Stops, masks and observing the LCROSS impact 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 optics." I understand that this recommendation may be dropped, but wanted to explore further what role aperture 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 used. 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: http://www.geocities.com/AlyaSerpens/OffAxisApertureStop.html .  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 - http://home.pcisys.net/~astrogirl/tips1.php  http://www.csastro.org/gallery/article4.htm   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: http://home.digitalexp.com/~suiterhr/TM/ApodDes.pdf  Suiter, H. 2003. Apodization. (Webpage). url: http://home.digitalexp.com/~suiterhr/TM/apodize.htm  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. http://home.digitalexp.com/~suiterhr/TM/ApodizeTut.xls 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. url:  http://legault.perso.sfr.fr/obstruction.html  Legault concludes that up to the larger 33% central obstruction of SCTs, "[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