H-alpha
H-alpha
[In a slim panel Black Hat is seen holding a small black device up in his left hand.] Black Hat: I got an H-Alpha filter for looking at the sun.
[In a broad frame-less panel Black Hat, holding the device down in his left hand while standing behind Cueball who is sitting in an armchair reading on his tablet.] Black Hat: It also works for nebulae. But that's about it. There just aren't that many hot blobs of hydrogen to look at, I guess.
[Back to a slim panel, Black Hat is seen holding his left hand to his chin, while he holds the device down in his right hand.] Black Hat: ...Unless...
[Another two slim panels follows. In the first Black Hat turns around and leaves, his head already partly outside the panel already, and he no longer holds the device in his hands. And then follows an empty beat panel.]
[In a broad panel Black Hat re-enters with the device held down in his hand. He is walking towards Cueball in his armchair. His black hat is somewhat out of shape. Cueball is still reading but is now hunched further forward and he has lifted his tablet so it is very close to his face.] Black Hat: Huh, did you know blimps all use helium now? You learn something new every day! By the way, we're out of fireworks. Black Hat: And some advertising company is real mad.

Black Hat has acquired an H-alpha filter. This is a special kind of optical filter used for scientific observations of the Sun's chromosphere . It is different from an ordinary solar filter, which is used to protect one's eyes or camera, as looking at the Sun bare-eyed will do damage to one's eyes. A camera using an ordinary (not H-alpha) solar filter was seen in 1828: ISS Solar Transit , and the consequences of not using such a filter were explored in 2227: Transit of Mercury .
Black Hat points out that the filter can also be used to look at nebulae , but doesn't see much further use for it; since the filter only transmits a very narrow bandwidth of light, one generated by hot hydrogen, it is not useful for looking at much else. This gives him an idea, and he leaves.
WARNING!!! A deep sky nebula H-alpha filter has a wider bandwidth than a solar H-alpha filter and WILL hurt the eyes if used to observe the sun!
Upon returning, his hat looks damaged. He casually shares with Cueball three seemingly unrelated observations which suggest what he was up to in the meantime: that most modern blimps use helium to keep them aloft, that their household is out of fireworks, and that an advertising company (or several, going by the title text) is upset. Early in the 20th century, most airships such as blimps and zeppelins used hydrogen as the lifting gas.  There were several incidents in which this gas ignited while the ships were in flight, resulting in spectacular and catastrophic fireballs, most famously the Hindenburg disaster . Taken together, the implication is that Black Hat tried to set someone's advertising blimp alight using fireworks; so he could use his H-alpha filter to look at the burning hydrogen.
In modern times, one of the most well-known uses of airships is blimps for advertising, as they are an unusual and hence attention-getting sight in the sky, offer a large surface area that can be used to show a slogan or logo, and can stay aloft for a long time at comparatively little cost. Modern blimps almost exclusively use helium as a lifting gas. While helium is significantly more expensive than hydrogen (and a non-renewable resource), it has similar weight and therefore similar lifting power to hydrogen, but is not flammable. (In fact, as a noble gas, helium is totally non-reactive under normal conditions).  Any attempt to cause a hydrogen fireball would, therefore, be doomed to failure.  Nonetheless, if Black Hat managed to set off sufficiently powerful fireworks near the blimp, it could potentially damage the skin, risking a loss of helium and possibly putting people in danger, which is likely why the advertising company is "real mad".  The joke is that Black Hat would do something as destructive as attempting to destroy a blimp in flight, potentially killing people aboard or on the ground, merely to have the opportunity to use his H-alpha filter.
Cueball "responds" by holding whatever he's reading closer to his face, apparently hoping to avoid further conversation (or consequences).
The title text references the insurance company MetLife, which until 2016 used the cartoon character Snoopy as an advertising mascot. In the Peanuts comics, Snoopy would frequently imagine himself as a fighter pilot in World War I in an aerial battle with the Red Baron , a battle he would frequently lose. The detail that Black Hat "dressed up as the Red Baron" might help explain another point: advertising blimps typically fly higher than the effective range of most fireworks. It would be entirely consistent with Black Hat's history to modify the stolen triplane mentioned in 496: Secretary: Part 3 to allow him to launch fireworks from the air, in mockery of an old-fashioned dogfight.
This comic was published shortly before Independence Day 2021, a US holiday that is often commemorated with fireworks. This may explain why Black Hat and Cueball originally had some fireworks around.
[In a slim panel Black Hat is seen holding a small black device up in his left hand.] Black Hat: I got an H-Alpha filter for looking at the sun.
[In a broad frame-less panel Black Hat, holding the device down in his left hand while standing behind Cueball who is sitting in an armchair reading on his tablet.] Black Hat: It also works for nebulae. But that's about it. There just aren't that many hot blobs of hydrogen to look at, I guess.
[Back to a slim panel, Black Hat is seen holding his left hand to his chin, while he holds the device down in his right hand.] Black Hat: ...Unless...
[Another two slim panels follows. In the first Black Hat turns around and leaves, his head already partly outside the panel already, and he no longer holds the device in his hands. And then follows an empty beat panel.]
[In a broad panel Black Hat re-enters with the device held down in his hand. He is walking towards Cueball in his armchair. His black hat is somewhat out of shape. Cueball is still reading but is now hunched further forward and he has lifted his tablet so it is very close to his face.] Black Hat: Huh, did you know blimps all use helium now? You learn something new every day! By the way, we're out of fireworks. Black Hat: And some advertising company is real mad.
