Now that you have selected an observing target that is suitable for your time, location, conditions, and equipment, your next chore is to find it. Not just knowing where it is in the sky, but actually getting the telescope pointed at it accurately enough that its light falls on your camera’s sensor in a location suitable for the composition you have in mind.
Finding objects is hard
You may find it harder than you expect to get your selected target into your camera’s field of view. There are several reasons why finding imaging targets is hard.
Finding objects you can’t see
First, you can’t see them. It’s no surprise that, with a small number of exceptions, you can’t see them with the naked eye; but you probably can’t even see them in your telescope’s finder, and many targets can’t even be seen in the main scope with an eyepiece.
There are some exceptions of course. Most Messier clusters can be seen in the finder scope, as can some nebulae. Few galaxies can be seen in the finder scope, but some of the brighter Messier galaxies can at least be seen in the main telescope with an eyepiece.
Despite these exceptions, the majority of your likely targets cannot be seen with your eye, even through the telescope. After all, one of the reasons for imaging is to capture objects that are too dim to see otherwise.
Even with brighter objects that you could see with an eyepiece, you may come to prefer not using an eyepiece. Mounting an eyepiece to find your target, then removing it and replacing it with a camera, will change the aim, collimation, and focus of your telescope. That’s fine at first, as a beginner, but your eventual goal will be to find your targets while leaving the camera firmly mounted in the scope.
Small field of view
Next, your telescope and camera combination may have a very narrow field of view, depending on your telescope’s focal length and the size of the imaging sensor in your camera. When you first started out in visual astronomy, you quickly discovered how much easier it is to find objects with a very wide-field eyepiece. Mounting some older, or lower-priced, cameras in a telescope is like using a very narrow-field eyepiece. The chip covers a very small area of the sky, leaving little room for error.
To estimate this effect, use the standard formula for calculating magnification:
But substitute the diagonal dimension of your camera’s sensor chip for the eyepiece focal length. Some example camera diagonal dimensions are:
|DX format DSLR||28|
|APS-C DSLR or Mirrorless||27|
|Micro 4/3 “point and shoot”||20|
|8300 chip camera||22|
|SBIG ST-2000 series||15|
|SBIG ST-7 series||8|
|Orion Starshoot (original)||4|
|Orion Starshoot G3||9|
|Meade DSI (original)||6|
As you can see, some older cameras that are available on the used market for bargain prices have very small sensor chips, which is like visual observing with a high-power eyepiece, and can make finding your target difficult. However, more modern cameras with large chips (and DSLRs) provide a field of view similar to what you’re used to with a moderate to wide-angle eyepiece.
Repeating the search on multiple nights
Finally, you will often build up astrophotographs by combining many exposures, often taken on different nights. This means that you not only need to be able to find your target, you need to be able to find it again, and on the second and subsequent sessions, place it in exactly the same location in the frame, with exactly the same rotation of the camera, in order to be able to combine new images with those taken on previous nights.
So, how do we find our targets?
Just like visual astronomy, finding targets for astrophotography is done in one of two ways: star-hopping or go-to systems. Let’s discuss those, and then bring up one interesting technological accessory that can help with many aspects of finding target objects for astrophotography.
Star-hopping for astrophotography
The classic way to find objects to observe or photograph is star-hopping. This separate article discusses star-hopping from a visual perspective, but the same general approach can be used for locating imaging targets. For imaging, there are some additional considerations:
Your camera’s field has a very well-defined size, and so it can be used as an accurate distance measure for star-hopping.
However, your camera’s small field of view can also be a disadvantage. If you are hoping to use test images instead of an eyepiece for the last, detailed, search while star-hopping, you will need to learn how to make very small and measured movements with your telescope. You can use test exposures with your camera to help with this. Using test images, learn how much 1/8 turn of your manual slow-motion control, or a one-second press of the motion button on your electronic control, moves your frame.
As we’ll discuss in a moment, most mounts well-suited to astrophotography are go-to mounts, and most imagers use go-to. If you are doing astrophotography without go-to and with manual sighting through star-hopping then, at least as a beginner, you should either stick with target objects that are bright enough to see in the finder from your site and use star-hopping with the finder; or use one of the available techniques to have an eyepiece available on your scope and star-hop using the eyepiece. (See this article for a discussion of two methods to swap an eyepiece for your camera without losing focus: parfocal rings, and flip-mirrors.)
Go-to systems for astrophotography
Most astrophotographers use go-to systems
I’m sure that statement will draw much criticism, and countless examples of excellent astrophotographers who do not use go-to. Nevertheless, I do believe that most astrophotographers do so. It is certainly true that most high-end mounts that are designed for astrophotography are go-to mounts. For example, the two most-used high-end brands for astrophotography are Astro-physics and Paramount. On those brands, go-to is simply a built-in feature of the mount, not an option.
For me, I’m interested in using my limited available time to either improve my instrument calibration, or to gather data. I don’t want to use my limited time hunting for objects. So I use the go-to feature of my mount, and would do much less imaging without it. (If you want to collect an album of images of things you found manually, without go-to, good for you; you’re a better person than me.)
There are several advantages to having a go-to system when doing astrophotography:
- The most obvious advantage, of course, is that you can find things quickly, without the need for a finder or for star-hopping.
- Go-to can also help you automatically or semi-automatically collect images from adjacent areas in the sky, to build up a matrix of images for mosaics covering large areas.
- Combined with a camera and appropriate software, go-to systems can position the telescope extremely accurately by taking and responding to test images.
- Go-to systems are also a very effective way to re-position your telescope to precisely the same location when collecting data on multiple nights, especially when combined with a computer-controlled camera rotator, which can automate ensuring you have the same camera rotation between sessions.
