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HST colour palette

p1taylor

By p1taylor
in Imaging - Deep Sky

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GordonH

GordonH

Posted
Posted

Hi

The HST palette or Hubble Space Telescope palette is a colour palette made popular by the telescope of the same name. You are quite right in that separate images are taken through Ha (Hydrogen alpha), OIII (doubly ionised oxygen) and SII (ionised Sulphur). With normal RGB imaging you take separate exposures through red, green and blue filters and then combine them together to create a colour image that we are used to and remember the human eye sees everything in RGB. Hydrogen, Oxygen and Sulphur in emission nebula emit light at different wavelengths, in the case of these most common elements Hydrogen and Sulphur both emit in the red end of the spectrum except that Sulphur is a deeper red and Oxygen emits in the blue/green end of the spectrum. In order to create a colour image that we can understand the images taken through Ha, OIII and SII filters must each be assigned to one of the RGB channels. So which one should they be assigned to? This is where the HST palette comes in that the Ha, OIII and SII data is assigned in strict order of wavelength so SII=Red, Ha=Green and OIII=Blue. This is not the end of the story as hydrogen is so dominant in space that if no compensation was made in the processing the end image would be mainly green so sometimes a weighting factor is put on the filters when colour combining such as a weighting of 6:1:3 for SII:Ha:OIII or the individual master frames can be stretched differently so that they all contribute equally.

You would never be able to see objects in the HST palette in space even if you have a telescope powerful enough but that is not the point of imaging with these filters. The point is that hidden structure and details within nebula can be shown that would otherwise be hidden when using traditional RGB, also it can be done despite light pollution. There are other palettes used such as CFHT (Canada France Hawaii Telescope) palette which is Ha:OIII:SII = R:G:B but it is less common than the HST palette.

You are right in one aspect and that is that imaging with these filters is more expensive as each filter can cost up to £700 depending on size and bandwidth

Hope this answers a few questions

Best wishes

Gordon

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GordonH

GordonH

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That is no problem, I'm glad to be of help. A friend of mine in America called Richard Crisp started the ball rolling of narrow band or emission line imaging for the amateur in 2001 and I consider him to be the expert on everything to do with this subject. His website http://www.narrowbandimaging.com has a wealth of images and information on the subject. He took his inspiration from the famous "Pillars of Creation" image from the HST and managed to find a company to make the filters and although when he first started posting narrow band images most people in the imaging community poo pooed the idea and said it wouldn't catch on, well now all those people have jumped on the band wagon and every filter manufacturer worth their salt make narrow band filters. The colours aren't to everybody's taste although I like them but the detail that can be shown in images is staggering and some very spectacular results can be produced, it can show nebula in a completely different light

Best wishes

Gordon

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ollypenrice

ollypenrice

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To second Gordon there is another advantage to narrowband; tiny stars. Small amateur instruments cannot produce really tiny stars but narrowband keeps them much smaller (though colourless) and this helps the nebulae to show. Also Ha is easy to shoot in the moonlight.

Olly

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Dangerous-Dave

Dangerous-Dave

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That is no problem, I'm glad to be of help. A friend of mine in America called Richard Crisp started the ball rolling of narrow band or emission line imaging for the amateur in 2001 and I consider him to be the expert on everything to do with this subject. His website http://www.narrowbandimaging.com has a wealth of images and information on the subject. He took his inspiration from the famous "Pillars of Creation" image from the HST and managed to find a company to make the filters and although when he first started posting narrow band images most people in the imaging community poo pooed the idea and said it wouldn't catch on, well now all those people have jumped on the band wagon and every filter manufacturer worth their salt make narrow band filters. The colours aren't to everybody's taste although I like them but the detail that can be shown in images is staggering and some very spectacular results can be produced, it can show nebula in a completely different light

Best wishes

Gordon

Good back history, thanks Gordon. Richard Crisp was my inspiration for trying Medium Format Pentax lenses with my SXV-H9.

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Ben Ritchie

Ben Ritchie

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Be aware that [O III] and [s II] integration times are long - there's often not much signal. Avoid H-beta, as IMO it's effectively a low-transmission H-alpha filter.

I'd recommend avoiding the older Astronomik H-alpha, as I had a lot of problems with halo effects around brighter stars. Baader were good, the issue is apparently better with newer Astronomik too but I don't have first-hand experience.

