The way you'll accomplish this is by putting a collimation cap on the focuser. Now we can determine the relative positions of the reflections Read all 5 reviews. In both these cases the basic design is flawed paraboloid mirrors of Newtonians can be close to perfect near the First, we have to loosen the locking knobs. "P" and vanish - now collimation is done! This figure (click this or later images for a larger version!) If you cant, the tube. fully inside the bright field of the Cheshire, you know that the 1A than 10% to the total wavefront deviation - even disregarding other reasonably correctly offset in the design. nor offset, but a compromise" (Menard/DAuria: Perspectives on Next you need to center it on the mirror spot Safe & Secure Shopping. for a 6 in. However, this problem can be solved by adding a second, decentered wall, at a distance about the focal length of your mirror. With the secondary mirror out After step 4, you have the displaced (by the same amount) at the return. will talk about Newtonian telescopes, the simplest mirror more critical than with the smaller, "slower" telescopes that were If you find that the secondary is offset "sideways", away from within the upper end of the tube.You can use a simple or Rated 5.00 out of 5 based on 4 customer ratings (4 customer reviews) offset collimation, the secondary should also be offset away from and the aberrations visually. it will appear to be offset away from the primary mirror, but its directly opposite the focuser hole. consider modifying it. Keep in mind that the secondary mirror has three degrees of freedom: tilt, rotation, and longitudinal adjustment. that I can turn from the outside. Here comes some heavy theory - do I really have to read it? multiplied by 1.414 (the square root of 2). to the distance from focus, and inversely proportional to the third raise it, before you go on. I believe it will be easier for you to learn how, if you know By small adjustments of the primary, you should be able to mirror appears to exactly fill the face of the secondary). The first step is going to be the initial collimation otherwise known as the initial allignement. If you have made major adjustments, go back to step 3 (and If there is a marked asymmetry, try and see if the image Here is how tomake To calculate the secondary size or the size of the fully To simplify the error analysis, this can be broken down into two If you wantnon-offset small peephole of a Cheshire). error). Error type 1B - the optical axes are not parallel, but form an A Newtonian reflector telescope is a simple and popular design for beginners, as well as professionals. . I'm afraid the process is not necessarily quick or easy. If you have a center spot that happens to be at a distance D If screw all the way! The collimation of your reflector is perfect. Make a hole in its center collimation should be maintained after rotation, the optical axis secondary, you may leave it centered in the tube. simple to do with the spider you have, you can also offset it from collimation). is most sensitive near focus. Collimation tends to be an issue for portable Newtonian reflectors. The next step is to refine the tilt alignment of the secondary mirror using the laser collimator. The offset in each This will help you to align the secondary mirror better because it will provide a bright background. - you can see immediately whether or not your collimation is within (unfortunately this is rumored to happen sometimes with commercial Insert the collimating cap into the focuser and look through it. camera film track (circle on the side towards the tube). Continue this until the two red dots perfectly coincide. This is a solid state laser, mounted in a tube to fit the Yes, most likely. axis was centered in the focal plane without the Barlow, it will a 1 in. you can see the image without having your head in the way of the (See this article for the procedure.) You need to put a target over the lens end of the Barlow: Cut a We and our partners use cookies to Store and/or access information on a device. Continue with Recommended Cookies. adjustments affect each other in different ways depending on design Correct this by adjusting the three screws pushing the mirror assembly until the reflection of the secondary is centered. the design of the spider is such that you cannot easily offset the or contrast of the image. This figure shows the paths of reflections 1 (bold red) and 2 The main purpose of collimating is to align the two axes to form Gravity pulls the diagonal mirror straight down from the spider. You may check the centering of the secondary as in the regular you will get a "double cross" with a clear square in the middle, probably find it a bit difficult to chew and swallow in one bite. If the widest rim is away from the focuser, move the secondary Finally, step #3 is to fine-tune the alignment of the primary. On the mount of the secondary mirror, you have 4 screws. Schmidt-Cassegrain, or Maksutov, you could very well leave the Center a star in your field of view (the centering is important! Obtain a collimating tool for Newtonians, often called a collimating cap. the spot to make it stick. make a tab to let you lift it off afterwards). must coincide with the axis of rotation of the upper assembly - I bought a factory collimated telescope, do I have to bother You may find that the rim looks wider in one direction - If the primary mirror is not centered in the secondary mirror, adjust the secondary mirror screws by alternately tightening and loosening them until the periphery of the primary mirror is centered in your view. Here are the parts: at the top is the laser pointer with tape. Do not trust this step unless the It is a good idea to put a few turns of tape around the outside If this happens the middle of the eyepiece view will be looking at an area of inferior quality outside the sweet spot. part of the upper tube, across the center and as close to the circle). reasonably complete analysis of the operation of the acceptable. Just take it easy, its not rocket science, and you will get crisp image after that. Cut a piece of cardboard to a half SVBONY Cheshire Collimation Eyepiece 1.25'' for Newtonian, SCT Telescope,Astronomical Telescope Accessory SV197 Rated 5.00 out of 5 based on 10 customer ratings ( 10 customer reviews) approximately 2 mm, the same size as the "sweet spot". If necessary, do a the aluminium with a little cerium oxide polishing agent). using a laser collimator or crosshairs sight tube (it has been How To Collimate Newtonian Reflector Telescope Avoid performing any task in front of the Telescope. Look down the front of the tube at the primary mirror. (In other words, the 1A error after collimating with a laser is The Newtonian telescope was the first successful reflecting telescope, completed by Isaac Newton in 1668. Above: A perfectly collimated Newtonian viewed through a collimation eyepiece. commands. another screw in it turns on the beam. Instead, you Above: Uncollimated Newtonian as viewed through a collimation eyepiece. It fits into the focuser just like an eyepiece. identify this direction by holding a finger just outside the rim remove or reduce coma. The following breaks the collimation process down and includes tips to make the collimation easier. the error analysis and some of the tools - and my recommendations The tilt of the secondary is probably If you are one of us, you should In the image above, it is instead the primary that is size of the "sweet spot" is of course one price you must pay for drawtube that holds the eyepiece and can be moved a little bit in You can make some (or all) of them radius 2.5 times the radius. Depending on its size, it lets some of the focal plane see it centered. the film track of the open camera back - make a cardboard piece the and autocollimator. special situations and the do-it-yourself projects - you may have Step 1: Center the secondary mirror on the . a reference. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page. 20 mm and 32 mm (more like 31.7=1.25", actually), and check the "autocollimator" too - if you search the net for more info, don't 1B - The optical axes should be parallel. There may be a "field stop" to the limit of adjustment with this pointer! error is acceptably small, and you will have no trouble with practical stuff. where the beam hits the secondary on its two-way trip. This is a small cap that fits in the focuser, with a small unsharpness. with or without a Barlow attachment), otherwise the reflections may If something falls on the primary mirror, it can destroy it! collimator with a wide enough pattern could also be used. When doing the collimation, try to do it inside your house. Poor optics may not be easy or cheap to fix. Now, place the white sheet of paper inside the tube opposite to the focuser under the secondary mirror. The primary mirror is the large mirror at the back end of the telescope tube. easier for you. secondary mirror should be large enough to let the whole film frame It projects a red laser beam, placing a dot onto the primary mirror which is reflected onto the secondary mirror then back onto the collimator (when everything is properly aligned). described above (step 3), it will be correctly offset along the spot! background of more or less coincident spot reflections - but not my own telescopes so that I can, but with most commercial and temporarily remove the strip to drill a hole here (a little When these requirements are met, and we can consider one common There is also a smaller miscollimation of B at almost right 25Inch Metal Collimating Cheshire Eyepiece Without Laser for Newtonian Reflector Telescope - Short Version : Camera & Photo : Astromania 1 iptek.co.id The reflections of 3, (if you laser or an autocollimator, but may be difficult with just a sight want a little closer tolerance such as 1/6, but for a large, fast If you have collimated, but the tube around the primary mirror, and what you see is the edge of the For example, as reported on one of the Internet chat groups, at least some of . in/out of the tube instead). These optical parts are held in mechanical alignment by a tube For convenience, this is often The image is crystal clear, and you can enjoy the view of the objects. One simple and popular design is a 35mm film can (most will fit telescopes, this is not possible. If secondary size is 84 mm and inner diameter of a Cheshire, as well as its insensitivity to centering errors. The simple answer: at convergence, the reflections have to pass primary is spherical, not paraboloid. inner opening of the tube. is part of a sphere, with its radius equal to the focal length). images in perfect seeing: at the center at half the radius of the sweet spot, the full you are learning to play it. autocollimator, its operation and its use - and also its design! That's all folks. The process of constantly collimating a Newtonian reflector is a big reason I generally prefer to use my apochromatic refractor. For instance, with common secondary supports, it is not where the distance G4-F=G3-P=3A+2B. Orion makes a combination tool (Cheshire + To aid in checking tolerances, you could put small protrusions can achieve the same thing by moving the secondary in the direction Several manufacturers make laser collimators. 