Dust and a noob question

[baileys2611]

Orion was a jerk...he made this lovely nebula that's so hard to process!!! :painting:

Anyway, I'm having a problem with dust. I attached this shot to give an idea, higher resolution is at http://www.flickr.com/photos/skyslab/10502169026/sizes/h/ . Every time I try to clean the filters and other components, I just get more dust settling on them to the point that my latest shots are literally polluted.

Does anyone have any tips on how you stop this?

Next noob question - We get nice pictures by stacking a processing, got it - but why? Conceptually if I take a 30 minute shot and then another and another etc etc, Aren't I just going to get the same information over and over again? Why wouldn't I just take the one shot I've done and stack it many times over? Is this a case of getting subtly different bits of information from time to time or am I missing something?

Anyway, help & advice would be welcome.

Thanks
Simon.

[chances]

Hi Simon,
Im only fairly new to astrophotography but I believe if you take Darks, Flats and Bias frames along with your main photos (light frames) you will be able to get rid of the dust specs, ISO noise and ... something else ;) hehe ... cant remember.

It took me a bit of reading to figure out what actually had to be done - youtube also helped alot - check the DSS FAQ (http://deepskystacker.free.fr/english/faq.htm)for some basic info.

If you have any questions id be happy to help - or at least try ;)

Cheers.

[baileys2611]

That's fair enough - but what happens when you subtract the information from the image? As in, I take a shot, subtract the noise and bias using my dark, then get rid of the dust motes using a flat - but it doesn't replace the information, it still leaves 'black holes'.

Am I missing more?

[Rex]

Hi Simon, I will try to answer your questions, however someone with more knowledge may have better answers.
Q1/ If you simply copy the same image 20 times and try to stack it you will end up with a real mess or at best, the same image you started with. The reason we stack many images is to increas the SNR (signal to noise ratio). There are different types of noise, thermal noise can be removed from your pics by subtracting dark frames. Signal noise is actually a random phenomena and cannot be removed easily. Because it is random it shows up differently in each image. When we stack our images using say the median settings, your stacking program looks at the signal value of each pixel in each image and nominates the median value of all the images for each pixel. (median is basically half way between the maximum value and the minimum value). So in this way you remove some noise because the noise is in a different spot on each image. If you stack a heap of copies of the same image, the value of each pixel on each image will be exactly the same, so the median will be the same as the value of each pixel. Did that make sense?

[Rex]

Q2/ As for removing dust motes from your pics. You need to take flats. I'm not sure of the exact algorithm used for flats but it uses the inverse of the value of each pixel in the flat and adds or subtracts that from each pixel value in your image and in this way your original data the dust mote will be removed from your image. Like I said what i have stated above is probably not 100% correct but it gives you an idea of how it works. Hope these have answered your questions and not confused you more.

[baileys2611]

Aha! Thanks Rex. What I didn't understand was that the noise is randomly different in each image, makes sense now. Thanks. :thumbsup:

[chances]

Good luck ! :)
I was quite amazed after I did all the extra work - the blacks look black! ... and the image pops out much nicer.

Looking forward to your pics!

[baileys2611]

This I think is the source of my confusion. How is it possible to remove a dust mote, which is a dark spot on the image, & end up with an image in it's place?

In other words, how do you take a space which has no information and replace it with information?:shrug:

[multiweb]

You dither. Meaning you offset each subs by a number of pixels. When the star registration is done the dust will be in different spots and should be rejected. Flats will get rid of dust motes as well but won't make up for missing data. Although usually the dust is out of focus so star light still goes through. Unless you have a big dark blotch on your sensor.

[Rex]

unfortunately Simon, like I said I am not sure of the exact algorithm, but I do understand what you are asking. Someone with more knowledge in the area will have a better answer. I know it is to do with the value of the pixels and some mathematical algorithm that is applied to the comparison of the values. the other thing I know is that it works.

[Rex]

There you go Soimon, what Marc said. :thumbsup:

[baileys2611]

Hey...have you been spying on me at work? :rofl::lol2:

I see, it sounds like this sacrifices some of the sharp detail for an overall better effect? But the process of averaging that Rex mentioned should bring that detail back a bit as each sub will have enough information to create a higher signal to noise ratio on a given pixel, after dithering or dither then average...hmm

I see I have some experimenting to do!

Thank you kindly for the help Chances, Rex & Multiweb. :thanks:

[rcheshire]

Simon. Dithering is your best friend. But it is not a replacement for proper calibration - bias darks and flats. Dithered images are flatter and cleaner. While calibration frames remove unwanted signal, fixed pattern/bias, thermal and optical train, dithering increases sampling over the image and hides imperfections in calibration. This is particularly relevant to DSLRs without any form of temperature control. Random noise is also ironed out. I dither by about 10 - 15 pixels.

You will notice that colour calibration is difficult with uncalibrated images, which makes post processing difficult as well. Hope that helps.

[Tony_]

I think the dust specks are on your sensor - that's why they are in focus. Maybe you could blow the dust off with a lens blower? You can process the dust specks out - but it would be better to get rid of them, if you can do this without damaging the sensor. The dust is not on your filters, lenses etc - otherwise it would not be in focus. If they are on the sensor it will prevent most of the light from hitting that part of the sensor.

