It has been a while since a review. Here is a quick writeup of the Shelyak LISA Spectrograph, a product that has been available for some time now, but only within my reach this month. It has been a pleasure working with it, and it’s not my first experience with Shelyak Instruments, a company based in France and run by some pretty amazing people. Visit their website for more of their offerings: https://www.shelyak.com/ . I have used their eShel echelle spectrograph, a fiber-fed system that gives R=10000 resolution. That unit is attached to our school’s 0.7m CDK-700 telescope and is quite a specialized system. The LISA ha sa lower resolving power at about R=1000, so it provides a full visible wavelength range spectra in a single spectra from near UV to near IR without having to deal with the multiple spectral orders of an echelle design.
We obtained this unit as a “Pack” containing the spectrograph, a guiding CCD, an imaging CCD for spectra, a calibration lamp unit, power supply and cables. The system has a 2″ OD nosepiece for mounting onto the telescope. All of this fits into a very nice, solid shipping case for transport, though I would not recommend transporting this system a lot. It is fragile with many potential physical components which can slip out of alignment or focus. One should not expect a spectrograph to be particularly robust, so this is not a negative in any way. Take care of precision instruments: my mantra.
Setting up the LISA requires a table, chair and a room that has darkness-on-command… and access to windows to capture some blue sky photons. Some logical thinking on the user’s part is also needed. There are two CCDs, one for guiding, the other for data capture. You need to have software that can handle this. I opted to use MaxIm DL for the guiding imager, and the relatively new offering from Shelyak, the Demetra software, for capturing spectra. Demetra is free and handles all the details of capturing and calibrating spectra for the user, a very nice package. It will capture the darks, flats, biases, calibration frames and the spectra, then process them to a final presentable data set and plot. MaxIm DL, is not free, but it handles guiding very well. You could also try PhD Guider, a free package but not as robust. I had no success with PhD Guider, but MaxIm DL worked instantly for me. Reason? Star images off the guide mirror in the spectrograph are not perfect, round PSFs… they can be a bit elongated, and if you are taking rapid integrations, the image might vary considerably due to seeing conditions. MaxIm DL handles this well.
Once the unit is up and running you will have:
- The spectrograph with calibration lamp unit attached.
- Four power cables coming from a 12V supply to the two CCDs and one each for the two lamps: tungsten and Neon/Argon.
- Two USB cables running to your computer (running Windows).
While on the test bench, one needs to test connectivity to software first and cool the imagers. The guide CCD is then adjusted in and out of the unit to achieve focus on the slit. The slit is easily seen when the system is exposed to some ambient light with short exposures taken. Care needs to be taken to also align the camera so that the slit is either running horizontally (my preference) or vertically through the guide CCD field. Next in line is the focusing and rotational alignment of the spectra capturing imager. This is more tedious: first a rough focus is obtained. One can use the calibration lamps (Neon/Argon) which presents a set of fine emission lines… or one can open the nosepiece and allow blue sky light into the unit to capture a solar spectrum. Rough focus is achieved via moving the imager in and out of the unit. Rotation is then done by loosening three set screws on the adapter which holds the CCD onto the spectrograph. Once done, fine focus is achieved by opening one of the spectrographs access panels, a series of well-placed small doors which are held onto the side via thumbscrews. Once these are open, all sorts of ambient light leaks into the unit, of course, requiring that you work in a darkened space with the calibration lamps as your focus choice. Inside is a lens which can be slowly rotated in and out of focus. The adjustments are fine but easily done with patience. Once focused, the FWHM of the emission lines will be at their finest.
Next to the software. It’s always best to test the running of spectroscopic software in a lab and not outside being cold or mosquito-bitten! Shelyak’s Demetra software did all that it was supposed to do until it got to calibrating the spectra for wavelength. I stumbled around with that for quite some time until I realized that the necessary emission lines were off the CCD field of view by just a small amount. Plenty of lines were visible, but not the extreme red end…. this meant that the grating needed to be rotated slightly to bring that portion of the spectrum back into the CCD’s field. Back to opening a side port on the unit, there is a small handle, that when very carefully raised or lowered, made tiny adjustments to the gratings angle. This was a bit tedious, but, again, with patience, not impossible. I should note that all the tools you need to do these operations are provided with the spectrograph: a nice touch.
