Rigid Camera Mountings

Tripod mounted cameras are limited to relatively short exposures of 15 seconds or less and a lens of no more than 100mm focal length is recommended. The reason for this is that longer focal length lenses produce less field of view on the film and therefore more magnification. As magnification goes up, we start running into the problem of "trailing" of the stars due to the earth's rotation and the apparent movement of the stars across the sky. The stars will no longer appear to be "dots" but will begin to resemble streaks which will get longer if the magnification goes up or the exposure time increases.

Tracking the Stars

This introduces a problem. To successfully make these longer exposures, we will have to keep the camera aimed at the same piece of sky as it is slowly drifting to the west due to the earth's rotation. The simplist form of camera "tracker" is the barn door tracker. A simple example is nothing but a couple of wooden boards fastened together at the ends with a hinge. One of the boards has a 1/4" hole drilled through it so that a camera can be mounted with a short 1/4" bolt and a second hole drilled and threaded so that another longer bolt can be threaded into the hole with the bolt head projecting downward and butting against the second board. The second board is in turn fastened to a wooden wedge that has to be carefully cut to a degree of angle which corresponds to the degree of latitude north or south of the equator from which the photograph is being made. For example, if your position is 30 degrees north latitude, then the wedge has to be cut to a 30 degree angle. The purpose of this is to align the hinges' axis to the north celestial pole. With this hardware assembled, one only has to find a SOLID and LEVEL surface onto which to clamp the assembly. Using a straightedge against the center of the hinge line, adjust the barn door tracker and camera until the straightedge points at the star Polaris (the North Star). If the mounting is level and the hingeline aligned with the north star then the photograph can be begun. By carefully rotating the tracking bolt a specified fraction of a revolution at predetermined intervals, the camera can be made to "track" the stars without trailing them on the photo. The earth rotates 1 degree every four minutes and that translates into 15 degrees per hour. Determine the number of revolutions required of the tracking bolt to move the hinge angle 15 degrees. With a little math, you can easily determine how much the tracking bolt should be turned every 15 seconds or so. Exposures of up to 15 minutes can be made with this device and still get nice round star images.

Since this is only a very simple example of a barn door tracker that does not have a "panning" head built on the board that mounts the camera, we can only make exposures of objects no higher than about 35 to 40 degrees above the horizon. If you want to get a bit fancier, a swiveling camera mounting will allow imaging on any part of the sky.

Piggyback Photography

Before you give up and decide that this is all too confusing, let me explain exactly what is meant by an equatorial mounting. In the previous section, remember where I described a basic barn door camera tracker and the wedge you would need to fabricate that would correspond to your latitude either north or south of the equator? Well, by mounting your barn door tracker to the wedge, you are in effect setting your camera up on an equatorial mount where the rotational axis (the hinge) of the barn door tracker is aligned with the rotational axis of the earth (this is an imaginary line between the north and south poles of the earth). If you simply mounted the camera on a vertical swivel so that the camera could be aimed at the star during the duration of a long exposure (which incidently would be called an alt-azimuth mounting) you would end up with a photo where any star that was in the exact center of the shot would be a nice round dot but every other star which is not at the exact center would exhibit itself as an arc. These arcs would seem to be arrayed all around the center star with the arcs being progressively longer the farther away they are from the center. This effect is called field rotation and is caused by the axis of the camera swivel NOT being aligned with the rotational axis of the earth.

Therefore, to get around the problem of field rotation, we have to swivel the camera on an axis that is parallel to that of the earth's rotation about it's own axis. By doing so, we can now make exposures (in theory) for as long as we like and get nice round star images provided the slewing rate (tracking rate) exactly matches the rotational period of the earth.

Any telescope or imaging device on such a mount is said to be equatorially mounted. Most telescope manufacturers and some aftermarket companies make a piggyback camera mounting bracket for purposes of mounting the camera to the scope. This is accomplished by attaching the camera to either the declination axis of a German equatorial mount (GEM) or to the optical tube assembly (OTA) of the scope itself. In the case of a fork mounted Schmidt Cassegrain or Matsutov the piggyback camera mount is attached to the OTA. With this arrangement, the camera can be aimed by moving the scope tube itself in right ascension or declination to the object to photographed. There are a couple main subdivisions of piggyback mounts and several variations of each. One is the standard and cheaper rigid mount which most manufacturers offer and a more versatile swivel mount which lets the camera be aimed independantly of the OTA. Either will work great so long as they are will designed and not susceptable to vibrations or slippage. If the utmost in precision is desired, one can even use the telescope with a reticle eyepiece as a "guider" to ensure that the camera always points exactly to the same area of sky if extremely long exposures of an hour or more are desired. This is done by monitoring the tracking of the scope drive while viewing the positon of a guidestar in the reticle eyepiece and making corrections to the scope when needed.

