Feasibility of the Detroit Area Schools Supernova Survey

Nick DavisResearch Advisor, Prof. David Cinabro

Wayne State University Department of Physics

August 10, 2009

Abstract

The Detroit Area Schools Supernova Survey hopes to create and maintain an ever-growingnetwork of part-time supernova searchers, mainly local high school science teachers and theirstudents. At the beginning of this project it was unclear whether or not there was a reasonablechance of success due to the weather and sky conditions in Detroit. Initial testing had beenpromising despite the complications of working with low cost equipment. The equipment canbe improved slightly for a minimal increase in cost. Targets must be chosen carefully becauseof sky conditions and limitations of the equipment, but due to the sheer number of galaxies inthe sky I believe that a successful supernova search can be accomplished.

Introduction

Perhaps people are inspired by a genuine in-terest in contributing to the fantastic mass ofastronomical data collected each year that as-tronomers, astrophysicists, and other space sci-entists depend for their work; or maybe their rea-sons are a bit more selfish, such as the allure ofwitnessing for oneself a rare cosmic event thatoccurred many millions of years ago. Whateverthe reasons may be, the seductiveness of super-novae have attracted thousands of would-be ob-servers, both professional and amateur. Super-nova searches are being conducted from dark skysites all over the world on every imaginable scale.Even the very smallest attempts using a singlecamera and telescope have been successful. Butmost of these searches have one thing in common– they are conducted under good conditions fromsuburban or dark rural skies. The Detroit AreaSchools Supernova Survey (DASSS) is going totry something di!erent.

The Detroit Area Schools Supernova Survey

will be performed under some of the worst ob-serving conditions that North American viewerscould expect to find. Detroit is one of the coun-try’s cloudiest cities and it is located in a heav-ily populated metropolitan area, not the kind ofplace you would expect to be attempting any sortof astronomical observation. It is no coincidencethat serious astronomy observatories are usuallylocated in the desert or high atop a mountain,or both. The dark skies and dry air allow forthe very best views that can be enjoyed from anearthly locale – Detroit is humid and bright.

But there are educational benefits to hav-ing an active local search. The fact that mostmodern astronomy is done with data collectedat the non-visible wavelengths of the electro-magnetic spectrum coupled with the realitythat astronomers virtually never look directlythrough a telescope might have made astronomyseem less attractive to young students. Rarely,if ever, are we as awed and inspired by a radio-spectrograph of the sky as we are by a beauti-ful photograph taken by the Hubble Space Tele-

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scope. A local search allows students to get ac-tual time with the telescopes, and the ChargedCoupled Device (CCD) camera can display im-ages at a fast enough rate to allow for ”real-time”viewing of a deep sky object.1 This more hands-on approach will hopefully provide the studentswith a feeling of involvement that would other-wise be lacking if they were to just analyze datagiven to them by a teacher or a book. Of coursehands-on learning requires equipment to be pur-chased for one to place his or her hands on, whichmay be a reason why it is not done more of-ten. Fortunately enough time has elapsed forthe technology, formerly available to only the fi-nancially privileged programs, to trickle down tothe more modestly priced equipment.

Robotic telescopes, capable of pointing towithin 1 arc-minute 2of an object are read-ily available at consumer price levels. Thesetelescopes are equipped with tracking motorsthat can track an object during the exposuretime necessary to image it with a CCD cam-era. A wave of new, economical, CCD camerashas recently swept the astrophotography market.These cameras employ low resolution, highly sen-sitive CCD chips originally designed for secu-rity video applications. Not only is their pricetag equal to a fraction of the cost of a ”tra-ditional” astro-camera, their greater sensitivityprevents the need for the long, often many hourslong, exposure times that only the most expen-sive telescopes are capable of. These two items,a robotic, so called GOTO, telescope and one ofthese new sensitive CCD cameras, along with alaptop computer are all that is needed to be-gin a search for supernovae. I had access tothese items and to an observing location on therooftop of a building located in one of the worstobserving locations one could imagine: all I hadto do next was cross my fingers and pray to theweather gods.

Figure 1: The Celestron Nexstar 8. c!Celestron Instruments

Equipment

The telescope used for this project was an 8”schmidt-cassegrain optical assembly mounted toa robotic fork mount. More specifically, it was aNexstar GPS model made by Celestron Instru-ments that the Wayne State University PhysicsDepartment already owned. The current marketprice for a similar telescope is around $2700. Ascope of this size is just about perfect for a non-permanent installation because a heavier scopewould require two people to set up, and in alight polluted city more aperture is not neces-sarily a good thing. The camera I chose to usewith this scope is the Deep Sky Imager (DSI)Pro II by Meade Instruments. I chose this cam-era for several reasons; the first and most impor-tant, is that it is fairly small and lightweight so it

1Actually, the camera software cannot display a truly live picture due to the fact that the exposure times neces-sary to image deep space objects can be fairly long even with very sensitive cameras. Also, the photons hitting thecamera’s CCD have been traveling through space for millions of years so we are in fact seeing an image of the past;still, it beats watching TV. Sometimes.

