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Wednesday, October 15, 2008

Star removal technique


Star removal example animation (kilck to start)
-
In narrowband color imaging star control is essential.
I have developed some time now starremoval technique.
It turned to be very accurate.
Please note, that this is a GIF-animation and image has a lower quality, than original.
Nebula in dense starfield can seen better without stars and
more faint details and large forms comes visible.
_
Images details, please, look previous posts.

6 comments:

matts.sporre said...

Terve J-P Metsävainio,

I really like your starless images!

I would like to experiment a little myself with it and was trying to find a description somewhere in your blogg but could not find it?

Would it be possible for you to share some of your "secrets"?

BR
Matts

J-P Metsävainio said...

Hi Matts,

I haven't had time
to write tutorial about that, sorry.

Fast way to remove stars is by using "dust & scartches" filter under PhotoShop.
Start with radius ~8 and set threshold to the value where
some fragments of stars is still visible and no details in image destroyed.
Repeat this with lower values,
untill no stars are left and all the details are still in the image.
You might need four to six iterations.

Then add the image with stars top of the starless one as a layer.
Set mode to "Darken" so all dark details are restored, flatten and
enjoy.
There is a even more accurate way, but it's complex
to explane.

J-P

matts.sporre said...

Hello again J-P,

I did now about dust and scratches, however using it iterative was new to me.

However it does not help me with a viel image I have. It might be a hopeless case, or I need some special techniques.

You can find it here
http://www.istarion.net/Picturepages/Vielrgb.htm

There are four more images processed slightly different on the bottom of the page. I managed to reduce the small stars in the top picture by masking of the nebula and using nois ninja on the small stars outside of the nebula.

It is however a bit risky since I might use noise ninja on faint parts of the nebula that I did not mask off.

Maybee I should use your dust&scratch iteration technique between each curve stretch in order to "deal" with the stars as they appear.

The problem seem to be that the fainter part of the nebula and the faint stars have about the same signal.

Also note that I (living in Stockholm) do not have the nice dark skies, I assume you have, so I have to take shorter exposures.

What kind of Vmag do you have?

BR
Matts

J-P Metsävainio said...

Hi,

All my images in the Blog
has been shot from heavily
Light polluted Center of
city of 130.000 inhabitants.
I think Vmag is about 3...
I do mainly NB imaging for
the reason.

Br.
J-P

matts.sporre said...

Ok, it is encouraging that you can take images this excellent from a VMag 3 site.

Without filters I can shoot ca 1,5 min sub-images (that is when the histogram on the back of the camera is reaches ca 40% to being over exposure). I have not checked what I can achieve with filters even thou I did an H-alpha image of the Cygnus area (I think I used 3 min subs).

Anyway – thank you for taking your time. I will pop into your blogg every now and then to look for the star removal technique description and to look at the nice pictures of course.

BR
Matts

J-P Metsävainio said...

Hi again,

You have a very good astro lens!
Fast optics collects LP fast as
well. I think, you can go 10-20
min. with H-a.
An other great filter to shoot emission areas is a UHC-filter.
I have a Baader UHC-s.
It cuts much more, than the filter
you are using now, IDAS-LP is good
for broadband targets, like galaxies, though.