Stops, masks and observing the LCROSS impact

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Sep 19, 2009, 9:32:16 PM9/19/09
to LCROSS_Observation
A prior version of the NASA Citizen Scientist About page recommended
an occulting disk in SCTs to reduce stray light. Such a disk
effectively increases the size of the central obstruction.

"The plume, if observable, will be very faint compared to the
illuminated waning gibbous Moon, so any technique that can reduce
stray light in your optical system should be used. On Schmidt-
Cassegrain type telescopes, one trick is to make a circle of black
paper about 30 percent larger than the secondary mirror mount that is
temporarily taped to the mirror mount, preventing stray light from
entering the baffle tube inside the tube assembly. Newtonian
reflectors telescopes are a bit more difficult to shield from stray
light, so the best approach is to just have clean, well collimated

I understand that this recommendation may be dropped, but wanted to
explore further what role apeture stops and apodizing masks might play
as useful observing accessories for the LCROSS impact.

The general conclusion and recommendation of this note is that
apodizing masks will increase resolution with acceptable brightness
losses for SCTs and Newts in the 14 inch class. Apodozing masks and
aperture stops may not be a good option for 10 inch Newts and SCTs.
Apodizing masks probably confer no benefit for Newts or for refractors
smaller than 10 inches in aperture. Occulting disks should not be

These conclusions are based on a web-based page and amateur literature
review. They are not based on experience, since I only rarely use a
aperture stop and have not constructed an apodizing mask. However, my
quick look has inspired to try and build an apodizing mask for lunar
observing with my 10 inch Newt.

The following is a discussion focusing on the use of apodizing masks
and off-axis aperture stops with respect to the LCROSS impact. I have
not personally built an apodizing mask and their effectiveness and use
is the subject of much conflicting information on the internet.

Here, I focus on the comments and opinions of three noted members of
the amateur and professional community: John Westfall, Harold Suiter
and Thierry Legault with some historical refernece to Sidgwick's
Amateur Astronomy Handbook.

The purpose of masks and stops for Newtonians and SCTs above 200mm in
aperture relate to seeing found at the time of observation. Larger
apertures are relatively more susceptible to distortion from
atmospheric distortion than smaller refractors of 130mm or less
because the larger scope's apertures subtend a greater angular
dimension of the sky. Conversely, smaller apertures have larger
larger Airy disks and inherently less resolution than a larger
aperature Newtonian or SCT, i.e. a larger Dawes limit.

Average or below average atmospheric turbulence robs larger Newtonians
and SCTs of their superior performance relative to smaller refractors
because there large apertures gather light from a relatively larger
angular diameter. However, in skies with excellent seeing, the larger
aperture Newt and SCT can access their inherently superior optical
resolution - if for brief moments of still air - and out-perform their
smaller refractor cousins. Application of these principles can be
seen in the modern DSO imagers preference for smaller 135mm or less
short fl refractors for imaging. Because they exposure for long
periods in average seeing air, the seeing disk stays around 1-3
arcsecs in diameter, their digital cameras can accumulate light over
long periods of time, and large aperture scopes are harder to make
track accurately. These performance characteristics weigh in favor of
a small aperture refractor for hobby DSO imaging.

In such average or less than average seeing, smaller refractors give
the visual illusion of having a better image. This is because their
resolution is lower and because of this lower resolution, the smaller
scope is not capable of transmitting the details of "boiling air" that
one sees at higher magnification in the larger Newt or SCT.

Because of poor seeing's effect on larger aperture Newts and SCTs,
amateur owners of such scopes apply the rule-of-thumb that "stopping
down, when seeing is poor, gives a better image." Sidgwick at 473.

Aperture stops and/or apodizing masks convert the 10 inch Newtonian
into a smaller aperture scope that is less susceptible to the effects
of atmospheric distortion. The stop-down does not increase the
inherent resolution performance of the larger aperture Newt scope.
Counterintuitively, it creates the illusion of a higher resolution
image by creating an inherently lower-resolution image that is less
able to reproduce the effects of atmospheric disturbance.

