Day 71

Time is flying by. I confirmed with Shelyak that we are good with the spacing on the imaging camera. Next was the guider set up. For the Lhires spectrograph, the guider is really the critical sensor for acquisition because as we discussed in an earlier post the camera that is recording the spectrum is totally blind to anything beyond the slit. It only can deal with the light coming through the slit. That’s why focusing the lens inside of the spectrograph is so key because the resolution of the spectrum really depends on that more than anything. The guider I am using is a ZWO ASI 174. It has a 1936 x 1216 pixel CMOS chip, field of view 6 x 10 arc min. The chip size is a little bigger than other options for the guider on this setup. The QE (quantum efficiency) is pretty high, 78%, so this should make things easier for plate solving and finding guide stars, at least theoretically. The camera is light weight and there is no cooler because all we are doing is plate solving, finding stars and guiding. Nothing else. Plate solving, by the way, is a frequent function of astronomical optical platforms. What happens is your system takes an image of the field of view and tries to match what it is seeing with known stellar databases. It is one of the most remarkable technological accomplishments of the early part of the 21st century in my opinion because now amateurs can do what the professionals do! Once the image is “matched” to the known stellar database by the software, the image is considered “solved” and the system now “knows” where it is in the sky! Many software programs are capable of doing this now.

The guide port on the spectrograph is tiny! 1/2 inch and not much more. The thread on it is called a “C” mount. I’ve heard of T-threads, M42 and several others but not this one. It just so happened that ZWO had this exact adaptor for the camera so I was in luck there!

I screwed the camera onto the port and it looks like everything is good in terms of focusing the slit, which is the dark straight line. The backfocus on the camera is 6.5mm and the distance from the spectrograph to the camera face was measured with the caliper at around 45mm so it appears that the total distance is correct within a mm or 2 if you look at the diagram below. Now the illuminated field looks weird. I get that, but I’m thinking it may be vignetting or internal reflection or something, maybe in part due to the larger chip size. Bottom line is we have a slit line of a few pixels in width that is centered on the chip shown by the cross hairs below!

What we have so far is a focused spectrograph doublet lens which we accomplished by using the internal calbration lamp Day 89. ,and now (hopefully) a centered slit in the guide camera. We are now ready to go to the telescope!

This is the guide port on the Lhires spectrograph. The opening is maybe 1/2-3/4 inch. The threads are male “C-mount” threads and actually are at the end of a small lens which is used to bring the spectrograph slit into focus

This is the lens which fits into the guide port sleeve and serves to focus the slit onto the guider chip

 

Technical drawing for Lhires showing the optimal distance to the guider ccd is 53mm. Subtract backfocus of ASI 174 camera which is 6.5 and that leaves 46.5mm distance between the spectrograph base and the camera face. I am at about 45mm for that so I think we’re good!

 

Recall the guider is seeing reflected light from the slit. It is the only camera that can actually see the optical field of view so in essence it is the eye of the spectrograph!

 

This is a 0.2 sec unbinned exposure in a room with what I thought was pretty dim lighting! I rotated the guider until the slit (black line) was dead center in the cross hair. Orientation of the slit is not critical as long as it is centered. The illumination is strange and not sure why it appears asymmetric like this but we will soon find out if this is correct or not!

 

2 camera configuration with guider on top, imager on the bottom

 

A Star Explodes!

On November 24 a supernova was discovered in the galaxy M77. Named “SN 2018 ivc” it is the only supernova to date discovered in the brightest Seyfert galaxy we know! M77 is an active galaxy with a quasar-like nucleus. It is pretty bright for a galaxy, about 9th magnitude, and is located in the constellation Cetus. A supernova discovery in a Messier object is pretty rare, and as it just so happened I had started an imaging project at Orion’s Belt Remote Observatory on the galaxy when one of the Astronomical Society members alerted us to the discovery! The supernova is a “Type II” which is distinguished by the presence of hydrogen in the spectrum. (Perhaps a spectroscopy project for a future event!) I was able to locate the star in one of the 15 minute frames I had taken of the galaxy (see below). Initially it was around 14-15 magnitude, located pretty close to the core . In the image I obtained nearly 2 weeks later it does not appear to have changed significantly in brightness.