As far as stacking multiple exposures. (I think): Every exposure is slightly different. ie. photons are collected differently and noise (some) is different. By stacking these images you are combining the collected data (photons). If you just stack the same image multiple times you are not adding anything different. Increasing the exposure of subs and the number of subs increases the signal to noise ratio improving the overall image and ability to process it. There are others who could give a much more technical answer than me.

Regards,
Tony.

[baileys2611]

Thanks Rowland, Tony.

I took the imaging train apart yesterday and gave it a very careful clean. Google is my friend here :google:

I didn't see any dust or marks on the sensor, but gave it a clean using a very soft brush and a blower thing. I've got some special liquid, lens cleaner, polypropylene & ethanol, as well as UHTC coating cleaner from Bintel. Should I use that also?

On the dithering, I tried playing around with it in MaximDL but don't think I've got the hang of it, I end up with pictures that have bizarre black pixels mixed with very light pixels around the stars. Google isn't my friend here as a quick search tells me I need more theory understanding before practical results improve.

So...Understand what dithering is meant to do thanks to this conversation, but don't yet understand how to implement it correctly.

[5ash]

Hi Simon ,
Just wondered what telescope and camera you were using as with some images black spots can be caused by removal of hot pixels during the stacking process . Regards philip

[baileys2611]

Hi Philip,

I'm using a qhy8 one shot colour with heater and modified front glass to use a CLS t-thread filter instead of the normal ir blocking filter, efw2 filter wheel (or filter is one of 5 that I use), orion thin off axis guider, thin camera angle corrector, sbig st-I guider, mounted on a paramount MX. Everything is automated to a core i7 windows 7 64 bit pro computer using the sky x, nebulously, MaximDL, phd tracker etc.

The scope is a celestron 9.25" XLT SCT. I've had Bintel do a collimation and check the optics, which are very good :D

[Tony_]

I'm not sure about using solvents on the sensor? Does it have a filter in front of it. Sometimes the solvents don't dry evenly and can cause smearing, which can be hard to remove. Maybe someone else can give better advice on cleaning the sensor. If you scratch the sensor or cover filter (if there is one) you will get a permanent line in all your images.
If there is dust on the sensor you should be able to see it with a magnifying glass if you do it under a light.
Occasionally I get an small insect in the camera that leaves spots when it lands on the sensor - but they are usually the same shape and not as many. Check your subs - see if the spots are in the same place on every exposure.
I think it is unusual to have so many dust spots - how old is the camera and how is it stored?

Regards,
Tony.

[baileys2611]

Hi Tony, fair enough, I understand.



Yes, that's what I thought also, and they just appeared after I put a new off-axis guider in place, weren't there before.

The camera has been stored in a bag with an air-tight zip-lock. I keep it with the desiccant that came with it (plugs into the power point to dry out again). It's about two years old with me, but I bought it second hand.

I'll try some more subs after I swept it clean and see if there's a difference.

[Rex]

Hi Simon, mate you said they appeared after installing a new OAG. they aren't on the prism of the OAG are they? Just a thought.

Sorry mate on second thought, then they would be in your guiding frames not your main image. :screwy:

[baileys2611]

No problems Rx :D

[Tony_]

G'day Simon,

Looking at the spots closer - I still suspect an insect. It must have been a very active one - I usually only get 4 or 5 spots.
(I still think if it's dust on the sensor you should be able to see it.)
Attached is your image - spots removed. (It could be done better but I hope you like it).

By the way if you want to get a better image of M42 - you need to take subs of shorter length also (maybe 10 -20 seconds) to avoid saturating the core. You then use layers and masks in photoshop to join the images of different exposures. Some people take subs of 4 different lengths to even out the difference in brightness. There are tutorials on youtube that explain how to do it.

Regards,
Tony.

[baileys2611]

Sensei! How did you do that magic??

[Tony_]

It's actually fairly easy.
Open the image in photoshop. Make sure the image is flattened (layer flatten image).
Open the brush tool. Choose a brush size (top menu) about the size of the spots. (smaller will give a better result but it takes longer).
Move the cursor to an area next to the spot you want to remove, where the color matches the color you want the spot area to be.
Then "alt" "click" - this selects the brush color. Then click on the spot 2 or 3 times (or hold it down) until you are happy with the color match.
Do this to all the spots.

You can use this to remove blemishes in non-astro photos too.

Regards,
Tony.

[baileys2611]

Well I'll be! You're painting over with a sampler.

It looks like you've done some magic and got the gas to even be in the right shape.

I'm amazed. Definite eye opener for me and a large learning curve. :thanx:

[baileys2611]

Thanks everyone for your advice and help. I'm still learning but my latest attempt is attached (low res).

Higher res is at: http://bit.ly/16TUZxs

[Rex]

Hey nice repro Simon, well done mate. Great result, looks very natural.

[baileys2611]

Thanks Rex,

+ Thanks for your help and others too in furthering my knowledge on imaging & processing. I'm still having trouble drizzle combining subs but that's just down to experimenting with settings and applications.