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StDr56 is a newly discovered planetary nebula (maybe). link1 link2

It was found by amateur astronomers Marcel Drechsler and Xavier Strottner.

According to the above articles, it's pretty big:

With an extension of 44 x 36 arc minutes, StDr56 is not only the largest probable PN in the region, but in the night sky it covers more than half of the area of the famous Triangulum Galaxy M33, which is located only a few degrees from Strottner-Drechsler 56.

Or

It’s about the same size as the full Moon on the sky.

So, if it's so big, why was it only discovered now? Wouldn't it make sense if all large objects were already discovered a long time ago?

uhoh
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Gimelist
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    No absolutely not. Have you ever tried to find a planetary nebula by hand? Their surface brightness (not total brightness, which is only measured after they are found) is incredibly low. I suspect objects as large as this can only be found with long integration times and large field of views. – AtmosphericPrisonEscape Jan 21 '21 at 23:07
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    Well it’s terribly faint. The image you link to (link 2) has been exposed 59.3 hours, and it even says in link 1 that “this object is /faint./” (their italics). It has to do with surface brightness, as mentioned by @AtmosphericPrisonEscape. Imagine the light of a single star, but spread out over the area of the Moon. Even the light of Sirius, the brightest star in the sky, would be washed out completely over that area. There’s a point where once you dilute the paint enough, your floor is not painted at all… – Pierre Paquette Jan 21 '21 at 23:15
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    Interesting question! example of a (different kind of) faint object, the back story there is that refractors were better suited than reflectors because nano-roughness of silvered mirrors produces a faint haze of scattered starlight that competes with natural skyglow to "fog" exposures and hide objects with low surface brightness. At least that's my reading of the sources there. – uhoh Jan 22 '21 at 00:00
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    @uhoh Do you have more references on the "haze" of refractor telescopes, please? The topic seems interesting. – B--rian Jan 22 '21 at 13:56
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    @B--rian there are three links in the beginning of the linked example. The third one doesn't work but I'll look for a replacement. I think I might have researched the diffuse scattering from nanoroughness of mirrors further at the time but four years later I don't remember. This is probably worth a new question, and aspects may also be appropriate in an answer to Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light) – uhoh Jan 22 '21 at 15:38
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    @uhoh If you don't mind, I will ask tonight. – B--rian Jan 22 '21 at 15:41
  • @B--rian Excellent! from here I found these: 1, 2, 3 and they all seem to contain some discussion of the limits of reflective optics, and refractive optics when the antireflection coatings are sub-optimal. – uhoh Jan 22 '21 at 15:47

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Large objects can be very faint if they are far enough away. So large objects wouldn't necessarily be discovered a long time ago. The object you mentioned is very faint and required a long view time to acquire enough photons to "see" it. As stated in a comment by @Pierre Paquette, the object was viewed for over 59 hours.

Natsfan
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    So as a follow up question, is there a systematic sky survey project where they look at a portion of the sky for 60 hours at a time to see if there's anything in there? – Gimelist Jan 22 '21 at 04:13
  • @Gimelist Will / Did you ask that question? – B--rian Jan 22 '21 at 13:57