2An arc-minute is a unit of angular measurement equal to 160 th of a degree.

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wouldn’t create any problems for the telescope’smotor drives. The second is that this camerahas a great reputation for ease of use and perfor-mance, and for only $500 it is a hard camera topass up. The camera needs to be tethered to acomputer via a USB II cable in order to function.For this I used my Apple Macbook Pro, which Ihad to install Microsoft Windows XP on becauseunfortunately the Meade camera software is forWindows only.

Setup and First Images

The very first night of the project, the first clearnight anyway, I setup the 8” Nexstar scope onthe roof of the Physics Building. It took lessthan 20 minutes to get the scope into position,connect the camera to the computer, and pre-pare the scope for star alignment. Before thestar alignment can be performed it is necessarythat the scope be level and pointing towards ge-ographic North. Once this is accomplished, thetelescope’s hand controller will guide the userthrough the two-star alignment process. Thesoftware will automatically choose the brighteststar in the region of sky that the telescope ispointing towards and ask the user to manuallyslew the scope to it using the electronic handcontrols. Since the guide stars that it will chooseappear big and bright, it is possible for someoneto setup the scope who does not have any priorknowledge of the sky. After the alignment wascomplete, I directed the telescope to point to-ward the double star Albireo 3 in order to test thepointing accuracy. As I had hoped, Albireo and

its companion were visible near the center of the25mm eyepiece that I had inserted into the scopefor the alignment process. Once I was satisfiedthat the telescope was pointing correctly, I in-structed the scope to slew towards the WhirlpoolGalaxy M51. With a visual magnitude of 8.4 andan angular size of approximately 10 arcminutes,M51 is one of the largest and brightest galaxiesvisible in the Northern Hemisphere. Still, all Isaw through the telescope’s eyepiece was a lightgray fuzzy patch of light with no structure tospeak of. This was not really discouraging sinceI knew that The Andromeda Galaxy M31 wasbarely visible from Detroit despite being nearly100 times brighter than M51. So I took the eye-piece out and replaced it with the Meade DSIcamera and booted up the software package thatwas included with the camera, and there it was.The fuzzy nucleus of M51 along with the moreprominent spiral arms were clearly visible in thelive view window on my laptop. Using the slowmotion controls I re-centered the galaxy’s corein the image window. I was now ready to takea picture. But first, I needed to record a darkframe. A dark frame is an image made at theexact same exposure time, and under the sameatmospheric conditions, as the photo that youplan on taking, but with the lens cover over thetelescope so that no photons can hit the CCD.The resulting image will not be totally black likeone would expect. Bright spots from hot-pixelsand dark rectangular smudges of noise will becovering the frame. The camera software willsubtract these areas of noise and bright spotsfrom the uncovered photos that it captures fromthen on.

3Albireo, also known as ! Cygni, is the fifth brightest star in the constellation Cygnus. It, along with its smalleryellow companion, form a true binary star system.

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Figure 2: Seen here are two images of the Whirlpool Galaxy M51 with identical exposure times. The only di!erence is the

image on the right has had a dark frame subtracted from it to reduce noise and eliminate hot pixel spots. Exposure

142.5s - 25 stacks. Exposure times are given in a total exposure time - # of stacks format, such as ”142.5s -25 stacks”.

In this example 25 images of 5.7 seconds each were combined since 5.7 ! 25 = 142.5

The dark frame must be taken before the nor-mal photographs for the software to perform thissubtraction automatically. So, I placed the capover the telescope and exposed a series of darkframes at 1, 2.4, 4.7, 5.6, 6.3, and 8 seconds sinceI did not yet know which exposure time wouldcreate an image that would reveal the most de-tail of M51 without being washed out by the lightpollution. Finally it was time to take a picture.In my excitement I forgot to change the expo-sure time in the camera control software win-dow so I ended up capturing an image at thedefault exposure time of 1 second. Watching thecapture software in action is really quite impres-sive. The camera starts taking 1 second expo-sures and it won’t stop until the user tells it to.While these images are being captured the soft-ware compares each one with the last and dis-cards any that are too blurry or slightly shiftedfrom the others. The remaining good exposuresare ”stacked” on top of each other. This stackingprocess works similar to the dark frame subtrac-tion except it only removes the noise from theimage and not any bright spots. The result isan image with much higher contrast and far lessnoise than what could be obtained with a singlecapture from a camera like this. I was excited

by this first image, but I knew that M51 was tooperfect a test subject.

