Best case scenario (as of 9/03/09)

[Adam Block]

Hi all,

I am trying to better understand what, under reasonable conditions, might be observed with respect to the plume. I would like an idea of its brightness, duration, and expected angular height above the limb of the moon.

It is my understanding that the best case scenarios for these values would make Earthbound observations very difficult. My understanding is

Integrated Magnitude: 8th at best, but perhaps 12th- 13th
Duration:   30 seconds to 120 seconds
Height in arcseconds: less than 5?! Perhaps just 1 or 2?

My understanding is that this mission's specifications are biased (understandably) towards the orbiting platform's observations and not the Earth. And in addition, the recent spacecraft corrections (fuel usage) may further limit favorable outcomes of observing much from Earth. One of the things I have not seen addressed is that the material thrown up must rise above the crater walls first and if a deep crater is chosen, do we still see anything?

Are recent quantitative values for the above published anywhere?

Thanks in advance for the information!
Adam

[Adam Block]

Please respond to abl...@as.arizona.edu .
For some reason...I do not see this posted to the google groups.
Adam

[cano...@yahoo.com]

On Sep 3, 12:59 am, Adam Block <ngc1...@gmail.com> wrote:
<snip>

> I am trying to better understand what, under reasonable conditions, might be
> observed with respect to the plume. I would like an idea of its brightness,
> duration, and expected angular height above the limb of the moon.

> . . . . My understanding is

> Integrated Magnitude: 8th at best, but perhaps 12th- 13th
> Duration: 30 seconds to 120 seconds
> Height in arcseconds: less than 5?! Perhaps just 1 or 2?

<snip> Are recent quantitative values for the above published
anywhere? <snip>

> One of the things I have not seen addressed is that the material thrown up must rise above the

> crater walls first and if a deep crater is chosen, do we still see anything? <snip>

I. Curtain brightness

LCROSS Team. 2009. Average and Edge Brightness of Ejecta Curtain
(Figure). LCROSS Observation Campaign website. url:
http://lcross.arc.nasa.gov/impact.htm and http://lcross.arc.nasa.gov/observation.htm
Image: impactexpectations1.gif on second url or on first url (last
accessed 26 Aug. 2009)

I suggest downloading impactexpectations1.gif to disk and then use a
paint program to blow it up. The scale is such that the average
curtain brightness between 40 to 60 secs reads about 3.0 mpsas. Even
then the figure has poor resolution.

In the last day or so the official LCROSS Citizen Science site has
just posted imaging recommendations on their website.

http://apps.nasa.gov/lcross/about/

They quote without citation an apparent brightness figure of 4 to 6
mpsas.

The ejecta curtain is an extended object - like a galaxy. The surface
brightness of such objects is expressed in magnitudes per square
arcsec (mpsas) because telescopes translate the apparent brightness of
point objects like stars differently than for extended objects, an
hence are quoted in integrated magnitudes. It is possible to
mathematically translate mpsas into an integrated magnitude. That is
the way that the apparent brightness of extended objects like galaxies
are printed in amatuer catalogues.

The mathematical conversion of 3 mpsas or 4 to 6 mpsas into integrated
magnitudes is not an 8th magnitude star.

The LCROSS Citizen Science About page suggests calibrating your
exposure time for the ejecta cloud using 8 to 9th magnitude stars:
"8th to 9th magnitude stars can just begin to be imaged with a
particular telescope/camera and exposure combination . . ." The About
page notes this "will of course over-expose the lunar limb
features."

I am uninformed as to the physics behind the LCROSS Team's 8th or 9th
magnitude recommendation.

A naked-eye magnitude 6 magnitude star equates to 20.8 mpsas; a
magnitude 5 naked-eye star equates to 19.3 mpsas.

By calibrating to an 8th magnitude star, I assume the LCROSS team is
trying to assure that your camera will have sufficient gain to pick up
any apparent brightness of the ejecta curtain with an adequate margin
of safety.

Published studies of the average apparent brightness of lit features
on the sunlit side of the terminator are known to have an apparent
brightness between 4 and 6 mpsas. The average surface brightness of
the sunlit part of the 71% illuminated Moon (65° phase angle) has been
measured to be about 3.8 mpsas. So, the cloud will probably not be
visible if as it rises, the line-of-site background is any brightly
lit feature. The brightness of the sky above on the sunlit side of
the terminator within a few degrees of the pole is predicted by
modeling at about 15mpsas. The dark limb "earthshine" apparent
brightness is between 12mpsas and 17 mpsas. If the ejecta cloud rises
off the crater floor and the background is the night sky above the
limb or the dark limb itself, the curtain may have enough contrast to
be visible.

