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Narrowband

NGC 6888 (Crescent Nebula) in glorious technicolour....


Tim

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But where is the BLUE ???

I was enchanted by the NASA APOD this week of this object http://antwrp.gsfc.nasa.gov/apod/ap080813.html

What an amazing object, and awesome photograph.

So when I saw a clear patch heading my way the target of choice was The Crescent Neb. Amazingly, the goto placed it absolutely dead centre screen (there's a first time for everything!) and I rattled off exposures for around three hours.

What surprises me, I have to say, is that there is none of that gorgeous blue envelope to be found on my frames, not a trace that I could find. Why is that??? :hello2:

The only things I can think of are these;

1) Moon at 91%

2) Heavy Dew. The OTA stayed free of dew but it was easily the heaviest night of dew I have ever seen. It might as well of rained!

3) CLS filter. To cope with light pollution I use a clip in CLS filter.

4) DSLR cmos not sensitive to whatever narrowband type the blue falls into?

Any help or ideas gratefully recieved!! I'd love to have another crack at this one if it meant I could grab some of that blue. Even the 600 sec sxposures dont show it.

So this is around 3 hours of data, plus darks, DSO at zenith, C9.25 @ F6.3. 300-600 sec exposures, stacked in DSS, processed in CS3. Guiding via WO72/DSI Pro/PHD.

Hope you like it.

Cheers

TJ

Larger Version : http://ukastronomers.com/Tim/image/7757_large.jpeg

7756_large.jpeg

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tj

your camera has a decent response at 500nm O[iII], but strarts to decline after this. The WR star is very hot, and so this blue cloud seen in the APOD may not be purely of O[iII], so your camera may not record that well.

More to the point, the outer halo is exceptionally faint...this is the only time I have seen it. You would require quite a few hours to pick it up. There is also a massive range of intensity betwee the red crescent and the faint blue, so correct processing of the crescent will mask the blue. This image was taken with some narrowband data, blocking out red, assuming the NB filter was intended to show the blue.

The moon will have brightened the background sky flux, and could very well have masked the blue also. I am not sure if atmospheric water adds to extinction. It will certainly causes scattering, but it will only absorb discrete wavelengths. The scattering is usually stronger in the blue (Rayleigh Scattering)

Although F6.3 is reasonably fast, it wont pick up the faint blue stuff that fast. Perhaps an F5 or less would produce the results you are after.

I hope this answers your question

It was a very nice image by the way. Lots of detail

Best Wishes

Paul

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PS

what is your CLS transmission spectrum like for the blue region <500nm

It may cut blue...and then you are fighting a loosing battle. But most filters try to pass Hb and O[iII] with high transmission. The blue cloud is most likely to be O[iII] or Hb as these filters are common. But there is a chance that it was an He II filter...Richard Crisp has taken a few images with his He II. They work well with WR objects because of the excitation caused by the hot star.

Best Wishes

Paul

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Hmmm. I dont know much, rather, I dont know ANYTHING about wavelengths and such, I can only give you observational evidence.

When I take a pic of M27, or M57, WITHOUT the CLS, there is plenty of blue showing, and very little red in the ring.

When you add in the CLS, there is a lot of red, much of the blue is replaced with red, but there is still SOME blue there ( see my M27 pic on UKA) which is why I was so surprised to see nothing at all. AFAIK, the CLS only blocks wavelengths towards the red end of the spectrum, and is geared towards the two colours produced by most artificial lights.

Thanks

TJ

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the human eye cant always be trusted. I look through my Orion Skyglow, and things appear green...I use my SLR and things appear fine. I will check your spectrum on line...

the filter should take away the hideuos orange Sodium doublet and the very bright mercury emission line in the green. This is fine because nebula have very little emission in the green, its mainly blue or red.

paul

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the CLS has good transmission to 450 nm but drops off as you go bluer. The image APOD was taken in Ha and O[iII], and I suspect some RGB for star colours.

It is probably a combo of the moon, the camera, the filter, the scattering from dew, and the extreme faintness of the halo itself.

Dont beat yourself up, Tony Hallas is an experience imager, with a large scope and observatory with clear nights, with a monochrome sensitive CCD (probably the STL11000m as usual). Your image is very good, and I would be proud of that

Best Wishes

Paul

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You have created a nice Image TJ, but the Important ingredient is not in this single image, It's in the overall advances you have made in the relatively short space of time you have been doing this.

You will continue to improve I'm certain, but don't cover yourself with dissillusion because you think your not achieving what Tony Hallas can do. He has many advantages, the main one being vast experience as Paul just said.

Ron.

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There's not much wrong with what you've done there TJ :hello2:. But like everyone else, I've never seen that blue either. It would appear that it's OIII, see here for the original, look especially at the OIII shot ( http://astrophoto.com/NGC6888.htm ). Fact is, you can't compare what premier league imagers like Tony Hallas and Rob Gendler do to our own efforts. For starters, look at the amount of exposure time they have obtained, none of this 'I'll grab a couple of hours before the rain starts/clouds roll in/some twerp turns on his 'security' light. I would also assume that they don't suffer from light pollution issues like we do, and look at the gear they have. They've spent 5 figure sums!

So unless you can buy a place in Arizona, spend a LOT of cash and have the time to not only get the data but to process it as well (I bet he's spent a few hours getting that just right), then look and adore because you can't compare, it's a different league these guys operate in. That's my thoughts anyway...

Tony..

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hehehe, you guys have the wrong impression totally :lol:

I am really pleased with the finer details captured on this pic, especially as it was under a full moon with a dslr, despite the wife saying she didn't like it because it looks like a placenta... :?

I dont expect or even hope to get anywhere NEAR the quality of those guys, (or even YOU guys :lol: ), the only point is, I was really surprised that there is absolutely no trace of any of that lovely blue, and I am just curious as to where it all went :hello2:

I'd be interested to see what somebody who knows what they are doing could do with the data... any takers :clouds1:

Another thing, would the skylight filter make any difference to it do you think????????

Cheers

TJ

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Thanks for that link Tony, that really shows the OIII data.

So the next question, is, Do DSLR's have the ability to pick up that data? If so, what does it take? Is it necessary to filter out all other light except OIII?

Thanks for the help guys,

TJ

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Thanks guys, I'm just processing the files again, based on the tutorial given elsewhere by someone, and this time, including flats.

Also, I am endeavouring to do all the processing without blurring, sharpening or dust & scratches filters etc, so we'll see how it turns out. I'll post it later for your honest critiqué :hello2:

TJ

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