Complications

There are only a few galaxies that appear as bigand bright as M51. In order for DASSS to be suc-cessful we must be able to reliably image muchfainter targets, so I spent the rest of the sum-mer attempting to do just that. As expected,capturing usable images proved to be more di"-cult when attempting to photograph galaxies. Iencountered a number of problems that, whilethey could be overcome, did add considerabletime onto the average viewing session. I will de-scribe all these problems here in this section andin the next section I will describe how they canbe dealt with.

There are a number of galaxies that are sim-ilar in brightness to M51, and even a few thatare brighter, but with few exceptions most ofthese appear much smaller in the sky. Rememberwhen I mentioned earlier how I had to re-centerM51 in the eyepiece of the telescope? If the tar-get I had been hoping to see was much smaller,then I might not have seen it in the eyepiece atall. This is exactly what happens; requiring the

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user to slew the telescope back and forth andup and down until the galaxy becomes visible inthe center of the eyepiece. It is a minor incon-venience but one that does add a few minutes tothe task of locating your target. The problembecomes even worse when hunting for fainter ob-jects. I have found that galaxies dimmer than9th magnitude are impossible to see through theeyepiece, though observers in the suburbs maybe able to make them out. This means that noamount of slewing back and forth can help youbecause you are looking for something that youjust cannot see. But capturing images of theseobjects is still possible.

Despite this problem, I was able to make im-ages of 9.8th magnitude M109 and 10.2 mag.

NGC3191. The way this was accomplished seemssimple but in practice it takes a considerableamount of time. I searched the internet forphotographs of these objects that included thenearby star field and printed them out. Armedwith these prints I returned to the roof andlooked through the eyepiece trying desperatelyto recognize a particular pattern of stars thatwas visible in the printed photo. Because theorientation of an object is arbitrary since thereis no defined up or down in space, the patternscan be di"cult to find. All the other problemsI dealt with were either weather related or thetypes of problems one would expect to encounterin a large city, such as the search lights in frontof the Fox Theater that were on every weekend.

Figure 3: The image of M109 on the left was used to help locate the object in the eyepiece so that the image on the

right (85.5s - 15 stacks) could be made. One can see in this example that the orientation of the image in

the eyepiece may not be what you expect.

Solutions

While there really is nothing that can be doneabout the Fox Theater’s search lights, or the lo-cal light pollution, there are a few things thatcould be done to improve the overall chances ofsuccess. Something must be done to improve thepointing accuracy of the telescope’s GOTO sys-tem to reduce, if not eliminate, the need for man-ual searching of the target. This improvementcan be accomplished in a number of ways. Thebest improvement would be achieved by mount-ing the scope equatorially. Leveling the scope

would no longer be an issue but the precise polaralignment necessary would increase setup timeby as much as an hour.[1] Equatorial mountingwould require the purchase of an altitude wedgeand increase the overall weight and height of thewhole setup which may make setup for some peo-ple more di"cult. Also, when performing thetwo-star alignment the user should center thestar in the eyepiece and then swap out the eye-piece for the camera and center the star again inthe live window of the camera software. Sincethe viewing angle through the camera is muchsmaller than that through the eyepiece this al-

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lows for a more accurate centering of the star,and therefore a more precise alignment of thetelescope. It may also be beneficial to considerpurchasing an 8” Meade LX200 series telescopeinstead of the Celestron Nexstar 8. The Meadehas two features that the Celestron lacks thatcould aid in achieving better accuracy. Thesefeatures are High Precision Pointing mode (HP),this slows down the motor drives to allow theoptical encoders on the motor shaft to achievebetter resolution, and the Sync command, whichallows the user to manually re-center a targetand then re-sync the telescope to that objectwithout starting over and doing another two-staralignment. These two features, when used to-gether, should improve the telescopes pointingaccuracy to near 1 arc-minute.4[2] The MeadeLX200’s are, however, currently more expensivethan the Nexstar.5 Regardless of which scopeis used, I think it is necessary to use an equa-torial wedge and an accurate polar alignment.The extra hour or so invested in the initial setupeach night could be recovered by reducing thetime spent blindly hunting for dim targets, aprocedure that may no longer be necessary onceaccurate polar alignment is achieved. Accuratepolar alignment will also allow for longer expo-sure times. I found that the telescope was inca-pable of tracking precisely for more than 6 sec-onds. This is directly do to drift in the RightAscension (RA) axis which is virtually unavoid-able with ALT/Azimuth mounted scopes. Andit was this drifting, not the light pollution as ex-pected, that was to be the limiting factor whenchoosing exposure times.

