New Starlight Xpress spectrometer

Callum thanks for bringing this to our attention.  Spectroscopy is something I’d like to venture into next year.

The french Shelyak  range of spectroscopes seem to be the ones favored by many.  The new automated range by starlight Xpress, which apparently will be announced in a couple of days,   appear seems to be a comprehensive all in one instrument at a good price.  However, I don’t know enough to make a sound judgement.  It would be good to have a comparative review or at least some comments by the experienced users.

Eric 

The lack of understand of what this instrument does compared with the slitless (Star Analyser?)  spectrum of M42 shown there was amusing considering the ~30x difference in cost.

That spectrograph (in non automated form) has been around a few years (Maurice Gavin did a youtube review on it).  I’ve yet to see a published spectrum using one though. There have been quite a few failures by companies bringing instruments into this field in the past few years. I would say to succeed it needs to get into the hands of a few observers who can show what it can do and get some results out there. That is why the Star Analyser and the instruments from the leading supplier in this high end field succeeded where others have struggled. Their latest UVEX spectrograph, also using reflective optics is coming out in commercial form later this year but is already well known from the results produced using 3D printed kit built prototypes. Getting potential customers directly involved in product development has proved to be a good  way of ensuring a market for the final products.  

Thanks for sharing this Callum. I heard of this spectrograph a few years ago but didn’t know they were adding automation.

It is great that SX are entering this field though I didn’t see anything that tells me how well the spectrograph performs. The idea looks great, but they really need it in the hands of someone who knows how to produce good quality spectra to see how well it performs in the real world. It could be fantastic but they only showed the most simple of results, a bit like demonstrating a new telescope by taking single images of the Moon without proper processing.

I would want to know how stable is it on the back of a telescope in an imaging run. It appears the design should be very stable but theory and practice don’t always match. I also want to know what the resolution is as that defines what you could use it for, along with how that matches to CCD pixel size. Interesting that the camera can be shifted across the spectrum though I’d want to know how much of a spectrum you can fit in a single exposure, and how stable that movement is. It has to be absolutely rock solid, otherwise things like wavelength calibration and response correction needed to create good quality spectra would fail. If someone could obtain spectra of more challenging targets and do the full end to end process including response correction that would go a long way to demonstrating they have a high quality product.

I do not agree that they are coming in cheaper than their competitors. There are only a couple of modular spectrographs I can think of, most come as complete units. Of the modular spectrographs one is cheaper and the more expensive one is a top end spectrograph designed for large research telescopes. They appear to be coming it at about the same price or possibly more expensive than complete unit spectrographs. That is not to put their spectrograph down, just I did not agree with their comments on the price. This might not be a negative as they have automation, though they are not the only spectrograph with automation capability.

I don’t mean to be too negative as I think SX produce great CCDs and accessories, and I want this to be a great spectrograph. I am just hesitant as this is their first foray into spectroscopy and I’ve not seen any results that demonstrate the quality of the spectra it produces.

Cheers,

Andy

I think Robin is right that they need to get some established spectroscopists to use it and publish results.

However, I would have to say i think all commercial spectroscopes (and my home made ones) have issues of one kind or another.

I had a Lhres III which had light leaks, was mechanically unstable, required focusing if you shifted wavelength via a crude mechanism etc.Then I had Lisa, it had fishtails and a flat lamp that produced flats with strong gradients etc. I will pass over the issues with mine!

I like the idea of air slits and all reflective oprics on the sx spectroscope but don’t like the beam splitter and not being able to see the star on the slit etc.

Regards Andrew

Yes you can but you have to establish what coordinates on the inage corresponds to the star being on the slit as unlike the reflective slit spectrographs you can’t see the slit in the guide camera.

Regards Andrew 

Hi Grant,

There are several good reasons why beam splitter guiding was abandoned in favour of the more expensive mirror slit guiding option by all the other manufacturers (SBig, Baader and Shelyak). Focusing and positioning the star on the slit is absolutely critical. You cannot guarantee long term the splitter image will remain parfocal and in the perfect sub pixel alignment with the slit you need and realignment is problematic working effectively blind, particularly with a spectrograph like this showing astigmatism where you have no idea if the star is in focus on the slit for best throughput. (Terry stated in the video that the XY slit position they supply with the spectrograph could not be guaranteed and the user would need to tweak this to place the star precisely on the slit but this is far from straightforward in practise.) The symptom is  low throughput which was seen by users of the  LHIRES copy designed by Ken Harison for example and eventually replaced with a mirror slit guider in the commercial version. (This also failed commercially for other reasons). Additionally when guiding on field stars, these need to be 2-3 magnitudes brighter compared with a mirror slit because of the 10% split

Cheers

Robin

Here is a link to the handbook. (dated feb 2016)

https://www.sxccd.com/handbooks/Handbook%20for%20the%20SX%20Spectrograph.pdf

The spectrum images there show very bad astigmatism, which is claimed to be an advantageous feature ! (It is not for serious use) though the spectra in the video did not look as bad as this so perhaps some improvements have been made.

Robin

The mirror slit does have a couple of drawbacks too though. Bright stars are normally guided when on the slit using the overspill. (Typically the slit width is chosen to match the star FWHM so there is always some light which does not pass through the slit) The resulting image is “Hamburger” shaped though, split by the slit, which some guider programs struggle with. Also because unlike an air slit, the light has to pass through glass this can limit measurements at the UV end of the spectrum. (The beam spliiter has the same problem though.)  The original not for profit LHIRES kit had jaws lovingly hand polished by volunteers but that would be expensive to do commercially.  Christian Buil has been experimenting with reflective air slits (from the base ALPY module) for the UVEX, along with using all reflective telescope optics and a different glass for the camera window to reach a remarkable 3200 Angstroms. Beyond that atmospheric ozone absorption becomes an issue.

http://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2277&start=10#p12459

Cheers

Robin