This benefit of stopping down is not, however, without a trade-off.
First, Dawes limit still applies and the now smaller apertured stopped-
down large light bucket scope has larger Airy disks and a lower
resolution. Blocking the larger Newtonian scope also reduces the
light collected of the mirror (light grasp) and the image is
relatively dim as the large Newt's smaller refractor cousin. But as
Westfall notes discussing apodizing rings this "creates some light
loss, but with the Moon this is scarely a problem."

Newtonians and SCTs are typically stopped down using an off-axis
mask. Construction of a typical mask can be found on the web. An
example can be found at: .

The aperture off-axis stop-down mask is a common-sense tool for the
owner's of large Newts and SCTs. If you set up and seeing is so poor
it cannot support your larger aperture, you can pull out a light-
weight mask and convert your large aperture 10 inch scope into a 5
inch scope. An equivalent, but somewhat heavy gear option that I use
is two own (and sometimes haul) a second 5 1/4" refractor. If I
arrive at the site and seeing is bad, I can set up the smaller
refractor instead of the light-bucket Newt. (With age, the light-
weight mask looks more attractive.)

Another variant of a stopping mask is the apodizing diaphram mask.
The apodizing diaphram mask differs in basic construction from the off-
axis small-holed stop-down mask. Web examples of amateur construction
of an apodizing diaphgram mask include urls -

The apodizing mask consists of a series of mesh screens that are
concentric and on-axis.

Ancedontal web opinions on how well these masks work either (a) to
inherently improve resolution in good seeing, or (b) to work as a stop-
down mask in average or less-than average seeing, or (c) to work on
refractors as opposed to Newts and SCTs widely differ.

Harold Suiter, author of the widely respected _Star Testing Your
Telescope_, feels that apodizing masks do not offer a benefit to SCTs
that have a large central obstruction, while they will improve the
image of a lower central obsructed Newtonian.

Suiter, H. 2001. Apodization for Obstructed Amateurs. url:

Suiter, H. 2003. Apodization. (Webpage). url:

Suiter also notes that such rings _must be engineered_ based on the
telescope - they cannot be constructed from rules-of-thumb.
Construction of the screen is a simple matter for amateurs and
involves simple measuring, cutting cardboard or masonite and window
screen material.

On his Apodization website, Suiter provides an Excel spreadsheet by
which an amateur can determine the proper measurements for an
apodizing ring.
The spreadsheet is not easy to follow and requires study.

Suiter notes that _informally engineered_ apodizing masks that he
obtained from local amateurs and that he tested usually did not confer
benefit assumed by the owner. This was because the masks were not
formally engineered and were created using rules-of-thumb.

The second view by John Westfall, a widely respected professional
lunar observer who has been active with the amateur community, is that
apodizing masks will help both SCTs and Newtonians in their inherent
resolution performance on lunar targets and during periods of average
or less than average seeing. Westfall discusses apodizing rings in
Westfall's _Atlas of the Lunar Terminator_ (2000) at 13.

Westfall discusses the reasoning behind his use of an "apodizing ring"
on obstructed telescopes. The circular slot in Westfall's ring for a
280mm SCT was at 72 and 86 percent of aperature.

Westfall concluded that the ring inherently improves resolution for
telescopes with large obstructions (SCTs or Newts) when lunar
observing, but not for unobstructed telescopes.

Westfall performed ray tracing analysis of unobstructed and obstructed
telescope types. He computed that the performance of highly
obstructed scopes (32%) changed when an apodizing ring was used:

Ring? 50% Airy Disc arcsecs 80% Airy Disc arcsecs Transmission
No 0.39 0.59 90%
Yes 0.17 0.55 68%

There was less of a performance increase for unobstructed scopes.
Westfall's conclusion for unobstructed refractors is similar to
Suiter, who concluded that the improvement confered on unobstructed
scopes by apodizing masks "was subtle."

Westfall concludes:

"In summary, central obstruction removes light from the central Airy
disk and places it in the diffraction rings, while an apodizing ring
has the opposite effect. An apodizing ring can also improve the
performance of an unobstructed system. Using an apodizing ring
creates some light loss, but with the Moon this is scarely a
problem." _Atlas of the Lunar Terminator_.