Prime Focus Photography and the Horror of Focusing

Prime focus shots using a T-adapter usually are constrained to bright objects such as the moon, open clusters, and possibly some of the planets. The reason for this is due to the very long focal lengths you are now using. As the focal length increases, you are taking in a smaller and smaller area of sky and the light from extended objects such as nebulae and galaxies is spread over a larger area of film. You are getting as much light from the object as you did with the shorter length lenses but the key is that it is spread out more on the film (the object will appear larger). Because of this, longer exposures have to be made to allow the film more time to accumulate an image on a larger area of film. With longer exposures, problems arise that so far you haven't had to be concerned about and they are scintilation (unsteadiness in the atmosphere), a less than perfect polar alignment of the scope (the norm), and periodic errors in the scope mounting drive. These problems will manifest themselves as trailed stars, wiggly lines or tiny arcs. Since we have no method while using the T-adapter to correct for this, we must limit exposure times to no more than a few seconds or minutes and select objects bright enough that will imprint the film adequately in this short time.

To make photographs with this method, you look through the camera viewfinder at the subject and focus it using the scopes focus knob. Of course you've got the remote shutter cable connected to the cameras shutter button so as not to jar the camera and ruin the shot, right? OK, you've got everything set up and are ready to focus the camera using the scope focusing knob. Hey! I can hardly see anything at all in the camera viewfinder and how can I get this thing focused if I can't see anything to focus on? (See? I told you this is not easy) But for every problem there is a solution.

The simplist and cheapest in my opinion is a simple aperture mask over the front of the scope that has two holes cut 180 degrees apart. The size of these holes varies depending on the aperture of the scope you are using (about 2" diameter for an 8 inch aperture and maybe 3" for a 10"). Unless you have a computerized scope such as an LX 200 or Ultima 2000 with "Go To" capability, you must use the finder scope to make an accurate mental or hard copy map of the position of the scope whille it is on the object to be photographed. You now move the scope to a bright star nearby and look through the camera viewfinder for the star. Most likely, you will see two stars with the same brightness. Move the scope focus in or out and you will notice the stars will begin to converge or diverge. What you want is to converge them into a single stellar image as accurately as you can and after doing so, you are in focus. Now move the scope back to the photographic object and using the locator map you made previously, position the scope EXACTLY as it was before. You can now make the photograph.

Another and perhaps better method is to change the focusing screen of the camera to one with less mattng to make the views brighter. Some cameras will allow you to do this and some won't. My camera (a Pentax K1000) won't.

Other methods are more drastic. A friend of mine who makes exceptional photos (you've probably seen them in the magazines) removes the top cover of his camera and the viewfinder prism. He places a homemade focusing magnifier directly on top of the focusing screen to get very precisely focused shots. Another method uses a device called a "knife edge focuser" to get a precise focus. This employs the fact that a well focused star makes a small pinpoint of light on the film plane of the camera. The knife edge focuser in simple terms is a very thin edge that when placed at the precise prime focus of the scope will cause a well focused star to quickly vanish from view when the scope is moved in a directin that will cause the knife edge to occulude the star. An out of focus star will not quickly vanish but will appear to slowly dim out as it is occuluded.

Guided Prime Focus Photography

Guiding requires the scope drive to have the capability of being adjusted or "corrected" in right ascension to compensate for drive errors caused by either manufacturing inaccuracies in the drive components or variations in power voltage. In addition, the capability to correct in declination in addition to right ascension, is highly desireable. Other errors that have to be corrected for are caused by atmospheric scintillation and positional errors resulting from refractive effects of having to photograph through less or more of the atmosphere as the object moves toward or away from the horizon in the east or west as the photographic object "moves" with the earth's rotation. Also, a less than perfect polar alignment of the scope can cause north-south or east-west drift errors over the duration of a long exposure.

