[LCROSS] Obs window notes - Moon less than 38% illuminated fraction

[cano...@yahoo.com]

The moon is now less than 38% illuminated and is outside LCROSS
mission parameters. Hermite A still remains positioned for libration
imaging for another day.

The illuminated fraction of the Moon next will exceed 38% after the
next New Moon (Feb. 24) and on or about 3-2-2009 4UT, at which point
an imaging window for the southern Faustini region will open. But
Faustini will first be lit and on the nearside under very low
illumination and outside LCROSS mission parameters as early as Feb. 27
or 28.

Clear Skies - Kurt

[Derek C Breit]

Clear Skies - Kurt
---------------------------------------------

Ummmmm... March 3rd between 6 and 7 hrs UT, unless I don't understand the
'38%'...

I would suggest imaging the moon with an eye towards occultations before
this time however. From as soon as you see it in the Western Sky, until
close to full Moon.. A couple goals would be in mind..

How faint of a star can you image how close to a bright limb feature, and at
what phase???

How do the changing lighting conditions affect your setup and what might you
do to improve things, such as reducing glare with flocking and baffles, or
using an occulting bar?

Derek

[cano...@yahoo.com]

On Feb 18, 8:16 pm, "Derek C Breit" <breit_id...@hotmail.com> wrote:
<snip> Ummmmm... March 3rd between 6 and 7 hrs UT, unless I don't
> understand the '38%'...

We first discussed the illuminated fraction concept on Feb. 8.

The NASA LCROSS Image Specifications Press Release states that "[t]he
LCROSS mission plans to impact the moon when the lunar phase is
approximately between 76 degrees and 150 degrees and between 210 and
284 degrees." The constraint is intended to protect space-based
telescopes from being inadvertently being pointed at the Sun and
damaging their sensors.

The LCROSS impact target "lunar phases" are probably a reference to
_lunar age_ or the cycle of the Moon's orbit expressed in degrees
between 0 and 360 degrees. If so, the constraints correspond
approximately to the following terminator positions, lunar days and
illuminated fractions:

http://lcross_observation.googlegroups.com/web/LunarPhasesHeldmann2007Slide18.png

Phase | Lunar Day | Terminator longitude | Terminator Type
|
Quarter | Illuminated Fraction

76 to | 6.2 | E14 | Rising | First | 38%
150 | 12.3 | W60 | Rising | Second | 93%

and

210 to | 17.2 | E60 | Setting | Third | 93%
284 | 23.3 | W14 | Setting | Fourth | 38%

The terminator crosses each LCROSS candidate target twice each month.
From the perspective of amateur Earth-based imager, the optimal impact
dates and times will place the terminator on or just past the
terminator. But the LCROSS Team has other planning priorities related
to optimally imaging from the shepherding spacecraft that may or may
not occur at the same optimal times for Earth-based imaging.

Three simulated images at the limits of the illuminated
fraction percent are provided here:

3-12-2009 Northern region libration +6.2 illuminated fraction 93% -
Third Qtr
http://lcross_observation.googlegroups.com/web/20090312_93IF6_2Lib3rdQtrSimulated.jpg

7-27-2009 Northern region libration +7.0 illuminated fraction 38% -
1st Qtr
http://lcross_observation.googlegroups.com/web/20090727_38IF_7Lib1stQtrSimulated.jpg

5-29-2009 Northern region libration +3 illuminated fraction 38% - 1st
Qtr
http://lcross_observation.googlegroups.com/web/20090529_38IF_3Lib1stQtrSimulated.jpg

See also the discussion on the Atlas Page and Finders Page -

http://groups.google.com/group/lcross_observation/web/atlas-of-polar-images-above-lat-80
http://groups.google.com/group/lcross_observation/web/finders

NASA LCROSS Image Specifications Press Release
http://www.nasa.gov/mission_pages/LCROSS/news/image_specifications.html

> I would suggest imaging the moon with an eye towards occultations before
> this time however. From as soon as you see it in the Western Sky, until
> close to full Moon.. A couple goals would be in mind..
> How faint of a star can you image how close to a bright limb feature, and at
> what phase???
> How do the changing lighting conditions affect your setup and what might you
> do to improve things, such as reducing glare with flocking and baffles, or
> using an occulting bar?