To my knowledge, there is no data on the apparent brightness of the
dark shadowed portion of a crater on the sunlit side of the
terminator. If Caebus A or B are the final selected target that will
be the observing condition for the impact.

Mars (3.9 mpsas), Jupiter (5.6 mpsas) and Saturn (6.9 mpsas) have all
been imaged at or near the point of occultation by the bright limb of
the Moon.

Doing some quick internet search, here are some amateur images of a
crescent Moon occulting Jupiter:

In 1990
http://www.icstars.com/HTML/JupiterMoon/MoonJupiter.html
In 2004
http://www.astro.umd.edu/openhouse/gallery/planets/occultations/09nov04.html

From what is currently known, if the curtain rises against a
background of the night sky and perhaps against the background of the
dark shadowed portion of a crater on the sunlit side of the
terminator, it should be visible.

But like all uncertain one-shot events, modelling and experience-based
guess work all has its limits. All that can be done is to take an
attitude of cautious optimism and see how the LCROSS Team's experiment
runs out.

II. Curtain top diameter

With respect to curtain size, the LCROSS Citizen Science About page
quotes an ejecta curtain size of: "We expect the plume to extend
approximately 10 km in height (mass weighted maximum vertical extent)
and extending about 30 km across by the time it becomes too diffuse to
observe. At its maximum predicted extent, the plume will be about 1/3
the apparent size of Jupiter as viewed through a telescope."

Recently released finder maps from New Mexico State University include
arcsec scales for the south polar region:
http://astronomy.nmsu.edu/rthamilt/LCROSS/media.shtml

See the "medium image" at url:
http://astronomy.nmsu.edu/rthamilt/LCROSS/media/NMSU_LCROSS_medium.png

The "Impact" page cited above and the "About" page talk about the top
of the plume being about 10km in diameter. The image scale on the
NMSU finder is 1.78km/per arcsec. So, the top of the plume would be
about 17.8 arcsecs across. Newton E, which is marked on the NMSU
image, has a catalogue listed diameter of 17km. Note the 20 arcsec
scale line on the NMSU image and the catalogue size of Newton E do not
match. I'll leave you to make your own conclusions about the
approximate visual size of a 10km curtain.

The NMSU chart is a fine image and is sufficient to get a rough idea
about how big 10km and 30km will appear in the eyepiece or camera on
the morning of the October 9 impact.

Malapert E, also appears on the NMSU finder, but is not marked, and
also has a catalogue diameter of 17km.

III. Curtain height

The LCROSS team's ejecta curtain model suggests a 5km tall by 10km top
diameter curtain. As noted above, the LCROSS Citizen Science page
also suggests a more robust height scenario, i.e. - "extending about
30 km across by the time it becomes too diffuse to observe." As the
curtain gets bigger, the curtain gets dimmer.

The LCROSS team's crater targeting selection criteria includes that
the ejecta curtain (min. 5km high with a 10km top diameter) must
extend above a 2km deep crater wall so sunlight can reach it. That
leaves 3km of ejecta curtain illuminated by sunlight. See LCROSS Team
"LCROSS South Pole Candidate Impact Targets" dated July 10, 2009 (file
LCROSS-SP-Targets-071009.ppt) at url:

http://01227941410742638900-a-g.googlegroups.com/web/LCROSS-SP-Targets-071009.ppt?gda=LT2WLU4AAACwPjh7SItssvxptLN2TxYMYp4Tc1hvx997dD42_g9XUEQpOLOI--TVBAsoswPtQESo0fYDf7Z7pQIgx6_mWRYJ47Cl1bPl-23V2XOW7kn5sQ

http://tinyurl.com/lpl4d7

I'll leave it to you to compare that information to the NMSU image
with the arcsec scale lines and to make your own judgment about
roughly how tall and big the curtain will appear in the eyepiece or
camera.

Hope the above provides more help than confusion. Further background
citations can be provided if needed.

Clear Skies - Kurt

[Adam Block]

Thank you very much for the detailed reply. I am also happy to see my post show up (24 hours later!).

