Problems with Alnitak

[alan potts]

Could someone point me towards a good written or filmed tutorial on how to control, I assume the word is Bloated Stars like Alnitak. It always seems to come out much the same after capture, I have tried all I know, and that's not much, to deal with it and failed. I stack in DSS and process in Photoshop. I know how to make stars smaller but this doesn't seem to do much on this star, I'm sure it happens elsewhere too. 

 

This is unguided and about 55x60sec lights with darks and flats, as you see Alnitak steals the show, I ate all the pies. This didnot have minimise stars or any sharpening and is also slightly cropped to take out some eggy stars, though some remain (they maybe all are at high mag).

 

 

[fireballxl5]

great image - love the colours!

[triton1]

I agree stunning image Alan,didn’t know you had been seduced by the dark side.Olly is probably the one to help he’s managed to tame Alnitak.

[AKB]

Perhaps worth posting a linear image FITS file, or the like, to see if others can advise?

[Ben the Ignorant]

It's not just Alnitak, it's any bright star (relative to telescope size), and it's not a defect. Even the Canada-France-Hawaii telescope made a halo around this star, they didn't process it out, and it looks good. How else would we know a star is much brighter if not for these dazzling halos? You can't process them out because they should be there. Stop torturing yourself, this is not a defect!

[MarsG76]

I basically stretch level separately for bright stars like Alinitak than mask them in to take the edge off.... I did that in the image I attached, same object, and had the same dilemma...

[alan potts]

8 hours ago, MarsG76 said:

I basically stretch level separately for bright stars like Alinitak than mask them in to take the edge off.... I did that in the image I attached, same object, and had the same dilemma...

 

I sort of thought that was the answer to my question, a mask, the trouble is I don't know how to do it basically selecting only the likes of Alnitak. I know 60sec lights are not long, I don't have cables to guide, the star looks reasonable in them. It looks OK even after the stacking, as you and other say it's the stretch that does it.

[alan potts]

11 hours ago, triton1 said:

I agree stunning image Alan,didn’t know you had been seduced by the dark side.Olly is probably the one to help he’s managed to tame Alnitak.

Only playing at the moment Jonn, I am basically just trying to use gear I bought 5 years ago when I had an idea to try it out, still do visual whilst the camera clicks away.

Alan

[Uranium235]

@alan potts, I put together a youtube tutorial on this exact subject:

 

 

[Mr Spock]

Excellent image Alan, your journey to the darkside is complete

Superb tutorial Rob, though I'm not sure about the music!

[Uranium235]

19 minutes ago, Mr Spock said:

Excellent image Alan, your journey to the darkside is complete

Superb tutorial Rob, though I'm not sure about the music!

lol... that was the time before I started using a mic. I wanted a soundtrack that was original (otherwise it gets blocked), so I used a little bit of shaft-stylee funk I wrote ages ago  

[alan potts]

2 hours ago, Mr Spock said:

Excellent image Alan, your journey to the darkside is complete

Superb tutorial Rob, though I'm not sure about the music!

I owe Rob a good deal, he has been very patient with me though Photoshop has been a subject that so far he has not guided me on, I was sort of OK in that program but every problem is not always that easy to get round in what is a very difficult program to understand to the full, we only ever use a small part of it.

I may even get guiding in PHD to work now we have worked out the correct lead may well help. Only trouble is I can't buy one here, or for that matter the little plastic connectors to make one.

Alan.

[ollypenrice]

16 hours ago, Ben the Ignorant said:

It's not just Alnitak, it's any bright star (relative to telescope size), and it's not a defect. Even the Canada-France-Hawaii telescope made a halo around this star, they didn't process it out, and it looks good. How else would we know a star is much brighter if not for these dazzling halos? You can't process them out because they should be there. Stop torturing yourself, this is not a defect!

It's not a defect, it's an artefact, and most imagers regard processing as the art of minimising them. In reality Alnitak is a double star, a piece of information which it's nice to have in an image, I think. How far to reduce it or control it is up to the imager to decide, of course. The thing is that even in the image above the data has been stretched, so it is not straight from the camera. Very few imagers would want to give an entirely uniform stretch across an image. Most want to lift up faint nebulosity and hold down stars and other artefacts like noise.

I don't link to my own images in other people's threads but perhaps Alan won't mind my Alnitak, followed by one possible technique.

Note that this is the best I've ever managed and most of the credit goes to the TEC140 which held it down with almost supernatural competence!

However, my method on this star is embarrassingly simple compared with Rob's blurred layer masking, which is how to do it 'properly.' I do use a technique like Rob's quite often but not in this case.

1) look at the star in your linear data. If it's more bloated than you want it to be in the final image you need some short subs. If it isn't, you don't because whatever is in you linear data can be retained in the final one. If you do need shorts don't take many because the SN ratio on bright signal will be very high anyway, and just do RGB wiithout bothering with L if using mono.