Two of the most popular and almost universally used techniques of guiding are (1) the use of a seperate guiding scope and (2) the use of an off-axis guider. both methods use either a reticle eyepiece for visually monitoring the scope's tracking or they use a CCD autoguider to automatically do this in place of an eyepiece.

An off-axis guider is a close cousin to the T-adapter mentioned previously except for one important difference. Instead of the empty projection tube of the T-adapter, an off-axis guider (OAG) employs the use of a "pick-off prism" that is located about midway in the projection tube part of the off-axis guider body. An eyepiece barrel is manufactured into the body of the OAG at a perpendicular angle to the axis of the scope and camera and this eyepiece barrel is situated precisely over the pick-off prism so that a small fraction of the light coming through the scope can be diverted into the eyepiece barrel. The pick off prism extends down into the light path just enough to intercept some of the off-axis light coming through the scope's optics (this is of little concern since most of this light would be wasted anyway since it is so far off-axis that it would never get to the film plane of the camera anyway). A reticle eyepiece (one with a crosshair pattern or similar arrangement) is inserted into the eyepiece barrel of the OAG and is used to monitor the position of a "guidestar" and thus allow the user to make appropriate corrections to the scope drive if tracking errors are detected. The camera is attached to the off-axis guider body utilizing a T-ring in exactly the same manner as is done with the T-adapter.

A suitable guidestar is selected by first framing up and centering the object to be photographed. The reticle eyepiece is inserted at this time and is focused by sliding it in or out of the barrel until focus is achieved. It is then secured in focus with a thumbscrew. If a suitable star is not located, a radial search must begin. With the simpler made versions of OAGs, this is done by loosening the threaded slip ring (secures the OAG to the rear cell of the scope) and radially moving the entire OAG-camera assembly while looking through the reticle eyepiece until a star is located. After doing so, the slip ring is securely tightened. Other (and more expensive) versions of OAGs provide the capability to make radial searches without having to loosen the slip ring and possibly screw up a nice composition in the camera. A single thumbscrew is loosened and the eyepiece barrel can be rotated independantly of the guider body and camera combination.

The main advantages of using an off-axis guider are that first, it is the cheapest way to guide and second the OAG utilizes the optics of the imaging scope and thus avoids the major disadvantage of "differential flexure" that is often encountered with the next method of guiding which we are about to discuss. When one is photographing with an SCT, the off-axis guider is almost mandatory because of the mirror "flop" problem inherent of scopes that physically move the primary mirror to focus.

This second method involves the use of a seperate guidescope that is mounted on top of or alongside the imaging scope. Both scopes are driven on the same mounting and theoretically any variance seen in the tracking of one will also be seen by the other. There are major advantages to using a seperate guidescope. The main one is that the acquisition of a suitable guidestar is much easier since the guidescope can be independantly reaimed (within limits) of the imaging scope so that the compositon of the shot won't be ruined. The second most important advantage is that the images of the guidestar will be far superior to those provided with an OAG. I need to digress a little here and explain this: most off-axis guiders (though not necessarily so) are used with Schmidt Cassegrains. SCTs use spherical primary mirrors that, by nature of their figure, have a lot of spherical abberation and coma. Spherical abberation manifests itself with stellar images looking more like little "comets" rather than the nice tight pinpoints of light that excellent star images should have. This is the reason that SCTs have corrector plates at the front of the optical tubes. The corrector plate eliminates most of the spherical abberation so that it won't be seen in the eyepiece or camera. SCTs also utilize an internal baffle tube to block off any off-axis light from reaching the eyepiece. While the internal baffling of the scope does an excellent job of preventing off-axis light from reaching the eyepiece or camera, it does not stop the off-axis light from getting to the pick-off prism. Sadly, this is the very light that off-axis guiders have to work with and thus the images seen in the reticle eyepiece are not exactly "pretty". A little further digressing needs to be done here: if one plans on using a CCD autoguider at some point in the future, he should be aware that autoguiders prefer nice pretty star images much as your eye does and the task of configuring the adjustable parameters of an autoguider is much tougher if the star images are smeared with spherical abberation and coma. I'm not saying that autoguiders cannot be used with OAGs but rather that the set-up times can sometimes be frustrating. Now we can get back on track.

