Finding a bright planet or the Sun or Moon is usually pretty easy for visual observing: find it in the finder, center it with a low-power eyepiece in the main scope, and switch to a high-power eyepiece. The challenge of high-resolution planetary imaging is getting your subject into the tiny field-of-view of the camera at high-magnification. At thousands of millimeters of focal length, this can be "not so easy." There are a couple of different basic approaches you can take. Tips Non-Tracking Mounts If you are using a non-tracking Dobsonian, focus first on Polaris. Then acquire your subject first in a Telrad or finder. Turn on the camera's Live View and see if the object is in the frame. If it's not, you will either have to use a higher-power finder with cross hairs, or push the scope around and hunt for it by trial and error. Even once you find it and get it in the field of view, it won't stay there long with a small field and high magnification. Using a non-tracking mount for high-magnification work can be quite difficult. Tracking Mounts A tracking mount will make life much easier for planetary photography. A Dobsonian can be used on an equatorial platform. Even altazimuth computerized mounts can track an object across the sky, although they will suffer from field rotation. Equatorial tracking mounts are the easiest mounts to use for high-resolution planetary photography. Computerized altazimuth and Go To mounts must be correctly initialized and aligned/calibrated so the mount knows where it is pointing and so that Go To's are accurate. Polar Alignment An equatorial mount must be correctly and accurately polar aligned, or the object may suffer from significant drift at high-magnification. If your polar alignment is off, an object can drift completely out of the field of view in an amazingly short period of time at high magnification. Polar alignment is also critical for accurate Go To's with a computerized mount. Finders Whatever type of mount and telescope you are using, at the high-magnifications required for high-resolution planetary photography, an accurately aligned finder is a necessity. A 1x-power finder like a Telrad or Rigel is good for finding the object initially and rough centering, but a higher-power finder with cross hairs may also be required to get a planet into a small field-of-view at high magnification. After I have the planet centered in the finder, I use my DSLR with Live View at 1x as an electronic finder. This gives a larger field of view than at the magnifications required for 1:1 pixel resolution which we will use when actually shooting. Be sure to set your shutter speed to Bulb and your ISO to its highest setting. This gives Live View its maximum sensitivity. If your scope is grossly out of focus, you may not see anything with Live View, so try to roughly focus it first. Go To If you have a telescope equipped with computerized Go To, you can use it to find the planet you want to shoot, if you are accurately polar aligned, and if the mount is correctly initialized and synced. Once you find your object, it usually won't be perfectly centered. It's a good idea to center it, and then re-sync the Go To mount so the pointing accuracy will be better. This can be very helpful if you swing over to a nearby star to focus, and then go back to the planet. Don't forget to then set the exposure so it is correct for the planet by adjusting the shutter speed and ISO. Tracking Rates Normally a telescope used at night will be set to track at the sidereal rate, the rate that the stars move across the sky due to the Earth's rotation. The Sun and Moon, however, do not move at the sidereal rate, they move at different rates. Most electronic telescope drive systems have separate tracking rates for the Sun and Moon. Most of the planets also don't move at exactly the sidereal rate because of their orbital motion. The outer planets of Pluto, Uranus and Neptune track at close to the sidereal rate. Saturn, Jupiter and Mars don't generally move at a rate that is different enough to worry about, but Venus can move considerably depending on its location in relation to the Earth. Most drives do not have special tracking rates for the planets so you will have to use the sidereal rate. It will usually be close enough so that tracking is not a problem if you are correctly polar aligned with an equatorial mount. If you are shooting something unusual like a fast-moving comet or asteroid, none of the standard rates will really work well. You will either have to track on the comet or asteroid with a separate guidescope, or use software that lets you set a custom rate with an ASCOM driver for your mount. Finding the Sun You would think that finding the Sun during the daytime would be exceptionally easy. And it usually is. But actually centering it in a telescope is harder than you would think. The telescope will be filtered so you won't see anything but a black field until the Sun is actually in the frame, so you can't star hop to it. You will usually be at high magnification, so it has to be pointed very accurately. You might think you could just use Go To, but that has to be synced and initialized first so the scope knows where it is, and there really aren't any stars or planets you can sync on in the daytime except the Sun, but that's what you are trying to find.