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NickH

NickH

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The new astronomiks do fix the "reflection" issues...we have both in Spain and UK sites, and the new ones don't have the problem

OIII not as long as you think with some objects. Doing the Veil a week or so ago, I was amazed how much came out in OIII

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GordonH

GordonH

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Hi

Generally there is always plenty of Ha signal and less of OIII and SII with SII generally being the weaker signal, this is one of the reasons why there is a "standard" weighting of 6:1:4 for SII:Ha:OIII when colour combining as mentioned earlier. It is not generally a good idea to use longer sub frames for the OIII and SII data as although you will increase the signal in the nebula itself you will also increase the signal in the stars which can then lead to problems with very garish magenta stars, also there is the problem of having different length dark frames to match. The more common approach is to take more sub frames for OIII and SII as this data has to be stretched further in the processing in order for all three filters to contribute roughly the same when comparing the individual histograms.

The use of Hydrogen beta (Hb) is of more benefit for observational use but does has some use in scientific areas of imaging but is rarely used in astro imaging. The best filters to use are Ha, OIII and SII. There are other more exotic narrowband filters such as Helium (He) and Ionised Nitrogen (NII). The Helium filter has been shown to have a beneficial effect when imaging some planertary nebula such as M57 (Ring nebula) and again I would refer you back to Richard Crisps website for further information on this and images. It is a little known fact that the Ha filter also passes the NII wavelength as their band width is very close to each other, this is the case until you go to a band width filter below 5nm at which point the Ha filter does not pass the NII so if you are using 3nm filters for example you would have to take separate images through a NII filter to get the NII although the difference without it is small.

As regards filter choices if you are on a budget then go for Baader or Astronomik and as mentioned if you get the newer astronomik filters then halos shouldn't be a problem. I spoke to Gerd Neumann from Astronomik at Astrofest this year as he was working with me on the Widescreen Centre stand and he said that if you can provide image proof of halos with their filters they will replace them, ie send an image to them. As far as I am aware Orion in USA are now making narrow band filters but I haven't heard any reviews as to how well they perform. Then you come down to the big money filters such as those from Astrodon, Custom Scientific and FLI. Astrodon have the biggest choice and I use them, I am happy with the performance but I do think they are over priced.

When deciding how long to image and how many sub frames to do for each filter it does pay to do a bit of research on the internet, I always check on Richard Crisps website as a first point of call and if I am unsure I ask him

Hope all this information helps

Best wishes

Gordon

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NickH

NickH

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Fantastic info Gordon. I missed those ratios in my article, so if you are happy to allow, I will link once it comes out to this thread here, as I think your summary is perfect!

Garish Magenta - yup...been there...done that...got the t-shirt!

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Ben Ritchie

Ben Ritchie

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The use of Hydrogen beta (Hb) is of more benefit for observational use but does has some use in scientific areas of imaging but is rarely used in astro imaging.

The reasons here are that under effectively all astrophysical conditions H-alpha and H-beta trace the same gas, but the line strength of H-beta is about a third of H-alpha due to the relative populations of the two upper levels of the lines. So you're getting the same signal but at lower efficiency. Unless you've got very strange CCD efficiency or an (unmodified) DSLR with high cutoff at the H-alpha wavelength there's not really much sense in using H-beta. The eye is quite unsensitive to H-alpha, so H-beta is beneficial here too.

Generally for science use you're not using H-beta for imaging, you're using things like the H-beta/[OIII] 4959 ratio as a nebula or abundance diagnostic - as the two lines are nearby they're relatively unaffected by reddening.

It is a little known fact that the Ha filter also passes the NII wavelength as their band width is very close to each other, this is the case until you go to a band width filter below 5nm at which point the Ha filter does not pass the NII so if you are using 3nm filters for example you would have to take separate images through a NII filter to get the NII although the difference without it is small.

There's also a singlet [N II] line at 5754A that can be used. Tends to be very target-specific - fully CNO-processed material is N-rich (and C/O poor), more evolved object which have undergone He burning tend to be N-poor

P.S. on a point of pedantry, many of these lines are forbidden and have square brackets around them - so [O III], [s II], [N II] etc.

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GordonH

GordonH

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I deliberately left out the bit about forbidden lines as I didn't want to cause too much confusion as to why some of the emission lines are shown with [ ] around them in texts. In a nutshell they are called forbidden lines as they can only exist out in space where there are very specific conditions that allow their existence. That is a simple explanation but i am sure it is far more technical than that, again i would advise anybody who wants to look into this fascinating branch of astronomy/imaging to look up further info on Richard Crisps website

Best wishes

Gordon

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GordonH

GordonH

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Hi Peter

Just a couple of things before you do. You will need to pay particular attention to the following

1. Much longer exposure times ie in the region of 20 minutes plus

2. A very accurate polar alignment to cope with same

3. Very good tracking and guiding

Best wishes

Gordon

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