With respect to the LCROSS experiment, there is uncertainty regarding
plume brightness and a basic observing strategy assumes that observing
the plume is _brightness limited._ As such, smaller aperture scopes
of 10 inches or smaller probably should not use an apodizing mask -
even if one could construct a properly engineered one within the
remaining time before impact. Losing 68% of light-grasp to an
apodizing ring would reduce the probability that the plume could be
observed if it is less bright than the LCROSS Team model predicts.
Observers may want to maintain a margin of safety with respect to
light grasp should the plume not reach the modeled apparent brightness
of 4 mpsas.

Owners of larger 14 inch class Newtonians and SCTs might benefit from
an apodizing mask while at the same time preserving margin of light
grasp safety equal to a 10 inch telescope. If Westfall's reasoning is
correct, an improved resolution resolution benefit will be confered in
both good seeing or if seeing is less than average.

With respect to occulting disks - which are simply larger artificial
central obstructions - Thierry Legault's "Obstruction" web page
analyzes the effect of central obstruction on lunar-planetary images.

Legault concludes that up to the larger 33% central obstruction of

"[T]he resolution power is not modified on high contrast structures:
Moon, double stars, Cassini division, shadow of a ring or a satellite,
edge of a planet,....

[T]he resolution power may be lowered on low contrast objects:
surfaces of Mars, Jupiter and Saturn. . . ."

Placing a larger than 33% occulting disk at the center of a larger
Newtonian or SCT will probably both reduce resolution -
dispropotionately more than would occur with an apodizing mask - and
image brightness.

In summary, a literature review indicates that apodizing masks for
larger 14 inch class Newts and SCTs may improve observation of the
LCROSS impact plume without causing unacceptable light losses. The
improvement may be inherent and work in both excellent seeing skies or
when seeing is less than average.

Clear Skies - Kurt

Jim Mosher

Sep 21, 2009, 3:57:05 PM9/21/09
to LCROSS_Observation

I have been hoping someone more expert than me would respond, making
this less of a two person conversation; but since no one has, some
quick comments seem in order.

The suggestion on the "About" page to experiment, between now and
impact night, with techniques to reduce instrumental glare, and to
extract small time varying signals from it, seems like a wise one to
me. Unless the impact plume is remarkably bright, we will most likely
be looking for a faint signal seen against a shadow whose darkness is
limited by the veiling glare from much brighter features nearby. The
more that glare can be reduced, the more evident the plume will be.

Customized setups designed for a minimum of instrumental glare might
also be considered. For example, in the instrument solar astronomers
us for observing faint light adjacent to the bright Sun (the
"coronagraph"), instrumental scatter is minimized by, among other
things, simplifying the telescope to a very high quality singlet lens
objective and blocking the main solar image with an occulting disk at
the prime focus. To fully exploit this, a coronagraph has to be used
at exceptional observing sites where the aureole of brightness around
the Sun caused by atmospheric scattering is equally low; but lunar
observers would seem well advised to try to reduce instrumental
scatter to a level lower than the environmental scatter at their site
-- something that is probably true for very few of us on clear

What helps will be very dependent on the particular setup one is
using. The suggestion to increase the central obstruction size is
certainly one worth exploring, even though it may not be of benefit
for most. I have also seen suggestions to hide the inner wall of the
conical SCT baffle from view by placing a circular stop at the exit
end of the baffle (rather than in front of it), to place black velvet
on the inner wall of a Newtonian tube opposite the eyepiece, to add a
lens to re-image the entrance aperture to an exit pupil (the exit
pupil, normally found a little beyond the eyepiece is visual observing
setups, is the image of the entrance aperture and any light outside it
is unintentional, and therefore bad) around which an appropriately
perforated occulting stop can be placed , and many others. Probably
the most basic is to look in, without the eyepiece, at a bright light
source, to see where unwanted light comes from, particularly as the
light source is placed off-axis at angles that might be encountered
from the sunlit Moon will looking for the LCROSS cloud (up to ~0.5°).
One also wants to look with the eyepiece to find the angles and
directions at which a bright source produces ghost images. Those more
knowledgeable about glare reduction can perhaps offer better

The suggestions to use off-axis stops and apodizing screens seem less
well advised, and (at least the latter) are, I believe, directed more
towards improving the ability to separate equally bright, and closely
spaced, objects as opposed to providing the improvements in contrast
needed to see faint objects adjacent to bright ones. The window
screen apodizing masks seem particularly ill-suited to lunar

