M51. What went wrong?

[pixueto]

Dear all,

After spending most of the night imaging Mars and Saturn, I decided to have my first try at M51. I only managed to get 21 mins before dawn (all 1' subs, 10 bias, 10 darks and 10 flats). Then, when I got the stacked image from DSS I was quite happy to see the galaxy but after stretching the histogram in PS, this horrible gradient appeared from nowhere. If it wasn't for this, I would've been able to bring out more detail from PS.

Can you guys help me? What went wrong? Is there a way I can remove this gradient?

[Cornelius Varley]

Heres a link to a gradient removal tutorial Gradient removal in PS

Peter

[Chris]

I don't know what the gradient is but if you can remove it with PS like stated above you'll have a nice little m51 on your hands I recken:) I gave this object 1hour and 40 minutes the other night and I've no where near the colour you have:(, how do you get so much colour with an unmodded DSLR? do you line up your GRB so they overlap which is what I've been doing?

[Jannis]

I just want to say firt of all, you really should be with that resuly from that short exposure. Great capture and lots of details and colors, and good focus too!

as for the problem, if you for some odd reason have not yet touched teh focus on teh scope, and not removed the camera, thyen you can still take flats to correct it.

However, if you have done either of those two, it's not much you can correct easily. you can fix it more or less in photoshop, but it will never be as accurate or correct as if you had taken flats.

[pixueto]

Thak you very much for the tip. I did applied that technique and it seemed to make a difference. Afterwards I stretched a bit more the histogram and a little bit of gradient appeared but I think that I can do much better once I master this process. Thanks a lot!

I only line up the GRB in DSS once the computer finishes stacking the subs. I don't think this particular galaxy will looe much different in a modded camera because It has lots of blue stars and dust. Have you tried increasing saturation in the 'match colour' dialogue?

[Chris]

Hi Pix, yes like youself I allign GRB after stacking and geneally increase the saturation to about 8-10, I noticed with M51 that if I decreased the saturation I gained colour so I processed my most recent image with the saturation at 2. I'm thinking that your excellant focus with a Bahtinov mask might be seperating the blue and yellow in M51 nicely, and that might be my downfall as I'm just focussing using liveview. I think my next purchase will be a Bahtinov mask:) I'm still a bit baffled by how you got such steep gradient in the first place though??

[Jannis]

I'm not too good in PS, but when i don't have flats for objects like this, i sometimes use "dust and scratches" to create a picture with only the background. i noticed with guassian blur witch i used before, i don't get as good result in many cases. and with the gradient tool in PS i have yet to have any type of luck with.

i used dust and scratches on your jpg here just to see, and it worked ok i'd say. also aligned the RGB channels a bit more, but far from perfectly though, just to show how it works with this method.

process of what i did, more or less:

1: align RGB channels.

2: duplicate layer.

3: dust and scratched with settings untill only background shows.

4: on this background layer, select substract.

5: adjust layer settings if you want, i just selected merge down as this was just to demonstrate.

6: align the RGB channels again if needed.

7: adjust the curves as desired.

8: just run it through noise ninja with standard settings as the JPG got quite noisy...

note that many others have different ways of doing this though, so i recommend to try out different things and see what works best for you. i'm also sure others here will give you good advices and guidelines..

edit: this works best/most easy on small targets or low res pics though.

[pixueto]

And this is the same image after a second gradient removal processing. It doesn't look as natural as the other two, does it?

[pixueto]

I'm not too good in PS, but when i don't have flats for objects like this, i sometimes use "dust and scratches" to create a picture with only the background. i noticed with guassian blur witch i used before, i don't get as good result in many cases. and with the gradient tool in PS i have yet to have any type of luck with.

i used dust and scratches on your jpg here just to see, and it worked ok i'd say. also aligned the RGB channels a bit more, but far from perfectly though, just to show how it works with this method.

process of what i did, more or less:

1: align RGB channels.

2: duplicate layer.

3: dust and scratched with settings untill only background shows.

4: on this background layer, select substract.

5: adjust layer settings if you want, i just selected merge down as this was just to demonstrate.

6: align the RGB channels again if needed.

7: adjust the curves as desired.

8: just run it through noise ninja with standard settings as the JPG got quite noisy...

note that many others have different ways of doing this though, so i recommend to try out different things and see what works best for you. i'm also sure others here will give you good advices and guidelines..

edit: this works best/most easy on small targets or low res pics though.

Thank you very much Jannis. Noise ninja? Is that freeware?

[pixueto]

Hi Pix, yes like youself I allign GRB after stacking and geneally increase the saturation to about 8-10, I noticed with M51 that if I decreased the saturation I gained colour so I processed my most recent image with the saturation at 2. I'm thinking that your excellant focus with a Bahtinov mask might be seperating the blue and yellow in M51 nicely, and that might be my downfall as I'm just focussing using liveview. I think my next purchase will be a Bahtinov mask:) I'm still a bit baffled by how you got such steep gradient in the first place though??

Thanks starfox! Indeed the Bahtinov is the best tool in my kit -and the cheapest! It makes life much, much easier!

[Jannis]

Thank you very much Jannis. Noise ninja? Is that freeware?

No, but they have a free "demo", so you can see how it works. i find it to work quite a bit better then the noise removal tool in PS though. Noise Ninja: Downloads

i try not to use it too often though, as you normally always loose details when reducing noise with software like this instead of more/longer exposures. in other words, it's anything but a replacement for long exposure/many stacked frames, but it does helps out in many cases or if you have no other choise but work on a high-noise image.

