étendu) is mainained.
A better comparison for LHIRES III is perhaps with SBIG SGS, but even then it is far highly resolving since it is a Littrow design.
[Olivier Thizy] I do not know which grating is inside the DSS7 but I measured the resolution on mine and it's around 400. So compared to 17000, it's much lower resolution. With a DSS7, you have a broad (4000A) domain and a dispersion around 5A/pix. This allow to see the visible spectra in one glance. Very useful to measure star colors, to look at different lines at the same times (great to see all Balmer serie for exemple).
Check for exemple my web page on DSS7 and specially at the bottom the Nova V5116Sgr:
http://www.astrosurf.com/thizy/dss7/dss7.htm
And compare to high resolution spectra taken by Christian Buil with a Lhires prototype:
http://www.astrosurf.org/buil/us/nsgr6/nsgr6.htm
Note that he used a 1200lines/mm grating so it's not the full resolution (the line profile was too large for the camera!). This is also an advantage of Lhires III! you can easily adapt your resolution. It's designed for high resolution, but you can lower resolution with few compromise.
High resolution has a price: limiting magnitude! I have been able to record spectra of a bright quasar with a DSS7; Lhires III is limited to very bright objects. Check out the optical design webpage and there is a Exposur Time Calculator that help you to calculate signal/noise ratio depending on your configuration, star type/magnitude, & exposure time.
Note that you will be limited also by your guiding camera. A webcam is not very sensitive. A Watec-120N (or AstroVidII) camera will improve your guiding a lot. But there are so many this to see at high resolution even for bright stars!!!
With high resolution, you can see radial velocity of stars (and Earth: most of your spectra will have to be calibrated and corrected of Earth's movement!).
You can see the difference of width of star line versus interstellar ones (check for exemple Lhires monitoring of Beta Lyrae at HeI 5876: http://www.astrosurf.org/buil/pic2005/mission.htm - look at fine lines of interstellar Sodium compared to broad lines of the star!). You can see matter ejection in Be Stars, spectra evolution in Hot Stars, etc...
One is not better than the other. You will have to adapt spectrograph with your projects. If you want to monitor comets or SuperNovar, you will need a very low resolution spectrograph (even DSS7 is too hig for comets or SuperNovae - well except if you own a 24" telescope!). If you want to look at details in stars, you will need a Lhires III!
There is also a big difference between the two. DSS7 is a commercial product and SBIG correctly charge their R&D, design work. They also charge the time the spend to build the spectrograph. and they also make profit of course. Lhires III is an amateur tool: all time spent by people (specially Christian Buil, François Cochard) is not charged, we will put all parts in shipping box for free, and this is sold a cost through a non profit association. Difference is that if you want to buy a DSS7 next year, you will certainly be able to. Lhires III kit is a one time offer (as we can't spend our all time to do it!) except if an industrial takes over for us - but then price will be much higher. Note that all schematics & part list of Lhires III isavailable so you can reproduce if you want. It's "open source"! But it will cost you more than the kit today as we get several discout on parts due to large quantity we buy this time.
We really hope an industrial will take over because our interest is to have more people to use the same spectrographe. This has several advantage:
[Olivier Thizy] It's close. SGS is comparable with Lhires III & a 400 lines/mm grating. But there are other differences: