Processing tips

[Imd]

Hi,

I'm looking for some processing tips using GIMP.

I took some pics of the eastern veil nebula at the weekend and stacked them in DSS. The sky was quite bright and I would like to darken the it on the image to show a better contrast. Also there is a bright glow surrounding the veil and I don't know what caused it but when I eventually start to guide with perhaps longer exposures I expect this kind of glow can only get worse,

I tried using gimp:

I took a copy and despeckled it then I tried cloning parts of the background to make it much darker then I subtracted.

It did work but if I look closely at the background I can see the circular blocks of black spray where I have edited it.

I have attached the image before gimp and after DSS so that you can see what I need to deal with. It was taken using 46x59 sec lights 4*darks 800 ISO using an 1100D and SW200P on an NEQ6 and no guiding.

I would say that this is the hardest object I have tried and so I am really stretching myself.

Please could you let me know what causes the glow around the veil, did I do something wrong when taking the snaps?

Also please could you send me any useful links that you have used to improve DSOs using GIMP?

Any tips which would help me progress would be much appreciated.

Thanks for looking

Ian

[Astrosurf]

The light gradient is called vignetting. See here: http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2211

If you used an imaging camera with a small chip you wouldn't get it. I get it with my 600D. I recently bought GradientXTerminator for Photoshop, but I don't know what you can use for GIMP. I've not used the sw.

The veil is dim so you're not going to get a lot with short exposures, but you've done well there! It's nice and crisp. I think the subs for my veil image (not at home so can't check) were 5 mins.

Try guiding next time. You'll be amazed at the difference!

Alexxx

[dph1nm]

The glow is vignetting. You need to take flats to get rid of it properly.

You might try subtracting a circular gradient in GIMP. It is a bit trial and error - you have to get the centre correct and adjust the foreground and background colour to match the gradient between centre and one edge. You can also try blurring very heavily after despeckling/cloning. That sometimes helps reduce the 'blocks'.

NigelM

[dph1nm]

Just remembered the astronomy plugins for GIMP http://registry.gimp.org/node/2352

There is a gradient removal plugin there which fits a polynomial to the background. Never tried it myself though.

NigelM

[Astrosurf]

The glow is vignetting. You need to take flats to get rid of it properly.

NigelM

Thanks Nigel. I forgot about flats. I haven't tried them yet. Silly me for not remembering!

[ollypenrice]

Flats take out the problem at source and are ten times better tan any post processing fixes. You really do need them. We all do! The sky on either side of the arcs of nebulosity is not the same so only flats will allow you to bring out this interesting fact.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

[Imd]

Thank you for these replies.

I have just had a look at how to create 20 flat frames (making every photon count) and this doesn't seem straight forward. I don't have an observatory. For the cheapo method It seems I need to take 20 shots of the twilight sky through a white T-shirt and keep moving the scope to avoid stars but the focus needs to be the same as the focus used for the lights. As the objects I want to shoot have not yet appeared I won't know the focus setting for my objects yet.

Also once I have the flat frames, I don't suppose I could just keep using the same ones because changes in dust, that would be just too easy

Can you recommend any other simple methods of creating flat frames?

[Imd]

I have realised, that there is another post explaining more about creating flats

http://stargazerslounge.com/topic/171079-light-dark-flat-bias-frames-with-dslr/

It all sounds very inconvenient, I don't think shining a laptop with notepad showing will help me make friends at Kelling

If you have any other clever ideas which doesn't involve leaving my camera and equipment outside and doesn't annoy any fellow astronomers, please let me know.

Thanks

Ian

[dph1nm]

Also once I have the flat frames, I don't suppose I could just keep using the same ones because changes in dust, that would be just too easy

Actually I do this with my DSLR and it works quite well provided you keep the camera at (roughly) the same angle on the scope. I guess it depends how often dust falls on your CCD. My Canon 1000D has a sensor clean function which works well, so I never really see any dust.

I always take twilight flats - and I don't use a T-shirt, just keep to a reasonably high altitude where sky gradients are minimal.

As long as you are reasonably close to focus flats should be fine. Just mark where focus was last time you used it.

NigelM

[mallorcasaint]

Using the Curves tool and Sample selection you can make the background darker. Unfortunately there isn't a lot of data in your Veil.

[Imd]

Using the Curves tool and Sample selection you can make the background darker. Unfortunately there isn't a lot of data in your Veil.

Thanks Leonis,

Do you know where I can find lessons on how to use the curves and sample point tool. I don't understand how you managed to add the colour ?

Cheers

Ian

[cgarry]

Using the Curves tool and Sample selection you can make the background darker. Unfortunately there isn't a lot of data in your Veil.

I would say that clipping the black level to remove the gradient is the last thing you want to do!

[Imd]

Actually I do this with my DSLR and it works quite well provided you keep the camera at (roughly) the same angle on the scope. I guess it depends how often dust falls on your CCD. My Canon 1000D has a sensor clean function which works well, so I never really see any dust.

I always take twilight flats - and I don't use a T-shirt, just keep to a reasonably high altitude where sky gradients are minimal.

As long as you are reasonably close to focus flats should be fine. Just mark where focus was last time you used it.

NigelM

Nigel,

How important do you find it is to keep the same orientation as would be the case when selecting your target?

Do I need to take a series of flats for each objects location that I plan to capture that night?

Do I need to maintain the same exposure as will be the case with my captures too?

Sorry for all these questions Nigel and everyone else who is helping

Ian

[Imd]

I have read some more about this topic and I think I understand my concern and how to resolve it. I believe the orientation is crucial and I have been quite lazy with my positioning of the camera. I believe that if I use an embroidery hoop and position the camera to point to the mount for taking shots I will always maintain the orientation and won't need to continually be taking flats every time a move to another object.

I will use av for autoexposure; a low iso setting; mark carefully my infinity focus with a pencil on the focus tube. I will try the white t-shirt at twilight method.

I now need to raid my nan's embroidery kit

Thanks for all your support

Ian