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Amateur Observations of
LCROSS Impact: October 9, 2009
See http://www.nasa.gov/lcross for time updates.
Lunar CRater Observation and Sensing Satellite (LCROSS)

OBJECTIVE:

The objective of this page is to provide the casual backyard observer useful information for observing the LCROSS impact event without having to go to too many other sites. However, you can get a much more in depth idea of what more serious observers are doing and learn a lot by going to the Observations tab on the main LCROSS page : http://lcross.arc.nasa.gov/observation.htm, and following the trails from here: http://groups.google.com/group/lcross_observation and here:
http://groups.google.com/group/lcross_observation/web/lcross-faq

The 'Tech Info' pages of the main LCROSS web page have great information on variables effecting what we will be able to observe, impact speed, expected crater size, impact location, etc., see http://lcross.arc.nasa.gov/rationale.htm. I would like to make sure the 'backyard observer' has an appropriate expectation of what they may actually be able to see.

To re-iterate, impact event visibility variables include surface topography (soft dust or hard rock) both at surface and 1 to 2 meters down at the location of impact, impact angle (we are controlling fairly well), velocity and mass (both pretty much fixed). Key elements for the backyard observer are how high will the visible ejecta plume go, and how bright will it be. What we can see will be some intersection of these two elements in the visible spectrum. Large aperture earth and orbiting instruments will be able to observe/resolve to very low altitudes where there will be much greater plume/dust mass/brightness (we can't resolve well to that low level with most backyard scopes but if sunlight is low enough, may be able to see a bright spot), and very dim high altitude spectra of the ejecta (we won't be able to see the low brightness parts of this very diffuse vapor cloud).

OBSERVATION:

The two most predominant questions then, are: Where do I look, and what will I see?

Q: Where do I look? : Now that the target area is pretty well defined (Crater Cabeus proper), we can zoom in on it. The Observations page on this website has good links showing you where specifically to look. More detailed info on this and clearer pictures are also starting to show up on various sites. Here is a quick summary approach:

  1. Start with the south pole at the sun light/shadow terminator (yes, the south pole on the moon is the same as on earth, so you'll be looking at the bottom edge). This is about what the moon (phase) will look like on the morning of Oct. 9:
  2. moon showing phase on 10/9

    Like shown on our site here: http://lcross.arc.nasa.gov/phase.htm

  3. Depending on your telescope, zoom in to the target area like these pics show:

  4. zoom variations zoom variation

    larger

    large & labeled

  5. Identify the Cabeus craters. Target is in Cabeus proper, slightly below A & B in picture above. Depending on how close you want to look, there is more good info here: http://ltvt.wikispaces.com/LCROSS+Impact

    Q: O.K., now I know about where to look, What will I be able to see?

    First: From LCROSS - Citizen Science Portal http://apps.nasa.gov/lcross/blog (Rick Baldridge Sept. 11, 2009) and LCROSS Science Team re a Cabeus A impact site:

    “For the amateur observing campaign, this crater is highly visible to Earth, but does provide some challenges. Now that the target crater is known, detail will be focused on precise viewing conditions and geometry. The plume will not extend above the lunar limb, and for the most part will not be situated against a dark background such as shadowed regions between craters. However, that does not mean the plume will not be visible. The initial phases of plume formation may be visible against the foreshortened darkened crater floor. Video and photographic observations must now focus on bringing out the brightening caused by the eject plume in front of a lit lunar surface.“

    We have since re-targeted a spot in Cabeus proper, and the plume background as seen from earth may now be at least partially in the shadow of the crater wall, which is real good. Things change.

    All of the following is based on an assumed earth visible ejecta cloud rising to 6Km above the lunar surface and crater wall. Latest estimates of the Cabeus proper crater impact site indicate the first two or three kilometers of that plume height (the brightest parts) may not be viewable from earth, but that the plume will hopefully have crater wall shadow behind it to help us see it. Impact design location is to maximize the amount of this in sunlight, but variables here will determine how much of it is actually illuminated, and it may be that only the high power instruments will see good contrast. But we don't know for sure.

    Ref: Moon = 3476 km (dia.) = approximately 0.52 deg viewable from earth = 31 arc min. = 1860 arc seconds (scale ~1.86 km/arcsec., or 3.2 arc sec. for a 6km event size). Or, using trig, at 385k Km away from earth the 6Km impact event will be approximately 0.000893 deg. or, 3.21 arc sec. So, these two estimate techniques give close results. 6km is only about 2 tenths of 1 percent of the moon's dia. (for reference, pretty small, but if you slice the moons diameter up into 100 strips, you can get the idea).