Yes, now that I'm done with ephemeris things, I hope to catch up with
you on the exposure calibration questions.

- Kurt




>
>
> Derek

[Derek C Breit]

On Feb 18, 8:16 pm, "Derek C Breit" <breit_id...@hotmail.com> wrote: <snip>
Ummmmm... March 3rd between 6 and 7 hrs UT, unless I don't
> understand the '38%'...

We first discussed the illuminated fraction concept on Feb. 8.

----

That doesn't change the fact that the Moon becomes 38% lit on March 3rd
after 6 hrs UT, which happens to be just about Moonset from the Western
USA..

Speaking of which.. For the SF Bay Area, I have all the lunar occultations
posted on my website..

The further you are from Morgan Hill you are, the more the times will
differ.

http://www.poyntsource.com/New/Lunar.htm

Derek

[cano...@yahoo.com]

Thanks for the heads up. That eps Gem egress on March 6 comes up in
Occult 4 for my op. I'll have to give it a try. - Kurt

[Jim Mosher]

> That doesn't change the fact that the Moon becomes 38% lit on
> March 3rd after 6 hrs UT, which happens to be just about Moonset
> from the Western USA..

I hesitate to say anything, but yes, you are correct Derek: according
to the conventional method of calculating such things (somewhat
different from Kurt's), the Moon as viewed from the San Francisco Bay
area will next be 38.00% illuminated at around 07:25 UT on March 3,
2009, and it will be descending in the sky and only about 10 deg above
the horizon at that time. For a geocentric observer, 38.00%
theoretical illumination (by the conventional measure) would have
occurred slightly earlier: at about 05:45 UT, but definitely not on
March 2 (UT).

I can't speak for Kurt, but he may be using a kind of mixed date/time
system in which times are given in UT, but dates are described using
the calendar date at his observing location in the western US. His
mention of the next New Moon (2009 Feb 25 - 01:35 UT) as coming on
"Feb. 24" may be an example of this. But I'm not sure of this, and
even less sure of where the "4h UT" value came from:

http://groups.google.com/group/lcross_observation/msg/5546e061977e2754

In any event, I think Kurt may be a bit over zealous in his
enforcement of restrictions based on percent illumination. The actual
mission requirement seems to have been originally stated in terms of a
rather different concept (available hours of observation before/after
dawn/dusk -- something that depends not only on the Moon's latitude
and elongation from the Sun but also the observatory's latitude), but
the LCROSS scientists may indeed choose to restrict the impact dates
to ones on which the Moon's distance from the Sun -- and therefore its
illumination -- is in a certain range, or even to the range of
geocentric angles in the ecliptic plane that Kurt prefers. Even so,
the logic of restricting amateur observations to dates on which an
impact would be allowed is much less clear.

NASA's original Call for Images :

http://www.nasa.gov/mission_pages/LCROSS/news/calling_amateur_astronomers.html

asked for an atlas of photos with a variety of lightings and
librations -- including ones with lighting opposite to that at the
expected time of impact -- and even if only observations duplicating
conditions at the moment of impact were wanted, the percent
illumination of the Moon as a whole is a very poor indicator of dates
on which particular features are in light and shadow at the poles (or
anywhere else). The attachment shows a typical example of two photos
with nearly identical percent illuminations, but very different
lighting at the Moon's south pole. The lighting patterns are enough
different that it seemed helpful to identify some corresponding
features:

Registration point (blue "+") : Pitiscus
Yellow dot : Maurolycus
Red dot : Barocius
Green dot : Manzinus

The complete LTVT screenshots from which these images were cropped can
be found (along with additional examples) at:

http://ltvt.wikispaces.com/Understanding+Polar+Lighting

The reason for the large difference in appearance at the same percent
illumination is a roughly 8.2 deg difference in longitudinal
libration. This is actually a rather mild example, since the
difference in longitudinal libration, and therefore the difference in
terminator position at a given percent illumination, can be twice as
large as that shown here.

