Horsehead Ha & HaRGB

[Carole]

3 hours of Ha with the Atik383L Mono using the Skywatcher ED80 Pro 12 x 900secs cooled to -10 degrees.
I also took Oiii and Sii binned which came out OK, but despite aligning the filters with both Maxim and Nebulosity there a something not right when I try to combine them together so for the time being I have used my DSLR image from a year ago to combine an HaRGB image.  The DSLR image was done with the ED120, so the Ha image is slightly larger.

I am very pleased I changed the Atik314 for the 383 from the point of view of the FOV.
Ha image.



Combined with the DSLR image from November 2011.  There is also something a bit odd with the alignment here too on the left side only.  I am sure there will be a scientific explanation, but at present I cannot think why it is on one side only. Images aligned in Maxim.

[Ivor]

Some good detail in there it is interesting to see the difference adding the H makes to the overall image. One comment I think the main star on the left could do with being reduced as draws the eye away from the nebula.

[MarkS]

The Ha looks good - you might find there's a bit more data in there.

Combined with the DSLR image from November 2011.  There is also something a bit odd with the alignment here too on the left side only.  I am sure there will be a scientific explanation, but at present I cannot think why it is on one side only.

It is possible the images are compressed (i.e the opposite to stretching) by different amounts depending on how high above the horizon they were - caused by atmospheric refraction.  Or it could be the telecope optics have different amounts of distortion in which case you need to correct the distortions before you combine.  I have 2 hours of Milky Way taken in July on an exceptional night when it was naked eye visible almost to the horizon.  I haven't managed to stack it yet because the part nearest the horizon has suffered different amounts of compression over that 2 hour period.

One comment I think the main star on the left could do with being reduced as draws the eye away from the nebula.

It depends on what you mean.  If you mean "Photoshopping" it out, I completely disagree with tampering with the data in such a manner.

[Ivor]

I didn't mean photoshoping it out completely just minimise it a bit more.

[Carole]

Thanks Mark and Ivor.

you need to correct the distortions

Blimey there is always some new problem to come up and hit you in the face.  I never knew you could get distortions like you mention Mark.  Nor how to correct them, is there a software that will do it?  I am assuming when you align the images that it doesn't do any stretching/distortion only positioning.  

I quite like Alnitak the core of which is greatly reduced, it's only the halo that remains large-ish and it doesn't both me enough to do anything about it.

What does bother me though is not being able to satisfactorily align the filters I have taken the same night.  The stars line up OK, but there seems to be an unalignment of smaller pixels (? hot pixels) spoiling the result.  Will post this up in a separate thread.  Until I can get this sorted, I am not going to be able to get any further with combining colour.

Carole

[Carole]

Just had a thought Mark, could the distortion be to do with the different flattener spacing on the two different cameras?

Carole

[MarkS]

There is nothing I know of that will correct for atmospheric refraction near the horizon, which is why I've still got nowhere with my Milky Way image. 

However for lens distortions, PhotoShop has plugins (PTLens or similar - do a Google for them) that allows them to be corrected.
It's a common requirement for photographers and lens profiles from various manufacturers can be uploaded into it.
Any different combination of scopes and especially flatteners could potentially cause problems similar to the ones you are seeing.
It's always a potential problem when trying to combine images taken with different equipment or even when trying to create mosaics using the same equipment.
Trouble is you won't be able to pick up standard lens profiles for them.  You'll need to generate your own which is probably not easy.  Iris allows you to do it

So, is your problem atmospheric refraction or is is lens distortion?  Unfortunately I can't answer this question for you.

[Carole]

I don't think either image was taken too near the horizon as my home site does not allow low imaging.  I suspect it's more to do with different kit.  I've just remembered there a thing called lens correction in Photoshop, I've only recently discovered this, will take a look at it, but not optimistic.

Carole

[Carole]

Well I've spent a good few hours on this, and after a lot of blood and sweat i have managed to improve this a bit, it's still not quite right, but a step in the right direction - just need a few more thousand hours to get it right  

[MarkS]

Looking slightly better.

If you do some stretching and squashing in Photoshop you'll probably be able to get the layers to line up quite nicely.

[Carole]

That's partly what I did Mark.

Thanks
Carole

[Carole]

Right, well being as the skies won't let us do any imaging, I decided to download Registar and see if it would help with aligning (or registering) images and filters.  So far so good.  It has certainly ironed out the distortion problem on this image.  

The more I think about it the more I am convinced it was caused by the flattener and differing cameras and telescopes.  The DSLR was done with the ED120 and the Ha was done with the Atik383L and ED80.

[Fay]

That looks a lot better Carole

[Carole]

Thanks Fay.  It looks like Registar is going to be a good tool.

Carole