    The brightness or magnitude of the event is of course dependant of what part you are talking about, but we estimate magnitude 6-9 for the best visible part and time. Depending on where we actually hit, there should be an approximate sun mask of 1 to 3 km., meaning the impact plume needs to rise up that far before the sunlight hits and illuminates it. Note, brightness falls off very quickly above that altitude, and the event will be V shaped. For the following examples, the big blue circle represents your field of view given a few telescope & eyepiece combinations. The smaller red circle highlights the Cabeus target. So, if you have (& a good calibration method which you can do anytime) an eyepiece that comes close to being filled up with a full moon, this is what the moon should look like:

    telescope view using 2010mm focal lengthTelescope view using a 2010mm focal length telescope (ex:16-inch f/5.6),
    13mm eyepiece with 82 deg. field of view
    (10-inch f/8 with 12mm, 80 deg. fov eyepiece would be similar)
    Moon Photo Credits: SkyTrip.de-Die Astronomie-Seiten von Mario Weigand



    telescope view using a 1625 focal length Telescope view using a 1625mm focal length telescope (ex:8-inch f/8),
    8mm eyepiece with 50 deg. field of view


    telescope view using a 1420 mm focal length Telescope view using a 1420mm focal length telescope (ex:10-inch f/5.6),
    5mm eyepiece with 50 deg. field of view


    same This is the same view as in previous example,
    just blown up so eyepiece field fills the image.


    Telescope view using a 2275mm focal lengthTelescope view using a 2275mm focal length telescope (ex:16-inch f/5.6),
    6mm eyepiece with 50 deg. field of view


    Telescope view using 2500mm focal length Telescope view using a 2500mm focal length telescope (ex:20-inch f/4.3),
    3mm eyepiece with 50 deg. field of view\


    enlargement of impact location

    You can try to enlarge the above photos to get a better idea on what you might see.
    If you can't do that, this picture is an enlargement of the target area with an example of what the event might look like. It is enlarged slightly from the field of view in the previous example. Note, the red identifying circle is the same size in all of these pictures--ie; it is enlarged the same amount as the moon.

    Note: The above rendition is meant to only approximate location and size. Location should be close, but actual location, size and brightness can only be estimated.

Here are some calculated estimations of event size for some perspective:

  • The following table is a sample listing of what to expect to see using different telescope/eyepiece combinations. See a larger table with user input here [scopeviews2.pdf and scopeviews2.xls ].
  • Focal length, if you don't know, it is = primary optic size x speed or f/ ratio of the scope.
  • If you use a correcting lens or a 'doubler' in your eyepiece holder, apply that factor to your telescope focal length. If you have a doubler, and it does not make things too fuzzy, it is recommended to use it here as it will greatly increase the odds of you being able to see the event.
  • For combinations not listed, you can ratio fairly easily. - This table assumes an event altitude of 6 kilometers or 0.000893 deg. = 3.21 arc seconds
  • Reference:      Magnification= scope focal length / eyepiece focal length.
  • Reference:      Real FOV in degrees = apparent FOV / mag
  • As you can see, no matter what size telescope you have, this event will be a very small portion of the field you will be looking at. Other than higher magnification, the main advantage of a larger telescope is the greater light gathering capability. So while small, the event has a better chance of being observed in a larger telescope because the larger telescope has a better ability to see low brightness objects.
  • Telescope Size
    (in.)
    Optical Focal length
    Eyepiece focal length (mm)
    Mag (ref)
    Eyepiece Apparent Field of View
    Event size as a % of viewing field
    6
    1219.20
    12
    101.6
    82
    0.11%
    6
    853.44
    6
    142.2
    50
    0.25%
    8
    1625.60
    12
    135.5
    82
    0.15%
    8
    1625.60
    6
    270.9
    50
    0.48 %
    10
    2032.00
    12
    169.3
    80

    0.19%

    10
    1422.40
    5
    284.5
    50
    0.51%
    10
    1092.20
    5
    218.4
    50
    0.39%
    12
    2438.40
    8
    304.8
    52
    0.52%
    12
    1706.88
    6
    284.5
    52
    0.49%
    12
    1962.91
    6
    32702
    52
    0.56%
    14
    1991.36
    6
    331.9
    50
    0.59%
    14
    2290.06
    6
    381.7
    50
    0.68%
    16
    3251.20
    8
    406.4
    52
    0.70%
    16
    2275.84
    6
    379.3
    50
    0.68%
    16
    1747.52
    5
    349.5
    50
    0.62%
    18
    2560.32
    6
    426.7
    50
    0.76%
    20
    2844.80
    6
    474.1
    50
    0.85%
    22
    2402.84
    4
    600.7
    50
    1.07%
    30
    5334.00
    4
    1333.5
    50
    2.38%