--

A much better indicator of the lighting pattern is terms of what
features are in light and shadow is the traditional measure of the
Sun's position in terms of "colongitude" (90 minus the eastward
longitude of the sub-solar point) and latitude. The colongitude (and
solar latitude) specifies the location of the rising and setting
terminators relative to surface features. The colongitude is 8.3 deg
larger in the second photo in the attachment, and the Sun is also more
to the south.

--

When the longitudinal libration is zero, the colongitude of the
terminator is roughly related to phases/solar elongation as shown in
the following table:

------ -----
Colon. Phase
------ -----
270 : New

345 <-- 75 deg elongation (start of LCROSS impact window)
0 : First Quarter
60 <-- 150 deg elongation (end of LCROSS impact window)

90 : Full

120 <-- 150 deg elongation (start of LCROSS impact window)
180 : Last Quarter
195 <-- 75 deg elongation (end of LCROSS impact window)

But, as illustrated in the attachment, a difference in longitudinal
libration will shift the terminator position relative to the surface
features at a given phase. With +/-8 deg of longitudinal libration,
an impact occurring at the minimum lunar elongation of 75 deg (and
therefore roughly 37-38% illumination for the full lunar disk) could
have a terminator position expressed by a colongitude anywhere in the
range 345 +/-8 = 337 to 353 deg.

The first of these possibilities, a colongitude of 337 deg, will next
occur at 2009 Mar 02 - 4:03 UT. The similarity to the date/time
quoted by Kurt at the start of this thread is probably coincidental,
since this would be on the evening of March 1st Pacific Time, and the
percent illumination at that time would be less than he allows: the
geocentric percent illumination calculated according to the
conventional method would be 26.9%, and 26.3% as observed from the Bay
Area.

A colongitude of 353 deg (the other extreme) will occur at 2009 Mar 03
- 11:34 UT. Both the Moon and Sun would be below the horizon as seen
from the Bay Area at that moment, but the theoretical geocentric
illumination would be 40.7%. Either of these possibilities, more than
a day apart, could correspond roughly to the lighting seen on an
otherwise unspecified LCROSS impact date with a solar elongation of 75
deg and an illumination of ~37-38%.

--

It seems to me that pronouncements about when observing "windows"
start and end need to be predicated on a clear statement regarding
what the objective of those observations is.

1. If the objective is simply to collect Earth-based images of the
Moon's poles at a variety of lightings and librations, as NASA
requested, any time is equally valid.

2. If the objective is to obtain an observation matching the
conditions of an announced LCROSS impact time, then it will likely be
necessary to search historic records, since finding an opportunity to
match all four parameters (sub-observer lon/lat AND sub-solar lon/lat)
in any short time frame is highly improbable. If only a match in
lighting is sought, without insisting the librations be the same as
well, then one should, I think, seek pictures with the same solar
colongitude (and, ideally, the same solar latitude), rather than ones
with a particular percent illumination.

3. If the objective is to practice for what the observers in Hawaii or
Chile will see on impact night by observing on nights when, from one's
own location, the observing conditions meet the LCROSS guidelines,
then those guidelines need to be carefully examined and applied to
one's own location. My reading of the mission requirements is rather
different from Kurt's, and I will try to address that in a separate
posting.

4. If the objective is photometric measurements and calibrations
related to future observations of the LCROSS impact, the requirements
may be still different.

Organizing photos by, or restricting observations to, particular
values of geocentric percent illumination has no particular value I
can think of, except perhaps as it relates to objective (4).

-- Jim