Plan Your Imaging
By: Don Selle
“Fail to plan, plan to fail” is an old adage that applies very well to most activities but especially well to astrophotography. Dark time in the field is a precious commodity and it can be very easily wasted. When you finally get all the equipment working together, and learn to avoid the operator errors, you will find having a plan and working it will help you make the most of your time under the stars.
You should know what objects you plan to image, and how you are going to image them (framing, exposure time, sequence of exposures, etc.) before you leave for your favorite dark site. Here are a few ideas for developing your plan that can help make your dark time more productive.
Selecting your targets
You should select your targets well in advance of heading out to image. While this might not be possible for a transient opportunity (such as a comet in the same field as a DSO), the last thing you want to be doing is searching through your planetarium program as it is getting dark to figure out what object you want to image. If you can take the time to create a list of targets in advance, you will increase your imaging efficiency. If you have also envisioned the image of each target that you want to create, you will find that your finished images will become better faster.
Crafting an excellent astro-photo has many similarities to taking an excellent daylight photo. In order to create the image you envision, you need to match the subject with the appropriate lens and camera exposure.
In a sense when you create an astro-image, you are showing the viewer the unique and beautiful “landscape” of the night sky. Selecting your targets in advance allows you to take the time to envision and plan for the images you want to create and ensure that they are best suited to your imaging set up.
Your telescope is essentially a fixed aperture and focal length camera lens, and the sensor in your camera is a fixed size so your field of view is fixed. So is your image scale which is based on the size of the pixels on your camera sensor*. A focal reducer can add some flexibility, and for planning purposes you should you treat this combination as a separate imaging system.
You want to select targets which roughly fill one half to two thirds of your field of view. This will ensure that you have sufficient resolution so that your object is well presented. Even though you can crop and resize an image to increase the size of the target, you will typically suffer a loss of detail when you do.
Seasonality of targets is also a factor to consider in target selection. When during the year will your target be best placed? Transit time of your target is the key here since you want to image when your target is high in the sky.
Transit times advance by nearly two hours per month. A target that transits at midnight on July 1st will transit at about 10 pm on August 1st. As a result, you want to choose targets that will be well placed in the sky to maximize your imaging time.
A good rule of thumb is that you should avoid imaging a target until it is above 30 degrees altitude, and imaging is best when it is near the meridian. At 30 degrees altitude, you are imaging through twice as much atmosphere as when your target is directly overhead. Seeing is typically worse at lower altitudes, and both light pollution and sky glow are also much brighter near the horizon.
I will usually select targets for a given date that are rising (in the eastern sky from a terrestrial perspective) and will transit about 2 hours after the end of astronomical twilight. This will give me ample time to polar align my mount, synch where my telescope is pointing in the sky with where my planetarium program thinks it is, get initial focus on both the telescope and guider, calibrate my autoguider, then slew to and frame my target, refocus and then start imaging when it is fully dark.
Since for most targets, my mount (a GEM) will track a couple hours past the meridian, I can get in about four hours of imaging before I have to flip the mount. During the winter months when there are more dark hours in the night, I will usually have a second “late” target selected that will be rising in the eastern sky and well placed shortly after I finish the imaging run on my first target.
Another factor you should consider when selecting your targets is their surface brightness, which is different than the visual magnitude of a DSO. Surface brightness is generally stated in magnitudes per square arcsecond. Like star magnitudes, the lower the number the brighter the object appears.
Here, the focal ratio of your telescope is the main consideration. All other things equal, a telescope with a higher focal ratio will require a longer exposure time to achieve the same exposure level on the same target**. If your telescope has a higher focal ratio, say f/8 or greater, you might want to consider imaging brighter targets. While you could spend enough total exposure time to image the “integrated flux nebula” around Polaris with that f/8 scope, you could complete the project in about 1/8th the imaging time with an f/2.8 scope.
One good way to establish a target list is to find examples others have done that you want to take on. An online search can be a good place to start. If you can find the details of how the image was captured – camera, telescope and exposure times are most important. This is where an online service such as Astrobin comes in handy. With the appropriate subscription, the whole of the Astrobin database, which includes all of the information needed for you to put together your own image plan can be searched.
Your target list should also include an initial exposure plan. This would list a minimum level of exposure for your OSC camera or for each filter you plan to use for your monochrome camera. Example images you have found which include info on the equipment and exposure times will be a big help in developing these exposure plans until you get sufficient experience using your own imaging set up and processing workflow.
A couple of other things you might consider doing, which are not critical, but will help you and potentially save you some time would be:
- Put together an equipment check list you will use when loading your equipment in your vehicle to transport to where you will image. Not only will this save you time but it could save you a whole trip because you will be less likely to leave a critical piece at home.
- Set up a standard set of computer files you will use for each target. This would include folders for your calibration frames plus folders for exposures for each of the filters for your monochrome camera.
- Keep a log of your imaging so you can go back and know what you did at a later date. Consider keeping a note of this in the actual file with your raw image files.
- When you do process your images, keep all of your raw files intact, including calibration frames. It is not uncommon that at some time in the future when your processing skills are improved, you will go back to these files and reprocess the image. You will find that with your new skills, the image is even more awesome the second time around,
Making a target list not only will save you time, but it will prepare you for when you decide to step up to automated imaging software like Squence Generator Pro, Voyager, or CCDAutopilot. You will have all of the information you need to jump right into these programs already assembled.
The beauty of pre-planning your imaging is that it can be done at any time. A little bit of thought, and pre-planning when its cloudy outside will pay you dividends when the weather finally turns good and you can be out imaging under the stars.