登录  
 加关注
查看详情
   显示下一条  |  关闭
温馨提示!由于新浪微博认证机制调整,您的新浪微博帐号绑定已过期,请重新绑定!立即重新绑定新浪微博》  |  关闭

天色轨迹-冷冰川的柴火间

所谓柴火间,就是杂七杂八。这里漫聊着属于普通人的科学和跆拳道哲学^.^

 
 
 

日志

 
 
关于我

●本人热爱自然科学,具体说来~天文,植物,地质,化石,矿物。。。从小就都喜欢。喜欢收集它们。 ●本人喜欢摄影,这是随着我的天文附带起来的,尤其喜欢记录天空中的颜色。 ●本人喜欢跆拳道,它不仅仅是运动,里面还充满了实践的哲学。 ●本人喜欢做饭,自己做便宜又吃得饱。 ●本人喜欢骑自行车,从初中就喜欢骑着车去野外。 ●本人喜欢做实验,只听书上讲的太没意思了。

【资料】对焦的方法  

2008-04-12 23:31:51|  分类: 作为资料留着 |  标签: |举报 |字号 订阅

  下载LOFTER 我的照片书  |

Focusing Methods for Astrophotography

There are many different methods of focusing, and some work better than others.

The better ones are also more consistently repeatable, which is an important consideration because of the time and effort we spend on our hobby.

These different methods range in accuracy from the least accurate - focusing with the unaided eye on the groundglass of the camera through the pentaprism, to the most accurate: examining a star in an actual test exposure.

Many of these methods listed below can yield good results if the problems of focusing are taken into account, and the limitations and idiosyncrasies of each method are understood.

We can get away with a lot if our optical system has a slow focal ratio. On the other hand, fast optical systems and high-resolution digital sensors demand critical methods for good results.

The following methods are presented, more or less, in a general order of accuracy, from least accurate to most accurate.


By Eye

It would seem that the easiest way to focus is to just look through the viewfinder of the camera and try to focus.

It may be easy, but it is not necessarily accurate with repeatable consistency.

When we get to the faint stuff, we find that it is very difficult to focus this way. It's not that accurate for the bright stuff either.

When we look through a telescope to visually observe we usually don't have any trouble at all focusing the eyepiece, unfortunately the same cannot be said for focusing a camera.

The trouble is, with a camera, the image is projected onto the ground glass of the focusing screen first. Most screens, which seem quite bright for daytime work with fast lenses that focus with the diaphragm wide open at f/2.8 or so, suddenly seem quite dark at prime focus of an f/8 telescope.

We can pick a bright star to try to focus on, but the light from a bright star irradiates and spreads in the ground glass of the focusing screen making it difficult to tell when it is in focus.

If you do attempt to focus with your unaided eye through the viewfinder, at least try to pick a star that is not too bright. If you are trying to focus on an extremely dim deep-sky object or the field you want to photograph does not have any bright or moderately bright stars in it, some experts recommend you slew the telescope over to the nearest bright star, focus on that, and then slew the telescope back. Problems can occur with this method though, especially for users of Schmidt Cassegrains, if the primary mirror moves during the slewing after focusing.

Focusing through the camera's viewfinder also assumes that the viewfinder optical system of the camera is in collimation, that the focusing screen is in the correct position, and that the mirror is not out of alignment.

This method can also lead to some very uncomfortable positions just trying to view through the camera, for instance when refractors are pointed overhead, and when Schmidt-Cassegrains are pointed anywhere near the north celestial pole.

If you do try to focus by eye, take the time to rack the focuser back and forth through the point of best apparent visual focus. Do this several times. Become familiar with what the point of best focus looks like as you rack through. Try going through it slowly. Try going through it quickly. Once you know what the point of focus looks like, practice coming directly to focus and then stopping dead. Do this several times.

Wear an eye patch on the eye not doing the focusing and keep the eye under the patch open. The eyepatch will prevent light and distractions, but preserve your vision. This simple procedure will help in preventing strain in the eye you are using. For some reason, probably physiological, the brain can tell when you have one eye closed and vision in the other eye suffers. It will also help preserve your vision in the eye under the eyepatch.

Fatigue, allergies, astigmatism, and anything that can affect your eyesight, can affect your focusing.

There is not much good to be said about trying to focus a camera through the viewfinder with the unaided eye. It does not have a very high percentage of success for the vast majority of people, and even those with exceptional eyesight find it is not consistently repeatable.

This method is not recommended.


Magnifier / Right Angle Finder

A better way to focus by eye is through the camera's focusing screen with the aid of a magnifier.

Any additional magnification that can be used will help, but a range of 15x to 25x is usually optimum for attempting to achieve the most accurate focus.

Canon's Right Angle Finder
In the days of film, the magnifier could be placed directly on the focusing screen for those cameras with removable finders. Unfortunately, today's DSLR cameras don't offer removable prisms. Our only option is to place a magnifier behind the eyepiece.

Most high-end camera manufacturers make accessory right-angle magnifiers that attach to the eyepiece. This 90 degree viewing attachment makes it more convenient to focus when the scope is aimed high overhead.

The problem with these devices however, is that they make the view extremely dim. Don't expect to be able to see any deep-sky objects with them. Placement of the eye is also critical with these finders. There is a "sweet spot" where the eye must be placed for critical accuracy. If you accidentally move your eye just a little bit you won't be in the sweet spot anymore, and this can be difficult to ascertain while trying to focus on a star.

These right-angle finders also usually offer a higher magnification that can be accessed by flipping a lever that puts a small barlow lens into the optical system. This helps by giving more magnification, but makes the field of view smaller and even more dim.

If these devices are used, it helps to try to focus on a double or multiple star that is not too bright, such as Alcor and Mizar in the handle of the Big Dipper, or the Trapezium in the Orion Nebula.

These right-angle finders can help by making focusing possible in a more comfortable viewing position, but the dimness of the view is a serious drawback.

B&K Products makes an accessory for focusing a camera that uses your own diagonal and eyepiece to focus through the viewfinder.

Stellar Technologies makes a focuser that takes the place of the camera and uses a ground glass and 22x magnifier to help with visual focusing. Once the scope or lens is focused, the camera is replaced and the images taken. This can work well because it provides a bright image at high magnification. The problem is with accessing the eyepiece when a refractor or SCT is pointing overhead.


Hartmann Mask, Schnier Disk

Hartmann Mask or Scheiner Disk

The Hartmann Mask is a simple device consisting of a set of holes in an opaque lens cover.

The out of focus images generated by each hole merge when the telescope is in focus. They operate much in the same manner as an optical rangefinder found on rangefinder cameras, such as the Leica M series cameras.

The device goes by two different names, with Hartmann mask describing a mask with multiple holes and Scheiner disk describing a mask with 2 holes. It was apparently invented by Christoph Scheiner in 1619, but is most commonly called a Hartmann mask today.

Hartmann Mask on a camera lens

A disadvantage to these masks is that you are faced with the same problems that plague any device that uses a focusing screen and optical components of a camera. Many focusing screens are dark, camera viewfinders lack magnification, and the viewfinder can be an an uncomfortable angle to view through.

If magnification can be used on the viewfinder, this will help in focusing a Hartmann mask.

Remember to remove the device before actually taking the photographs!

You can very easily manufacture one of these masks yourself simply by cutting two holes on a piece of cardboard that is the same diameter as your dewcap.

They are also available commercially in products such as the Kwik Focus from Jim Kendrick Studio or Pocono Mountain Optics.

Ron Wodaski has made an excellent suggestion for making a Hartman mask. He suggests using triangles at an angle to each other instead of circles.

Each triangle creates 6 diffraction spikes that help gauge where the center of the star image is and when they overlap, the image is in focus.

Make the holes as large as possible so that you are not stopping down the scope too much, making the view dimmer.

One problem with this method is that the viewfinder of the camera may be at an uncomfortable angle when the scope is pointed overhead with a refractor, SCT, or Mak.

Another problem with using a Hartmann mask is that as the images merge as you get close to focus, the exact point of best focus can sometimes be difficult to ascertain.


Diffraction Spikes

A method similar to a Hartmann mask employs diffraction spikes. If you use a Newtonian telescope, your secondary mirror supports will already supply the necessary spikes. If you use a refractor or other scope without secondary supports (also called spider vanes), then you can simply place some wire, string, or wooden dowels at a 90 degree angle to each other, in the form of a cross in front of the aperture of the scope. When focusing is finished, they can be removed.

With this method, a fairly bright star is examined. Outside of focus, you will see two spikes. As you get closer to focus, the spikes will merge into one and get brighter and longer.

Problem with this method are similar to the Hartmann mask: the viewfinder of the camera may be at an uncomfortable angle when the scope is pointed overhead with a refractor, SCT, or Mak, and when the images merge close to focus, the exact point of best focus can be hard to determine.


Star Trail Test

This method was described by E. S. King in his 1931 classic book on astrophotography, A Manual of Celestial Photography.

Focus can be tested by making a series of time exposures and varying the focus during the exposure. A simple tripod can be used instead of a tracking mount as the stars are allowed to trail. A record is made during the exposure of which trail corresponds to a scale on the focusing ring of the camera lens, or focuser knob or draw tube on a telescope.

A black card placed in front of the aperture of the lens or telescope briefly between focus changes will help to differentiate different settings. Each trail should be of a sufficient exposure to leave a suitable trail in the image for the focal length of the lens being used, and a longer exposure can be used on either the first or last exposure to indicate which trail is the first or last focus setting.

It is a prerequisite that the focus of the telescope or lens must be repeatable in relation of the markings of the focus indicator scale. Telescopes that focus by moving the mirror can be problematic due to mirror slop. Auto-focus lenses that decouple the internal focusing mechanism can also be non-repeatable.

For both lenses and telescopes, focus should be approached by turning the focuser in the same direction during actual focusing as when the tests were preformed.

If this method is repeated on nights of different temperature, a chart can be compiled plotting focus versus temperature change, and exact focus can be accomplished on nights where temperature variations would change the focus.

For a telescope with a rack and pinion focuser, an inexpensive dial micrometer can be used to indicate focus position that is accurate to 1/1000th of an inch, or 25 microns.


Hybrid Method - Star Trails with a Hartmann Mask

A variation of the star trail focusing method combines it with a Hartmann Mask.

This method was described in detail in an article in Sky and Telescope magazine by Chuck Vaughn in February 1991. It is the method Chuck used to focus his Olympus 350mm f/2.8 telephoto lens for film astrophotography, but it can be used with digital cameras as well.

The method is pretty much the same. The mask should be oriented with the center of the holes on a north-south line so the double images don't overlap in the image.

Hartmann Mask Star Trail Test

For this test, a focusing scale was constructed with the finest divisions that could be made with an extremely thin and sharp pencil point, and the divisions at the smallest separation possible so that the lines did not merge. The scale was then attached to the focusing ring of a Nikon 300mm f/2.8 lens.

The focusing scale was examined with a high power magnifier and the lens ring turned the least amount possible each time, always approached in the same direction.

You can see on the image the difficulty that a star that is too bright presents. Try to pick a starfield with stars of different brightness and realize that the best ones will be on the fainter side.

Only half of the star trail that is numbered is presented here in order to give the image a sufficient scale so that it could be meaningfully evaluated. The other half not shown on the left side is pretty much a mirror image of the right side of the image.

We can see that the double trails are widest and that they get closer as correct focus is approached with the best focus being the trail numbered 1.



Autofocus

Most DSLR camera systems have auto-focus mechanisms built into the camera body that work in conjunction with auto-focus lenses and are sensitive enough to focus on a star with a sufficiently fast optical system. These body / lens combinations can be used to auto-focus on a bright star or planet (1st magnitude) or object with sufficient contrast, but should be tested first for reliability.

Some of these systems also offer a focus indicator that will work when attached to other optical systems such as telescopes if the f/ratio speed of the optics is bright enough for the auto-focus detection system in the camera body. It has been reported by Wil Milan that the Nikon system, will work with f/ratios as slow as f/6. Again, tests should be undertaken to determine the reliability of such a method with your particular equipment.

It may be difficult to correctly place the star exactly on the auto-focus detector because the detector is relatively small, and although usually well marked on the groundglass, it can be very difficult to see against a black sky in the dark. Shining a red flashlight down the tube of the telescope will illuminate the groundglass and the star can then be correctly positioned on the auto-focus detector. Once correctly placed, the flashlight should be turned off so as to not compromise the auto-focus system which works on contrast detection. Some bodies like the F5 illuminate the focusing rectangle that is active when the shutter button is partially depressed.


Parfocalized Eyepiece

It is possible to parfocalize an eyepiece with the focal plane of the camera.

The problem is making the eyepiece, in fact, parfocalized with the sensor of the camera. To do this, the camera must be focused exactly first. This can be done with another more accurate method, such as software metric-assisted focusing, or knife edge, or trial and error. Then the eyepiece is focused by sliding it in and out of the focusing tube and locked with a parfocalizing lock-down ring. The next time the telescope is used for astrophotography, it is focused visually with the parfocalized eyepiece and the camera is then substituted.

The problem again is the eye's accommodation, it can make up for tiny differences in the true focus, and the image will not be exactly focused.

If this method is used, the shortest focal length eyepiece possible (3 to 5mm) should be used to overcome the eye's accommodation.


Groundglass and Magnifier

The ground glass is at the same distance as the sensor in your camera. Since the image is focused on the ground glass, problems with the eye's accommodation, such as in a parfocalized eyepiece, are avoided.

Stellar Technologies makes a focuser called the CVF that takes the place of your camera and uses a ground glass and high-power magnifier to focus.

This method will also work with extended objects such as the Moon and Sun, in addition to stars, something a knife edge or Ronchi Screen can't do.


Knife Edge

Knife edge focusing is a simple concept that works remarkably well. It is not dependent on excellent eyesight and can be accomplished with or without glasses. It is a technique that is not easy for a beginner to use however.

Knife edge focusing

A knife edge is placed in the exact same position that the film's emulsion will occupy in the camera. The eye is placed slightly behind the knife edge and the image of a bright star is examined. You must use a point source like a star to knife-edge focus. You can not use knife-edge focusing on extended objects such as the Moon or Sun or galaxies or nebulae.

Since the knife edge is at focus and the eye is behind this point, the eye will see the star as a disk of light as the light cone expands again after focus. The knife edge is positioned so it is perpendicular to the scope's movement, either in right ascension or declination, and the telescope is moved so the knife edge cuts into the light cone.

If the telescope is exactly at focus, the star's disk will wink out all at once. If it is not at focus, the shadow of the knife edge will be seen moving into the disk.

For purposes of this discussion, the point of focus will be considered an actual point. Although in fact, it is not, due to diffraction (see Definitions and Formulas).

If you look at a star that is out of focus, you will see a disk of light. This disk wil be larger the more out of focus it is. If you insert a knife edge into the light cone where it is out of focus, you will simply see the knife edge start to silhouette the disk, entering from one edge and moving across the disk until it is completely obscured. This is the same as if you held your hand out at arms length and moved it in front of a light not far away.

Now imagine that the light source is only a point. What would it look like if you moved something in front of it? It would simply vanish immediately when something moved in front of it.

As the light cones comes to focus at the focal plane in a telescope it narrows down to nearly a point. If a knife edge is exactly at the focal plane, the star will wink out all at once as the knife edge moves into the light beam from the star.

If you work with a scope that has a slow focal ratio, the depth of focus will increase. For these scopes there will be a range where the star will grey out all at once. This range is the depth of focus and for a scope with a large depth of focus, the middle of the range should be chosen.

The type of blade used in the knife edge must be taken into account. If the edge is beveled on both sides, as most razor blades are, the focus will actually be 1/2 of the blades thickness from where you think it will be.

The knife edge must be where the sensor's focal plane will be. This is not trivial to achieve if you want to construct your own knife-edge focuser. You need to have some type of skill at machining and the ability to measure the distance exactly.

There are commercial solutions for knife-edge focusing. Hutech sells a knife edge focuser made by Mitsuboshi. If the knife's edge is calibrated to the focal plane of the camera, this can be an easy and convenient way to focus because no computer is required in the field. Stellar Technologies International also makes knife edge focusers for their Stiletto series.