Figure 4: In this image of M101 (184.5s - 45 stacks)

the stars appear as long trails; an e!ect of

drifting. The jagged structure apparent in

the trails is due to periodic error, which is a

problem inherent to all gear driven telescope

mounts.

Weather

Weather determines the success of an observingsession so much that it deserves its own sectionin this article. Clouds are an observer’s mainconcern but many other factors must be perfectin order to get sharp images – the most impor-tant of these being wind speed. Early on in theproject I was frustrated by successive weeks ofcloudy skies at night. In order to get the mostout of every opportunity I would watch the skyon partly cloudy nights waiting for a break inthe clouds. On nights such as these where theclouds were moving fast enough that there washope of them breaking up or moving o! past thehorizon, wind was a big problem – and it doesnot have to be blowing very hard either. Lateron in the project I decided on a simple test toperform before I went through the hassle of get-ting the telescope out. I would set my target listand star field photographs, about 7 pages stapled

4The newer model Celestron Nexstar also has these features but Celestron has opted for a single arm mount, ratherthan the double arm fork used by the older models and the Meade LX200 series, that makes the scope undesirablefor astrophotography.

5The price of $2700 I quoted in the Equipment section of this article was for the Meade LX200.6Stronger winds can even prohibit the telescope’s ability to track reliably.

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together, on top of my table (actually it was acontractor’s outdoor storage box) and stand backa little so I would not be blocking the wind. Ifthe papers blew o! the table then I knew it wastoo windy to expect sharp images.6

Another problem that I experienced later onin the summer as the temperatures rose was hu-midity. On days that were slightly hotter thanthe days before, moisture in the atmosphere andcondensation on the telescope’s front correctorplate put an early end to any observing ses-sions. On the really bad nights the humiditywas easily seen as halos around distant streetlamps. On these nights I knew better than toeven try. But the most frustrating nights werethose where the humidity was not bad enoughto see the halos, yet high enough that dew wasa problem. Simply wiping it o! is not a solu-tion since it would rapidly reappear, making dewprobably the biggest annoyance of summer ob-serving. This occurred most often on days whenthe temperature exceeded about 75 !F.

Nights that were absent of clouds, wind, themoon, or halos around the street lamps, werefew and far between. During the 10 weeks ofthis project I spent 26 nights observing on theroof. The first six or seven of these was spentlearning, through trial and error, which settingsin the camera software would yield the best im-ages. I was blessed with exceptional atmosphericconditions, and an unusually cold 55 !F temper-ature, on the third night. This led to the captureof M51 shown in the right side of figure 2. Afterthat night I had only 8 truly successful nightsout of the remaining 23 but that was largely dueto the fact that I spent a lot of time searching fortargets that were beyond 10.2 magnitude, whichI later on concluded to be ”out of reach”. In theend I was able to capture nearly 50 images butonly 16 of these were not streaked by poor track-ing, wind, and vibrations on the rooftop causedby the buildings air circulation system. I wouldexpect a setup on the ground, where the wind isbetter shielded and vibrations are dampened bythe earth, to have a higher rate of success.

Conclusion

The goal of any supernova search is to repeat-edly and reliably image a list of galaxies in thehope that a supernova will be spotted in one ofthem. The more galaxies imaged in the search,the greater the chances of seeing a supernova.This can mean imaging hundreds or thousandsof galaxies in a week and then repeating thatsame list the week after, and the week after thatand so on. But you need more than just oneimage of each target per week.

Supernovae are discovered visually by com-paring two images, taken perhaps only daysapart, and looking for a new ”star” to appearin one of the images. This is accomplished fairlye"ciently by using a blink comparator tool suchas the one included with the Meade camera soft-ware. Two inverted images, similar to the one inFigure 3, are displayed on a computer screen oneat a time. The software will display one imagefor a short time before then displaying the other.This process will continue, cycling back and forthvery quickly, until the user stops it manually.Any di!erences in one image are very easy tosee during this process. If you are lucky the newbright spot may be a supernova, but it could alsobe that is is nothing more than a hot pixel, or adistant asteroid with a slow orbit. To be certainthat it really is a supernova the observer shouldhave multiple images of the target made within afew minutes of each other. Due to slight driftingbetween exposures, the stars recorded will not bein exactly the same position on the two di!erentimages but a hot pixel will. Asteroids will appearto move during the time between the exposurescausing them to appear in di!erent places rela-tive the the galaxy: a supernova will not do this.