Here are my follow-up comments:

1. Curtain Brightness

 I understand the idea of mpsas- however I wonder if for the sake of approximation for small apparent sizes of the curtain (say the first handful of seconds above the limb) that a stellar magnitude could very well be an appropriate measure. The curtain would only be arcseconds in size and for all intents a stellar-like object. Furthermore, I am guessing that the edge/limb brightening affect we observe by looking through a cone of material would only further make it more stellar-like in geometry since only the edges, two lines, would be most easily visble. I don't know why, but I feel like rebeling against the idea of this being a "diffuse" thing due to its small angular size on the sky as viewed from Earth. Regardless, I will try to detect the faintest stars I can near the limb of the moon and see what I get.

2. Curtain Diameter (size).

Cited below is 1/3 of Jupiter's angular extent (approximately 40"). This is around 14 arcseconds and the answer I was looking for. The problem I have is...a good source that has passed information to me that this value- even 5 arcseconds, may be too generous. If there is new/better information, even if it is not favorable for us Earthbound folk, I would really like to know it now! Of course if confidence is high for at least 5-10 arcseconds- then I am quite pleased!

I will be imaging at approximately 0.15" per pixel. A 10 arcsecond curtain that is reasonably bright should extend 15 pixels beyond the limb of the moon. If I halve this value due to the over-exposed moon limb on the detector eating into this value, then I end  up with 5-10 pixels. That is not much bigger than the capital letters at the beginning of these sentences... but it is observable I think.

3. Seeing

I have not seen much (any) discussion on the effects of seeing. However, I can venture to guess that most ground-based systems will be terribly oversampled when trying to observe this. If the seeing is poor, the light will be blurred across many pixels on the detector and read noise will dominate most people's view. I just wonder if there is a prefered plate scale (assuming the sky is dark enough, and the aperture large enough) for observing this event to avoid losses due to the specific detector being used.

Anyway, the most up-to-date value on #2 would be greatly appreciated.

Thanks,
Adam
(abl...@as.arizona.edu)

[Adam Block]

Hi again,

I just realized, I am mixing up curtain diameter and curtain height. So ignore my #2.
I suspect the curtain height is the critical value for Earthlings...

So, let me ask this, will any part of the curtain/plume extend beyond the limb of the moon as viewed from Earth? Below you state that the curtain should rise at least 3km above a crater. This is almost 6 arcseconds. But all of these craters are more than this distance from the limb. Is this not true on October 9th? 6 arcseconds is a tiny number...

Adam

[Chas Miller]

Hi Adam,

Glad to see you are taking an interest in LCROSS observations. I just
wanted to point out that given the factor of 1.78 km/arcsec at time of
impact, to convert an estimated plume height in km to arseconds, one
should divide by 1.78. So, for an estimated plume height of 10 km, this
would translate to 10/1.78 or 5.6 arcseconds. A plume only 3 km high will
be 1.7 arcseconds (tiny!).

Given a diameter of 17 km for crater Newton E, the angular diameter will
be 17/1.78 or 9.6 arcseconds. That is about half the 20" ruler in the
images we posted (the scale was drawn in Powerpoint - not precisely
accurate, but should be fairly close).

Hope this helps and good luck on your observations.

Chas Miller

[cano...@yahoo.com]

I get the same thing as Chase - 5.6 x 1.7 arcsecs for a 10 x 3 curtain
- essentially a linear object.

For an object located at 0E,0S on the Moon or for the longitude
dimension of an object located at the north or south lunar poles:

1 arcsec = ( 1/ 206265 ) * Moon_dist in kilometers

Mutiplying by 10km for the object size gives the object size in
arcsecs. This is the small angle approximation formula.

On Oct. 9 11:30UT, the JPL ephemeris gives a distance from the lunar
center from my o.p. of 367284 km.

Omitted are adjustments for foreshortening.

If the cloud declines in visibility until it is not visible at 30km
(see discussion in the LCROSS Citizen Scientist About page), then the
max scenario is let's say 20km x 10km or 11.3 arcsecs x 5.6 arcsecs.