This means you can't just sight along the side of the telescope tube to center the Sun. You can't look at it in your Telrad, or finder, unless you have proper safe filters over them also. One thing you can do to roughly aim your scope is to look at its shadow on the ground. The shadow will be smallest when the scope's profile is minimized the most when it is pointed correctly at the Sun. Unfortunately, this usually isn't accurate enough to actually center the Sun or a sunspot group if you are at high magnification. You can get a special 1x power solar finder, or make one yourself. These are simple devices that have a pinhole that projects the Sun's image onto a small flat target aligned with your scope. TeleVue makes a nice one that is about the size of a D-cell battery. The best solution is to simply get a safe solar filter for your regular finder. Baader makes an excellent solar filter material that is aluminized mylar. It is relatively inexpensive and can easily be cut to different sizes, one for your main scope and one for your finder. You can also make a simple cell to hold it. Don't forget to securely tape it down onto your scope and finder so that it can't accidentally be knocked off, or blown off by the wind. Focus is absolutely critical for high-resolution planetary photography. Focusing at high-magnification can be difficult, especially if the seeing is not good. Trying to focus by eye on low-resolution planetary detail can be especially difficult. If the seeing really makes focusing difficult, that is usually an indicator that the seeing is not really good enough to do critical high-resolution work. Don't give up however, because the seeing can change and get better at a moment's notice. If the seeing is awful because the jet stream is overhead and not going to move anytime soon, then that bad seeing will usually not get better. I recommend focusing on a relatively bright star that is close to the planet you want to shoot. With a star, we can use FWHM focus metrics in software like Astro Photography Tool, Backyard EOS, and Images Plus. Software metrics gives us an objective way to evaluate focus instead of subjective guessing. The specifics of using each software's particular type of metric are detailed in the previous sections about using that software. In general, the star forms a disk that can be measured for size. When a star is best focused, this disk has its smallest size. We simply adjust the focus on our scopes while watching the numbers. When the FWHM is lowest, the star is at its smallest diameter and is best focused. All three software packages also allow averaging a number of frames to smooth out seeing variations. This is a very useful feature that helps achieve accurate focus. We want to use a relatively bright star that is as close to the object we want to shoot because moving a large telescope around can move the primary mirror and affect collimation. If you slew a great distance across the sky, the primary mirror, especially in an SCT, will probably move enough for it to affect the collimation and image quality. Focusers Most Schmidt-Cassegrain telescopes focus by moving the primary mirror. This can cause serious problems for achieving and maintaining critical focus. My Celestron 11 Edge has the capability of locking the mirror down, but this doesn't do you any good while focusing by moving the primary. When you change the direction that you turn the focus knob while focusing, the image shift is so great that the planet can move entirely out of the field of view of the camera at high magnification. You can hardly focus on something that is not in the field of view. Refractors and Newtonians generally do not have this problem because the objective is fixed, and you focus by moving the camera in the drawtube of the focuser. It is very hard to judge exact point of focus if the object shifts when you change direction on the focus knob and the mirror shifts the image. If you have to move the mirror to focus in an SCT, always end by turning the knob counterclockwise to load mirror upwards on the baffle tube so it is less likely to shift and change the focus. The solution to the problem of image shift with an SCT at high-magnification is to lock the primary mirror down and install a rack and pinion or Crayford focuser on the back of the telescope. This will also help tremendously with collimation issues. You can also get some image shift in a poorly designed or manufactured rack and pinion focuser, but a Crayford focuser should eliminate these problems by design. That's not to say you can't get a Crayford focuser with problems, but the Crayford's design itself mitigates most of them. For focusers in general, as with telescopes, you usually get what you pay for. It is also possible to motorize a focuser so you don't have to touch the telescope at all to focus it manually. A motor on a good Crayford focuser can be an absolute delight to use. You will have virtually no image shift. This is important because you really need to go through the point of best focus to the other side before you can actually determine where the point of best focus was. This necessitates changing the focus direction and going back to the point of best focus, which is what causes so much problems when moving the mirror to focus an SCT. A motorized focuser also allows you to focus without physically touching the telescope. Even on a good mount, at high-magnification, touching the focuser can cause the image to move around, making accurate focusing difficult. Under good seeing conditions with a rock solid mount and a motorized Crayford focuser, it is also possible to automate focusing with some software packages like Images Plus. Focus Shift Due To Temperature Change Focus can shift due to dropping temperatures during the night. This is usually because the metal in the telescope tube shrinks and contracts as it gets colder. This can be especially problematic with Catadioptric telescope designs because of the magnification factor of the secondary mirror, usually by a factor of 5x, in an SCT. A 10-degree temperature change over the course of the night can be quite common, and will almost certainly cause focus to change. Some telescopes are designed with materials that counterbalance each other's thermal expansion and contraction characteristics, and this lessens the problem of focus shift due to temperature change. It is best to closely monitor the temperature and check the focus periodically through the night until you get a feeling for how it affects your particular telescope and you learn what you can get away with. Special Considerations for Solar Photography You can't use a star to focus during the daytime when you want to shoot the Sun, so what is the best way to focus on the Sun? Another problem you will have in trying to use Live View to focus on the Sun, either on the LCD on the back of the camera, or on a computer monitor, is that it is so bright in the daytime it washes out these displays. It is exceptionally hard to focus on a computer screen in the daytime. The solution is to isolate yourself and the computer screen in the darkness. You can do this primitively by just putting a winter coat over your head and over the computer. This can get very hot in the summertime however!