The down side is trying to use one with a scope like a refractor where the camera end of the scope is pointed down to the ground. You can't use a knife-edge on a Canon lens that electronically focuses because they won't focus without a body on them. Also, you cannot knife edge focus on an extended object, such as the Moon, Sun, planets, nebulas or normal daytime scenes. In this case you can move the scope to a bright star, knife edge focus on it, and move the scope back to the object of interest. Bad seeing will make knife-edge focusing problematic, the worse the seeing, the harder the focusing.


Ronchi Screen

Focusing by Ronchi screen is exactly the same as knife-edge focusing. Instead of a single knife edge, a Ronchi screen has multiple lines, each of which can act as a knife edge.

This makes focusing a bit easier because the star does not have to be positioned exactly to a single edge, you have many to choose from. It is also very easy to see when you are heading in the right direction towards focus as the shadow of the lines become larger and less lines are visible. Very close to focus, only a single line is visible and this is the one that will act as the knife edge to finalize focus.

As with a knife-edge focuser, the Ronchi lines must be at the exact same distance as the Camera's sensor, so care must be taken in the construction of a Ronchi focuser.

Ronchi screens can not be used to focus on an extended object such as the Moon, Sun, galaxies, nebulae or normal daytime subjects.


Hybrid Ronchi Screen

Stellar Technologies makes a Ronchi screen focuser that uses a Ronchi screen and magnifier eyepiece in a hybrid focuser called the "Stiletto". The Stiletto takes the place of the camera for focusing. It has a convenient right-angle diagonal which makes focusing comfortable for when a refractor or SCT is pointed overhead.

In use, the astrophotographer sees the Ronchi screen lines when the image is out of focus. More lines are seen when the image is more out of focus. As focus is approached, the number of lines reduce and the individual bars of the Ronchi screen grow larger. When the lines disappear completely, and you see a shimmering whitish/gray area, the scope is at focus.

Once in focus, the scope focus is locked down, and the Stiletto is removed and replaced with the camera body.

The accuracy of this method depends on the size of the Airy disk that is formed by the telescope and the size of the grating in the Ronchi screen. If the size of the Airy disk is considerably smaller than the line screen, then the position of exact focus must be interpolated between when the lines disappear on one side of focus and when they disappear on the other side of focus.

The Stiletto can not be used on extended objects such as the Sun, Moon, planets, or normal daytime scenes.

Stellar Technologies also offers different screens for the Stiletto, including a standard knife-edge.


Digital Zoom

【资料】对焦的方法 - 冷冰川 - 天色轨迹-冷冰川的柴火间
Stars of the Trapezium enlarged on the camera's LCD display.
Take short exposures and zoom in on a star and examine with the LCD display on the back of the camera. Then you change the focus slightly and examine the image to see if the star looks smaller. Through a process of trial and error, you will eventually go through the point of best focus. When the star starts getting bigger you will realize this. The problem is going back exactly to the point of best focus. Unless you have some type of dial calibrator on your focuser, it can be difficult to know exactly where the point of best focus was and to return to it.

Another problem is the location of the LCD screen on the back of the camera when the telescope is pointed overhead with refractors and SCTs. This problem can be solved by outputting the video display from the camera to an auxiliary monitor. Then the image can be examined in relative comfort.


Software Metrics

Computer programs like Images Plus and DSLR Focus contain focusing modules. You'll need a computer or laptop to use them at the scope. With these programs, a star is selected in the field and just that portion of the frame is downloaded. The computer then continuously downloads short exposures, and the star is examined as the telescope is focused. When the star is at its smallest diameter, the scope is in best focus. Some programs examine the star and give a numerical readout of the stars diameter or FWHM (Full Width Half Maximum) or maximum brightness value to assist in determining focus.

Using this type of software is an excellent way to determine focus. It takes into account all of the variables involved in focusing because it examines the actual final image on the chip.

The downside is that you have to be setup to use a computer next to the telescope, which can be somewhat of a hassle if the scope is used in the field at a remote observing location.

On nights of bad seeing the focus star's diameter can change during scintillation, making determination of the point of best focus problematic. In this case, a fainter star can be used with longer exposures. This will help average out the effects of seeing, but will take longer. Or the software make take a number of exposures and average the values.