The following gives some additional perspective on event size in relation to some familiar objects:

One arcsecond is the width of a dime as seen from 2 kilometers or 1 1/4 miles away (o.k., that sounds pretty small, and also maybe is not so familiar, but we are using telescopes) Inner planetary size is a function of where the planet is with respect to the earth when we see it, so these are rough estimates, but outer planets, due to the greater distances won't change much in visual size.
Rough optical telescope atmospheric seeing limit = 0.6 arcseconds
Angular separation of the gravitationally lensed quasar 0957+561 = 8 arcseconds (o.k., that one's a little tough for most of us to find).
Ring nebula: Magnitude 8.8, Apparent Size 1.4' x 1.0' (84 x 60 arcsec)
M40: Double Star in Ursa Major, (magnitude 8.4), Separation 0.8' = 48 arcsec.
Mercury: Apparent size: 5.00 arcseconds
Mars data: Apparent size: 4.23 arcseconds
Jupiter data: Apparent size: 34-36 arcseconds
Angular diameter of Europa = 0.8 arcseconds
Saturn data: Apparent size: 19.74 arcseconds
Neptune data: Apparent size: 2.21 arcseconds
Crab Nebula: Rough radius = 150 arcseconds

So, as a ruler, this event will be 1/4 the size of Saturn, twice the size of Neptune (if you have observed it before), and roughly the size of Mars this October. Suggest looking at Mars (depending on your horizon, it will rise at approximately 1:00a.m. on October 9 in the constellation Gemini and not far behind the moon) to calibrate your size expectations.

Q: Where do I look?

A: South pole. For most small to medium telescopes, this is where you need to point. If you can magnify sufficiently to zero in on the general crater area, then go to this site now for a good description of where to look and note again, wait for the project to define the exact impact location: http://groups.google.com/group/lcross_observation/web/lcross-faq. This link also offers additional answers to often asked questions by folks very knowledgeable in moon observations.

Q: I live in Texas. Will I be able to see this event?

A: Yes, but it will be just starting to get a bit light out. Impact time for your time zone is 6:30 a.m., and I think Sunrise at about 07:30 by October. This will still be reasonably dark/early dawn which complicates viewing of low light events a bit but it still should be possible if you can keep your eyes dark adapted as best you can.

East of the Mississippi will be more of a problem due to the rising sun making it very light out. Viewing through the telescope however, you should still see reasonably dark background behind the moon, so it still may be o.k. East coast viewing is even more difficult because the sun will be up. When it's this light out, 'dark adapting' your eyes is very difficult when trying to see something of this low brightness.

Remember:

  • This event only lasts a couple minutes. Timing is key. Be ready. Know where to look and give your eyes some time to 'dark adapt' prior to impact time.

  • If you are watching through a telescope, you've only got one eyepiece. If there are a number of friends & neighbors out viewing using just the one telescope, it will be tough to share 2 minutes between 5-10 people. Suggest also having the website up or going to an event that can project the impact event for many people to view. Star parties are great because of the large number of scopes out, and you'll easily 'hear' when the folks with the bigger scopes see the event, but the same people-to-eyepiece ratio can exist & may be even worse.

  • This event is short (20-120 sec. for best part), low (2-10 km height) and dim (magnitude 6-11).

  • This event will be relatively dim and hard to see, particularly in relation to the very bright moon that you will be looking at and whose bright terminator is so very close. Do your general moon crater viewing in advance and then move your eyepiece view to mostly darkness on the south pole & let your eyes adapt for a couple minutes prior to impact. Keep the very bright sunlit surface of the moon out of the field of view as much as possible -- you will get a much better experience of this low brightness event.

TELESCOPES/OPTICS:

Q: What kind of telescope should I buy to see this?

A: First, unless you plan to do future astro viewing (something I fully recommend), I don't suggest you spend the money on a telescope to view this 2 minute event. Would hate to have you spend a fair amount of money & then just put the scope in the closet for the next 5 years & then sell it.

Second -- If you pass this first checkpoint, the best view for the buck may be a Dobsonian telescope of the 10-14 inch variety. While this event will still be very small and hard to see with this telescope, it can serve you later very well for a large amount of celestial viewing objects. There are numerous manufacturers of these & you should shop around for a good deal. You'll want eyepieces in the 12-18mm range for general viewing of the heavens, but because of the moon's brightness and proximity/size, you'll want eyepieces in the 4-8mm range for this event. These smaller focal length eyepieces magnify more, and are also good for viewing Mars, Jupiter and Saturn. Telescope purchasing assistance is beyond the ability of this site, so we recommend you do some research and talk with retailers and go to some start parties and ask the folks at them -- they are typically very willing to share much info on their telescope experiences and can tailor responses for your expected future needs. They are a great information resource.