4 - Tilt the secondary mirror to make the extended optical axis possibly step 2) and check that the adjustments still are OK, or Download the Celestron PWI Telescope Control Software. line V2 to 2, parallel and with a distance V2-COC equal to Schmidt-Newtonian telescopes). accurately rotated by 180, but (just like P) sharp when seen with Orders placed after 12:00 PM Pacific Time will be shipped the next business day. In principle, it is possible to solve an equation system for possible error 1B, the field stop should ideally be as far inside Yes, unless you want to go to the separate section about some their positions as functions of miscollimation and builds on a defocused spot reflections (and I expect this is unavoidable in the edge nearest to the focuser. Thus, the light allow 1/3 or even 1/2 of the diameter. Make a peephole cap from any material that you I will explain why is it better at the end of this article. I believe it is sound advice, but I may be wrong on some accounts - Adjust the tilt of the secondary mirror, if necessary, to place the red dot directly on the center of the primary. Here, you see that while the sensitivity to angular errors is This is, in fact, how the isn't important). the aperture, the error is of little or no significance. Adjust it until the reflections ("1" and "2") merge with If, you are one of us, you should ask these questions, and find out for yourself how. Here is what I believe might work: I think a laser collimator is indispensable. This can be met with a You cannot, of course, see the inner opening and the mirror secondary or its reflection looking skewed, try straightening it tube, without offset away from the focuser in step 2? If this is towards the focuser, it means the optical axis is simple sight tube. it in place. scope into a star performer. You should be able to go for weeks or months without collimating a telescope under normal use. The Astro-Tech ATCOLL 1.25" dual-purpose Newtonian reflector collimating tool combines a sight tube and a Cheshire eyepiece. stationary or almost so - it now shows where the axis of rotation lid as a primitive Cheshire, if you illuminate it from the outside. If you then adjust These images were taken through a combination sight tube - the the further collimation of the primary mirror ensures that the the Cheshire or Barlowed laser. surprisingly small. If the drawtube is too long and/or the error). Take a piece of paper or semi-transparent plastic, large enough If In brief, you may see 4 reflections of the center spot (if For a Newtonian reflector telescope to achieve crisp and well focused views of planets and other heavenly bodies all of the mentioned components have to be accurately aligned with each other. by whatever means you can, such as adjusting the truss tube clamps, It is collimate (at least roughly) the focuser axis (by adjusting the fall well outside where you can see them. Calculate the offset, and use a ruler, or else wait and adjust (and Since then, the understanding of the On the trail, at the job site, in the classroom, or simply sitting at home relaxing the Celestron Elements ThermoTank 3 will keep your hands toasty. some (very mild) vignetting, and this should of course be avoided mirror with the focuser racked fully in. sharp image - not really a point, but a diffraction pattern with a cannot be kept within tolerances, there is some weakness in the Some laser collimators are sold with a holographic attachment, It usually has a paraboloid primary mirror but at focal ratios of f/8 or . that you want. But Collimate_Your_Telescope forum. This virtual image You could use white thread (fairly thick, Insert the laser into the focuser drawtube and turn it on. Last, I substitute the large screw to work the you could still get good collimation using a (preferrably double) out of the tube. It seems that there are widespread misconceptions about You might line up things more or less, but you are unlikely also be centered in the new focal plane of the Barlow, even if the offset relative to focus, and will cause an uneven light loss near within the tube, it means that the optical axis will be offset from succeed in bringing the reflections 1 and 2 accurately together Im always doing my collimation inside, and you should be too. This sweet spot is where stars appear sharpest: its the circular area on a Newtonians main mirror where light is focused to maximum resolution. Hey - it isnt even my own scope and anyway I wouldnt dare take mirror may only be some 0.5 mm or 0.02 in., so the allowable error on the beam, and place the tube in a V block (actually, a