Another difficulty is that during the focusing process you really want to go past the point of best focus to be sure you have, in fact, seen the point of best focus. Then you have to go back and find that point of best focus again. It helps to pay attention to the numerical readouts in this case and note the numbers for best focus, and then try to find that position of the focuser again by trail and error. Note that these numbers can change due to seeing, even if the focus is not moved!

Make sure that a star that is selected for use in focusing is not saturated, especially if you are going to use the brightness readout to determine best focus. You may have to change the ISO or exposure time to ensure that the star is not overexposed, depending on the brightness of the star.


Real Time Video Display

Video from the sensor in the camera is displayed on the LCD on the back of the camera in real time. This is a feature that is available on almost every point and shoot digital snapshot camera, but has not been available in the past on most DSLR cameras except the Fuji FinePix S3Pro and Canon 20Da. With the lastest generation of DSLR cameras, such as the Canon 1D Mark III and 40D, and the Nikon D3 and D300, live focus is becoming standard.

【资料】对焦的方法 - 冷冰川 - 天色轨迹-冷冰川的柴火间
Canon 20Da setup for live video out sent to a hand held monitor for focusing.

The Live View video can usually be enlarged which greatly facilitates focusing on a star. In my experience, I find it is best to focus on the faintest star that you can see on the Live View video display.

In some cameras it is possible to change the brightness of the display by changing the ISO. Other cameras automatically adjust the brightness to give the best display, although this is really intended for daytime scenes. This live display can be accessed through the camera's video out plug and fed to a monitor or computer. Any information that is displayed on the LCD on the back of the camera, such as menu items and picture review, can also be viewed through the video-out plug.

  • Live Video Out to a Television - The live video feed out of the camera can be fed to an auxiliary monitor or television where it can be viewed in a more comfortable position if the scope is pointed overhead where viewing the LCD on the back of the camera would be in an awkward position. You don't need a computer in the field to use this feature. I use a 30 year old Walkman hand-held TV. Recently I acquired a $15 black and white television that runs on D-cell batteries and 12 volts. Others use DVD players and hand-held video game players. Any device that has an input for analog video can be used to display the live video out of a DSLR.

  • Live Video Out to a Computer - It is also possible to view this live video in real time on a computer. Several devices are made for PCs for taking a analog video feed and inputting it to a computer through a USB or Firewire connection, such as a Dazzle Digital Video Creator or Belkin Video Bus II.

    Once you have the hardware to get the live video feed into your computer, you'll need a software program to view it. You can do this with several different freeware programs, such as VirtualDub, or Hocus Focus.

    Hocus Focus is a program designed by Gregory A. Pruden especially to focus a digital snapshot camera for astrophotography, such as a Nikon Coolpix that has real-time video-out capabilities, but it will also work with the Canon 20Da. This program is very useful for focusing a star because it has several different metric displays for focusing, such as FWHM (Full Width Half Maximum), maximum brightness, and radial sum value.

  • USB Out to a Computer - The latest generation of DSLR cameras (Canon EOS 40D and 1D Mark III, and Nikon D3 and D300) are now feeding the video out of the camera through the USB cable which also control the functions of the camera such as shutter speed, ISO, etc. These cameras also allow remote viewing on a computer through the camera control software that comes with the camera.


Focusing Camera Lenses

Most modern auto-focus camera lenses will focus a little bit past infinity. This can present difficulties in focusing on stars at infinity, especially in the dark at night.

Some of these lenses are capable of focusing on a star with the camera's auto-focus mechanism, but the accuracy goes down as the focal length of the lens gets shorter, and the aperture of the lens gets smaller.

To auto-focus on a star, it is also necessary to have the star placed exactly on one of the small auto-focus points in the camera's viewfinder. These can be especially hard to find in the dark. Some cameras illuminate these auto-focus points, but then they can be so bright as to make the star difficult to see. In these cases, it may be necessary to shine a red flashlight into the camera lens at an angle that illuminates the field enough to make the auto-focus areas visible so a star can be placed on them, but not bright enough to make the star invisible.