Getting accurate go-to results
As we mentioned above, one of the reasons to use go-to systems is to point to objects that you can’t see, even with an eyepiece in the telescope. In such a case, you really are relying on the go-to system, so it needs to work. “Close” isn’t good enough; the selected target needs to end up on your sensor chip after the go-to.
There are several things you can do to achieve highly-accurate go-to results.
Next, practice doing the appropriate multiple-star setup for your go-to system. Use multiple alignment stars if your system permits that, and use a high-power reticle eyepiece to centre the alignment stars. Even better, use a camera instead of an eyepiece, and use either a crosshair overlaid on the camera view, or plate solving, to calibrate your alignment stars. Ensure that at least one of your alignment stars – preferably the last one – is near your imaging target in the sky.
Finally, most mounts have some kind of additional feature to enhance the accuracy of the go-to system. Unfortunately, these features vary from mount to mount so I can’t give you specific advice here. However, there’s a good chance you have access to one or more of the following techniques:
- On some mounts you can continue to add synchronization data points to the mount’s sky model throughout the night. On such a mount, a good technique is to go-to a recognizable star near your target object, then synchronize this star into the sky model, then go-to the target. The synchronization to a known star near your target will usually give you enough accuracy to get the target on the chip.
- The control software for some mounts contains a feature to survey a large number of stars in the sky and build a very accurate pointing model that takes into account how the components of your system shift and flex as the telescope moves around the sky. TPoint, which is bundled with TheSkyX Professional Edition, and MaxPoint, an add-on to MaximDL, are examples of such software. On some systems, such software can be combined with your camera to automatically survey the sky and build an accurate pointing model. This can be a completely automated, hands-off process, taking less than an hour, and it can be done between dusk and true darkness, when it’s not dark enough for imaging anyway.
- Finally, “plate solving”, discussed below, can be used with some control software to automatically calibrate your position in the sky and refine the precision of your go-to operations.
A tip: several of these techniques can involve taking test emerges with your camera to refine your pointing location, and this will require that you be in focus. So, form the habit of first doing a go-to to a suitable focusing star near your target, achieving good focus, and then doing a go-to to your target. Since many targets are large, diffuse objects, focusing with them in the frame can be difficult. Use a nearby isolated star instead.
I’d like to end this article by discussing one more advanced, but extremely useful, software feature: Plate Solving.
Plate refers to photographic plates, a reference to the early days when photography was performed using glass plates coated with photographic emulsion. But despite the historical reference in the name, plate solving is a rather modern innovation, and uses sophisticated computer technology.
Plate solving is either a standalone application or a feature of many telescope control and planetarium programs; usually at extra cost. You hand, to the plate solving program, an image of a field of stars. The system then compares the relative location of the stars in the image with a comprehensive database listing the location of all known stars above a certain brightness. When it works, the plate solving application can tell you the exact location in the sky of the image you provided.
Two plate solving applications that I use are:
- The image link feature of the camera control add-on for TheSkyX Professional by Software Bisque; and
- Pinpoint Astrometry, a plug-in for MaximDL.
In addition to the above, the following two packages are extensively discussed in various online groups I frequent, although I have not personally used them:
- AstroTortilla (an open-source application which works with Nebulosity, MaximDL, and others); and
- Elbrus by Main Sequence Software.
Generally, you provide the software with hints by telling it your approximate imaging scale (in arc seconds per pixel), and approximately where in the sky you think you were pointing when you captured the image. The software will then return the Right Ascension and Declination coordinates of the centre of the image, the rotation angle of the camera, and the exact image scale.
You can then use this information for a variety of purposes:
- to adjust the position of your telescope: automatically with some control programs, and by trial and error with any;
- for the synchronization step of setting up the alignment of some go-to systems, instead of manually centring a star in your eyepiece;
- to return the telescope to the exact position of an image captured previously, to continue gathering data across multiple nights;
- to measure your actual image scale, not just a calculated theoretical value (which will use usually differ from reality because of the length of additional optical elements such as extenders in your imaging chain).
Slew the telescope to the approximate location of your desired target;
Take an image of the field where the telescope is now pointed;
Plate solve that image to determine precisely where the telescope is actually pointed;
Calculate the difference between actual and intended location, and move the telescope to correct the error.
The result is automated perfect pointing. (In TheSkyX this feature is called Closed Loop Slew.)
The final confirmation of your pointing is to take a test image, with an exposure just long enough to see your dim target in the frame, and to check your composition.
If you are using a dedicated monochrome CCD camera, use high binning for your aiming test exposures – at least 2×2, and 3×3 if your camera supports it. This will give you faster downloads and smaller files, and the resolution is perfectly adequate for alignment, finding, and framing purposes. Most important, binning increases the sensitivity of your pixels, so a short exposure will be enough to see what you are pointed at. I find a 20-second exposure binned 3×3 is enough to make nearly any target visible enough to confirm aim and composition, and often as little as 10 seconds, at 3×3, will do.
If you’re using a DSLR, binning won’t be an option, so you’ll just have to use longer exposures. DSLR users should not use the camera’s built-in “long exposure noise reduction” feature for test shots. That doubles the amount of time needed for each exposure and isn’t necessary when you are just confirming your aim. (In fact, in a later article I recommend against using this feature even for your main exposures, collecting dark frames manually instead.)
Note: If you are using a “one-shot colour” CCD camera, it probably will have a binning option, and you should use it for your test shots, as mentioned above. However, the results will probably not be in colour. Don’t be surprised. The way a 1-shot-colour camera works (with a Bayer Mask of coloured filters in front of the camera pixels), binning removes the ability to produce colour.
Now that we have found our target (or before, as part of finding it) we need to get into good focus.