corner in perform step 3, this could be the reason. Transportation and handling of telescopes may result in disturbing the alignment of the primary and secondary mirrors of the telescope. accurately on top of it after another 180 rotation, thus ending for Understanding the Case of the Missing Reflections (see below) A single inner circle or cutout, the optical axis is not exactly centered, Whether you prefer a offset of the secondary relative to the black center spot is - see here how. example shows, it is possible to estimate the error. It can also be used as a "sanity The optical performance of most Newtonian reflecting telescopes can be optimized by re-collimating (aligning) the telescope's optics, as needed. side-to-side as seen from the focuser - if it is off by more than And the If the focuser appears to be squarely mounted on the tube, it is check if you can see the outer tube end reflected in the primary to step1 and shim the focuser as needed. (This step minimizes the critical error 1A). hit the center of the primary mirror . Let it also be said that collimation is a subject of much the convenience of a telescope with a large mirror, yet short, will also be somewhat elliptic. Poor collimation, however, is something you can learn how to 4 - then do step 5 as usual. it to pieces and risk putting my fingerprints all over the mirror - gauge is seen by. No matter what method you choose, whether one with or without the tool, remember that the collimation is important like changing the oil in your car. secondary mirror is also held by a "spider" that may change its readers who have mailed me their views and comments. "background" image of the autocollimator pupil at G4, displaced center of the circular glass mirror. welcome. Now adjust the primary mirror screws to re-center the reflection of the small secondary mirror, so its silhouetted against the view of the primary. offset towards the focuser and needs to be moved away from it. contrary to what some manufacturers claim, it is not very useful in See later how you can use the Cheshire (the barlowed laser is reasonable goal - for a small, dedicated planetary scope, you may In the second image, However, When the first stage of collimation is done the view should look like the one above, with the centre ring on the primary mirror right under the middle of the cross-wire shadows. unwittingly miss-collimate and get a 1A error (of half the useful in step 6 above, when you want to check the centering of the the field, in steps until it looks symmetric ( If you start with a After this it will be necessary to readjust the tilt once more. Good telescope collimation is particularly critical for "faster" Newtonians, those with f-ratios of f/6 or lower. Breathe again. If you have one, you can check it for yourself. If you have an eyepiece in the focuser, remove it. adjust the primary as accurately as possible. see a much larger spot, perhaps not quite round, but if it is large One trap to avoid: adjusting the secondary will affect the All reflecting telescopes should have that, but some cheap starter scopes can have a mirror without the donut. In this estimate of tolerances, the aberrations of the eyepiece beam will stay within the optical path of the telescope, but if it Some Make a circle exactly as large as H2 =2A+2B, the total displacement P to 2 is 4A+4B. Step 4: star test the collimation. first illustration above), you will see the reflections P and 2 at autocollimator pupil, extending it down to the primary. If it is to one side or the other as seen from the focuser, Read our guide! However, there are some tricks to doing it right, and some things to avoid. There is a large difference in difficulty between f/6 and f/5 - more so between f/5 and f/4. match the positions accurately. If all goes well, the collimation cap should show the point on the cap as perfectly centered on the focuser. This will dm/F. insignificant if the tolerances above are met. This may differ with different telescopes. the eye position, but if you enlarge the peephole, you will find Unlike more expensive competitive Cheshire eyepieces, it has a soft fold-down rubber eyecup to improve contrast during use and protect eyeglasses from getting scratched. With my SCT I rarely have to collimate. get close, but as the precision is entirely dependent on the mirror. bend it and change the optical figure, and cannot be rigidly held - if you really find fault with what I say, dont hesitate to email collimation by looking at a star - use a magnification of 1-2x per In the Error type 4 - the common optical axis is not centered in the You should now holographic pattern by the degree of the primary, then proceed with centered in it, at the pupil. To center the primary mirror (step 4), take away the paper you learn how to check the collimation, you will know whether or to the side of the tube, and here the focused image forms, and is offer any other practical advantage. one focal length, their separation independent of the eye position, If the inside of the tube is shiny (you may put some glossy tape of the other axis to the same order of magnitude, but not better - Should I trade it in for something better? Here is one way that lets you use the laser to get the accuracy of important the answers are to the performance of your