#21  
Old 26-01-2009, 09:53 AM
kinetic's Avatar
kinetic (Steve)
ATMer and Saganist

kinetic is offline
 
Join Date: Jun 2008
Location: Adelaide S.A.
Posts: 2,280
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
Reply With Quote
  #22  
Old 26-01-2009, 10:20 AM
renormalised's Avatar
renormalised (Carl)
No More Infinities

renormalised is offline
 
Join Date: Apr 2008
Location: Townsville
Posts: 9,698
Smile

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.
Reply With Quote
  #23  
Old 26-01-2009, 02:03 PM
Quark's Avatar
Quark (Trevor)
Registered User

Quark is offline
 
Join Date: Apr 2008
Location: Broken Hill NSW Australia
Posts: 4,104
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
Reply With Quote
  #24  
Old 27-01-2009, 02:21 PM
Craig_L
Craig

Craig_L is offline
 
Join Date: May 2007
Location: Adelaide, South Australia
Posts: 558
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
Reply With Quote
  #25  
Old 27-01-2009, 03:41 PM
bojan's Avatar
bojan
amateur

bojan is offline
 
Join Date: Jul 2006
Location: Mt Waverley, VIC
Posts: 6,932
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)
116.3 KB34 views
Click for full-size image (Horse.jpg)
25.6 KB28 views

Last edited by bojan; 27-01-2009 at 05:28 PM.
Reply With Quote
  #26  
Old 27-01-2009, 05:40 PM
Quark's Avatar
Quark (Trevor)
Registered User

Quark is offline
 
Join Date: Apr 2008
Location: Broken Hill NSW Australia
Posts: 4,104
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
Reply With Quote
  #27  
Old 27-01-2009, 05:49 PM
Quark's Avatar
Quark (Trevor)
Registered User

Quark is offline
 
Join Date: Apr 2008
Location: Broken Hill NSW Australia
Posts: 4,104
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
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 10:49 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Bintel
Advertisement