#21  
Old 26-01-2009, 09:53 AM
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kinetic (Steve)
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Hi Trev,

As Stuart said, it's probably the culprit.
Polarising filters certainly are good at reducing reflections from
dust in the air, so from a non-technical view point it could
follow that most of the relection light from the Witches Head is
polarised light.

It would be a good comparison to try and exactly duplicate your
first night with a second non filtered attempt, same processing
routine etc....to demonstrate just how 'polarised' the light from
the WH really is....if that's what caused it.

One thing buggers all this theory up though for me....why did the
Running Man and parts of M42 show up so well?
Just purely a much brighter reflection neb, or another reason?

Steve
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  #22  
Old 26-01-2009, 10:20 AM
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renormalised (Carl)
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Quote:
Originally Posted by Quark View Post
Hi Stuart,
Thanks for your thoughts on this. I think you may have hit the nail on the head, so to speak, regarding the effect a polarizing filter might have on a reflection neb, or at least one as dim as this one.

Regards
Trevor
Ah...you have a polarising filter...no wonder. Blue nebulae, as you know, are from the reflected light of stars off very small dust grains in the nebs. They preferentially scatter the blue light (Raleigh Scattering) at the expense of other wavelengths. What you may not know is that light is plane polarised as well. This polarisation depends on the size and shape of the grains in the nebula, the amount of light they scatter, the luminosity of the illuminating stars, etc. Your polarising lens is probably blocking all the light from the neb. I would take it off....however, here's a little experiment for you to try. Try changing the orientation of the polarising plane by rotating the lens and see what you get. You may find Brunhilde suddenly appears

Last edited by renormalised; 26-01-2009 at 02:09 PM.
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  #23  
Old 26-01-2009, 02:03 PM
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Quark (Trevor)
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Hi Steve & Renormalised,

I will give it another go without the filter.
I think M42 and The Running Man show up because they are intrinsically brighter objects.

When processing my image in IP after stretching it and seeing no sign of the old hag, I went no further with colour balance or noise reduction. So I am sure I could have done something for the emission nebs.

Regards
Trevor
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  #24  
Old 27-01-2009, 02:21 PM
Craig_L
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Hi Trevor,

I recently shot IC2118 with a Canon 450D and you'll need a lot of exposure. I took 7 X 10 min subs @ ISO 1600 at F4.5 and at a dark sky location. It was still pretty dim before processing. So 5 min subs @ISO 800 and F5.6 is asking a lot. Certainly take the polariser filter off and try 1600ISO and 600sec subs. Link here to my attempt - http://www.iceinspace.com.au/forum/s...ad.php?t=40414

Craig
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  #25  
Old 27-01-2009, 03:41 PM
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Hi Trev,
On Sunday night I went for Which Head (from Daylesford, pretty dark site).. just to see what that strange object on your photo is.. here it is, 10x stack (DSS) of 150 sec exposures with unmodified 400d, 50mm Canon f1.4 FD SSC, @f3.5, ISO1600, no darks or flats (camera long exposure compensation was ON), cropped and contrast enhanced (that is why it is so grainy, I did not bother to take perfect picture, I just wanted to try to get it.).

I too believe this strange thing on your image is internal reflection from Rigel, there is no trace of anything like that on my photo.

BTW, on the same image in the corner I found Horsehead too.. see attached :-)
Attached Thumbnails
Click for full-size image (Wich_Head.jpg)
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Click for full-size image (Horse.jpg)
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Last edited by bojan; 27-01-2009 at 05:28 PM.
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  #26  
Old 27-01-2009, 05:40 PM
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Quote:
Originally Posted by Craig_L View Post
Hi Trevor,

I recently shot IC2118 with a Canon 450D and you'll need a lot of exposure. I took 7 X 10 min subs @ ISO 1600 at F4.5 and at a dark sky location. It was still pretty dim before processing. So 5 min subs @ISO 800 and F5.6 is asking a lot. Certainly take the polariser filter off and try 1600ISO and 600sec subs. Link here to my attempt - http://www.iceinspace.com.au/forum/s...ad.php?t=40414

Craig
Thanks for that Craig,

Had a look at your image, very nice, look fine to me.

Out at Stockport would definitely offer darker sky's for capturing The Witch.

Regards
Trevor
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  #27  
Old 27-01-2009, 05:49 PM
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Quote:
Originally Posted by bojan View Post
Hi Trev,
On Sunday night I went for Which Head (from Daylesford, pretty dark site).. just to see what that strange object on your photo is.. here it is, 10x stack (DSS) of 150 sec exposures with unmodified 400d, 50mm Canon f1.4 FD SSC, @f3.5, ISO1600, no darks or flats (camera long exposure compensation was ON), cropped and contrast enhanced (that is why it is so grainy, I did not bother to take perfect picture, I just wanted to try to get it.).

I too believe this strange thing on your image is internal reflection from Rigel, there is no trace of anything like that on my photo.

BTW, on the same image in the corner I found Horsehead too.. see attached :-)
Thanks Bojan,

Spot on regarding the internal reflection.
Last night I had another attempt, removed the polarizing filter beforehand.

Took 11 x 7 min subs with 6 darks 16 flats and 16 bias frames at ISO 800
That blue feature is gone and IC 2118 appears very faintly on each of the Raw frames.

I will post the result on the Deep Sky forum.
Thanks for the reply.
Regards
Trevor
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