23 July 2020 at 4:01 pm #574682Callum PotterKeymaster
If you are interested in getting into spectroscopy (and have deep pockets) you might be interested in the new Starlight Xpress spectrometer which was previewed on the Woodland Hills YouTube webinar last night. It looks to be a very complete package.
scroll through to about 2:55 in for the start of the session with Terry and Michael.
No warranty expressed or implied 😉
Callum23 July 2020 at 5:05 pm #582901
Thanks for the headsup. Am watching it now.
Blue CCDs. Excellent. I can have a camera that matches my eyes! I don’t care what colour it is as long as it still does the business but some people like their kit to be pretty. At least its not red.24 July 2020 at 1:49 am #582904Eric WatkinsParticipant
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.
Eric24 July 2020 at 11:38 am #582905
I’m not really understanding what you don’t like about the new system. Concave grating corrected for astigmatism, on board ref, onboard flat and onboard focusing/guiding. I don’t do spectra, but that would certainly seem to tick some boxes.
Yep, I am ignorant, but I’m not seeing a lot to dislike beyond the price – but then again I struggle to understand why so many salivate over high end 4″ refractors when the same money would buy a 14″ Newt.
I don’t imagine the BAA will be subsidising purchase of these like they did for the Shelyak systems a year or two back.
What am I missing? 🙂24 July 2020 at 12:34 pm #582903
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.24 July 2020 at 12:46 pm #582906
Dont get me wrong, it would be good to have some competition for Shelyak who dominate the market currently and reflective optics are indeed a big advantage, mainly due to the lack of chromatism (ask David Boyd about his LISA) than the extended UV response, but designing one suitable for astronomy rather than for bench use is tough. The one Maurice reviewed was far from free of astigmatism though which limits its use. The field distortion looks suspect too in that video. The length of the slit is very short but the calibration lines were significantly curved even over that short distance, fine for a bench instrument but you are going to struggle to produce spectroscopic images of comets like this for example with this instrument
Availability of calibration lamps and guider modules is a given these days. The calibration lamp is just the same fluorescent lamp starter discovered by Swiss amateur Richard Walker and adopted by Shelyak for all their instruments and all other other offerings currently use a mirror slit guider rather than a beam splitter which has practical alignment issues and was abandoned by other manufacturers (This was touched on in the video where it was said that the user would be expected to tweak the guide position to position the star on the slit, critical for throughput and potentially challenging when you cannot actually see the slit in beam splitter designs so it remains to be seen how this will work in practise)
I did not intend criticising the instrument. Difficult really since I have not used one, though I do see some potential issues and did ask various questions about it when it came out which remain unanswered because of the lack of experienced users publishing results. (The limited knowledge of the dealer/customer? in the video was embarrassing). I agree with Eric, we are lacking an in depth practical appraisal by experienced spectroscopists. Contrast this with how much we already know about the design and performance of its direct competitor out in commercial form later this year.
Robin24 July 2020 at 12:51 pm #582907Andy WilsonKeymaster
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.
Andy24 July 2020 at 8:24 pm #582909
My take on it was that its self contained and very suitable for those lucky well heeled souls who have scopes at remote dark locations and so want the whole lot easily controllable and adjustable via a GUI. Similarly for university observatories to train up the students.
Thinking about it, there must be a market out there at some of the ITelescope type sites for a few of these. Look how long we had to wait for a spectrum of the new nova in Reticulum.
I too would be curious to see how it performed, preferably via a review done by someone not too bought-in to existing technology/manufacturers. People do tend to become tribal and sometimes don’t want to hear that something else is good in case it reflects poorly on their own purchase choices. I imagine we can all think of instances…24 July 2020 at 9:46 pm #582910Andrew SmithParticipant
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 Andrew24 July 2020 at 11:21 pm #582911
If the Lodestar x2 is being used for guiding, would it be possible to display that guiding frame in semi real time? Would really need to ask Terry or Michael I imagine.
Would make sense – the Lodestar x2 isnt a bad imager (SX7 without the cooling). But would you see where the slit was?25 July 2020 at 8:27 am #582912Andrew SmithParticipant
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 Andrew25 July 2020 at 12:27 pm #582914
I wondered if any internal reflections made it visible. But and x/y cordinate should be enough really.25 July 2020 at 3:03 pm #582917
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
Robin25 July 2020 at 3:07 pm #582918
I don’t believe the lack of remotely operating spectrographs is necessarily due mainly to the lack of suitable instruments. There are spectrographs such as the ALPY and DADOS which need no adjustment (In fact the SX spectrograph should not need adjustment in normal use with a suitable size camera really) and others like the Eshel which is capable of fully remote operation but you see very few operated this way. Remote operation of slit/fibre fed spectrographs is much tougher than imaging, particularly if you want to automatically acquire targets. (Andrew will be able to advise on that) Spectroscopy also needs a lot of telescope time which is expensive on rented telescopes.
Robin25 July 2020 at 3:37 pm #582919
Here is a link to the handbook. (dated feb 2016)
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.
Robin26 July 2020 at 1:28 am #582923
Thanks for the clarification(s) on why you seemed to object to the device. Nice to understand the rationale. I must admit I rather liked the idea of using the PHD slit option. Seemed quite sneaky but can see why a mirror system might work better.
As you say, it ideally needs someone impartial trying both and seeing what works best in practice.26 July 2020 at 1:49 pm #582931
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.
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