I have long been skeptical of the concept of small apertures "cutting
through" seeing. One of the most famous exponents of that concept
was, of course, Percival Lowell, who thought he could bring out
remarkable detail on Mars and Venus by reducing the aperture of his
Arizona refractor until the increasingly collimated light he produced
in that way generated spurious effects in his eyes. Many other
reports of cutting through seeing with reduced apertures may come from
inexperienced amateurs with imperfect (or imperfectly collimated)
optics who are actually seeing the reduction in aberrations caused by
using a small and more perfect subsection of a large and less perfect
lens or mirror, or are seeing the greater tolerance to imperfect focus
that a longer f/ratio provides. In my own experience with terrestrial
visual observing, I do not recall ever encountering seeing so bad that
stopping down a 100 mm Maksutov aperture to an unobstructed 35 mm off-
axis one made a road or building sign that was illegible in the former
case become readable in the latter. I *do* get the impression that
there is improvement in contrast for my particular telescope (which
perhaps a longer exposure image could exploit -- my telescope has very
poor contrast to start with), but visually I cannot actually see any
additional faint detail because the possible reduction in glare is
more than offset by the loss of light (about a factor of 7 in this

I do not personally know if there are instances in which a 10-inch
telescope can, under identical seeing conditions, resolve more than an
optically perfect and perfectly focused 14-inch telescope but I find
it doubtful. As I understand it, the rule of thumb in the old days of
film photography (when exposures were longer) was that a lucky image
through an amateur 10-inch telescope could record *just as much* as
could be detected through a 100-inch telescope; but that is not to say
that a 10-inch telescope routinely, or ever, sees *more* than a 100-
inch telescope under identical seeing. It was also claimed you could
see more (with your eye) than could be photographed under the same
conditions, but little of that unphotographable detail has proved to
be real.

I have a letter from Ewen Whitaker saying that lunar images taken with
the Yerkes 40-inch refractor in Wisconsin were commonly taken with the
lens stopped to 18 inches to obtain "the sharpest image", an effect he
attributes to seeing; but it does not appear the 61-inch Catalina
telescope used for the /Consolidated Lunar Atlas/ of the early 1960's
was ever stopped down. Instead, according to the booklet accompanying
that atlas, a smaller f/ratio (allowing shorter exposures) was used
during poor seeing, and a larger f/ratio (presumably obtained with a
Barlow-type lens near the focal plane) was used in good seeing. It
seems especially difficult to imagine that if no adjustment to focal
length were made, the improvement in seeing-limited resolution
resulting from stopping down the objective could be so great as to not
be more than offset by the longer exposure times required.

In the same vein, I am also puzzled by the suggestion that deep sky
objects are best imaged, at least by amateurs, with moderate to small
apertures. I thought that to obtain the shortest possible exposures
one wanted the lowest possible f/number (short focal length compared
to aperture), and that if one wanted a generous image scale as well
(controlled by the focal length), this required very large apertures
-- one of the primary motives for building monster-sized professional
telescopes to explore small faint fuzzy objects.

In any event, despite what it may say on the websites you encountered,
I don't think the purpose of apodizing masks is to cut through seeing
by reducing the effective aperture (for example, by changing a 14-inch
telescope into a 10-inch one). Instead, I thought the idea of
apodizing masks was to reshape the diffraction pattern from a point
source in ways conducive to splitting double stars, making a smaller
telescope as effective as a slightly larger ones under any given
(ideally, perfect) seeing. That is, rather than making a large
telescope perform like a small one, I thought one wanted to make a
small telescope perform like a large one.

On the websites you encountered the apodizing strategy seems to be to
be to lower the brightness of the first few rings of the Airy pattern
by reducing transmission from the outer parts of the objective. An
opposite strategy, suggested by Lord Rayleigh himself, is to reduce
the diameter of the first dark ring (and therefore the central bright
spot), by masking the inner parts and favoring the outer ring of the
objective. An intermediate strategy, suggested by Harold Suiter
(although not particularly endorsed by him) is to maximize the amount
of light falling in a fixed diameter (such as that of the original
first dark ring). Whatever strategy is adopted, the claims for
improvement seem modest, at best.

