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M27

astro mick

By astro mick
in Imaging - Deep Sky

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astro mick

astro mick

Posted
Posted

Hi.

An image of M27 taken last night,using an Celestron 80ED a set of Astronomic LRGB Filters,and a Starlight MX716 ccd camera.

Each exposure was for 30mins.Processed in AIP,and Photoshop.

Conditions: Quite clear,just some wispy cloud,little wind,and not really dark.

Mick.

post-17080-133877632602_thumb.jpg

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roundycat

roundycat

Posted
Posted

I missed this one earlier today or I would probably have replied. Not that my replies are necessarily what you would be looking for.

Overall it looks quite good but then the star colour seems to become very obvious. Also you have a slight alignment problem on the right. The nebula looks a bit soft, try a small High Pass on the whole thing having made a star mask first or just select the nebula and do it that way.

For a brightish object such as this I would prefer more but shorter exposures, it makes for a cleaner stack and less bloated stars.

Dennis

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SlipperySquid

SlipperySquid

Posted
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Hiya Mick,

You are over processing the image mate. I know this from the last data you sent me.

Start from scratch by stacking each channel and combining Then do gentle curve stretches in photoshop and then levels to set the black point.Do this alternately curves then levels until the detail starts to appear BUT don't overdo it.

The short cut in photoshop is usually Ctrl+m (for curves) and the Ctrl+l for levels.

I know the data will be in the subs you have it's just adjusting your workflow so that you don't over do it.

If I get chance I could always pop to yours and go through a simple work flow routine to get the best out of your subs (well the best I can do, I'm no expert) but it should put you on the right road as I find it hard to write down exactly what to do :D It's easier to be shown (well that's what I think)

Matt.

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astro mick

astro mick

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Dennis,Matt.

Thanks for the info,and especially the replies.You are both saying the same thing,yet on my PC it looks really good.I do use Noel Carboni,s Tools,so i,m wondering if i,m over doing it a bit here.

Matt your always welcome.I know i,m never going to be an OLLY.but onwards and upwards.

Mick.

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roundycat

roundycat

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Mick, just for the record I disagree with Matt and everyone else that says to use Levels and Curves alternately.

My experience with PS over the last fifteen years is simple; treat Levels as the axe and Curves as the scalpel. You would not go from using a scalpel back to the axe. Would you?

The point about this is that everything you can do with Levels you can do ten or twenty times more effectively using Curves. How many mid-range adjustment points can you have with Levels? Answer - one! How many with Curves - answer fourteen!

Watching the histogram is not the most effective or accurate way of determining where the black point is. Black point itself is something of a mis-nomer as you rarely if ever want black in an astro image.

If you want to know how dark the darkest bit of the picture is then look at the Info Palette, it is a darn sight more accurate than the Levels histogram. You can do this whilst making simple or complex adjustments using Curves. Make sure you set the second readout on the Info Palette to HSB. By default it is CMYK which is all but useless for astro/web work.

Your Curve stretches should start by setting the background to a neutral colour (most of the time), this is close to the bottom of the Curve. You may find it easier to adjust one channel at a time and flick back and forth through the History palette to see if you are making an improvement each time. Don't be tempted to make your major contrast stretches one channel at a time, use the RGB channel and then individual channels to get the 'bright' colours right.

Above all, don't be afraid to keep looking in the History. If you have not made continuous improvements - be brave! Go back and do it again.

More info here: Mono to RGB

Dennis

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SlipperySquid

SlipperySquid

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The it could be the PC monitor mate? I had the same problem with my lap top that I was pushing the curves in the image to much as the monitor wasn't set up correctly?

Or it could be an older monitor that's on it's way out?

Email me the subs again for the RGBL and I'll take a look Mike if you want?

Matt.

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SlipperySquid

SlipperySquid

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Dennis,

With ref to Level's and curves, this is what I have been shown from the off as a basic start in bringing the image out in photoshop from the likes of RobH, MartinB, PeterShah etc at various Star parties. Right or wrong it's a basic start.

As I wrote I find it easier to show than to put into words and your explanations,correct or not, don't actually take into account the amount of knowledge some one may have with Photoshop etc

I've read loads of tutorials and some of them go straight over my head and with loads of variables with what can be going wrong with an image some times I feel like just throwing the laptop and giving up.

Matt.

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roundycat

roundycat

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Matt,

I stand by what I said and I will argue my corner with anyone. Try the following with some of your old data. There are some provisos as I have no idea what your raw data looks like. Can I take it that you can use your aligning/stacking software to output a 16bit integer TIFF? Can I also assume that you are not one of those that allows the stacking software to make any adjustments of its own or deliberately makes any changes to the data using the aforementioned software. (No DDP,De-Con or fancy embedded screen stretches).

I say all this because I need to make sure your data looks like what I think it should look like. When the 16bit TIFF is opened in Photoshop it should be virtually black everywhere except for a few stars. If you are lucky enough to enjoy a dark sky and have gathered lots of exposure (let's say 6-10 hours for starters) then you may see some dim shapes from a bright nebula.

