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6" Refractor, Hydrogen Clouds and NV (Report)

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I've been an amateur astronomer for fifteen years, but I think only the last few nights I was priviledged to observe Milky Way's hydrogen regions in all their glory - thanks to NV. The setup was:

  • 6" TS f/5.9 Achromat on TS alt-azimuth mount 
  • PVS-14 with Harder 2100 FOM tube
  • 67 mm TeleVue Plossl, giving 13X magnification and 3-degree field of view
  • H-alpha filters: Baader 7 nm and Antlia 3 nm
  • APM Amici prism for right angle correct orientation view
  • Interstellarum Deep Sky Atlas
  • Bortle class 4 site at Wembury Point, England (SQM showed 20.60). Transparency 6/7. 




Initially started with the summer classics - M57, M13, etc, but once I tried the 3 nm filter on the Gamma Cygni region, I literally couldn't pull myself from roaming around this region surrounding Sadr all the three nights. It was mind-blowing. IC1318 and Crescent both photographic in appearance. Glowing, cury, textured emission in high contrast to dark patches and filaments. The dark triangle near LDN889 not only apparent in form, but exhibiting detailed structure. The eyepiece view was quite similar to this picture, if you squint your eye a little: https://bit.ly/3hJ4Ws4

The Crescent showed doubling front in the north and Y-shape in the center, mottled body, almost "electric," I guess, due to the scintillation. Visually was very similar to this picture:

Numerous DWB emission clouds and the Sh2-101 Tulip in Cygnus were bright - the whole region all appeared as pierced and surrounded by faint hydrogen emission, which was also easily apparent in the eyepiece. The extent of the nebula seen far exceeded what was shown in the Interstellarum atlas.

NGC 7000 North America's Gulf was very detailed and at high contrast, with bright knots in Mexico and in the north. The IC5067 Pelican was bright, showing three protruding "fingers" and irregular shape. IC 5068, the Cygnus Arc, was bright and apparent, split into multiple irregular regions. IC5146 Cocoon was pretty bright, without much fine detail inside, and apparent central star. Looking at the Interstellarum Deep Sky Guide, I now see that on those emission nebulae I saw about similar level of detail to what Uwe Glahn saw in his 27" reflector. 

The wide field of the 6" showed the whole western portion of the Veil in its field, with irregular bright "claw" of IC1340. NGC 6974/9 with Pickering's Triangle, shown as visible from 12" aperture in the atlas, was pretty bright in the eyepiece. The eastern part had less detail, but its overall irregular shape easily seen. 

Ventured to IC1396 in Cepheus, it appeared very extended and pretty faint, I couldn't see much dark detail, although central round dark region was apparent. Sh2-132, Sh2-157 (bright), NGC7635 and NGC7538 all exhibited some degree of detail, but I am not too familiar with these objects, and left them for another occasion. I don't think I could see the bubble itself with certainty, but it's on a to-do list now.

Going south, M27 was bright and pretty compact, compared to vast hydrogen clouds of Cygnus. I won't mention myriads of well-known and obscure star clusters along the way. I think I could see into great "depth" and could observe less well-known NGC clusters well. (Dolidze-Dzimselejsvili 11 is a really peculiar alignment of stars!) Roaming along the Galactic equator, I have never seen so many stars in the eyepeice in my life. I own mostly Cassegrain designs, but f/5.6 reduced to f/2.3 is something else. Carpets, laces of stars!

I tried switching to "normal eyepiece", but literally couldn't see much, compared to the NV mode. The eye wasn't properly dark-adapted, and, to be honest, after the level of detail seen in NV it felt a little futile. I tried 7 nm H-alpha filter, but it showed significantly less detail than the 3 nm Antlia. From dark nebulae, Barnard's E (B142) in Aquila was apparent without filtering. Some dark nebulae, if overlaid on HII emission, were better seen with the hydrogen filter on.

The last night before returning home I explored Sagittarius, literally bumping into M22 very low on the horizon while scanning manually, and it was a sight to behold! Definitely a top globular in my book now, replacing M13, along with Sadr surroundings being visually more engaging and richer than even the great Orion Nebula! And then... M8 Lagoon was almost a live photograph - silky-smooth whirlpool, bright and detailed flower-like center with stars, dark patches - it's difficult to describe! Looking at monochrome photographs of the Lagoon, I think visually what 6" showed that night comes quite close. I could spend the whole night on that object alone (or the Sadr!) but that was my first overview "mission" so I had to move on. The M20 Trifid was very bright, almost laser-etched on the sky background with irregular patches dividing it easily seen as pitch-black filaments. This photograph is similar to what was seen in the eyepiece (although yes, there was noise in the tube, and it wasn't that deep, so squint to get an idea):

In that area further, M17, Omega, was very bright and high-contrast as well - I could see the Omega shape in it, so now I finally get why it's called that, and the center emission region was extremely bright and knotty. M16 Eagle appeared as an eagle with wings open, and although there was lots of texture, I couldn't see Pillars of Creation that night, but definitely worth a try at a higher elevation and with a barlow.

Suddenly, my visual astronomy life is enriched with so many objects, exhibiting so many details. Comparing what I've seen with Uwe Glahn's and Ronald Stoyan's sketches in the Interstellarum Deep Sky Guide, on extended emission nebulae a 6" aperture with FOM 2100 NV under Bortle 4 sky shows more detail than unaided 27" reflector under very dark sky, and definitely exceeds 14". Could it get any better? On the way home, I had the opportunity to visit the Stonehenge!



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Excellent report. I use 3nm filters almost exclusively for nebulae from the city, but interesting to read that your 3nm worked so well under darker skies too - and clearly improved on the 7nm. Thanks for posting - we need more night vision disciples on SGL!

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A really good combination, 6" Achromat and NV, no need to worry about colour correction when you have a black and white view of the sky. 😀

Must be very portable ( the scope is a little wide, however being fast not to long) which is great to visit those dark sites.

How do you find views of the Nebulae compare between London and  
Wembury Point when using the 3 nm H filter? From Gavster photos the background is really dark outside of London, you can tell when he has been to the Isle of Wight.

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Yeah, through the filter this 6" is really an apo! I wish the field was flat, but I think I'm fine with that instrument. Honestly, I haven't tried 3 nm in the city. I've tried 7 nm on Rosette with the 90 mm refractor and it was disappointing, and M42 was OK'ish, but the whole experience still a far cry from Gavin's pictures. Rural sky was a completely different story. 

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18 hours ago, Maxim Usatov said:

Yeah, through the filter this 6" is really an apo! I wish the field was flat, but I think I'm fine with that instrument. Honestly, I haven't tried 3 nm in the city. I've tried 7 nm on Rosette with the 90 mm refractor and it was disappointing, and M42 was OK'ish, but the whole experience still a far cry from Gavin's pictures. Rural sky was a completely different story. 

In London skies I'm told that a faster scope is better as it allows for turning up the gain in the NV device. All your refractor scopes needs to be around F6. 

Flat stars, getting those in a refractor is an expensive proposition, i.e. a Petzval design. Maybe adding TS flat would help you out there? Very much a hack. 

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Tried the TS flattener at various spacings, didn't do much good for this particular scope. It's fine, really. I can tolerate that for the views I'm getting and the correct orientation with the prism.

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@Maxim Usedv. Beautiful review, a great comet hunter, William Bradfield, used an achromat of this type. Very nice photography of Stonehenge, if one goes to the UK to see Stonehenge is a must!

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