NGC6888 Ha

[Carole]

I also took some Oiii to go with this, but had a lot of problems with it which I won't go into now except to say I think I got some damp on the filter.  So this is a "sort of combination of Ha and Oiii but mainly Ha.

7 x 900secs Ha and 8 x 900secs Oiii
Still learning to combine filters:

NB: I did this AFTER I went to the hospital and knew I wasn't having a heart attack, but too late to go to DSC.

full frame:
http://gallery.orpington-astronomy.org.uk/albums/userpics/10047/Crescent_combi_Ha_Red_Oiii_green_Full_frame.png



Close up:

[MarkS]

That's looking really good - really clean data!  Clearly not a 450D!   So you have two filters - you synthesized a third channel did you?

On the full size image it looks like you might have a slight registration problem on the stars inside the area of nebulosity - I don't know why that should be.

Nice one.  This narrowband stuff is a whole new ball game!

Mark

[Carole]

Hi Mark,
No I didn't synthesise a 3rd channel I just used two.

On the full size image it looks like you might have a slight registration problem on the stars inside the area of nebulosity

I think I know why.  It is possible the two different frames are not 100% aligned accurately, or it could be something to do with the different filters producing different size stars.  
The reason this appears only on the nebula is because I cheated a bit with the Oiii as follows:

Because of the problems I was having with the Oiii it was producing horrible vignetting and the flats did nothing to help so what i did was graft the Oiii nebula onto the Ha image and ,made a "semi Oiii layer" so the Oiii layer only truly has the Oiii in the nebula part itself.  I am sure this is not the right way to do things, but rather than waste a whole nights imaging I decided to cheat a bit.  That's why I said "mostly Ha".

As you say this narrowband stuff is a whole new ball game and currently I am struggling a bit with combining the filters as I get horrible colours and the thing you mention about the registration (I must admit I had to align the graft manually, so maybe I was fractionally out).  

Also flats are new ball game because you have to measure the ADU and with the Atik383L it has a shutter, so if you take too short an image you get shadow from the shutter.  So the trick is to get a longer flat and get the light not too bright, so I have decided to resort to a light box so hopefully this will be a constant for a given filter.  But again I had a very strange flat reading with the Oiii filter.  I spoke to Bernard today and he wondered whether there was moisture on the Oiii filter - yet to be resolved.

Carole

[MarkS]

No I didn't synthesise a 3rd channel I just used two.

What makes your stars white then?  To get white you need R+G+B.  So if, for instance, Red is Ha and Green is O3 then where does the Blue come from?

But then again, I've never processed a narrowband image ...

[Carole]

Because I used Ha as the base layer, then added it into the red channel and then added it as luminance as well.  The Oiii "layer" is in the green channel.  So no blue layer.  (I think?  I processed this a number of different ways until I got something I liked, but I think this was how I got there).

When you combine Filters Mark they are all Mono to start with and they only get their colour adding them into the channels. 

Carole