It can also be difficult to keep the star on the auto-focus point long enough to achieve focus if the camera lens' focal length is long enough to show pronounced drift due to the Earth's rotation if the lens or telescope is not mounted on an equatorial mount that is tracking the stars.

If you can't get an accurate focus with the camera and lens on auto-focus, you can try using some of the various methods listed above. A right-angle finder's 2.5x magnification will work better on lenses of long focal length and less well on wide angle lenses. This is pretty much true of Hartman masks, diffraction spikes, and examining stars on the camera's LCD screen. All of these methods work better with longer focal lengths. Software assisted focusing on a computer or laptop and live video-out focusing work well, even with short focal length lenses.

Another method that works with wide-angle lenses is to autofocus on an object that is very far away in the daytime, and then tape down the focus on the lens, and turn off the auto-focus mechanism on the lens or camera body. This can also work with lenses of longer focal length, but the object on which you focus on needs to be farther and farther away as the focal length of the lens gets longer.

Be aware that many zoom lenses are not par-focal throughout the focal lengths of the zoom lens. This means that each focal length may focus at a different point for infinity focus.

On auto-focus lenses, also note that the true point of infinity focus does not always coincide with the infinity mark on the focus distance indicator on the lens barrel. This is particularly true for zoom lenses, especially those that are not par-focal throughout the zoom range.

Note that the longer the focal length of the lens, the more prone it will be to focus shift due to temperature changes during the night.

Focusing with a Filter

Focusing through a filter can be a problem. A filter used inside the light path between the objective and sensor will definitely change the focus. Filters such as a hydrogen-alpha filter can be extremely hard to focus visually because the eye is not very sensitive at the hydrogen-alpha wavelength. Narrow-band filters can also be difficult to focus through visually because of the reduced overall amount of light that they pass.

Live view focusing through the Canon 20Da, Mark III, 40D and Nikon D3 and D300 is excellent for determining the focus when using a filter.

Be careful of temperature shifts. One way to do this is to determine the focus at a given temperature, and mark the focusing ring twice, once without the filter and once with it and record the difference. Then in the future, at any given temperature, you can determine the focus visually without the filter, and move the focus by the amount determined by the previous test for the filter.


Focusing Recommendations

  • The live video feed out of a DSLR is a very easy method of accurate focusing.

  • Autofocus can work if the optical system is fast enough and the star is bright enough and located exactly on the autofocus spot.

  • Knife-edge focusers like the Stiletto and Mitsuboshi can be accurate and easy to use if you are familiar with knife-edge focusing.

  • Hybrid methods such as the Stiletto Ronchi screen and magnifier, or CVF with groundglass and magnifier can be easy to use and accurate.

  • If you use a computer next to the scope, examine images of stars in test exposures downloaded to the computer with software metric-assisted focusing such as DSLR Focus or Images Plus.

  • If you don't have a computer, examine images of stars on the LCD on back of camera at high magnification with crossed dowels for diffraction-effect assisted focusing.

  • Be aware that many auto-focus camera lenses will focus past infinity.

  • At an absolute minimum, take a test exposure and examining the star at high magnification on the camera's LCD. Tweak the focus a little bit on each side of the best apparent focus to see if you can improve it. Adjust the exposure so the stars are not saturated and over-exposed.

  • The worst way to focus is by eye alone through the viewfinder without any additional magnification.

  • Once you have achieved focus, lock the focuser down. If it is a lens, tape it down. Be careful that the focus does not change in the process of locking it down.

转载地址:http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM

  评论这张
 
阅读(235)| 评论(0)

历史上的今天

在LOFTER的更多文章

评论

<#--最新日志,群博日志--> <#--推荐日志--> <#--引用记录--> <#--博主推荐--> <#--随机阅读--> <#--首页推荐--> <#--历史上的今天--> <#--被推荐日志--> <#--上一篇,下一篇--> <#-- 热度 --> <#-- 网易新闻广告 --> <#--右边模块结构--> <#--评论模块结构--> <#--引用模块结构--> <#--博主发起的投票-->
 
 
 
 
 
 
 
 
 
 
 
 
 
 

页脚

网易公司版权所有 ©1997-2018