More importantly, the devices used to produce apodization will
inevitably introduce additional scattered light (which is likely to be
the bane of LCROSS observations). Ideally the transmission of the
apodized objective is modulated by a non-scattering film of some sort
that rejects the unwanted light without introducing new sources of
internal reflection (it is a bit hard to picture how that is
possible), and which (as Suiter points out) must not alter the phase
of the transmitted light (or alter it in a desired way).

Looking at a light at night through a piece of window screen you will
immediately see the problem with constructing an apodizing mask from
materials of that sort: a single layer of rectangular screen produces
a starburst of diffracted spectra, primarily in a cross pattern, but
also at intermediate points in the checkerboard. This is acceptable
if one is looking only at the very center of the field (as in double-
star work), but not at all acceptable for lunar observing. As Suiter
points out, for an 18 wires per inch screen (1.41 mm spacing), the
"first order" blue spectrum (0.4 micron light) is seen about 1 arc-min
away from the source that produces it (with red further out in
proportion to its wavelength). The second order spectra are twice as
far from the light source and so on. Each original light source in
the sky thus produces numerous glare spots. If we were to look at the
LCROSS impact site through a window screen, we would see that point
overlain by the glare spectra from the sunlit parts of the Moon 1, 2,
3 and so on arc-minutes from that location.

This is very undesirable: window screen masks produce a very
significant reduction of contrast in return for a possibly
undetectable improvement in resolution.

-- Jim


Sep 21, 2009, 4:23:15 PM9/21/09
Jim and Kurt,

My guess is there are many people lurking and learning lots.

My 2 cents,


Kenneth Frank, Night Sky Network Administrator
phone: 415-337-1100 x 121
fax: 415-337-5205
Astronomical Society of the Pacific
390 Ashton Ave.
San Francisco, CA 94112
>> and Thierry Legault with some historical reference to Sidgwick's
>> image of a lower central obstructed Newtonian.


Oct 6, 2009, 5:16:26 PM10/6/09
to LCROSS_Observation
Glad you found the dialogue helpful. You are in a great location in
San Fran. Jim has fog, I have clouds. My parting advice is:

1) Keep an eye on weather -

2) Remind your imaging star party participants to synchronize the time
of their watches. If you are using your cell phone as your timing
instrument, compare the time shown on your cell phone to a reference
standard now and make a note of time index difference.

3) Remember to post images at the NASA LCROSS Citizen Science site

4) If you plan to image, imagers might want to consider registering at
the NASA LROSS Citizen Science site early due to possible server
overload on Friday.

5) There are now odles of finder charts and how to guides out there
targeted to various experience levels. Study them now instead of
waiting for Thursday night.

Clear Skies and Happy Imaging - Kurt

On Sep 21, 2:23 pm, KENNETH FRANK <>
> ...
> read more »- Hide quoted text -
> - Show quoted text -

jim phillips

Oct 7, 2009, 11:13:49 AM10/7/09
to, Lunar Domes,, Prof. Stephen Keene

I just read the following:

When the 2.4-ton Centaur hits at 7:30 a.m. EST, at a speed of 1.6 miles per second, it will throw up a plume of debris 6 miles high.

The LCROSS probe will then fly through the plume and analyze its contents with a battery of sophisticated instruments, before itself crashing into a different spot in the same crater four minutes later, to create a second cloud of dust and rubble


TWO crashes. I did not know that!





Jake Burkart

Oct 7, 2009, 12:18:52 PM10/7/09
We didn't have the budget to put brakes on the instrument portion.  The LCROSS probe itself is quite a bit smaller, so we may not see much from Earth.
Jake Burkart
Jake Burkart
Multimedia Director
The Collaborative

Deirdre Kelleghan

Oct 7, 2009, 11:19:27 AM10/7/09
to, Lunar Domes,, Prof. Stephen Keene
Wish I lived in the US of A , but will observe daytime moon if clear. Happy viewing to all.


2009/10/7 jim phillips <>

Deirdre Kelleghan
Vice Char IFAS
Outreach  IFAS

Deirdre Kelleghan

Oct 6, 2009, 5:51:25 AM10/6/09
Hi everyone

Perhaps I am a lurking person :-) if it is clear here in Ireland I will be watching


2009/9/21 KENNETH FRANK <>

Tom Munnecke

Oct 6, 2009, 1:51:24 AM10/6/09
for some reason, this message just arrived, even though it was dated Sep 21...

