Horsehead nebula 27.11.10

[mickw]

You've done a good job capturing it from crap skies and the reprocess works really well, looks like you worked hard on this version, the background is a lot cleaner also.

Much better than v1

[Carole]

Thanks Mick, I did some noise reduction as well.

Carole

[RobertM]

Yes, I'd agree with Mick.  You've done a really good job of reprocessing and the noise reduction has helped a lot.

Robert

[MarkS]

Hmmm.....

Overall I like the re-process but I don't like the star reduction.  It has a "gravitational lensing" effect - altering the positions of the nearby stars and nebula!

My own guess is that the star bloat can be improved by range scaling e.g. asinh, ddp, photoshop curves or something similar.  If you find the star bloat is less pronounced in a single sub then range scaling is the way to go.  Preferable to trickery!

Mark

[Carole]

It has a "gravitational lensing" effect - altering the positions of the nearby stars and nebula!

Yes I had a feeling someone would say that.  

I've tried curves but couldn't do much with that, and I am not familiar with

range scaling e.g. asinh, ddp

, looked also in Iris but could not find them there either.

I've done this one as a compromise as unless I can find a more satisfactory way of doing star reduction I think this is as far as I can go this time.  I have reduced the stars a little (not so much as before), and combined that with the noise reduction.

Mick I think it is "take you pick time" for "members images", but probably don't choose the first one.

[MarkS]

Take a look at a single sub and examine the size of the saturated area around the bright stars.   I can guarantee this will be fairly small (unless you were shooting with a ridiculously high ISO).  By using an improved processing sequence it will then be possible to reduce the star bloat in the final image back to exactly the same sized saturated area.

For instance, look at the size of the saturated area in this mosaic of 5 min subs at ISO 800, also using a CLS filter, in Sidcup:
http://forum.orpington-astronomy.org.uk/index.php?topic=6657.0  (look at the big version)

Mark

[Carole]

I'll have another go tonight then and see if I can combine the stars from the original stacked image (before stretching) with the stretched one and see what I get.  I'll try also a single image as well.

Carole

[Carole]

I'll have another go tonight

Sorry too tired after a horrendous four hour journey home from Lewisham.  Hardly any buses and ended up finally managing to get one to Grove Park and then no more, had to walk all the way home from Grove Park to where I live in Bromley Common.

Carole

[Carole]

OK Mark, almost one month later I have managed to re-process the image and I hope I have reduced the star bloat more satisfactorily and overall I think it is less noisy.  I certainly prefer this to my original process.  I still don't know how others manage to control Alnitak so well, I'd like to know how it is done. 

[MarkS]

Looking much better now but overall I think it's a bit too red. 

For Altinak, check out your subs - if the saturated area is the same size as in the image below then there's nothing you can do to control it other than to combine it with some shorter subs.  If the subs show a small saturated area then log or arcsinh scaling on the final stack should do the trick - you should be able to get the same small saturated area in your final image.  You won't be able to get rid of the diffused "glow" around Alnitak without very sophisticated processing (which I have never attempted).  Alnitak is so bright that it will always cause weird effects - a good analogy is trying to photograph clouds on a summer's day but catching the sun in your photograph - it will always cause unwanted image artifacts.

Mark

[Carole]

Thanks Mark.

[Carole]

One last bash at this.  Some-one suggested using feathering and curves to reduce the halo.



I think this is as far as I can go on this. 

Carole