Image Artifacts - How to fix?

[glend]

I have a small stack of subs of the Horsehead taken early this morning, subs are 4 x 270", stacked with darks and offset frames. These were take through my SKywatcher MN190 with the Canon 450D (full spectrum modified - but not the cooled camera). What concerns me about this image is the dark donut artifacts at various places on the image - which is really annoying. I had cleaned the front anti-alysing filter prior to the shoot, as I had seen something like this from this camera before. The cooled camera doesn't show these. I suspect there is something either on the rear of the anti-alysing filter or on the surface of the sensor itself. Is there any way of processing these out, would Flats do that? I know I will have to disassemble the camera to clean the sensor and back of the front filter.
Here is the Astrobin full size link (in jpg of course as that's all Astrobin supports in the Gallery):

http://www.astrobin.com/full/205820/0/

Small version is attached here.

[codemonkey]

Hey Glen,

Flats will definitely clean that up for you. Trick is that you need to do them at the same time as you do your imaging, since those little specs of dust can move around.

Having said that, on occasion I've forgotten to do them, and then done them the next night... I usually end up better off, but it's rare that it corrects all of them.

Cheers,
Lee

[Camelopardalis]

Glen, as Lee says the flats should clean that up for you. I had much bigger splodges on mine until I recently cleaned the sensor with acetone as per your recommendation :D these things seem to be dust magnets! But regardless of where the incursion is in the optical path, flats should sort it out.

[glend]

Thanks guys, I must get serious about flats I suppose. I have stripped that camera down this morning and sure enough found some dust partices on the inside surface of the anti-alysing filter, nothing noticable on the sensor surface but I cleaned them both just to be sure. Can't work out how the dirt got on the back of that filter in a supposedly sealed space - must be more careful.

[rustigsmed]

hi glen,

flats make the most difference out of anything i have found. i wish i got onto them waaaay earlier than i did. have to redo so many shots now, the quality difference you can get is phenomenal!

But there are still a few tricks you can do... but they are no substitute for flats.

1st photo photoshop clone tool, copying space next to blemish and copying over.

2nd photo based off photo above, but then tried to create an artificial flat by, first duplicating the image above then, applying dust and scratches to remove most stars, then clone tool to remove the bigger stars and some of the flame neb. then apply a gaussian blur (high number 150 or something). go back to 1st image above, go to heading "image", apply image, then apply this image, using subtraction method - play with sliders, think it was about 60% application with an offset of 12.

a slight colour boost using minimal LAB stretch.

Cheers

Rusty

[glend]

Thanks Russell for that sssistance with processing out the artifacts, I have to learn that.
I have now cleaned the camera and will re-shoot soon. I have taken down the Astrobin image as frankly it was embarrasing. Thread close now and thanks to all that replied.:thanx:

[rustigsmed]

no worries Glen, I saw your astrobin version was down. cleaning helps but also flats will get rid of the strong gradient/haze throughout the whole image - not just the dust bunnies - which I artificially attempted to remove in the second image. my HH from last year looks the same as yours because I didn't take flats back then.

I recommend twilight flats - as you are using a DSLR they are actually quite easy to take - in terms of getting the exposure right. when at dusk find an even patch of sky and select the Av mode on the canon camera and start snapping away - no focus required. make sure no stars are evident. this page will help too http://www.iceinspace.com.au/63-211-0-0-1-0.html just remember that flats only work for the camera orientation you have taken them at.

So looking forward to seeing more :thumbsup:

Cheers

Russ

[glend]

Thanks Russ, as it was a test I used the uncooled camera, thus the haze, gradient. With the cooled camera running the sensor at -10C there is no need for darks, just a few offsets to pick up any hot pixels. The uncooled camera just happened to be handy in the obs because I had been testing my RC08 collimation with it.

[rustigsmed]

I am with you on the darks - I don't use either even with a non cooled dslr (summer is a different story), but flats will improve your images massively. definitely give it a go :thumbsup:

[rally]

Glen,

Why would you not need darks at -10C ?

Rally

[glend]

Rally, testing my Rcheshire and myself on our cold finger cooled Canons showed that below 0C there was no noise apparent on long exposure darks taken with these cameras ( comparisons between Darks of 5 minute duration and Offset bias frames shot a 1/4000th of s second, reveal no difference - noise cease to be an issue at those temperatures for these cameras). Of course you need the modification and temperature controller system to hold the sensor at that temperature with little variance. Hence, dark stacking is no longer required, offset bias frame stacking is still necessary for hot pixel detection and elimination. Once you build a library of offset bias frames of different durations that you commonly shoot at, you can stop shooting them as well.
I do still maintain a dark and offset library that has been shot at -10C at my common sub durations but generally don't use the darks anymore.

For full detais on Canon Cold Finger modifications and temperature control system build see Rcheshire's thread on Cold Finger Modification here on IIS.;)

Here:

http://www.iceinspace.com.au/forum/showthread.php?t=132498

and here: http://www.iceinspace.com.au/forum/showthread.php?t=136524

[rally]

Thanks Glen,

That is very interesting - does anyone know what Canon has done to achieve this ?
I had always assumed that Dark current was a fact of life !

[OzEclipse]

Glen,
nice shot

There are only a few spots
Open a copy of the image in Photoshop
Duplicate the base layer
Gaussian Blur ~ 25 pixels
Blend mode on Lighten
Create a layer mask
select the mask
invert the mask or fill it with black
Get a blurry paint brush ~50 pixels 100% opacity. Dab white spots over each of the marks until they disappear

takes about 5 mins

cheers

Joe

[RobF]

Also see this great guide from Eddie T on calculating which optical surface is likely to have the dust.
http://www.iceinspace.com.au/63-211-0-0-1-0.html

With anything other than Newts you're pretty well stuck with dust motes no matter how fastidious you are, so flats definitely a good thing to conquer.