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Figure 5: M51 and its companion NGC5195 appear in

this inverted image. It is a composite of 17,

5.7 second images for a total exposure time of

96.9 seconds.

This means that for every galaxy that youintend to observe you will need to capture mul-tiple images of it in the same night. So for eachweek the survey must capture at least twice asmany images as the target list contains. Suc-cessful supernova surveys conducted with a sin-gle telescope, such as one conducted by amateurastronomer Thiam-Guan Tan in Australia, of-ten image thousands of galaxies a week.[3] Tanaccomplished this by writing his own scriptingsoftware to automate his task.7 Although thiswould be ideal, my experience with equipmentin our budget range leads me to believe that au-tomation would not be helpful since it would beimpossible to achieve the level of pointing accu-racy required. Tan is using a Vixen Great PolarisDX (GPDX) mount, a mount that is famous forits precision Japanese build and low periodic er-ror, to carry his telescope.

The alternative to having one telescope imag-ing 200 galaxies a night is to have 20 telescopesimage 10 galaxies a night: this is the approachthat DASSS aims to take. And for this task Isee no reason why the equipment that I testedwouldn’t work given the following,

1. Target galaxies are all close together, per-

haps in the same constellation, in order toreduce slewing time between targets.

2. Targets are no fainter than 10.2 Magni-tude, which was the dimmest galaxy I wasable to record.8

3. Targets must be high in the sky in the di-rection away from the lights of DowntownDetroit.

4. Images of each target showing the sur-rounding star field to aid in the finding ofthe target must be acquired prior to theobserving session.

5. The sky must be absolutely clear andpreferably moonless. Imaging on nightsof average clarity proved to be a waste oftime.

The original goal of having a single userrecord 10 galaxies in an hour has proven to beunrealistic. Equatorial mounting will likely re-duce the required observing time by a substan-tial amount but reducing the time needed downto even 2 hours would require more accurate,and more expensive mounts. The problem withthis is that a Vixen GPDX or equivalent Germanequatorial mount (GEM) would increase the re-quired budget for DASSS by several thousanddollars per telescope. Also, a GEM style mountis much harder for the inexperienced telescopeuser to setup. They are also more demand-ing of accurate alignment than an ALT/AZ forkstyle. Although they give better performancewhen properly polar aligned they are less forgiv-ing of inaccurate alignments, meaning that quicksetups are unsuitable for astrophotography. In alocation such as Detroit, where the weather dic-tates how long your observing sessions can be, afast set up time is essential. But if the user isexperienced in the operation of the telescope, aGEM mount will do two things: it will reducethe time it takes to find faint targets because ofthe higher precision pointing ability; and longer

7Tan’s autoscripts are available for download at www.skyandtelescope.com8This limiting magnitude may not apply in suburban locations.

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exposure times will be possible due to a moreaccurate polar alignment.

Despite the inevitable shortcomings of a lowbudget telescope, it should still be possible toemploy these scopes in a search that could provesuccessful. In order to maximize my chances ofrecording the faintest detail in each image, I re-stricted my test targets to those in or aroundthe constellation Ursa Major because it was ina prime location at the time of night that I wasobserving. In Ursa Major alone, there are 19galaxies brighter than the 10.2 limit that I sug-gested. Throw in a couple bright ones from the

surrounding area and one could easily have 25 ormore galaxies to image in just one part of the sky.If the remote observing sites are located at in-tervals located radially outward from DowntownDetroit then they could all be observing the sec-tion of sky that is opposite to Detroit from theirlocation. Assuming that each site has the samemagnitude restriction of 10.2, there should beenough targets available for each site to recordat least 10 galaxies per night.9 If each site doesits part and captures 10 targets per night we willhave the necessary number of images to succeed.

Figure 6: Also noteworthy are these images of 7.5 mag. M63 (25s - 25 stacks) on the left and 7th mag. M81 (85.5s - 15 stacks)

on the right. Taken under the same poor conditions, they are an example of what you can expect to capture on a

fairly humid summer night. Dimmer targets were undetectable under these conditions.

References

[1] Blaine Korcel, Polar Alignment using the Star Drift Method.http://www.Astronomy.net/articles/16/ , 1985.

[2] LX200 Instruction Manual. http://www.meade.com/manuals/lx200/index.html

[3] Thiam-Guan Tan, Searching for Supernovae on a Shoestring. Sky & Telescope Magazine, July2009.

9It is likely that sites located farther from Detroit would be able to record fainter galaxies, resulting in thepossibility of having more potential targets.

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