Major disadvantages of seperate guidescopes are few and simple. cost being the biggest and increased weight and less protability being the least. The guidescope doesn't have to be of the same quality as the primary instrument but does need to have a primary focal length of at least half that of the main tube. Ideally, it should be equal to the imaging scope but in most cases this is not practical because of cost, weight and protability considerations. Smaller and lighter scopes will surfice since their focal lengths can be increased with the use of a barlow or other lens. A rule of thumb is that the operating focal length of the guidescope should surpass that of the imaging scope by a factor of at least two to three times. The second disadvantage of seperate guidescopes is the requirement for a very sturdy attachment mounting that will keep the guidescope in perfect alignment with the imaging scope. This is more difficult to achieve than at first it may seem. When operating at focal lengths of 1500mm or more, even the tiniest bit of flexing from the weight of the guidescope in either the guidescope optical tube or in the mounting to the imaging scope will cause a loss of alignment of the two. Thus, as the main scope mount tracks the sky, slight changes in the center of gravity can cause the two scopes to get out of alignment with each other. The result will be "unexplained" trailing of the stars even though the photographer knew his or her efforts at guiding were near perfect. This effect is called differential flexure and is well known by many astrophotographers.

Differential flexure can and is avoided by experienced astrophotographers who use seperate guidescopes. The remedy is good engineering of the mountings between the two scopes and a healthy investment of cash to either buy or custom manufacture suitable mountings.

This technique is also the most difficult to do successfully. Due to the extreme focal lengths used, very PRECISE tracking and polar alignment is called for. These extreme focal lengths enlarge on the film surface a fixed quanity of light coming from a relatively small object (or area of an extended object) and thus require exposure times to be relatively long. I commonly use exposure lengths of 1/2 second to 4 seconds depending on the brightness of the object and the focal length of the eyepiece used. Exposures any longer will most likely result in blurred images due to scintillation or tracking inaccuracies.

Since I personally know of no way to guide eyepiece projection shots off-axis, I have to resort to those nights when the atmosphere is extremely steady and the transparancy is good. Even then, the failure rate is very high with only one or two decent photos out of a 24 exposure roll of film. The only way to get good shots is to pick your nights, shoot LOTS of frames, and bracket your exposure times by varying their lengths. There are charts, tables and PC software programs to advise you on exposure times for various objects, but in reality they serve only as guides to get you started. For some reason, what works one night will not work on another. Sky glo, light pollution from one night to another and site selection are all variables that affect the results. Experimentation with different films, exposure times, different eyepieces and meticulous record deeping will build a database from which you can increase your success rate.

Reticle Eyepieces

Illuminated reticle eyepieces are absolutely necessary for guided astrophotography and a few features they may or may not have will now be addressed.

Guide stars generally are tough customers to see. One would think that with all the stars in the heavens, surely a star adequate (meaning bright enough and well positioned) for guiding purposes would be easy to find. Well, sadly, this is not the case. I have spent as much as 30 minutes tweaking the scope position and making radial searches before one was found and even then the selection could be far from adequate. If one has ever leaned over a guiding eyepiece for a couple of hours trying to observe a star barely visible knows exactly what I mean. Ever heard the phrase "stare crazy"? Guide on a marginal star for an hour or so and you'll understand.

This brings up the first of the features I consider important in the selection of an illuminated guiding eyepiece and this is the capability to either dim or brighten the illumination of the reticle. When working with marginal guide stars, sometimes the reticle needs to be dimmed to near invisibility to keep from washing out or overpowering the guidestar. A too bright reticle can make the use of dim guidestars impossible.

A second feature I consider important is the capability to "blink" or turn on and off the reticle automatically while the photo is being made. Monitoring accurately the position of a dim guidestar is made much easier if the eye is alowed to rest from the glow of the reticle for brief periods. This allows the eye to stay much more sensitive to low levels of light and thus allow much dimmer guidestars to be successfully used. A few astrophotographers claim that a blinking reticle is nothing but a distraction and they absolutely will not use them. In answer to this, I can only say that from my experience, I have made several exposures of up to an hour while guiding on a guidestar that was so tenuous that there would have been no way I could have used the star without the blink feature.

