What is Plate Solving?
Plate solving is comparing an astronomical mage with a known database to determine the exact coordinates to which the telescope is pointing. Plate solving is done either with a local database on the users computer or by sending the image to an internet database.
As accurate as the pointing is for some high-quality telescope mounts, there is enough potential pointing error that the intended object may not appear on a small camera sensor. Or if it does appear, it may not be centered. Sometimes with dimmer objects, the short exposures used for framing may not show the object at all. But by comparing the image with known star patterns, the user can be sure that the intended target is in the image. It is not absolutely necessary, but the tools are so good, why not use it?
Years ago, when I started learning the hobby of astrophotography, image sensors were quite small. My SBIG ST7 camera, attached to my 10-inch Meade LX 200 telescope, combined to create a field of view of just 0.25 x 0.17 degrees. For comparison, the moon has a diameter of approximately 0.5 degrees. By today’s standards, the pointing accuracy of the Meade LX200 was not particularly good, and the sensor was not particularly sensitive. When you combine those two factors, there were many times when I pointed to an object in the mount’s database and saw absolutely nothing resembling the object in which I was interested, and only some brighter stars could be seen. I sometimes spent hours examining known star patterns from other photographs of my intended object, trying to match them with the stars on my sensor. Usually, I could use the star patterns to determine if I was pointing to where I wanted and to roughly center the object on the camera sensor. But there were times I could not match the star patterns on the image with known photographs of stars near my intended target, and I gave up trying to image for the night.
Enter plate solving! Today’s imaging software allows us to take an image and automatically compare the star patterns with a huge known database of the entire sky. Not only will the software determine the match, but it will also nudge the telescope until the intended object is exactly centered in the field of view! It will also calibrate the mount coordinates to match those of the known database coordinates. This automatic match is even accomplished in a matter of seconds, saving a great deal of time trying to do the match visually and manually.
ASTAP (Astrometric STAcking Program) is one of the most popular plate-solving software. It also does image stacking, but I don’t use it for that purpose. It also integrates into common Imaging software, including the free NINA software. ASTAP is my first choice because I have found the results usually take just a few seconds. When the software is installed, you have a choice of which database to download, ranging from a small database all the way up to a large database. I recommend the large database if your computer storage is adequate. The results are usually very fast because the database is local on your computer, the image has known approximate coordinates based on the image itself, and there is no need to “send” the image out on the Internet to be interpreted. Setup requires you to enter your focal length and pixel size, so ASTAP knows the size of the field-of-view to inspect. When I have a target in mind, NINA will take the image, use ASTAP to determine the actual coordinates of the center of the frame, nudge the telescope to center the object and then match the mount coordinates to the actual coordinates. This process may be repeated a few times to center the object precisely, but it usually doesn’t take more than a minute or so.
Another popular plate solver is astrometry.net. This is considered a blind plate-solver. An image is taken and submitted to the nova.astrometry.net website. You do need to create an account to use the service, but when local plate solving doesn’t work, blind pate-solving can really help. Only the image itself is uploaded, so astrometry.net has no other identifying information, such as catalog name or expected coordinates. It uses an algorithm to decipher star patterns and matches them to a known database. This process takes longer because the image needs to be uploaded, and the algorithm takes time to interpret the star patterns to determine the sky coordinates.
Plate solving has revolutionized the process of framing and precisely centering your intended target. This is great for all astronomical targets, but it is particularly useful for dim objects in brighter nighttime skies where the object may not be visible in the short exposures used to find and center the object. It can also fine-tune and calibrate the pointing in go-to mounts.