Single raw uncalibrated 15 minute image, unfiltered, of SN 2018 ivc. Dec 5 at 5:20 UT. From Orion’s Belt Remote Observatory, Mayhill NM. The supernova is located at the intersection of the 2 black tic marks

Day 89

Less than 90 days to the Spectro conference. We’re still not at the telescope yet, although collimation is done (see last entry). Looks like the spectrograph was NOT properly focused. I visited the observatory of my friend Joe D who is an expert amateur spectroscopist and has the same equipment. Sure enough the spacing on his optical train was totally different. The adaptor on his measured about 42mm. Mine was only 30 -35. Way off. I went back to the bench testing, looked at all of the adaptors I had stored in various drawers etc and could not come up with any combination of sleeves etc that were not either too long or too short. I assumed the adaptor that came with the Lhires was correct when in fact it was not. So I checked with Francois over at Shelyak Instruments and he confirmed that my set up was incorrect. The slit on the spectrograph for Lhires  is 23µ, and the camera pixels are 4.54µ (Atik 460 EX). Then the image of the slit should be ideally 23/4.54 = 5.06 pixel for 1×1 binning or about 1/2 that for 2×2. I was struggling to get just under 4 for binned images. What to do? Looked like there was an adaptor specific for Atik cameras on the Shelyak site with more appropriate spacing so I got one of those. The results are promising. For a 5 sec exposure of the calibration frames I am routinely getting 2.8 or so FWHM. Hopefully we’re good now. I have never had to fuss with spacing like this before for routine astroimaging. I mean we’re talking mm here! But apparently it is significant especially if in the future we plan to contribute to astronomical research!

Technical drawing for Lhires showing the optimal distance to the imaging ccd is 54.85mm. Subtract backfocus of Atik camera which is 13.5 and that leaves 41.35mm distance for whatever adaptor is being used

 

The idea is to focus the internal lens of the spectrograph using the calibration lamp. Make the line as narrow as possible. We have the camera open in the Sky X, 5 sec exposure and increased the resolution to 500%. We move the focusing ring on the lens by opening the side doors on the spectrograph and just reaching in there. Slight turns until the line does not get any narrower

In the processing program we can read the FWHM of a section of calibration line is under 3 pixels! In a perfect world we should be 2.5 but we’re 2.8. I think that is satisfactory but we will ask the experts!

So hopefully time to move on to the guider! Thanks for reading!

Dr Dave

Day 107

While we prepare the Lhires spectrograph for the telescope indoors, next step outdoors is to collimate the C14. There are a thousand ways to do this and much of this can’t be covered in a single blog entry. Everyone has their “take” on it. For myself I have used many different scopes and while lasers and other collimating instrumentation can work, they can be expensive and difficult to reproduce consistent results. Laser collimators generally work well on Newtonian-style telescopes with a flat secondary mirror. On Cassegrain-style telescopes, which have a curved secondary mirror, a laser collimator will work properly only if the secondary mirror is perfectly centered above the primary mirror. Unfortunately, telescope manufacturing tolerances are such that slight secondary mirror offset is inevitable. Under these conditions the laser collimator may give a false indication of collimation. Using a star is much more accurate. The bottom line is you are doing this to get the best possible STAR image you can so it makes sense to me to do the collimation while observing stars. My preferred method is to use a software program called CCD Inspector (CCDware.com) . You can use the program free for 30 days. Afterward the cost is $150. This is still cheaper than many laser collimators, and the program has many other applications for imaging. Definitely worth a look

The classic mass produced Schmidt Cassegrain telescope has 2 spherical mirrors, one primary and one secondary. The secondary is attached to a “corrector plate” with usually 3 Allen screws (see diagram below) These screws can be replaced with easy to turn thumb screws called “Bob’s Knobs” 

The collimation procedure is pretty straightforward. Several 2 second exposures of a defocused 2nd magnitude star are taken. While you take an exposure or 2 place your hand over the front of the telescope and observe where the shadow appears in your exposure. The software will tell you what the current collimation is in arc seconds (you have to enter your image scale in the program settings) and most importantly a line (arrow) will appear showing you what direction you need to move the star image to properly collimate the scope. For an SCT you have to move the corrector in the direction OPPOSITE the arrow or line to get the star movement correct. Position your hand over the scope as above until you see the shadow of your hand directly opposite the line on your 2 sec image. This is the direction you need to move the corrector. See where your hand is in relation to the Bob’s knobs and use the knobs to move the corrector exactly in this direction. I use the excellent diagram produced by Astro-Tech (see below) to figure out which knobs to tighten or loosen. Only move the knobs slightly at first to get a feel for it. Then take another exposure. You should see the star image move in the direction of the line. Repeat this process until collimation is reached, which for most seeing conditions, should be less than 5 arc sec. I started at around 26 or so! It looks like we’re good! Note that at first the line will stay in the same direction until you are close to being collimated, then will start shifting around to different locations. Then you know you are as good as you can be for those seeing conditions!