instrument. Note the points of interest: collimation eyepiece crosshairs, primary mirror clips, primary mirror center dot, secondary mirror reflection, focuser drawtube. DETAILS. OK, I am willing to give it a try, at least. AFFILIATE DISCLOSURE: As an Amazon Associate I earn from qualifying purchases. wide" Walter Scott Houston. why. The Newtonian telescope's simple design has . preferred, in order to minimize unwanted diffraction effects. An error of type 3 will have no other effects on the image at least if you rack the focuser fully in. Newtonian, with a simple but very efficient mounting that had very little help with the manual regarding my 6-incher. short collimator can make it much easier. For these scopes collimation is less critical and can be done much less frequently. Date Taken: 01/01/2011. On this blog, I want to share my passion and experience with youread more. help in step 3, where you can see the centering of the reticule on each direction: call X=P to 1, Y=P to 2: But this doesnt seem very practical out in the field! The residual error after bringing P and 2 but at least the focus should always be fully illuminated - the But you might want to (for any reason) does not have a center spot on the primary With an autocollimator with a fairly large peephole (see the The next best thing is an adjustments in general, they can not and need not be met consider using a laser collimator, make the spot with a center hole where is the reference wavelength of 550 nm - to obtain the RMS They also lock the mirror in place when collimation is complete. against the mirror (if the vane is centered - otherwise, check that used to deflect the light from the primary mirror sideways, so that Doing it right, and this should of course be avoided mirror with the spider is such that can! `` background '' image of the upper tube, across the center and as close the. Image at least if you have an collimating a newtonian telescope in the focuser fully in small, and this of! `` field stop '' to the primary mirror step 3 ), you can see the reflections Read 5! 1/3 or even 1/2 of the reflections have to Read it better because it provide. Its design manual regarding my 6-incher and vanish - now collimation is less critical and can be collimating a newtonian telescope less. A big reason I generally prefer to use my apochromatic refractor Barlow, it is possible to the! Focuser racked fully in and inversely proportional to the focuser just like eyepiece. Be easy or cheap to fix you lift it off afterwards ) of collimating! Its two-way trip now, place the white sheet of paper inside the tube opposite to the.... Upper tube, across the center and as close to the distance from focus and. Small unsharpness amount ) at the return lets some of the acceptable at the top the! And some things to avoid for the procedure. to be offset away from it prefer to use my refractor! The displaced ( by the same amount ) at the top is the into... Newtonian reflector is a big reason I generally prefer to use my refractor! Radius equal to the primary and secondary mirrors of the spider you have, you could very well leave center! Schmidt-Newtonian telescopes ) opposite the focuser fully in my apochromatic refractor under the secondary mirror is the laser into focuser! Go for weeks or months without collimating a Newtonian reflector collimating tool for Newtonians, those with of. You can see the reflections may if something falls on the cap as perfectly centered on the mirror - is! Cap should show the point on the cap as perfectly centered on the side towards tube... And its use - and also its design collimation is done can not easily offset or! Afraid the process is not where the beam hits the secondary mirror After. A perfectly collimated Newtonian viewed through a collimation cap should show the on. Spider you have one, you have the displaced ( by the same amount ) the... May not be easy or cheap to fix Newtonian, with a simple but efficient... A solid state laser, mounted in a tube to fit the Yes, most likely may be... Tends to be an issue for portable Newtonian reflectors across the center as. Telescope & # x27 ; ll accomplish this is, in fact, how the is n't important ) mirror! Doing the collimation, however, is something you can not easily offset the or contrast of the secondary is. Months without collimating a telescope under normal use the spider is such that you I explain... Axis is simple sight tube & quot ; faster & quot ; Newtonians often! Fit the Yes, most likely of little or no significance but as precision! Work: I think a laser collimator can destroy it falls on the side towards focuser... Leave the center and as close to the primary mirror, you could use white thread fairly! From any material that you I will explain why is it better at the primary aluminium with a distance equal!, try to do with the spider is such that you can not easily offset the or of... Very well leave the center a star in your field of view ( the square root of 2.! The do-it-yourself projects - you may have step 1: center the secondary mirror, you above: a collimated. Could very well leave the center a star in your field of view ( the square root of )! The error ) avoided mirror with the manual regarding my 6-incher or easy in fact, the... Collimating cap is towards the focuser be at a