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
On his Apodization website, Suiter provides an Excel spreadsheet by
which an amateur can determine the proper measurements for an
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."
"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
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