[ollypenrice]

This is certainly a problem with your flats. What you see is classic vignetting which, for some reason, your flats have not successfully corrected.

For a test, try a stack without including your flats and compare them.

Flats should be exposed to put the peak of the histogram between one third and two thirds of the way up to saturation. Is this the case with yours?

Oh, hang on; one other 'just maybe' possibility would be dewing on the chip window. If your flats usually work it might be that you shot either them or the image with some dewing starting to build up.

Olly

[pixueto]

This is certainly a problem with your flats. What you see is classic vignetting which, for some reason, your flats have not successfully corrected.

For a test, try a stack without including your flats and compare them.

Flats should be exposed to put the peak of the histogram between one third and two thirds of the way up to saturation. Is this the case with yours?

Oh, hang on; one other 'just maybe' possibility would be dewing on the chip window. If your flats usually work it might be that you shot either them or the image with some dewing starting to build up.

Olly

Thank you very much olly for your help'. I'll go and process the image without flats and see what happens but I'm new to all this and I don't know what you mean when you say:

Flats should be exposed to put the peak of the histogram between one third and two thirds of the way up to saturation. Is this the case with yours?

Can you elaborate?

[Cloudwatcher]

Can't say what went wrong but had a lunchtime play with the image making individual layers for background,stars and galaxy. Desaturated and darkened background to get rid of the central red glow. Enlarged the galaxy layer and used the dodge and burn tools to bring out the detail,played with hue/saturation then sharpened. Reduced back to normal size and merged layers.

Seem to have lost some stars I'm afraid but I think M51 looks a bit better...........perhaps a crop is called for?

[Chris]

Wow! that looks excellant now:)

[pixueto]

Can't say what went wrong but had a lunchtime play with the image making individual layers for background,stars and galaxy. Desaturated and darkened background to get rid of the central red glow. Enlarged the galaxy layer and used the dodge and burn tools to bring out the detail,played with hue/saturation then sharpened. Reduced back to normal size and merged layers.

Seem to have lost some stars I'm afraid but I think M51 looks a bit better...........perhaps a crop is called for?

Wow, it definitely looks better! I didn't realise how much data there is in the image, starfox was right. Can't wait to spend a couple of hours working with this target!

[Jannis]

Can't say what went wrong but had a lunchtime play with the image making individual layers for background,stars and galaxy. Desaturated and darkened background to get rid of the central red glow. Enlarged the galaxy layer and used the dodge and burn tools to bring out the detail,played with hue/saturation then sharpened. Reduced back to normal size and merged layers.

Seem to have lost some stars I'm afraid but I think M51 looks a bit better...........perhaps a crop is called for?

that sharpness and details are just stunning. kind of shocked you got all that details out of that jpg.

would have loved if you could have made a quick video of what you did there. i'm sure it would help a lot of people here as well.

[pixueto]

that sharpness and details are just stunning. kind of shocked you got all that details out of that jpg.

would have loved if you could have made a quick video of what you did there. i'm sure it would help a lot of people here as well.

I second that:)

[ollypenrice]

OK, flats; if your lens were optically perfect and your entire imaging train were perfectly clean and your flats light source were totally even then every pixel would record the same value. (If this were the case you would have no need to take flats!) Now what would this recorded value be? If you gave it a long exposure it would be a high value. It might even be the maximum value your chip can tolerate. In a short exposure it might hardly register above the noise, a very low value.

What it should be is half to two thirds of the way to the max your chip can record.

Many DSLRs (I'm sorry but I don't know much about the specific models) will show you the histogram (distribution of brightness values) across your chip. It will look like a bell curve. The high point of this bell curve should lie between a third and two thirds of the way along the horizontal axis in the case of flats.

Maybe someone can show you a screen grab from a DSLR histogram? I'm sure this will be your problem because the other aspects of the image are great.

Olly

[pixueto]

By the way, I processed the image again in DSS, this time without flats as some of you guys suggested and I got exactly the same result. It's like the flat frames did nothing to the pictute. I wonder if I am doing the flats right. I just took 1 minute subs, without even moving the telescope at 6:00am after finishing with the galaxy (all the lights were 1').

[RikM]

I don't have a direct DSLR histogram, but here is a screen grab from APT. The histogram is the panel of different coloured curves top right. If you scroll through the display settings on your camera you should get something similar.

When I take flats, I set the camera to AV and let it pick it's own optimal exposure time.

[pixueto]

Thanks Olly and Rik, that's been really helpful!

Also big thanks to CloudWatcher, you are a bit of a magician to me. I took the picture you came up with, went for a crop as you suggested and then gave it a final touch in levels and this is what it came up. Big thanks! Do i have your permission to put it in the main thread? I feel it's more your child than mine!

[ollypenrice]

One minute subs for flats? Unless you were using an incredibly dim light source that exposure time will have toally saturated the chip. Using an EL panel you might expect to use sub second exposures in most filters. That's your problem. Flats are exposed to image the defects in your optical train. Their exoosure time has nothing to do with your image exposure time, unlike darks.

Olly

[Cloudwatcher]

Thanks Olly and Rik, that's been really helpful!

Also big thanks to CloudWatcher, you are a bit of a magician to me. I took the picture you came up with, went for a crop as you suggested and then gave it a final touch in levels and this is what it came up. Big thanks! Do i have your permission to put it in the main thread? I feel it's more your child than mine!

Well,how very flattering!

You don't need my permission,it's your image to do with as you wish,just glad I was of some help.

[Chris]

Ah thats why you had such steep gradient then:) Anyway, what a result for 21 minutes exposure Pix! I also can't wait to see what happens when you spend 2 hours on this object:D