Open in PS and do the following.

Levels, mid slider to a value of 2 and OK.

Ditto

Ditto

At this point the background might be starting to get bright depending on the total exposure given. If so you may use the 'black' slider to re-set the background brightness to a lower value but don't bother looking at the histogram. Look at the Info Palette and aim for a background brightness of 15-18% in HSB. That equates to about 37-45 levels. Probably too bright but good enough for starters. Do not on any account push the black slider until it touches the left of the histogram. Leave the careful stuff to Curves.

These first three moves are very dependant on your data and total exposure. I sometimes use four iterations of Levels and, rarely, only two.

After this you go to Curves and do not go back to Levels adjustments for anything. Any need to change the lowest value (aka Black point) in the picture should be done with the RGB channel in Curves. If you have a colour bias in the background sort it out with Curves. Measure the value of each colour at some point that you want to be neutral. Add the three values, divide by three and set each colour channel to that value +/- 1 or 2. That will give you a neutral background without changing the brightness or luminance value. You should do that in RGB. When adjusting the colour channels to get rid of this background bias make sure you only move the bottom of the curve. The rest of it should remain straight.

The tricky bit, and the bit that gives you the most satisfaction as well as producing the goods is adjusting the contrast throughout the picture to get the whole thing to look right. As most people either ignore or are unaware of the detail in the shadows or highlights you can concentrate more on the mid range contrast. You can only set this properly once you have done the initial Curves stretch so you can actually see all the information in the picture.

I would be happy to try to demonstrate this in more detail if you think it worth while.

Dennis

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ollypenrice

ollypenrice

Posted
Posted

Dennis, this is very interesting. I use Levels and curves in the classical manner but will go through your posts carefully because I'm very intrigued and I know you know your stuff.

Just on this image though, I think the cutting back in levels has been too severe. Leave a bit more on the left, Mick.

Also, on an object like this, it is easy to do a separate stretch for the stars.

Layer them in later.

Olly

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Darth Takahashi

Darth Takahashi

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Ditch Photoshop altogether and use PixInsight, its processing takes a little to learn but it far superior since you can keep your data in the original FIT format at 32bit IEE754 level, using Floating-Point Arithmetic.

Nothing beats that for all those **** retentive imaging processors...:)

The difference between using curves and level is EXTREMELY subjective when applied to an image since the original quality of the image data is the overriding concern. However, treating this mathematically should show little difference.

The main issue with LEVEL adjustments in PhotoShop is that people can be HEAVY HANDED with it and the same is also true for CURVES but you just need to work a little harder and you have a nicer graphical display showing that your clipping the data!!!

But in the end NOISE is NOISE and not DATA.

Scalpel and axe no! more like kitchen knife and scalpel but switching to PixInsight is the better option in the long run.

Just my thoughts on this issue and I'm definately on safer ground with the MATHS.

Neil.

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roundycat

roundycat

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First, never mind the maths, show me an image where keeping it in either FITS format or 32bit or floating point is a distinct improvement on one worked on as a 16bit integer TIFF. As you will have to either print it and send me a copy or post it on the internet I can save you some time. DON'T DO IT, there will be no visible difference or improvement as it will be in 8bit.

I did say that assumptions would have to be made regarding the image data. No matter what techniques or software you use bad data will only make a bad picture.

The main issue with Levels adjustments in PS is that you only have the one adjustment point. With Curves you have fourteen. The graphical display might be sufficient for anyone brought up on computer games but with PS you can actually measure the value of any pixel on the screen. That is the only accurate way to ensure you have no clipping.

Dennis

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Darth Takahashi

Darth Takahashi

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The original images starts as a 16 bit integer file, agreed?

It doesn't matter whether or not its a FIT or loss-less TIFF, it can record 2 power 16 levels of grey, that's 0 to 65,536 tones.

Now apply a LOG stretch or and EXP stretch and you no longer have an integer! Of course, PhotoShop will happily coerce the new float back to an integer - this is a FACT and it results in an error.

Now in the typical image processing sequence you will perform many of these operations as follows;

Image stretching, transposition (Alignment & rotation), creation of RGB or LRGB or LLRBG image etc... Each step in the integer world will result in image degradation.

Many imagers realize this and create routines to limited the damage caused by each of the processes above.

PixInsight avoids this problem altogether by maintain the image in the highest possible format until the final output image is required.

Now the AXE comes neatly into play, since the image date is compressed into an 8bit JPEG file for display here.

I have said nothing contentious here, these are all facts and the limitation that we need to deal with / manage.

Stan Moore (CCD-Stack) said it best on the SBIG forum "Image processing time is cheap - scope time is precious" You can always come back to your data and apply new techniques but good data to start with is hard to come by.

Neil.

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Darth Takahashi

Darth Takahashi

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The image is very nice... A little over processed but still very good...

I have nothing but admeration for anyone who takes up astro-imaging as its blumming difficult even when you have all the kit that I have, just ask Matt!!!

Still it looks nice standing in the living room next to the TV;)

Yes, sorry the thread did get a little esoteric!!!:)

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