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The Lagoon Nebula (M8) in H-alpha Using a Kenko 200mm Lens

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#1 james7ca

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Posted 15 June 2019 - 04:08 AM

Taken last week over two nights of partly clear skies using an ASI178MM-Cool camera and a Kenko 200mm telephoto lens. Two hours and twenty-one minutes of integration time (71 x 2m) with a Baader 7nm H-alpha filter. On the camera, lowest read noise gain setting and temperature set point of -15C.

 

I usually post over in the CCD and CMOS forum but since these subs were taken with a $100 camera lens on a Celestron AVX mount (mostly, about one third were taken on a night when I was using a Mach1GTO) I thought it would be interesting to post here to show what can be done with fairly modest equipment (I'm talking about the Kenko lens, the AVX, and the ASI178MM camera, not the Mach1GTO grin.gif ).

 

Interestingly, the AVX was not even polar aligned, but I did use a guide scope. I was also working under red-zone light pollution with a target that was fairly low into the light dome of my home town.

 

Image processing done with PixInsight and Photoshop CC2019 (okay, that's not really "modest," but certainly commonly used). Full disclosure, I also added a few minutes of subs that were taken with an exposure time of 30 seconds (to help with the core of the nebula).

Attached Thumbnails

  • Lagoon Nebula with Kenko 200mm and ASI178MM (Small).jpg

Edited by james7ca, 15 June 2019 - 05:13 AM.

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#2 james7ca

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Posted 15 June 2019 - 05:07 AM

Oh, here is an image of the Kenko lens (the H-alpha filter was mounted internally to the camera, using this adapter, although the lens has enough back focus that it can be used with a filter drawer or filter wheel (I think it offers about 60mm of back focus, certainly a little more than the standard 55mm for a T-mount lens). I also used a front-mounted step-down ring to limit the aperture to about f/5.4 (the native on the lens is f/4). There is also an image of the front-mounted step-down ring (both on and off the camera, 55mm -> 37mm, and then back up to 37 -> 55mm for lens shade). Note, 37mm of total aperture.

Attached Thumbnails

  • Kenko Lens and ASI178MM.jpg
  • Kenko Lens Aperture.jpg

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#3 OldManSky

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Posted 15 June 2019 - 08:52 AM

Ah, stepped down to 37mm aperture -- that explains the nice round stars out to the edge :)

I have a 200mm f/4 Takumar (from 1977) that I use now and then, and do the same thing.

 

Really nice shot with "modest" equipment and light pollution! :)



#4 james7ca

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Posted 15 June 2019 - 09:14 AM

Ah, stepped down to 37mm aperture -- that explains the nice round stars out to the edge smile.gif

...

The final master integration had a median FWHM of just 2 pixels and an eccentricity of 0.26. You really can't do much better than that and still have sampling that will produce round stars. When I compared this image to others on the internet I was surprised to see that many of the images that had been done with larger scopes showed less detail than I got with the Kenko. Close double stars that were resolved in my image where sometimes blurred together from much larger scopes and some of the dark silhouette features where completely missing with larger instruments. I think some of that was from overly aggressive noise reduction or maybe poor focus.

 

However, one area where my image suffers is in the dark regions surrounding the nebula, not a lot of detail there and with fewer stars there isn't as much contrast between the very dark dust and the bright background of the Milky Way. More exposure would help and I noticed that there was more haze on the second night when I collected the majority of the subs (resulting in a higher median value and less contrast). Just a few miles from my location (at the airport) they were having ground fog for most of the night. In fact, the fog may have come to within a few city blocks from where I was imaging.


Edited by james7ca, 15 June 2019 - 10:18 PM.


#5 44maurer

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Posted 15 June 2019 - 03:32 PM

Beautiful.



#6 james7ca

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Posted 16 June 2019 - 09:12 AM

Brian, thanks.

 

Here is a copy where I've labeled a few "animal" figures that I see in the bright and dark features of the Lagoon Nebula. To my knowledge these are all my own "creations." smile.gif

 

The Godzilla is something that once seen can't be unseen (IMO), but I've included a detail that shows the various body part. The Pterodactyl is very large, a head in profile with the beak on the lower left, then moving toward the upper right, a dark eye, and then the crown on the top of the head.

 

There are other animal-like features that I could have shown (an otter and a very small stick figure man with a large club to name two), but further additions and comments are welcomed.

 

I also realized that I overstated the aperture since I'm using a nested set of step-down and step-up rings. The measured opening is 34mm and that would suggest an f-ratio of 5.9 (I say "suggest" since with the aperture stop so far displaced from the design point it's not really functioning as a pure aperture stop). 

Attached Thumbnails

  • Lagoon Animals.jpg
  • Godzilla in the Lagoon.jpg

Edited by james7ca, 16 June 2019 - 10:39 AM.

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