Turtle Nebula (NGC 6210)
This planetary nebula was first recorded on 22 March 1799 by the French astronomer Jérôme Lalande. He measured its position very accurately for his star catalogue «Histoire céleste française» which appeared in 1801. But Lalande did not recognize the true nature of what he thought was a star and hence Dreyer credited Wilhelm Struve for its discovery. Struve found the object in 1825 while he was searching for double stars with the 9.6 inch f/17.8 Fraunhofer refractor at Dorpat (Tartu) in Estland. He found 13 NGC objects which he published in 1827. 
John Herschel observed this planetary nebula on 25 May 1830. He cataloged it as h 1970 and noted: «Struve's fifth nebula in the list at the end of the Dorpat Catalogue of double stars. Very bright equal to a star 8 or 8.9 magnitude. 8" diameter, and of a uniform light, but with the edges boiling and ragged. A fine object like a star out of focus. Viewed between clouds. Struve's place.» 
The dying central star of the planetary nebula NGC 6210 is surrounded by a complicated structure, which looks like a «nautilus shell». The remarkable features of this nebula are the numerous holes in the inner shells with jets of material streaming from them. These jets produce column-shaped features that are mirrored in the opposite direction. The multiple shells of material ejected by the dying star give this planetary nebula its odd form. The fainter outer structure resembles a «tortoise» (sea turtle). This is how the PN got its nickname «Turtle Nebula».
Material flung off by this central star is streaming out of holes it punched in the nautilus shell. At least four jets of material can be seen: a pair near 6 and 12 o'clock and another near 2 and 8 o'clock. In each pair, the jets are directly opposite each other, exemplifying their bipolar nature. The jets are thought to be driven by a fast wind – material propelled by radiation from the hot central star. In the inner shell, bright rims outline the escape holes created by this wind. This same wind appears to give rise to the prominent outer jet in the same direction. The hole in the inner shell acts like a hose nozzle, directing the flow of material.
NGC 6210 is about 6'600 light-years away. The nebula measures 1.6 light-years from the very top of the turtle-shaped form to the tip of the bottom. The inner nautilus shell is about 0.5 light-years in diameter. 
|Designations||PN G043.1+37.7: NGC 6210, PK 43+37.1, ARO 5, EM* CDS 904;VV 82, VV' 143|
|Right Ascension (J2000.0)||16h 44m 29s|
|Declination (J2000.0)||+23° 47' 48"|
|Radial Velocity||-36.2 km/s ± 1.1 km/s|
|Expansion Velocity||21.0 km/s (O-III), 35.5 km/s (N-II)|
|C-Star Designations||AG +23 1564, AG82 216, BD +24 3048A, GCRV 9624, HD 151121, IDS 16403+23.59A, PLX 3808|
|C-Star Magnitude||12.44 mag (B filter), 12.66 mag (V filter)|
|C-Star Spectral Type||O6, O(H)|
The planetary nebula NGC 6210 is located in the constellation Hercules. The best observation time is from March to October.
400 mm Aperture: In the 21 mm Ethos eyepiece (85x), even without an O-III filter, NGC 6210 is well visible as a washed-out, irregular spot that gets larger as you look past. If you look directly at the nebula, the central star is well visible. In the 9 mm Nagler eyepiece (200x) the nebula shows a rectangular to oval shape and a slightly bluish color. The «flippers» of the turtle are not visible. — 400 mm f/4.5 Taurus Dobsonian, Hasliberg, 18. 8. 2023, SQM 21.1, Bernd Nies
762 mm Aperture: The planetary nebula shows a bluish colour and a roundish-rectangular shape without nebula filter in the Ethos 13 mm eyepiece, whereby the central star is also well and long lasting. The little legs, which give the object the name Turtle Nebula, are well visible in one direction and 180 degrees in the other direction only indirectly visible as pointed or narrow rays. This is only visible at higher magnification with the 7 mm Nagler. — 30" SlipStream-Dobson f/3.3, Hasliberg, 18. 8. 2023, Eduard von Bergen