ISO v post processing - what next?

[groberts]

Having recently improved my polar alignment, though still unguided and using an unmodded 700D camera, I am now starting to increase exposure times and have attempted some hitherto inaccessible objects. I have had some success with the North America Nebula at 90 seconds (previously 30 - 40 secs) and ISO 1600 and the other evening tried the Veil Nebula etc.  However, this time I found ISO 1600 produced a more bleached image and therefore went for ISO 800 instead.  The final picture, after tacking showed the Veil, which post processing in Photoshop has further enhanced but I am not happy and would like to get more detail, if possible.

Should I have stuck to ISO 1600 in order to gather more information, albeit at the cost of graininess, or am I missing something in post processing - which I am new to and finding (as with many of these issues) something of a black art?

Veil Nebula - original sub 90 seconds @ ISO

90 seconds @ ISO 800

Stacked (subs+darks+bias+flats) + post processing & cropped

  

[Knight of Clear Skies]

That's come out quite nicely, it's almost an OIII view of the broom due to the limited Ha response of the camera.

Boosting the ISO doesn't really increase sensitivity, the camera just applies a multiplier to the data, boosting both signal and noise. in theory you could just shoot at ISO 100 and stretch the stack more to get the same image (in practise you might get slightly different results due to the quirks of individual cameras, some are slightly better at different ISO settings). The main benefit of choosing a sensible ISO level for the exposure is that it makes it possible to check the exposures as they come in.

Your 90 second ISO 800 sub looks pretty sensible. The thing to avoid is boosting up the ISO too high and blowing out bright areas.

[nmoushon]

Knight is correct. ISO doesnt get you more data. It just "pre-processes" the data for you so it is easier to see in live view. Or on your pc screen if you dont have live view. When I used my DSLR I would use 1600 so that I could easily make sure I got the object framed correctly and then dropped it to 800 or even 400 depending on the outside temp. Higher ISOs will heat up your sensor more than lower ones and introduce more noise. I took 8hr of M42, 4hrs at 1600 and 4hrs at 800. The amount of data that I was able to pull out was almost exactly the same. (They were shot on different weeks so one was a better night than the other) The noise in the 1600 was noticeably higher though. Even with both having a full set of darks, bias and flats applied.

[dph1nm]

in theory you could just shoot at ISO 100 and stretch the stack

Hmm - personally I would stay away from very low ISO due to potential quantisation issues (i.e. needing more than one photon to give one count in the image). Depends on the specs of your camera though.  With many Canons the signal-to-noise is better at higher ISO, especially with shortish exposures, due to the lower readout noise.

NigelM

[Knight of Clear Skies]

Yes, like I said above, it might not actually work out that way in practise.

[Martin Meredith]

Knight of clear skies is right: it does look a little O-III-like. As it happens I have a stack of 3 x 30s taken with an O-III filter from a Lodestar-C guide cam with a (not well-collimated at the time) Quattro 8" f4 scope and the similarity is there:

Cheers

Martin

[groberts]

OK thanks.

Would a light pollution filter help or does that detract from the overall quantity of useful light i.e. the image?

[Owmuchonomy]

Well done on a difficult large DSO. Can I suggest you use GTX (Gradient Exterminator)?  It is a plug in filter for Photoshop.  You have quite a vivid gradient and the plug in will remove it.  There is a free trial before you buy (its not much anyway and a one off fee).  Doug German does a great You Tube video on how to use it.

I just started a thread on the Eastern veil as I imaged it last night.  I used up to 10 minute subs (6x) at ISO 800 with my Canon 60 D.

[nmoushon]

Yes a LP filter helps A LOT! I used a clip in one with my canon and worked wonders! Though you will have to retake your flats as it adds a blueish hue over the entire image. The flats will take it out without a problem though.

[glowingturnip]

honestly, so long as you've not clipped any pixels in the capture (ie so long as no pixel reads 255) then even in your first capture you've lost no information and it all comes down to post processing.

I agree with Nigel (dph1nm), use a high enough ISO so that you have suffient quantum capture (ie you want each arriving pixel to make a difference to your raw frame), then everything afterwards, even if it looks washed out, can be used to make a good image.  I recently did a whole bunch of captures at iso400 and think I lost too much, wil be shooting at at least iso800 from now on.

[groberts]

Yes a LP filter helps A LOT! I used a clip in one with my canon and worked wonders! Though you will have to retake your flats as it adds a blueish hue over the entire image. The flats will take it out without a problem though.

Are you saying that you take the subs / darks / bias with the filter and the flats without?

Does a LP filter in any other way impact on the capture quality, other than positively? 

[nmoushon]

You take all the images with the filter in.

I have not found/noticed any negatives with using the filter. The only downside is the blue hue but thats any easy fix.

[Atreta]

Great image and nice topic, made me want to learn more about how iso works and found a very informative article explaining about iso and unity gain.

http://www.allaboutastro.com/dslrs-and-unity-gain.html

There's always new things to learn