Classic SCT optical arrangement. On the left is the secondary which is mounted on a “corrector plate”, not a spider . 3 Allen screws secure the secondary and can be adjusted to tip tilt the mirror for collimation. Reproduced from the text book “Telescopes, Astrographs, Eyepieces” by Smith, Berry et al

 

Bob’s knobs. This is the front of the OTA. These replace the Allen screws so in the field you can collimate the telescope very easily without fussing with an Allen wrench

 

This diagram is excellent and shows you which knobs to loosen and tighten in order to move the star in the direction shown. For an SCT you have to move the secondary in the direction opposite the arrow in the CCD Inspector program using your hand’s shadow to figure out where on your scope that is. (see text)

 

And this is the result. 2.5 arc sec collimation! Took about 15 minutes to do!

 

Thanks for reading!

Dr Dave

Day 109

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!

Dr Dave

 

Spectroscopy update

Presentations from last February’s conference are now available! Sorry it took this long.

https://sites.google.com/view/spectroscopy-bootcamp/archived-presentations

And now the 2nd Annual Sacramento Mountains Spectroscopy Workshop has been announced! It will take place in Las Cruces NM February 22-24, 2019. They now have a dedicated website which is here: http://www.smswweb.com/

Due to the tremendous success of last years introductory meeting which was held up in Mayhill NM the meeting format has been expanded to accomodate the need. Any and all interested please take advantage! This is the only meeting of its kind in the US and its purpose is a practical introduction to spectroscopy to where after the meeting you will be able to understand spectroscopy, what it is, as well as basic acquisition and processing of spectral data. The faculty honestly is unparalleled and features Dr Stella Kafka, director of AAVSO, Francois Cochard, developer of amateur spectroscopes Lhires III and others, Christian Buil , optical engineer and French amateur who developed most of the relevant software for processing spectra, Dr David Whelan from Austin College and several others.

My job between now and then will be to have a functioning spectroscopy set up since the data acquisition part of the course is happening right here in my backyard! So over the next 4+ months hopefully you can follow the progress until the big event! Hard to imagine the world’s spectroscopy experts will be convening right here at the Talavera Space Hut on Friday Feb 22nd. Better get started!

Thanks for reading!

Dr Dave

Mars 2018

Of course we had to observe Mars’ closest approach to Earth in 15 years! That occurred on July 31 when it was a “mere” 35.8 million miles from us. I was able to capture it right at that time from the Talavera Space Hut!    Unfortunately it is not favorably positioned for Northern Hemisphere imagers as it’s maximum elevation is only 29 degrees. Seeing at this low altitude even in the best conditions is not that great. Consequently it was very difficult to obtain quality images, but loads of fun to record the event! Interestingly I compared the images I took in 2005 when it was at around 43 million miles (7 million miles further) and at that time the planet elevation was much higher. You can see how even with way more modest equipment the resolution is better!

Mars is at “opposition” now in terms of it’s orbital position relative to Earth, meaning we are between Mars and the Sun. When the Sun sets, Mars is rising and at sunrise Mars is setting.

Here is a schematic diagram of Mars in 2018 with simulated images  courtesy of the Association of Lunar and Planetary Observers website:

Of course this isn’t exactly what I saw but the other complicating factor was a dust storm that raged on the planet for months obscuring all surface features. Considering that and the low altitude position we didn’t do too badly. Note that the South polar cap is at the top here. One thing I did see that isn’t shown here is the North polar cap! You can barely see it in the fully processed images below but note the appearance in the blue filtered image. You can really see it there!

 

This is probably the best resolved image taken on the night of July 31. The blue glow at the bottom is caused by atmospheric refraction and very slight misalignment of the RGB channels. Equipment used was a Celestron 14″ SCT, Skyris 132M mono camera with filters. Each channel was about 30 seconds at 100 frames per sec.

 

Same equipment as the image above but just not cropped. It’s the same image scale. All images were obtained at prime focus around 4400mm. When I tried going up to 2x with a barlow the planet was “boiling” everywhere! Had to stay at prime

 

I thought this was cool! The Northern polar cap is very clear at the bottom of this blue filtered image!

 

These were images I took back in 2005. The lower image was at opposition. Equipment back then was a 10″ newtonian and at that time I was able to use a 5x barlow! The seeing was that good. From Western Massachusetts of all places! Camera was a simple color webcam called a Toucam pro! But again the altitude of the planet makes a huge difference. i don’t recall the exact position but I’m going to say it was at least 50 degrees or so from what I remember.

Well that’s about it for Mars opposition 2018! Until next time. Thanks for reading!

Dr Dave