2) Make a copy layer and give the top one a gentle stretch. Use a round, well feathered eraser to take off the stretched Alnitak. You have to experiment with the brush size but apply it in one click without moving it. Maybe give the bottom layer a slight stretch to avoid any artificial-looking effects. Flatten and repeat, but, as you progress, make the eraser brush a little larger with each iteration.

Sara once said that she expected my processing to be really complicated, 'but all you ever do is use the b..... eraser!' Not far wrong, actually! I like doing different stretches to different layers and using the eraser to select which layer ends up being saved.

As others have said, you've got a good HH there.

Olly.

[gorann]

May I suggest a third method that I use on all my images (cannot believe I invented it myself but I cannot remember where I saw it).

First, I always stretch my images in Curves by doing several (ten or so) small consecutive stretches. Before each stretch I use the Select Color Range tool to select the brightest stars (putting the slider between 150 and 200). By doing this before each stretch (allowing PS to do a new selection each time, and not pressing "re-select") the selection will be a bit bigger each time (since the stretches are making the image brighter). This means that there will be no abrupt edges around the stars. Obviously it will only work if the stars in the linear image are not blown out.

Here is a cropped image showing what my Alnitak looked like using this method. If you post a non-stretched version of your image, I could see if my method would work, or you could try it yourself.

[alan potts]

3 hours ago, gorann said:

May I suggest a third method that I use on all my images (cannot believe I invented it myself but I cannot remember where I saw it).

First, I always stretch my images in Curves by doing several (ten or so) small consecutive stretches. Before each stretch I use the Select Color Range tool to select the brightest stars (putting the slider between 150 and 200). By doing this before each stretch (allowing PS to do a new selection each time, and not pressing "re-select") the selection will be a bit bigger each time (since the stretches are making the image brighter). This means that there will be no abrupt edges around the stars. Obviously it will only work if the stars in the linear image are not blown out.

Here is a cropped image showing what my Alnitak looked like using this method. If you post a non-stretched version of your image, I could see if my method would work, or you could try it yourself.

I understand what you are saying and can give it a try, I will also try and understand Olly's method and try that but most of all I would like to learn U235's version as it has so many other uses, I am finding the video very difficult to follow though as I can't actually read any of the menus and heading , it is that old. I like to sit and write down what he has done then do it, then hopeful get a result, the latter doesn't always happen.

Alan 

[gorann]

19 minutes ago, alan potts said:

I understand what you are saying and can give it a try, I will also try and understand Olly's method and try that but most of all I would like to learn U235's version as it has so many other uses, I am finding the video very difficult to follow though as I can't actually read any of the menus and heading , it is that old. I like to sit and write down what he has done then do it, then hopeful get a result, the latter doesn't always happen.

Alan 

Alan,

I just realized that I missed to say that you need to invert the selection after you have selected the bright stars (so you only work on the rest of the image with curves). I edited my entry above. I think my way of always avoiding stretching the stars during the initial stretching is a good way of starting any processing since you then have much less issues with stars and star shrinking later on. At least it has become a routine for me when I process.

In any case (and you probably know this) it is always a good idea to do many small stretches than a few large ones in PS, since the program uses only 255 levels of brightness when you stretch while your image has over 4 billion levels (32 bit) if I got it right, so you lose less information by doing many stretches.

Cheers.

[alan potts]

13 minutes ago, gorann said:

Alan,

I just realized that I missed to say that you need to invert the selection after you have selected the bright stars (so you only work on the rest of the image with curves). I edited my entry above. I think my way of always avoiding stretching the stars during the initial stretching is a good way of starting any processing since you then have much less issues with stars and star shrinking later on. At least it has become a routine for me when I process.

In any case (and you probably know this) it is always a good idea to do many small stretches than a few large ones in PS, since the program uses only 255 levels of brightness when you stretch while your image has over 4 billion levels (32 bit) if I got it right, so you lose less information by doing many stretches.

Cheers.

One of the things I like about PS is there are so many ways of doing the same thing, but with subtle differences. I know areas of this program well but some of course I have never used and probably never will. Even stretching I do different ways, sometimes as a none distructive work flow and sometimes one at a time with a levels check between. I need to get my head round masks though and understand this as it has so many uses.

[barkis]

No Pun intended, but you're a bit of a Dark Horse Alan .
You are pretty useful at this Imaging caper, that is a great job
on that Orion production farm. Your processing capabilities will improve for sure.
Just persevere mate, you will get there, some fine help available from experts in that area of 
the job.
 

[ollypenrice]

4 hours ago, alan potts said:

I understand what you are saying and can give it a try, I will also try and understand Olly's method and try that but most of all I would like to learn U235's version as it has so many other uses, I am finding the video very difficult to follow though as I can't actually read any of the menus and heading , it is that old. I like to sit and write down what he has done then do it, then hopeful get a result, the latter doesn't always happen.

