A lot of people know about Sh2-264, but few know the name. It’s the huge, roundish nebulosity at Orion’s head.
Spanning a good four or five degrees across, this nebula has a very low surface brightness. I long assumed that it was unobservable.
The last few months I have began to wonder… is Sh2-264 actually observable visually? Some images show it as just a little bit fainter than Banard's Loop, and I can see Banard's Loop pretty well from sites that are dark enough. Searching the internet, I came across this long exposure image:
http://forum.ourdarkskies.com/index.php?act=Attach&type=post&id=3475
I realized my best shot at detecting this nebula would be to sweep back and forth across the nebular edge I have arrowed in the image, and see if I could detect the slight increase in background sky brightness.
A few months later, on January 3rd, I was driving down to the darkest site in Alabama I have access to, Conecuh National Forest, located on the Florida line. The skies are DARK there. M33 was naked eye- with the moon still up!!! Once the moon set at 9pm, the sky REALLY got dark. M33 was an easy, fuzzy glow. The Horsehead nebula was easily visible without the H-beta filter, and Banard’s Loop was bright. I knew that now was the time to try out Sh2-264!
Leaving the H-beta filter in the 27mm Panoptic (I had just gotten done gawking at my best-ever view of the Horsehead nebula- it looked better than some images I have seen of it), I moved the scope up to Meissa, then star-hopped to a location at about
5h 30m +12 20’. Sweeping the scope back and forth in a northerly-southerly direction, I could detect a very faint increase in sky background brightness right at the correct location. I moved the scope south west in a line that followed the arrowed nebular edge in the second image. I was able to trace a very faint increase in sky background brightness in a rough line from about 5h 33m +12 30’ down to 5h 24m +10 30’! This matched perfectly well with the long exposure image of the nebula I found on the net! Trying the UHC and OIII filters, I found that the nebula was dimmed to the point of invisibility or near invisibility. For a final sanity check, I removed all filters from the system, and checked to make sure a slight increase in the number of background stars could be responsible for the “nebula” I was seeing. If anything, the sky background at the nebula’s position was slightly DARKER than the surrounding sky with no filter in the system!
Putting the H-beta filter back into the system, I decided that the “brightest” of
Sh2-264 appeared to be around 5h 32m +12 30’. This area seemed to be signifigantly brighter than the other end of the nebular edge I detected, which faded out of view around the position where Uranometria plots the dark nebula B224.
After these observations I must accept that I have actually seen this ultra-faint nebula. I searched through the internet, and I cannot seem to find another visual telescope observation of this nebula, though one person claims to have seen it by holding a nebula filter up to his eye and seeing it naked-eye (if that can truly be considered naked-eye). Taking this into consideration, I certainly cannot be the first person to see it telescopically, especially since it is such a well-known nebula appearing in so many long exposure images.
It was very gratifying making these observations, and I can hardly wait till I return to this dark site and do some follow-up observing!