The selection of the reticle pattern is the third most important feature. My favorite is the double reticle type that forms a small box at the intersection point of the reticle lines. This box provides an excellent guidance tolerance when making the photo. By keeping the guidestar within the confines of the box or as closly as possible to it, a well guided shot can be made. Reticles that offer angular seperation grids are great for determining the position angle and seperation distance between two closely spaced stars but for guiding purposes, they are next to worthless. Any superficial clutter in the reticle pattern only makes for more difficult guiding when using faint guidestars. My advice is to keep it simple and don't go for the extra features.

Illuminated reticle eyepieces get their reticle illumination from a small light emitting diode (or "led") built within or inserted into the side of the guiding eyepiece. the led is powered either by it's own self contained battery or by the scope's power supply. The selection of either type is determined by the type of scope and its features or by personal preference. Both can work equally well but in my opinion the remote powered ones are preferred.

The selection of the optical type is really not important. Wide fields of view are unimportant unless the user has other special requirements of the eyepiece other than its use as a guiding eyepiece. Mine is a super Plossl type of 9mm focal length but this is unimportant for guiding purposes. The most important consideration, however, is that it be well made and of good quality. Prices range from about $50 for the most basic up to about $200 for the higher quality ones.

The first requirement of the camera is that it have the capability to hold open the shutter for extended periods of time. This necessitates that the shutter control be of the manual type rather than the electronic or fully automatic variety. Most modern 35mm cameras manufactured today have electronic shutter controls which makes them unsuitable for long exposure photography due to the fact that the shutter, while being held open, imparts a constant amperage drain on the camera's battery. The small batteries in these cameras cannot withstand very long periods of holding open the shutter and will quickly deplete their charge and go dead. In this case, the shutter will close since there is nothing to physically hold open the shutter. The best camera selection is one with a manual shutter control. With a remote shutter cable installed and the cameras shutter speed set to the "B" (or bulb) position, the shutter can be opened, and indefinately held open, by depressing the button on the cable and locking it down with the thumbscrew on the remote cable.

A second important feature to look for in a camera is mirror lockup. When the shutter button on a camera is depressed, two things happen in rapid succession. First, the mirror flips upward to allow the light entering the lens to get to the film and second, the shutter then opens. The motion of the mirror while retracting out of the way and the rapid sudden stop when it does, sets up a vibration in the camera and scope assembly that often will blur or create double images on the photo. The most desireable astrophotography camera is one that has the capability to "lock up" the mirror before the shutter is opened. This allows the vibrations to settle out before the film is exposed to the light and thus eliminate the blurring that sometimes occurs with the mirror movement. Purchasing a new camera that has manual shutter capability and mirror lock up these days is next to impossible unless one is willing to spend big bucks on some of the very high end models that offer manual as well as automatic operation. A better choice is to shop the pawn shops or used camera dealers for one of the older models that were made when manual control was the norm. There are several that are useful and a couple that come to mind were the Cannon F-1 body that was manufactured in the early 1970s and the Olympus OM-1.

Film and Gas Hyering Film

Commercially available film emulsions available today do a fantastic job on the purpose for which they were intended and that is to produce high quality photographs in daylight conditions or in low light conditions when used with a flash. Astrophotography, however, is an entirely different ballgame. Here we are usually dealing with very low levels of light that these emulsions were never intended to work with. The exceptions to this would include lunar photography and photography of the brighter planets. The moon, for example, receives virtually the same illumination from the sun as the earth does during daylight. Exposure times and film selection are nearly the same as those you would make photographing terrestrial subjects but can vary somewhat according to the phase the moon is in when the shot is made. The question of color balance can come into play since some film types seem to color the moon in an unnatural way and cause the sky to take on a "muddy" appearance in the photo.