Q: Can I observe this event using binoculars?

A: No. In general you'll need at least 200x magnification, and maybe only possible with 300x & above. More importantly, you will need the greater light collecting capability of a large aperture optic. Even some of the larger binoculars (25x by 125mm) still won't do the job. Zoom binoculars (such as 10x-40x by 80mm for example) at maximum zoom (2mm exit pupil in this example) are great for very bright terrestrial viewing of birds and hangliders during bright daylight viewing, but are not useful at that magnification for low light viewing which is required here.

Q: I have an old 6-inch Newtonian with a reasonably good mirror. What are my chances?

A: Not great.

Q: I have an 80mm refractor I bought for my son some time ago. What are my chances?

A: Refractors of that size are great for clear and sharp viewing of wide stellar fields, but this size does not have much magnification capability which is needed for viewing such a small event as this.

Q: I have an 8-inch Newtonian, with pretty old optics. Assuming the plume is bright enough, will I be able to resolve this event using my optics?

A: Good question. For those who don't know, but suspect, every optical set has a limit on what it can actually resolve ie; the ability to separate two distinct items. In general, because of good optics in even 100 year old telescopes, only very small telescopes would not be able to resolve this event size. That doesn't mean a two-inch telescope will see this (see table above), just means that the optics have the ability to resolve something of this size given all other factors working well.

IMPACT:

Q: How fast will you be going when you hit?

A: Approximately 2.5 km/sec (or 2500 meters/sec) = 5592 miles/hr., = 8202 ft./sec. See the mission rationale page http://lcross.arc.nasa.gov/rationale.htm for the answer to this and similar impact related questions.

Q: Viewing through a good size telescope (20"), will I be able to see the impact itself, or the 'flash' created by it?

A: Most probably, No, but with a caveat. The impact is designed to occur on a permanently shadowed crater wall (facing away from the sun and generally us as well), and is not expected to create enough of a 'flash' as to be seen on opposite crater walls (which may or may not be sun illuminated at the time). Seeing the flash from earth however is more likely than seeing the impact itself (possibly viewed only by the LCROSS shepherding spacecraft). Targeting accuracy however may not be sufficient for a guarantee of this type of a hit, so the possibility (we'll say low probability) does exist for a viewable impact or flash if the centaur hits at a spot that is directly viewable from the earth.

Q: How high will the impact plume actually go?

A: Depends a bit on what we consider the 'plume' to be. For us, we're interested in visible light spectrum, and the following gives an idea of mass vs. altitude & time. Best visible brightness for small scopes on earth is estimated somewhere in the lower altitudes at the 30 to 90 second event time. We're guessing 2-6 Km altitude for the brightest part of the event which is what amateur observers might expect to be able to see. The 'plume' will be bright at first as soon as it rises into sunlight (estimating magnitude 7 to 9), but will fade in brightness quickly as it grows in altitude.

ejecta curtain characteristics - 1% water content

IMAGING:

Q: What's the best way to image the impact event?

A: A digital astro camera mounted to a reasonably large telescope. These are not SLRs. Folks with these setups have a good idea already on how best to image this event. See links previously noted.

Q: Can I use my digital camera to capture the impact?

A: It may be possible, but it won't be easy -- Use focal plane or eyepiece projection through a sturdy, tracking mount. Biggest problem after setting this up may be focus and timing. Practice with your setup ahead of time if possible. This may only work with multiple pics 'stacked' together. Main problem with stacking a number of pics is you must take a series of them which (unless you have multiple optics to use) will take up all your viewing time, and you may miss the whole thing. Also, watching the event through a camera viewfinder may limit what your eyes can see, and won't be possible when the shutter is actually open. It's a choice you have to make, as you don't have much time to both image and view with your eyes.

Q: What kind of exposures should I use?

A: Again, 35mm or digital SLR pics are not likely to succeed with this dim & quick event. Generally, you'll want to use the highest ISO your camera can handle without producing too much noise. Use dark frame subtraction (post session if possible or you'll loose half your viewing time), and a number of exposures -- preferably stacked in some fashion. While a good exposure of the full moons bright surface might be F/8 at 1/250th sec at ISO100, this will be much, much dimmer than that -- on the order of many stops. In fact, when imaging a magnitude 8 galaxy, exposures of many minutes to an hour are often used.

