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ASTRO: vdB 128



 
 
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  #1  
Old July 29th 12, 07:42 AM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
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Default ASTRO: vdB 128

vdB128 is a mostly reflection nebula around a pulsating variable star in
Cygnus. I found little distance information but one source lists a very
iffy estimate of about 3600 light-years (1100 parsecs). I found revised
Hipparcos data saying 0.00057" for the parallax of the illuminating
star. If right that puts it some 5700 light-years away. The star is
V1768 Cyg to variable star observers. SAO 69362 or HD 190603 are more
general catalog entries for it. At magnitude 5.6 it was a major pain to
deal with! Fortunately my new filters eliminated most reflection issues.

Objects in the vdB catalog, while classed as reflection nebula often
have some or even a lot of H alpha emission as well. Note the red
streaks of H alpha throughout this reflection nebula.

Back on July 3, 2011 Tom Davis posted a wide field shot of the nebula as
part of his ongoing attempt to image all the vdB catalog. He considered
it rather boring being small and faint though the field it sits in is
quite spectacular. His image is at:
http://www.cloudynights.com/ubbthrea...Number/4671876

After seeing his image I put it on my To-do list. In fact I put it on
the list at highest priority so ended up taking it that very night. Now
I'm just getting it processed over a year later.

This object is a good opportunity for me to again address what I call
the f ratio myth. That is, that the faster the optics (lower the f
ratio) the less exposure time you need for faint objects using a CCD.
That is true of typical snapshots with an ordinary camera, digital or
film. With such a camera taking an earthly picture at f/8 you do need 4
times the exposure compared to f/4. It is false when applied to CCD
images of the night sky. Though many can't seem to shake this myth.

Tom used 200 minutes at f/3.8 for his image. I was using a telescope
running at f/10. Based on his 200 minute exposure if the myth were true
I'd need (10/3.8) squared = 6.9 times more exposure than he used. That
would be 200 minutes x 6.9 = 23 hours of exposure time. Yikes!
Fortunately its a myth. For the attached image I used my usual 40
minutes or only 1/34th the exposure time calculated by the myth. Yes my
signal to noise ratio is lower and is in line with using one fifth of
his luminance exposure time. It would have been comparable if I'd used
close to the 200 minutes he used.

Choose a fast system for its wide field of view not because it will save
you imaging time. Aperture is what will reduce your imaging time not f
ratio.

Sorry about the image size. When there are a ton of stars in an image a
JPG file suddenly blows up to over a megabyte in size.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME

Rick
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Prefix is correct. Domain is arvig dot net

Attached Thumbnails
Click image for larger version

Name:	VDB128L4X10RGB2X10R.JPG
Views:	233
Size:	1.08 MB
ID:	4205  Click image for larger version

Name:	VDB128L4X10RGB2X10R-CROP7.JPG
Views:	91
Size:	203.2 KB
ID:	4206  
  #2  
Old July 29th 12, 08:59 PM
WA0CKY WA0CKY is offline
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Default

After seeing the posted images I realized I posted the wrong images again. But I couldn't find the right ones in the vdB128 folder. Turns out I put them in a wrong folder so no wonder I got the wrong ones. These have less noise.

Rick
Attached Thumbnails
Click image for larger version

Name:	VDB128L4X10RGB2X10R2.JPG
Views:	173
Size:	1.04 MB
ID:	4207  Click image for larger version

Name:	VDB128L4X10RGB2X10R2CROP.JPG
Views:	160
Size:	262.1 KB
ID:	4208  
 




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