Definitely needs improving ...

[Demonperformer]

I would value any opinions on what is happening in my astropics. Any time I process anything with a 'fuzzy' in it, the 'fuzzy' bit turns out 'blotchy':(. This is a problem that happens regularly, so specific image-taking details are probably not significant, but I have attached an example from last night (200% of the original image size) so you can see what I mean. I can provide any details of what I used if that will help.

Several thoughts of what it might be:

• Just too little data - tons more data would solve the problem
  
• Problems because I am using GIMP, which only works in 8 bits rather than 16 - PS would solve the problem
  
• I am doing something incredibly stupid with the processing - it happens when I take the initial TIF from dss and do the initial level adjustment (histogram stretch?) - am I moving the slider too far - but, if I move it much less, I tend to lose the fuzzy altogether
  
• Something completely different
  

If anyone who actually knows what they are doing has come across this before and (equally importantly) found a solution, your help would be much appreciated.

Thanks.

[steppenwolf]

I am guessing that this is the Dumbbell Nebula? If so then I would suggest that your exposures are simply not long enough. How long and how many exposures did you take?

[Psychobilly]

It's "posterizing" does the histogram have discrete spikes on it? Usually a result of overstretching data thats two "shallow" - not enough exposure time.

What's the exposure detail for the stack...

Peter...

[roundycat]

It certainly looks like posterisation, always a result of too much stretching of insufficient data. In my opinion Gimp is useless, as is any other 8 bit program. Working in 8bit is analogous to a 256x stretch that you have no control over.

On the exposure front remember that if you have adequate exposure you do not have to stretch the image at all. No one 'stretches' a conventional landscape.

You can control posterisation to some extent by applying a small blur to the image, the interpolation that takes place 'fills in' the bits that are missing but it is only the software guessing what to do, there is no real data.

Dennis

[Demonperformer]

Steve, yes it is dumbbell nebula. [and Peter] about 20mins data (30 sec subs as that is about all i can manage on the altaz) using oIII filter. Really it was just an experiment to see the difference the new filter made compared to an unfiltered shot, but it is a problem i have encountered on previous sessions with more data, guess it is still just not enough ... must do more.

Dennis, appreciate your comments about 8bit processing. Someone has offered me a copy of PS, when i can get around to arranging collection. That will no doubt lead to a whole host of other 'how do i do this' questions. You have been warned:D

[Psychobilly]

The limit is likely the 30s subs then... simply not long enough to capture the faint stuff...

[steppenwolf]

I agree with Peter, the 30 seconds is just not enough for this sort of object and the amount you are having to 'push' the data you have collected is introducing artefacts like the posterisation shown here.

[riklaunim]

80 x 15 sec, UHC-S, or bigger.

Using short exposures can get good images. You just need a lot of frames. Could you post a raw stack in PNG or TIFF format?

- Check if Paint.NET (free) supports 16 bit images

- Use demo of Astra Image Astra Image - Image Processing Software to see what you can get when processing levels etc. with this software (or Nebulosity)

The size of the nebula is quite big. Try maybe with smaller focal length?

[Psychobilly]

What camera did you use DP ? ..

[dph1nm]

If you are using GIMP (which I do) you need to adjust the image in DSS first, so it looks reasonable to the eye, and remember to apply the changes when saving as a tiff. I wonder if you are doing this last bit? If you just save the 16-bit DSS image without applying any changes and open in GIMP you will get something like your picture.

NigelM

[the rev]

I have found with my OIII filter I need to take 60 seconds as a minimumto get a decent amount of data from my images to stack.

[Demonperformer]

Thanks for all your responses.

Peter, Canon 1000D

Nigel, I have not been making any adjustments to the image I get in DSS - when it creates the stacked image, I have just been saving it and then loading it into GIMP and processing it there - I'll have a look at doing alterations in DSS first

Rik, interesting image, same 20 mins of data but with only 15s subs - presumably something to do with the difference in sensitivity between the canon and your dedicated ccd cameras? I am using the 660mm focal length, my 80mm is 400, so could try that, but wonder if the reduction in aperture (from 102mm) may be counterproductive? I'll have a look at your software suggestions. I tried to upload one of the original files as a tif, but it seemed to have problems. Maybe because of the size (58MB)

[riklaunim]

Tiff can be zipped, or use PNG. My image was from 1260 mm focal length. Canon 1000 has smaller pixels, but not that smaller to make the nebula that large at 660 mm. We will have to look at the raw stack to see if it's capture or processing problem

[Demonperformer]

The zipped tif is still 49MB, which is way over the 2MB max upload.

Guess the way forward is to get as much data using the same setup as poss and then see if that makes an improvement. Since starting doing imaging with the Canon, I have been rather like a kid in a sweet shop, wanting to get half a dozen objects every night I am out. Maybe it's time to be a little more mature about it and concentrate on one object over several sessions.

Watch this space ...

[no pun intended!]

[dph1nm]

Nigel, I have not been making any adjustments to the image I get in DSS - when it creates the stacked image, I have just been saving it and then loading it into GIMP and processing it there - I'll have a look at doing alterations in DSS first

Just to explain: DSS stores things as 16-bit tiffs, but GIMP only works on 8-bit images. So I think that when GIMP reads a 16-bit image it scales it down to 8-bits, which is the equivalent of dividing by 256 and rounding to integers, before it works on them. So if the numbers in the DSS image are are small to begin with, there isn't much left by the time it gets into GIMP! Hence I recommend scaling up in DSS first to make sure it fills the full range of the 16-bit tiff. DSS also has an option to save the tiff with or without the scaling applied - I can't remember what the default is, but clearly you want the scaling applied.

NigelM