Main problem here is a low light, moving object and 1 minute of available exposure time -- very hard to image without specialized equipment (imagine/remember trying to get that picture of the bear moving away from you far away in the woods, in the shadows at dusk, and you were hand holding the camera. How did it come out? Kinda like that, but further away, smaller and dimmer). Experiment now if you don't already have a good idea.

Q: Can I get pics of this event using my digital camera 'piggybacked' on a telescope with tracking?

A: Generally, no. The event would be about the size of a pin head on a kitchen table. You'd need a very large (huge) focal length lens (a lens combination with 3000mm on a 35mm film camera only gets you about 60x magnification. Digital cameras with the smaller chips get you an effective boost of about 1.5 times this for a given lens, but you're still barely getting to 100x mag) to get the needed magnification. Advantage though is you can blow up a good resolution/high pixel digital photo & maybe get a good result with some software manipulation. If you do try, make sure the tracking is good and your scope is used to carrying the camera and large lens weight so the tracking stays on track. Eye-piece or direct projection would be the preferred method when using a normal digital camera. Short focal length 'reflector' lenses, while small and light, are not recommended. Better yet are the digital astro-cameras with a huge lens -- a different web site.

Q: I have been using an astro video camera attached to my scope to obtain many images of planets and then stacking them for a final image. Can I use this technique for imaging the LCROSS impact.

A: Possible, and with the right camera and setup, this may in fact be the best way to both capture this event as well as potentially show it to many people at once. Not sure on this one. Biggest unknown here is can the individual cam frames capture the low brightness of this impact event. Will likely require a camera that is very good at low light -- not all are -- even ones used for planet imaging. Best is to test on low light objects and set up viewing session accordingly. Also suggest (if not already familiar) making sure you use it/practice with the eyepiece you'll be using and making sure the scope tracking will not be a problem -- you've likely already made yourself familiar with these variables. If the event is bright enough, suggest capturing & stacking 10-40 images in sets and making a series of them to create a semi-video sequence. Good luck -- am curious to see how well it works.

Q: What about just a time lapse digital photo taken through a scope?

A: A time exposure (5 to 30 seconds for ex) through a good size scope may make an interesting pic or series and will capture/integrate more of the low light event. You also won't loose any or as much time as you would between many individual shorter frames. Again, make sure the tracking is dead on, and you have virtually none of the sun-lit moon's surface in the shot (very edge only) less you blow out the image you are trying to capture. You'll likely end up with a blur if it works, but it will be something good to see, especially if surrounding terrain also comes through and is reasonably sharp.

OTHER:

Q: Can I view LCROSS while it is in flight from now until impact?

A: Yes! See the Observations page http://lcross.arc.nasa.gov/observation.htm and you can follow what other folks have already been doing in tracking and imaging LCROSS in flight. Additional information on ephemerides and tracking info is also available on this page. These observers have been using Google groups: http://groups.google.com/group/lcross_observation and you can pick up a great amount of info if you explore what they have been talking about.

Q: Will the LCROSS impact damage or knock the moon out of it's orbit?

A: No. See the Comparing Crater Size page http://lcross.arc.nasa.gov/CraterSizes.htm on the Education tab of the main web page for answers to these questions. This page has a very good description of relative crater sizing & history of natural impacts on the moon. You can compare our 'hit' with those to get a good feel for what this event will do.

Q: Will you be contaminating the moon?

A: See the answer to this and other similar great questions on the Frequently Asked Questions page: http://lcross.arc.nasa.gov/faq.htm.

Q: What about unused propellant in the Centaur upper stage -- won't this compromise the science observed?

A: Great question, and an issue we have planned for from the beginning. Again, see the faq page for an answer to this question. Mission duration is designed to allow the Centaur to 'vent' off all of any unused propellant. The Centaur fill valves are not designed to be totally leak proof as this upper stage on normal missions is jettisoned before or soon after orbit insertion of its payload. We are actually monitoring this venting by watching the 'protuberance torque' which we have to counteract to stay solar array sun pointed. When we no longer have to fight such torques, we feel reasonably good that the Centaur has vented all its unused propellant. Operationally, we have also designed maneuvers to roll Lcross and Centaur so that the sun can 'cook off' water captured in the insulating foam on the Centaur. We've done a couple of these 'bar-b-que' sessions already, and can actually detect the effect by observing the spacecraft attitude when we roll the other side into the sun. A little more on this on the Water Detection page: http://lcross.arc.nasa.gov/water.htm.

Last update: 10/06/09

 

Visit the NASA Mission Site @ http://www.nasa.gov/LCROSS
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Editor: Brian Day
NASA Official: Daniel Andrews
Last Updated: October 6, 2009