Horsehead nebula 27.11.10

[Carole]

Desperate to try to sort out my guiding problems I went out last night minus 4 degrees, then found the skies were clear and how often does that happen?

It became hazy and so the stars are really bloated (must learn the tricks to try to suppress this in photoshop).

Anyway, on the 2nd try I managed to get guiding working again - THANK GOODNESS FOR THAT!!! Been a right PITA to sort out.  (Must have been moisture in the guide camera which has been in the airing cupboard for 2 weeks).

Just over 2 hours of 5 min subs, darks, flats and bias.
Modified DSLR Canon EOS 450D CLS filter guided.

(Note to myself, must remember to take the Bahtinhov mask off BEFORE I start imaging!!)

[Rocket Pooch]

Thats quite good Carole.

[The Thing]

I'm impressed.

[Carole]

Thanks Chris and Duncan.  Shame about the bloated stars, but very pleased to have got what I did as it's my first proper Horsehead, especially after all the problems I've been having.

Carole

[Mac]

See what persistence can do.
Nice capture.

so did you make a not of exactly what you did.
All you need to do now is to refine it.

Mac

[Carole]

so did you make a not of exactly what you did.
All you need to do now is to refine it.

Hi Mac, are you referring to what I did to image, or what I did to sort my guiding out?

I think the only thing I did differently this time to previous images was more subs (I normally only manage 1 1/2 hours), and I didn't go mad with the processing which I think I have overdone in the past.  I only used the Bahtinhov mask for focus and did not use Bahtinhov grabber as it looked to be spot on without.

Oh and of course I have done my flats properly this time. 

Carole

[RobertM]

Nice capture Carole.

[Carole]

Thanks Mac and Robert.

Carole

[MarkS]

Carole,

That really is surprisingly good!  You must be really pleased with this one. The subject is well chosen - filling the field of view.  Lovely horsie.

Two hours of solid imaging - is that a record for you?

The background noise is nice and even - it is caused by the light pollution - take the same image from a dark sky site and you would notice a dramatic improvement.

This must be your best image to date - well done!

Mark

[Jim]

That's really nice, well done Carole.

[Carole]

Thanks Jim and Mark,

Two hours of solid imaging - is that a record for you?

Yes, normally the clouds roll in or something and I have to stop.  This time I only stopped because the haze was getting so bad.  

The background noise is nice and even - it is caused by the light pollution

I'm glad to hear that as I was wondering what the cause was.  Some-one suggested doing longer subs, but I am not sure what a DSLR will go up to as my images look very washed out after 5 mins.  I think getting my flats right has also helped.  

The subject is well chosen - filling the field of view.

I'd be lying if I didn't say this was a crop.  Full frame below. There is a bit of shift as I bumped into the guide scope, whilst checking the objective lenses were not misted up (I had the dew heaters on), but wanted to be sure as I have never worked in such frosty temperatures before.  So had to stop guiding and re-start.  Might have shifted the whole set up slightly as well.  

This is the full frame.

[MarkS]

Some-one suggested doing longer subs

Longer subs are only useful when CCD read noise is the dominating factor.  But in this image the dominating factor is certainly the light pollution (even using a CLS filter).

Mark

[Fay]

That has come out ok Carole, at least you have seen the horse!

[Mike]

Very good considering the poor conditions. Well done.

[Carole]

Thanks Fay and Mike.

Some-one has shown me a neat trick to reduce the stars, however it doesn't reduce the halo.  I am not sure if this is acceptable.  Comments appreciated.

[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