What does it mean? It means 109 days until the Sacramento Mountains Spectroscopy Workshop. The second international conference, the only one of its kind in the US will be held right here in Las Cruces NM and on February 22, 2019, the world’s spectroscopy experts are going to convene in my backyard! The purpose will be to collect spectroscopy data using a live set up with the Lhires III. (Littrow High Resolution Spectrograph) So in other words, it’s on! I figured a formal countdown will help me to be prepared in time. We will see. While I have the astronomy degree and understand the science of spectroscopy, my only experience with actual astronomical spectroscopy is what is called a star analyser, which is a small screw on filter like grating that actually got me interested in this. With a cost of under $200 you can observe stellar spectra and do an amazing amount of science using just a video camera and this simple low resolution grating. Now it’s on to bigger and better. I have 2 friends who are fairly expert astronomical spectroscopists but their equipment is up in the mountains , 2 hours from here (where my imaging equipment is located) so they needed someone here in Las Cruces who had the same or similar equipment that could be used for this conference. That’s how this came about. My set up for spectroscopy is down here so now I have exactly 109 days to become an expert at spectroscopy data collection from being a complete newbie! That’s what we are going to focus on. The analysis of the data will come later. So let’s get started!
Thankfully imaging experience does come in handy because I will still have to do the basics of setting up my mount, polar alignment, pointing and tracking as well as collimating the scope. All of that mundane stuff which I have done a thousand times. Using a spectrograph of this type is quite a bit different than regular deep space imaging! You can see in the schematic diagram (courtesy of planetastronomy.com) that there are 2 camera ports, one for the imager and one for the guider. Note that the imaging camera is placed behind the diffraction slit so is basically blind to the star field you are observing! Because of this your guide camera has to function both as a guider and also in a way an imager because you will have to center your object with this. The imaging camera cannot see anything but the light coming through the slit. We will sort this out later.
The first step is to bench test and focus the spectrograph before going out to the telescope. I used an eyepiece a while back to look at the Sun (which you CAN do with this spectrograph since only a tiny fraction of light from the Sun gets through the slit) You then locate the hydrogen alpha absorption lines by turning the tall silver knob on the back of the spectrograph. It’s called a micrometer and has numbers etched into it. Write these down! Most of the beginning observations we will be doing on stars will undoubtedly involve H alpha lines so this is the only way you will know where on your grating they can be seen.
Next step is to test the camera operation on the spectrograph and focus the lens. What that means is you have to have the proper distance between the lens which is inside of the spectrograph (see diagram below) called a “doublet” (2 element refracting lens) and your ccd chip. Note this is NOT focusing the telescope. That comes last. Thankfully the system is designed to work well with certain cameras. I am using an Atik 460 EX as my imaging camera. One of my expert spectroscopist friends has the same set up. The adaptor for this camera is provided by Shelyak as standard so I am pretty certain I have the proper distance between the ccd and the lens for this set up.
Focusing the doublet in the spectrograph can be accomplished by using the calibration lamp inside the instrument. It is a neon lamp and produces emission lines when the light travels through the system. You can see the hydrogen alpha emission lines with an eyepiece and I did that before this, noting again the position of the micrometer. The doublet has a focusing ring which is accessed by removing the side doors on the spectrograph (see below). I actually had to take both doors off and manipulate the ring using 2 hands. The ring movement is fairly course. So the strategy was to take a series of exposures and try to get the emission lines as thin as possible. The other item of business was to adjust the orientation of the camera so the emission lines are vertical and by convention the blue end of the spectrum is to the left (Red Right- easy way to remember) . I’m not sure yet what the goal is in terms of pixel width on the emission line thickness. The experts seem to refer to Full Width at Half Maximum “FWHM” for the line resolution which is a little weird to me because I am used to that characterizing point sources of light but not this kind of thing. Anyway there is a software program that spectroscopists use to analyze their data and there is a simple way to measure the “FWHM” from the calibration image (see below). So the Atik 460 has a pixel size of 4.54 microns. My optical set up for this will be a C14 at prime which my imagers brain tells me will amount to an image scale of about 0.24 or so arc sec per pixel! That’s way oversampled for the seeing here which averages around 2-2.5 or so. My friend using the same camera routinely bins 2×2 so after experimenting with the emission line thickness it looks like binning is the way to go, although even at 2×2 the line thickness is 4+ pixels. Not sure I can focus the doublet any better. I will ask the experts on this to make sure it’s adequate. This is obviously different than an actual stellar spectrum obtained from space but also not sure how that translates.
So at this point I believe we have the spectrograph focused, but will get confirmation of that. We have identified the H alpha position on the micrometer. The camera is properly oriented and the adaptor spacing for the camera is correct.
I thought this was a pretty good schematic of the components of the Lhires spectrograph. Light from the telescope reflects off of the slit and also passes through it. The reflected light is seen by the guider and is the same field as the light that eventually reaches the imaging ccd but notice how the ccd will not see a star field , only diffracted light from a star. That light goes through the doublet and is reflected back off of the grating back through the doublet again and lands on the imaging chip!
Bench testing the Lhires. The Atik is attached to the imager port. You can see there is the smaller port on the top for the guider covered by the black rubber and the orange cover protects the telescope attachment port
Close up of the camera to spectrograph connection. I tried connecting the camera directly to the instrument first and was unable to reach focus so this spacer adaptor provided by Shelyak creates the proper back focus distance for this Atik 460 EX camera
The Lhires has a built in neon calibration lamp powered by a 12 volt conversion adaptor plug from a standard AC outlet. Note the side door is opened and you can see a white teflon screw which tightens the doublet once focus is reached. There is an identical door on the other side.
This was the first attempt at focusing without binning the camera. What you do is turn on the calibration lamp and take an exposure. In this case it was around 10 seconds and this produced about 50% saturation. Signal to noise looks pretty good.
However at 1×1 binning the resolution is 11.69 pixels. That seems way oversampled to me. How this works is you load your fits image on the image page and simply draw a rectangle around a portion of the emission line, then click on the “FWHM” button on the right. You’re trying to get the line as thin as possible by focusing the doublet
With the camera binned 2×2, the FWHM decreased to 4+. This seemed to make more sense. We will see if this turns out to be ok
Finally if you are beginning in this journey, this is the book you need! You can find it on the Shelyak.com website
Next up: guider set up, slit focusing, collimation of the scope
Thanks for reading!