You can also buy a special laptop "tent" that will cover the screen. It will be shielded from the brightness of daytime and will have a small opening that you can peak into such as these by Hoodman, or Lapdome. I find the best solution is a special focusing cloth made for large-format photography. These are used by photographers who use large-format film cameras in the daytime. In its simplest form, it is an opaque cloth. In its best form, it is an opaque cloth that is black on the inside and white or mylar on the outside to reflect the Sun and keep it cool under the cover. Good ones are made by BlackJacket and Camera Essentials. You drape it over your head and the computer and pull it in around you to keep out stray light. If you can't focus on a star, what should you focus on? In this case, you can try to focus visually by best guess on the detail in a sunspot or on the edge of the limb of the Sun. Or, you can use a standard deviation metric in Backyard EOS. The standard deviation of an extended object is sort of a measure of contrast. Contrast will be at maximum when the object is at best focus. The seeing is usually worse in the daytime due to solar heating of the atmosphere and environment, so having a metric can be really helpful. Craters along the Moon's terminator are usually high contrast so visually focusing on them is usually easier, but this standard deviation metric can also be used when focusing on the Moon.
If you are using a refractor to do Solar Work and trying to focus with the magnified view on the LCD on the back of the camera because you are not using a computer, not only can it be hard to see the screen in the sunshine, it can be hard to even get to it unless you have a camera with an articulated screen that can be swiveled in different directions. When the refractor is pointed up at the Sun, the camera and LCD screen are pointed down at the ground at a very awkward and inconvenient viewing angle. The same is true for an SCT. It's not like a Newtonian where the camera is up at the top end of the scope with the LCD at eye level. In this case, it can be helpful to use something like an auxiliary monitor or a GigTube. You can feed the video signal out of the camera and view it under a focusing cloth at a much more convenient angle. If you are using a laptop to control the camera and focus, this won't be a problem. Focusing Masks Focusing masks like a Bahtinov or Hartman mask have become popular for focusing for long-exposure deep-sky astrophotography. One problem is that they don't work as well on extended objects like the Moon and Sun. At night you could use them on a star, but not in the daytime. Another problem is that they make the image dimmer, and harder to focus on. Since the image is already fairly dim because of the high-focal ratio and high magnification used in planetary photography, this is undesirable. These masks also effectively stop down the aperture and increase the focal ratio even more, increasing the depth of focus. This makes focusing more imprecise with the mask on, and more critical with the mask off. An interesting new variation on the focus mask is the GoldFocus mask by Jeff Winter. It uses special software to analyze the images produced by the mask to perform both collimation and focus. One problem is that, again, it doesn't work in the daytime on the Sun. There is also software by Niels Noordhoek called Bahtinov Grabber that will analyze a Bahtinov mask image and autofocus your scope if you have a motorized ASCOM focuser. Other Considerations Some Dobsonians that are made primarily for visual observing may not come to focus with a DSLR camera. This is because the focal plane for a DSLR is several inches behind where the focal plane for an eyepiece is located. In this situation, you can try using a transfer lens to move the image farther out of the tube, or sometimes a Barlow can make the image come to focus in the camera. Usually, however, you will need to move the mirror up in the tube by a couple of inches. This can be done by simply re-drilling the holes in the tube for the primary mirror cell, or shortening the poles for a truss-tube Dobsonian. Some small refractors have the opposite problem. Because they use a diagonal for visual use with an eyepiece, the DSLR camera won't come to focus even with the focuser racked all the way out if a diagonal is not used. The solution in this case is to use an extension tube to reach focus. You can also try shooting through the diagonal, but this adds additional optical elements to the system and mirror reverses the image. If you shoot through a diagonal, or any optical system with an odd number of mirrors, the image will be mirror reversed. You will need to reverse it back during image processing to make it correct.
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