At this stage it was time to get the spectrograph outside and into the observatory for some real work. As we all know, when it comes time to work in the observatory with new equipment, not all things go smoothly the first time around, so best to commit to that aspect of the instrumentation life and give a few hours to setup and operations. I have found it helpful to take videos at key moments which act as note-taking for my later operations handbooks left at the observatory.
This spectrograph is for use on a flat field 16″ SCT which is on a Paramount. The system is almost always used for visual work and occasional planetary or lunar imaging, so spectroscopy brings this telescope’s capabilities up a major notch. The spectrograph was attached to the scope via a Van Slyke two-inch slide mirror system, which allows for easier target acquisition and focus. The power connections were made followed by hooking everything up to a small laptop for controlling the telescope, the spectrograph CCDs and the guiding. We use the Sky X software for mount control. At this time, the Sky X, MaxIm DL, and Demetra are all launched. The telescope has been homed and is aimed at Vega, a good solid standard star, easily found and bright. Goal number one was to get the system in focus and guiding to work.
Getting guiding going was a trick that took me just over a hour to figure out. I tried to start with PhD Guiding, a pretty nominal (and free) system that also also has a built in spectroscopy slit guide feature that would be nice…. but I could not get the software to keep the star centered. It kept losing the guide star. Sigh. You might have better fortune than I, so here is their link. https://openphdguiding.org/ Moving then to MaxIm DL, a not free software, the guiding worked flawlessly. There is also a very nice “move” command in their guiding which allows one to shift the target star from a location on the chip to another…. like the slit opening for the spectrograph: VERY nice. The guiding is done directly via a standard guide cable from the camera to the mount, so as long as your mount supports an ST-4 autoguider connection, you should be fine. Other methods would also work, such as an ASCOM connection through the Sky X or similar.
Now that guiding was going smoothly, data could be collected. Vega was target number one. A series of three 5-second integrations were taken. Anything longer and the system reached full well capacity. Here is the fully calibrated spectrum of Vega from that night:
Notice the Balmer lines well into the UV on the left side. This has been cropped, otherwise you would also see the telluric lines off in the IR on the right. Wavelength calibration was quick and easy, as was calibration for spectral response of the imager. Vega is a nice A0V star, so those processes were easy. Once a response curve is made, it cam then be applied to the other spectra you take on a given evening and in the same region of the sky (atmosphere matters).
Being so close to Messier 57 (Ring Nebula), I could not resist the temptation and though, “hey, why not?” and slewed the telescope a little bit to center M-57 on the guide chip. The results were quite good. The following is a series of three 120-second integrations all processed in Demetra automatically.
It’s good to know also that Demetra handles background subtraction as well as the standards (bias, dark and flat field images). The results are quite good! Shelyak makes note that this is a system designed for fainter objects, and I must agree. While it will be fun to check out Jupiter and its rotation (and methane!), getting spectra of galaxies and investigating their redshifts is now a part of the plan. Is this for everyone? Not at all. As I have been saying a lot this past year, astronomy is more than pretty pictures. Sometimes it’s pretty plots! This is an expensive instrument to get for a hobbyist unless you want to get into data collection for science use. The AAVSO (https://www.aavso.org/), BAA (https://britastro.org/) and others would love your variable star spectra and have whole active observing sections to help people get going. Is this the way to get started in spectroscopy: I’d say no to that. I’d recommend playing with a transmission grating threaded into your filter wheel first. Move to this when you are ready. More about this system at Shelyak’s LISA page here: https://www.shelyak.com/produit/pf0021vis-lisa-slit-visible/?lang=en