More images taken through the time filter
Although the brightness difference in U Gem is subtle over a period of just three nights, another "star" in the field became very noticeable! It turned out to be the asteroid (240) Vanadis. On the night that I took the first image, the asteroid was just outside the frame. Three nights later, the asteroid had moved into the field of view, below U Gem. The time filter gave very good contrast to the asteroid!
Like other filters, the time filter doesn't show you something that you couldn't see without it. It just makes it stand out better. If you've ever used a UHC (ultra-high contrast) filter to look at a nebula through the telescope eyepiece, you probably were impressed by how well you could see your target. And then afterward, you probably also discovered that when you took the filter away, the view wasn't all that bad without it. It's a common experience amongst backyard observers. Once you've been given a good glimpse of hard-to-see features, they're much easier to see the next time you go after them, even with lesser instruments, or under worse viewing conditions.
After examining U Gem, I followed up with another long-term variable star, R Leonis. This one is much more impressive than U Gem. It lives at a dim magnitude 11 for many months and then suddenly brightens to better than 5th magnitude. It's hard to take a short enough exposure without saturating the pixels of my camera on a 5th magnitude star. R Leonis reached its maximum brightness in March, and it's still brighter than 6th magnitude. It's going to take till mid-June to became faint again. I don't know if I'll be able to get the minimum shot before we lose it to the sun. It's still an impressive blink using the time filter!
I call it a time filter, but the original time filter was called a blink comparator. You can look it up on Wikipedia. Yeah, it's really a thing. Clyde Tombaugh used it to discover Pluto.
But "time filter" is a broader term, encompassing many different types of "blinking," or looking at photographs of the same object taken hours, days, weeks, months, or years apart. It doesn't have to be with a blink comparator, or by creating an animated GIF. It could be just two of your pictures compared side-by-side, or it could be a comparison between a picture you took tonight, versus a picture taken by someone else a long time ago that you find in a book or magazine article.
Amateur astronomers typically use a lot of filters. They have color filters for their eyepieces to help them see subtle details on the planets. And they'll use H-alpha filters to bring out faint nebulae in their astrophotos, or to look at prominences on the sun. As mentioned earlier, a UHC filter can help them see faint nebulae, like the Veil Nebula, for instance, through the telescope eyepiece. An OIII filter can help them see planetary nebulae better. One special set of filters for astrophotographers allows them to create images using the "Hubble palette" (Google it) of colors, which has become quite popular. They can make their pictures look they were taken by the Hubble Space Telescope.
Filters make certain features easier to see, and that's why I make the case for the "time filter." The time filter isn't a physical piece of equipment. It's simply a period of time. If you take a picture of something and then wait for some period of time and take another picture of it, you'll see any changes when you compare the two images. Some people refer to this as "change detection." I use it all the time to take pictures of asteroids. In my images, asteroids look like stars. I can't tell which one is the asteroid until I blink it. So, I'll take two pictures of the asteroid, about an hour apart. During that interval, the earth will have moved sufficiently enough in its orbit to give me some parallax for the asteroid, which is much closer to me than the background stars.
There are changes you might not notice without a comparison of old and new photographs. Unless, of course, you have a photographic memory! In backyard astronomy, we often look at the same targets over and over through our telescopes. But few of us can remember all the features perfectly. There are a lot of stars in an eyepiece field of view. If I looked at the Orion Nebula a month ago, will I be able to detect anything new if I take a look at it again tonight? Probably not. But if I took a picture of it a month ago and take a picture of it tonight, blinking the images will certainly make any new features "pop."
Of course, the Orion Nebula is an object in our own galaxy, along with all the stars you see with it in the eyepiece. It's not likely to change much over time, unless a nova occurs. On the other hand, spotting a supernova in a distant galaxy is the kind of change that you're more likely to detect, if you have an adequate telescope. Supernovae are visible from hundreds of millions of light years away, and you can view the entire galaxy in your field of view, rather than being limited to one small nebula within that galaxy. Furthermore, there are so many galaxies that your chances of spotting a supernova in one of them on any given night are far greater than spotting a nova in the field of view of the Orion Nebula. You just need to look at a lot of galaxies each night! I use the time filter every time I take pictures of galaxies.
However, I should mention that there are some nebulae that are variable, because they are illuminated by a star that is variable. One good candidate for the time filter is the well-known nebula, Messier 78. Amateur astronomers have been taking pictures of M 78 for over half a century. But apparently nobody noticed a strangely disappearing and reappearing feature within M 78, until 2004, when an amateur named Jay McNeil spotted it in a picture he took. It wasn't there in older images taken by either professional or amateur astronomers. It's not visible in the DSS image you can retrieve from the DSS website. That image, according to the FITS header, was taken in 1988. There was an image from 1966 in which McNeil's nebula was spotted. So, McNeil's Nebula was added to the list of "variable nebulae." But it's still unclear what its period is.
I somehow didn't hear about McNeil's Nebula until I read about it in a Sky & Telescope article around 2018. I've shot M 78 multiple times over the years, so I started looking through my pictures for a glimpse of McNeil's Nebula. My earliest CCD image of M 78 was taken in 2009, and McNeil's Nebula is visible in that image. I shot it again in 2014 and it was still there. A picture I took on February 8, 2015 was the last time it was visible in my images. I looked for it in 2019 and it wasn't there. Nothing there in 2022. Still invisible in 2024. Weird! The last time I shot it was a couple weeks ago and it's still a no-show. When will it appear again? Only the time filter will tell!
Below is a shot of M 78 through the time filter. In this case, I shot it with two different cameras, with different pixel arrays, so I couldn't blink it. Instead, I made a "side-by-side." McNeil's Nebula is located in the dashed circle.
When I was a kid, one of the magazines my parents subscribed to featured a monthly comic by Henry Boltinoff. Can't remember the magazine, but it could have been Reader's Digest or the Saturday Evening Post. The comic was called "Hocus-Focus," and it was two panels of the same picture, only there were 6 differences between the two images. You had to take a close look to spot all 6. I loved those things! Sometimes he got me. I couldn't find all 6 differences and ended up having to read the answer to the puzzle, which was written upside-down to avoid an easy spoiling. If you Google "Hocus-Focus" you'll probably find the following sample.
Of course, there was also the Sesame Street version of Hocus-Focus, which was the segment, "One of these things is not like the others." That one was a little bit easier, meant for younger children. But for whatever reason, I got the impression that grownups felt it was really important for me to practice finding differences. So, ever since then, I've been obsessed with comparing things and finding differences. Is it any wonder that, of all the special interests within the field of astronomy, I picked supernova hunting as my favorite activity?
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| Can you spot the supernova? |
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