Yes, I'm one of the lurkers, and learning a lot... just happy to watch the traffic go by and hope for clear skies on Fri.

Jones, Jane H (1862)

Oct 8, 2009, 4:21:44 PM10/8/09
to, Lunar Domes,, Prof. Stephen Keene

I’ll be doing what you’d like to be doing,  Deirdre.  Sketching the south polar area of the moon using my 17.5 inch reflector and enjoying the imaging, video, the LCROSS/LRO tweets and NASA TV coverage of this amazing mission.  Good luck to all , and Godspeed to LCROSS! I’m also showing the public how to just find the south pole of the moon, even in the daylight hours.  ;-)


On 10/7/09 8:19 AM, "Deirdre Kelleghan" <> wrote:

Wish I lived in the US of A , but will observe daytime moon if clear. Happy viewing to all.


Jane Houston Jones
Senior Outreach Specialist, Cassini Program
JPL - 4800 Oak Grove Drive, MS 230-205
Pasadena, CA  91109  818-393-6435
2009 What's Up podcast archives

jim phillips

Oct 8, 2009, 4:42:04 PM10/8/09
Good Luck Dierdre and everyone else as well!

Jim Phillips

Date: Wed, 7 Oct 2009 16:19:27 +0100
Subject: [LCROSS_OBS: 1170] Re: TWO Crashes into Capeus


Oct 8, 2009, 5:12:02 PM10/8/09
to LCROSS_Observation
The best seat in the house will be on NASA TV which will be streaming
video from the shepherding satellite.

It will be about lunchtime at impact in Ireland.

On Oct 7, 9:19 am, Deirdre Kelleghan <> wrote:
> Wish I lived in the US of A , but will observe daytime moon if clear. Happy
> viewing to all.
> Deirdre
> 2009/10/7 jim phillips <>
> > I just read the following:
> > When the 2.4-ton Centaur hits at 7:30 a.m. EST, at a speed of 1.6 miles per
> > second, it will throw up a plume of debris 6 miles high.
> > The LCROSS probe will then fly through the plume and analyze its contents
> > with a battery <,2933,561502,00.html#> of
> > sophisticated instruments, before itself crashing into a different spot in
> > the same crater four minutes later, to create a second cloud of dust and
> > rubble
> > TWO crashes. I did not know that!
> > best,
> > Jim
> --
> Deirdre Kelleghan
> Vice Char IFAS
> Outreach  IFAS Hide quoted text -

Deirdre Kelleghan

Oct 8, 2009, 5:21:39 PM10/8/09
to, Lunar Domes,, Prof. Stephen Keene
Yes, I did a rough pencil sketch last night with  my 8 inch dob,( stacks of cloud) I  was hoping for a better go this evening.
The moon is low enough at the moment ( 21:15 UT )  seeing is bad , much worse than last night at the same time.
Lots of cloud around , but have two scopes out   in  hope , and will check on conditions as the evening goes on. My goal would be just to improve my sketch of this area , that may well go down in history as being the well on the moon. :-)

Had a good look at a very clear wonderful early morning Cabeus , and a mid morning Cabeus area .
Spent the afternoon explaining it all to people , some who figured NASA were bombing the moon.


2009/10/8 Jones, Jane H (1862) <>

Deirdre Kelleghan

Oct 8, 2009, 5:34:26 PM10/8/09
Yes NASA TV will be my lunchtime view from here.


2009/10/8 canopus56 <>

Oct 8, 2009, 6:20:20 PM10/8/09
I had question about the impact location but on a larger scale.

From the central US, say Kansas, assuming visablity, as facing the moon with a normal upward gaze where will the impact be, I am guessing about the 6:10 position, and from what direction is it being impacted, I'm guessing again at about 6:10 position and paralell with the equatorial plane about 2/3rds of the surface towarw the facing side.

How far off am I ?

Thanks again,

Robert Sobotor
Sent from my Verizon Wireless BlackBerry

Deirdre Kelleghan

Oct 8, 2009, 5:30:28 PM10/8/09
Thanks Jim , enjoy , it is really cool to be able to share it via NASA TV


2009/10/8 jim phillips <>
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