I prepared an image last month that pasted scaled verisions of those
physical sizes based on the longitudinal diameter of Malapert E (17km
dia) on to south polar image:

http://members.csolutions.net/fisherka/astronote/observed/LCROSS/20090811_0937UT/20090811_0937UT_OPW111d52mN40d46mKAF0_SP_Illustrated.jpg

http://tinyurl.com/prdaku

If visiblity ends at a height of 30km, then the max scenario cloud
height is 16.8 arcsecs.

Clear Skies - Kurt

> >>http://www.astro.umd.edu/openhouse/gallery/planets/occultations/09nov...

> ...
>
> read more »- Hide quoted text -
>
> - Show quoted text -

[cano...@yahoo.com]

On Sep 3, 10:59 pm, Adam Block <ngc1...@gmail.com> wrote:
<snip>

> 3. Seeing
> I have not seen much (any) discussion on the effects of seeing. However, I
> can venture to guess that most ground-based systems will be terribly
> oversampled when trying to observe this. If the seeing is poor, the light
> will be blurred across many pixels on the detector and read noise will
> dominate most people's view. I just wonder if there is a prefered plate
> scale (assuming the sky is dark enough, and the aperture large enough) for
> observing this event to avoid losses due to the specific detector being

> used. <snip>

Re: 3. Seeing

On Oct 9 11:30UT at my o.p. (in SLC at 42N) the Moon's altitude will
be about 70 degs or about 65 degs are your o.p.

In practice observing under similar altitude and lighting conditions
(and a similar time during the early morning hours - 5:30am local
time) in early August, seeing were fairly normal for lunar observing
- at the time analogous to impact.

However, with retreating astronomical twilight - about 15-20 minutes
after that - it was like someone flipped a switch. Seeing went to -0
on the Pickering scale. The entire south pole image was smeared
continously.

This was in August during summer conditions and big gradients between
night and day temperatures. I suspect conditions in fall will be
better in terms of a safety margin between the impact time and when
morning atmospheric disruption starts.

However, if the LCROSS Team slips the impact time forward to better
favor Hawaii, Keck and the IRTF, that will close my observing window
at 111W longitude.

Re: Detector size and sampling

I'm still thinking about the image scaling recommendations to prevent
oversampling on the LCROSS Citizen Science "About" page, e.g. an efl
of 4000mm for "big chip" DSLRs and 2000mm for "small chip" ccds.

Clear Skies - Kurt

[Adam Block]

Ah... yes. Went the wrong way.
Indeed, this is a very small value. This is looking more and more like Lunar Prospector type event.... :( I think these kinds of values (the small end of ultimate plume height) are more strongly affected by the local topography and moon phase (shadows of surrounding area).

Again, it appears to me based on these values and potential target craters, the plume may not get to the sky. This adds a bit of complexity to me, because I might have otherwise chosen to set my detector to saturate the apparent moon limb (and still detect recommended star magnitudes). However, if the event is INTERIOR to this apparent limb on my detector... then my gain is determined NOT by stars I can detect... but instead by non-saturated detector values for this interior crater. This is potentially a different answer in my mind.

Ultimately I guess the final crater choice will really put some limits on some of these values. It is my understanding this decision will be reached on Tuesday or Wednesday?

I do hope this conversation is helpful and not a regurgitation of material. (if it is, I will cease and desist)

Adam

[cano...@yahoo.com]

> Ultimately I guess the final crater choice will really put some limits on
> some of these values. It is my understanding this decision will be reached
> on Tuesday or Wednesday?

That's the remaining constraint question to be answered on the Sept 9
or 10 announcement of the final target. Will the final selected
target be:

1) On the dark floor of an off limb crater like Cabeus A or B with the
background being the dark floor only?

2) On or just inside the dark side of the terminator with the cloud
rising into sunlight with a background of the dark night sky above the
lunar limb (with less lunar glare and a relatively lower sky
brightness) or with the dark limb itself as the background?

3) On the bright side of the terminator in a crater just on the
observable limb with cloud rising directly against a background of the
night sky on the bright disk side (with relatively more lunar glare
and a higher sky brightness)?

> I do hope this conversation is helpful and not a regurgitation of material.
> (if it is, I will cease and desist)

From the view of a general group member, as far as I know you are
fine. Nothing final can be worked out until the LCROSS Team
completes target selection. It was always expected that there would
be surge of interest and flushing out of imaging techniques within the
last 30 days before impact. Group membership ran from about 80 in
February-March, to 120 in May, to 170 after launch, and now is at
about 230.

- Clear Skies, Kurt