Whats with the brush strokes

[johnb]

Hi

So this is 3 hours worth of 300 sec exposures, darks and flats, there is plenty more histogram to stretch but no matter how gentle I am the brush stroke effect just gets worse and worse

Pointers appreciated

Regards

John B

[billhinge]

I had a fiddle in pixinsight (you can download a trial copy but there is a bit of a learning curve)

I'm just a novice at this but I noticed there was a lot of blue/green noise when stretched.

I converted your image above to 32bit floating point

ran the dynamic background extraction to remove gradients

ran the repaired HSV and backgound enhance scripts a few times

tweaked the histogram manually

ran ACDNR & SCNR noise reduction

did deconvolution

probably get a better result with a larger file to start with though

[billhinge]

heres another with slightly more contrast and noise reduction

[Jessun]

The 'brush strokes' IMHO is due to drift, warm sensor, and no dither.

- Drift is due to misalignment with the north celestial pole.

- Warm sensor is due to not allowing the DSLR to cool between subs. 20-30 seconds minimum is often quoted.

- Dither is the practice of moving the telescope a fraction between each sub to make sure each pixel isn't constantly aimed at the same patch of sky. Software does this for you.

All in all this helps a long way in combating the streaks. Just 2p.

/Jesper

[Scott]

Well I think it's pretty damn good John. I'm not really experienced enough to offer any advise though.

[johnb]

heres another with slightly more contrast and noise reduction

thanks, it looks really odd on my PC ?

[johnb]

The 'brush strokes' IMHO is due to drift, warm sensor, and no dither.

- Drift is due to misalignment with the north celestial pole.

- Warm sensor is due to not allowing the DSLR to cool between subs. 20-30 seconds minimum is often quoted.

- Dither is the practice of moving the telescope a fraction between each sub to make sure each pixel isn't constantly aimed at the same patch of sky. Software does this for you.

All in all this helps a long way in combating the streaks. Just 2p.

/Jesper

thanks, I will add a delay between shots of 30 seconds, PHD which I use for tracking has the dither option so will active that as well ( i assume that takes a few seconds so I guess that combined with a built in delay between shots should really help)

John B

[billhinge]

thanks for telling me when people don't give you feedback you wonder what they are seeing, have I posted something strange looking?

Could you explain what you mean by odd ? it is a bit overemphasised granted since I started with 37K jpeg but the intent was to show there was a lot of detail that could be brought out in a full size file with more processing

seriously though I thought it looked OK on my screen, in fact yours looks OK on my screen other than being a bit less contrasty, I couldn't see the brushstrokes you mentioned under normal conditions

[russellhq]

Billhing, on my screen the background is really blotchy, it seems to be most pronounced around the stars. It does look a bit odd. Maybe its a difference in the way our monitors are set up.

[Jessun]

John, do you use software to control the camera? A few options out there and I think APT for instance can talk to PHD to allow dithering between subs.

http://www.ideiki.com/astro/Default.aspx

[billhinge]

probably monitor differences, it doesn't look blotchy to me, as I said I didn't see the 'brushstrokes' on the original either?

[Luke]

I did see streaks that looked like that in one of my stacked images once. I restacked them again, and then it was fine. I *think* that I used the wrong set of dark frames on them the first time around, i.e. the darks I used first were taken at a different temperature to my subs. That's my best guess anyway!

[russellhq]

probably monitor differences, it doesn't look blotchy to me, as I said I didn't see the 'brushstrokes' on the original either?

Try using this graphic to see how your monitor is set up.

http://www.photofriday.com/calibrate.php

[billhinge]

Hmm

That page looks fine to me, I have seen these before but they are only approximations. I calibrated my monitor with a GretagMacbeth EyeOne, admittedly an old one that I had for 6 years

Not sure whats going on then (I am wearing glasses and I have had my eyes tested ). Perhaps we'll agree to differ

PS what browser are you using? I'm using Safari which is colour managed - and what colour space

[Jessun]

I agree that the images do have a clotted cream look to them, and not just slightly.

/Jesper

[billhinge]

I was just fiddling with the contrast setting on my monitor and as an experiment I turned the contrast up to full. I think I can see what you are referring to

Personally I don't like my monitor so bright so each to their own, but thanks for the heads up just shows that not everyone sees posted internet images the same way

[Jessun]

That's true billhinge, and the blacks are always under scrutiny dealing with ap. But as you said, not much to work on with a small jpeg.

/Jesper