mod 300D in camera n/r v Iris dark subtraction

[tornado33]

Hi all
heres a bit of an interesting comparison
both images taken with 50mm lens on modded 350D with 2inch uhcs filter on front of lens.
First one is 2x 300 sec shots @ISO 400 lens at F4 with in camera n/r off and darks subtracted in Iris by strictly following the instructions. In the original result before jpeg compression theres small green pixels all over, seems that because theres so many stars it for some reason didnt optomise the dark subtraction properly. But worse, the colours lack the richness of the originals as they appear on the canon software.

2nd pic is a SINGLE 250 sec pic with in camera n/r on (taking another 250 secs to do the DF. Lens at f4.5 Total down time for the image 500 secs. I think its time well spent, as the colours are far richer then I get by turning them to CFA monochrome and turning them back in IRIS after dark subtraction. Its a bit "rougher" because its only a single 250 sec shot, and that was through some high cloud! If I can do several more of these shots with n/r on and stack them, I will get a much smoother result with the richer colours. I will try the same comparison with galaxies. If the same results then I will exclusively use in camera n/r. The taking of the darks gives me a break from the manual guiding anyway, what I normally do is take a 1 or 2 min pic with no n/r just to see ive got the object framed properly and that focus and tracking is okay, then turn on the n/r and do the longer shots. (Wouldnt it be great if the camera could store a dark and subtract it to every image of same length and ISO)

One thing, I wish the camera would "beep" when the dark is finished. I set a timer for the length of the light exposure and start it again when the camera starts taking the dark to get around that.

What do Images Plus users think? Personally, (most of the time) Iris does a good job of df subtraction, with its "optomise dark" function where a dark of unequal length and/or temperature will be accurately subtracted leaving very little noise, but the colours I can get are rarely as good as using the canon software to extract the Raws (n\r on) as tiffs and using Iris to just do the stacking
Scott

[Itchy]

Scott

Of course, not having a 350D myself, I really know nothing about the in camera noise reduction. That said, the thing that worries me about it is that something other than dark subtraction is most likely happening and that process may remove some important data. I guess I feel that I have a little more control if I start with the RAW data.

Do you create a master dark to use for dark subtraction with iris or are you using a single dark?

Also, I noted in another thread that you used a 1/400sec offset. Did you mean a 1/4000sec shot?

To really check this out I would like to have a go with your RAW data in ImagesPlus. Any way we could organise this? I work at Tighes Hill.

Cheers

[tornado33]

Howdy
Ive tried using a master (of 2 darks) and a single dark, seems to make no detectable difference.
For everyone else, yes I will send some Raw images to Tony and see what He and Images Plus can do
Scott

[davidpretorius]

great comparison!

[tornado33]

Thanks.
An update. here is an image based on the first image (2x 300sec ISO 400 shots, 50mm F4). This time I sort of split the difference. That is I opened the RAWS in the canon software (Digital Photo professional 2.0), saved them as 16 bit Tiffs, did the same with a 300 sec dark. then opened them in Iris and subtracted the dark, expecting to get the dreaded coloured spots on the nebula as one gets that unless you open the RAWs in iris as monochrome CFA. However the coloured spots were not very apparent, nor were the ugly bright green spots, so I then saved the result as a Tiff then opened that in PS CS and neutralised the sky background colour (using Curves Midpoint and clicking on a starless area) and stretched the levels slightly. Thats It. I cropped the Eta carina part and put it here.

Note, theres a little "star bloat" mainly around brighter red stars (I dont yet have an IR/UV cut filter) I actually thought that without the filter the star bloat would be worse than it is
Scott