Luckily, some of these emulsions are adaptable to our hobby. What I personally prefer is to select a film that is moderately fast with an ISO of at least 200 to no more than 1000. This seems to be a good compromise between speed and grain. The faster the film is, the more quickly it will record the object being photographed at the expense of more grain. There is one particular film that I feel is fantastic for the beginner and that is Konica's 3200 speed flim. Although the ISO is much greater than ISO 1000, this stuff will do miracles with 10 minutes exposure at f/6.3 or 25 minutes at f/10 from a dark site. It does, as I mentioned, have a good bit of grain but the speed benefits and correctness of color make it well worth considering. My personal favorite is Fuji's Super G 800 Plus because of its relatively fast speed, color balance and respectable grain. It also responds to gas hypersensitization quite well which I will get into latter. Its not bad right out of the box either without hypersensitization and I frequently use it that way. New emulsions that have recently hit the market are Kodak's Ektapress Multispeed 640 and PJM. I personally have had little experience with either of these and will have to investigate them further before making any coments. All the previously mentioned films are color emulsions but we also shouldn't ignore black and white films. I will mention only one and that is Kodak's Technical Pan 2415. This emulsion was developed several years back as an emulsion to use on glass plates at profesional observatories. Since then, it has been used on conventional 35mm and larger mediums. TP 2415 has the finest grain characteristics of any film available and if extremely fine detail is desired, this is the one to use if it is shot gas hypered and processed in D-19 developer.

OK, you've heard the term gas hypering several times now and I'm sure you are wondering what the heck it is. It's time to digress again and discuss the concept of reciprosity failure of film. As I mentioned earlier, commercially avialable films are good at what they were designed for, and that is making "normal" photographs under "normal" conditions. Long exposure photography of the night sky puts requirements on film that it simply wasn't designed for. This particular use can hardly be called a "normal" environment. When the shutter is opened for long durations, film emulsions have the characteristic of recording the few available light photons from the object being photographed quite well initially but after a period of a few minutes it begins to "resist" accumulating any more image density and as even more time passes, this resistance seems to increase. Areas of the emulsion that don't receive as many of the photons (areas where the sky should be more or less black) will continue to brighten due to the exposure of sky glo and light pollution. The result is that the image density will only get to a certain point, begin to slow down and eventually stop getting any denser while the "dark" areas in the frame will continue to get denser (lighter). The latter effect is called sky fog and if allowed to continue, the photograph will come out with a bright sky (instead of the dark one which is desired). This has the effct of washing out the object that was photographed due to the decreased contrast between the two.

I seem to remember reading that during the days when the old TP2415 emulsion was used on the glass plates at the professional observatories, someone discovered that if the emulsion was treated with a pure anhydrous form of hydrogen gas under pressure and low heat, the emulsion would tend to accumulate image density of the object photographed for longer periods of time at a fairly constant rate and thus stay ahead of the sky fog levels. In addition, the relative "speed" of the emulsion was increased as an added benefit. Strangely, the act of gas hypersensitization actually causes a slight amount of fog on the unexposed film. Hydrogen was and still is used for this purpose but it has one major drawback- it is extremely flammable and is hazardous to store. Accidents have and do happen and this makes the use of pure hydrogen a hazardous pursuit for the amateur user.

Sometime latter, it was discovered that a non-flammable mixture of 92% nitrogen and 8% hydrogen would do the job almost as well without the inherent risks of pure hydrogen. This gas mixture is called forming gas and its use is quite safe aside from the hazard of handling and storing pressurized bottles.

The process of hypering with forming gas today is almost universally accepted among amateurs. Commercially manufactured hypering kits are available from Lumicon which seem to be the most commonly used along with a few homebuilt units. The technique of gas hypering involves putting the film in an airtight cannister, pumping the chamber interior to a very deep vacuum, purging the interior with forming gas and revacuuming for two or three cycles. The chamber is then pressurized with forming gas at about three to six psi and left to "bake" at about 50 degrees Centigrade for periods of from 4 hours up to 100 hours depending on the baking pressure, temperature, and film type used.

The action of hypering film does two important things: it removes nearly all the moisture that is trapped in the film emulsion and various other impurities that cause reciprosity failure.

The drawback to using hypered film involves its shelf life after being treated. It can be stored in a freezer under airtight conditions for a few days but that is about it. Generally when the film canister is opened and the treated film exposed to the atmosphere, it must be exposed as soon as possible and then processed. It degrades rapidly and usually can be used for only one night. This means that maybe only two to four shots can be made on a roll before it will have to be developed to stop further degradation. Moisture and heat are the prime enemies of gas hypered film. The processing or developing of hypered film is pretty much stock and can be done commercially at one hour photo labs with the standard C-41 process.

Getting Your Film Developed and Printed

Some prefer to do their own processing and I suppose to get the most out of the negetives, that is exactly what one should do. However, if you are like me, you would rather have someone else do the hard work and instead concentrate on the easy task of making the photos. HAH!