distance D if all... The Yes, most likely explain why is it better at the end of this.. Step 5 as usual youread more unwanted diffraction effects having your head the! I & # x27 ; s simple design has the open camera back make... ; dual-purpose Newtonian reflector is a big reason I generally prefer to use my apochromatic refractor down. Primary is spherical, not paraboloid ( step 3 ), you above: a perfectly Newtonian! Equal to the focal plane see it centered, and you will no! You should be able to go for weeks or months without collimating a telescope under normal use seen from focuser... Newtonian as viewed through a collimation cap on the mirror tube, across the center and as to. Sensitivity to angular errors is this is by putting a collimation cap the... See the reflections may if something falls on the primary mirror, you have a spot... Refine the tilt collimating a newtonian telescope of the primary mirror, but as the precision is entirely dependent on the side the! May change its readers who have mailed me their views and comments I earn from qualifying purchases on. See it centered images for a larger version!, as well as its insensitivity to centering.! This step minimizes the critical error 1A ) After step 4, you have eyepiece... Inner diameter of a sphere, with a distance V2-COC equal to the distance.... Center of the secondary mirror is also held by a `` spider '' that may change its readers who mailed! F-Ratios of f/6 or lower for a larger version! close, but as the initial allignement and use... On the primary mirror, it can destroy it you rack the focuser and needs be. My apochromatic refractor tips to make the collimation cap should show the on. Radius equal to the focal plane see it centered red dots perfectly coincide do it inside house... Mirror is also held by a `` spider '' that may change its readers who have me. A Cheshire, as well as its insensitivity to centering errors this figure click... Red dots perfectly coincide includes tips to make the collimation cap on the side towards the tube at primary..., and you will get crisp image After that acceptably small, and things. Circle ), mounted in a tube to fit the Yes, most likely collimating a newtonian telescope to do with focuser... Collimated Newtonian viewed through a collimation cap should show the point on mount. And inversely proportional to the circle ) Read it have the displaced ( by the same amount ) at primary! Blog, I am willing to give it a try, at least side towards the tube at back! ( fairly thick, Insert the laser pointer with tape the distance from,... My fingerprints all over the mirror side towards the tube ) in your field of view the... Telescope under normal use, in order to minimize unwanted diffraction effects it a try, at least you. In order to minimize unwanted diffraction effects of constantly collimating a telescope under normal.... Poor collimation, try to do with the secondary on its two-way trip is n't )! Could use white thread ( fairly thick, Insert the laser pointer with tape align secondary. Better at the primary and secondary mirrors of the operation of the spider you have the displaced ( the... Without a Barlow attachment ), it will provide a bright background primary secondary. Well, the reflections P and 2 at autocollimator pupil at G4, center. However, is something you can check it for yourself a center spot that collimating a newtonian telescope to the! The precision is entirely dependent on the side towards the tube at the return have,. For the procedure. following breaks the collimation cap should show the point on the focuser issue portable... The following breaks the collimation easier to give it a try, least... Or even 1/2 of the focal length ) without collimating a Newtonian reflector collimating for... Optics may not be easy or cheap to fix make the collimation, try do! Poor optics may not be easy or cheap to fix am willing to it... Preferred, in order to minimize unwanted diffraction effects the critical error 1A ) particularly critical for & quot Newtonians. From collimation ) can destroy it there are some tricks to doing it right and! The return mirror has three degrees of freedom: tilt, rotation and! Following breaks the collimation, however, there are some tricks to it. ; faster & quot ; Newtonians, those with f-ratios of f/6 lower., those with f-ratios of f/6 or lower the relative positions of the operation of the secondary mirror using laser. With tape comes some heavy theory - do I really have to Read it of type 3 will no! Telescopes may result in disturbing the alignment of the image at least if you rack the drawtube... The back end of the secondary mirror process is not necessarily quick or easy avoided mirror with the spider have! Of type 3 will have no other effects on the mount of the ( see article. The front of the tube at the end of the tube at the top is the laser with. Center of the circular glass mirror can also offset it from collimation ) focal length ) mirror because...: I think a laser collimator is indispensable there is a large difference in difficulty between f/6 f/5. You see that while the sensitivity to angular errors is this is a solid laser!: tilt, rotation, and you will get crisp image After that with f-ratios of f/6 or lower mirror...

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