Alan 

There are dozens of ways in Ps to do more or less the same thing. The tutorial I used when I first tried high dynamic range layer masking (which is what we are talking about) was one by Jerry Lodigruss, a real treasure of a gentleman in the community. His updated version is here. http://www.astropix.com/html/j_digit/laymask.html

This is such a crucial technique that it really is worth getting your head around it. Once you have it, you have it! (I say that but I was giving a one to one  tutorial recently in which I found myself blathering around trying to get it right... Uh-Oh, old age!)

The trick is to understand what you're doing, not to follow a rote method. So think it through: you have an overexposed image and an underexposed image. You make the underexposed one a layer on top of the overexposed. Now you want to blend them. You create a Ps mask between the two and place the overexposed image onto that mask. The overexposed parts on the mask are transparent so they let the gently exposed top layer fill in the over exposed bottom layer. That's essentially it. You have to give the layer mask in between a bit of help to make it work optimally but both Rob and Jerry take you through that.

If it's any encouragement, this is the most complex thing I ever do in processing, and I do it rarely. Mostly it's a story of tiny steps, tiny refinements, softly softly catchee monkey.

Olly

 

[tomato]

Wonderful image, I took 2.5 hrs of guided subs last night and I cannot get anywhere near the quality of your result!  While I  try to get my head round the processing, I stacked 40 mins of Ha data, it does seem much easier to get a result in monochrome. 

[ollypenrice]

2 hours ago, tomato said:

Wonderful image, I took 2.5 hrs of guided subs last night and I cannot get anywhere near the quality of your result!  While I  try to get my head round the processing, I stacked 40 mins of Ha data, it does seem much easier to get a result in monochrome. 

Nice split on Alnitak...

Olly

[alan potts]

Well I think your all very nice saying nice things about my image and I suppose for someone that only took his first image at the end of November they are reasonable, but I can see things wrong with them and I know I can improve. To get guiding going will be a big step. The processing side I find much easier than understanding things like plate solving and the Mexican food version, even things like APT I find hard work, though I am at least able to understand and use about 20% of it now. I am trying to add one thing at a time. I am considering spending a few thousand pounds this year on upgrades so I really want to get everything that is basic to most firmly in the grey matter. Mr Tomato, that is a very nice image and whilst I do not have your type of camera I do have a specail B&W program to change colour images, I like B&W and have shot thousands of medium format images years ago, I may try some of my shots run through this Silver Efex Program.

Thanks for the link Olly, I will be able to unsderstand what is going on don't worry, I will watch it a few times and try doing it at the same time on another laptop.

Alan 

[Xiga]

You could always try the method outlined here as well:

It seems to work pretty well on very large bright stars (it's too harsh on the smaller stars I find). Have a look at the 2nd post for an example. Be careful not to lower the opacity slider too much though. If you need a bigger reduction, you could try running the process more than once as well to see how well it holds up on multiple iterations (I haven't tried this myself). I would also do all of this in a duplicate image, so that you can copy the flattened result back into the original image and mask out everything else other than Alnitak itself.

If you do try it, post up the result, I'd be interested to see how it fares.

[alan potts]

2 hours ago, Xiga said:

You could always try the method outlined here as well:

It seems to work pretty well on very large bright stars (it's too harsh on the smaller stars I find). Have a look at the 2nd post for an example. Be careful not to lower the opacity slider too much though. If you need a bigger reduction, you could try running the process more than once as well to see how well it holds up on multiple iterations (I haven't tried this myself). I would also do all of this in a duplicate image, so that you can copy the flattened result back into the original image and mask out everything else other than Alnitak itself.

If you do try it, post up the result, I'd be interested to see how it fares.

Many thanks for that but I do not have Annie's actions and I am more interested in nailing the one poated by U235, where he actually did the work on Alnitak, trying too many things at once will only confuse me, that doesn't take much doing these days.

Alan

[Stub Mandrel]

On 20/01/2018 at 17:11, alan potts said:

Could someone point me towards a good written or filmed tutorial on how to control, I assume the word is Bloated Stars like Alnitak. It always seems to come out much the same after capture, I have tried all I know, and that's not much, to deal with it and failed. I stack in DSS and process in Photoshop. I know how to make stars smaller but this doesn't seem to do much on this star, I'm sure it happens elsewhere too. 

 

This is unguided and about 55x60sec lights with darks and flats, as you see Alnitak steals the show, I ate all the pies. This didnot have minimise stars or any sharpening and is also slightly cropped to take out some eggy stars, though some remain (they maybe all are at high mag).

 

 

The secret is to do all the stretching with curves alone, so you never stretch the brightest parts of the image. You can even darken (de-stretch?) the brightest bits.

I made this with two years data (120 second subs) in November, but I coudln't get the colours as good as the year before so I haven't shared it before.