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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 30 Mar 2010 (Tuesday) 13:15
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M81

 
DonR
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Mar 30, 2010 13:15 |  #1

I'm not really pleased with this one, but it was the first time I have been able to get out and image in quite a while. Shot last Friday night with a bright moon. I was only able to expose for 3 minutes at ISO 400 due to the moonlight. 42 exposures plus darks, flats, dark flats and bias frames, at 1000mm (newtonian reflector) with a 350D. I really had to stretch it to see the galaxy at all, and the noise is quite apparent.

Those vertical bands look like read noise, which the bias frames plus dithering the aim between exposures should have reduced more than they did. Perhaps I need to take new bias frames, or perhaps I just got unlucky with my random dithering. I look forward to shooting this subject again on a darker night.

Don


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lloydsjourney
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Mar 30, 2010 17:52 |  #2

I like it.




  
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NovaTJ
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Mar 30, 2010 18:01 as a reply to  @ lloydsjourney's post |  #3

Good subject DonR! I like it even though it has the banding noise which reminds me of some of my images with the 40D. I did several re-stacks with DSS and was able to reduce the banding, but wound up with some blotchiness in the background. I thought about trying M81 last Friday, but the moon scared me away....glad you did do it!

Greg


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VIGER
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Mar 30, 2010 18:37 |  #4

Well done


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DonR
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Mar 31, 2010 18:51 |  #5

Thanks for the comments.

Yes, I know what you mean about the blotchiness in the background, NovaTJ. I used Iris, not DSS, but trying to maintain the background and eliminate the noise was a lost cause on this one I'm afraid, which is why I ended up clipping the sky background so much.

Still, I was surprised by the stark vetical bands. I have never seen them before to that extent, and theoretically dithering the aim of the exposures and using bias frames shouldn't have left that much fixed pattern noise. Dithering, though, is a random process, at least the way I do it, and if I were unlucky I could have accentuated the fixed pattern noise not removed by the bias frames instead of reducing it.

Anyway, I guess I remember now the reason I don't try to shoot faint objects on nights with a bright moon!

Thanks,

Don




  
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tkerr
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Apr 02, 2010 10:40 |  #6

I like it even with the banding.

When you were taking your images(subs) of this object, did you dither the mount between exposures?
Do you use Noel Carboni's Astronomy Tools Action Set? (external link)
Dithering can help with some of that banding and background blotchiness, and Noel's astronomy tools has actions for removing both Vertical or Horizontal banding noise, and color blotch.


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DonR
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Apr 02, 2010 13:30 |  #7

Hi Tim,

Thanks for the comments.

I did dither the aim between exposures, which normally minimizes the banding caused by fixed pattern noise. For some reason, it didn't this time. Possibly I just had to stretch so much due to the bright moonlight that the residual fixed pattern noise crept through.

But dithering needs to be done systematically to be effective, and I haven't established a system. I try to dither so that the aim is at least a few pixels different each time, but I find myself forgetting which way I dithered last time, etc., so it ends up pretty random. In the worst case, random dithering could end up enhancing rather than reducing fixed pattern noise. I guess I either need a fail-proof system or to get a few years younger in order to improve my short term memory.

I use PHD Guiding, which must be interrupted during dithering. I understand that Nebulosity now works with PHD to suspend guiding, perform a systematic dither, then resume guiding. But last I heard there were some problems with it. I've never used Nebulosity for exposure control, but if those problems are resolved perhaps it would be worthwhile.

No, I don't use Noel's actions. I've been using Photoshop for many years, though, and I know I could have gotten rid of the banding in post-processing. The problem is that too much manipulation in Photoshop results in distorting the image beyond reason, especially if it is borderline to begin with. Had that visible banding been the only problem with the image I would have tried carefully to fix it, but IMHO the image is not that great anyway. The next few days promise dark evenings, so if the sky conditions hold I will try it again while M81 is favorably positioned.

Don




  
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tkerr
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Apr 02, 2010 14:13 |  #8

DonR wrote in post #9920636 (external link)
Hi Tim,

But dithering needs to be done systematically to be effective, and I haven't established a system. I try to dither so that the aim is at least a few pixels different each time, but I find myself forgetting which way I dithered last time, etc., so it ends up pretty random. In the worst case, random dithering could end up enhancing rather than reducing fixed pattern noise. I guess I either need a fail-proof system or to get a few years younger in order to improve my short term memory.

LOL! I can relate to that.

DonR wrote in post #9920636 (external link)
I use PHD Guiding, which must be interrupted during dithering. I understand that Nebulosity now works with PHD to suspend guiding, perform a systematic dither, then resume guiding. But last I heard there were some problems with it. I've never used Nebulosity for exposure control, but if those problems are resolved perhaps it would be worthwhile.
Don

I also run PHD for autoguiding and I also have Nebulosity 2. I no longer use Nebulosity for capturing image data. One reason is because it makes my images an ugly looking green that is hard to correct. And the dithering capability does have problems. It gets stuck and cannot regain lock on your guide star, or just plain fails and has to shut down the program. Then while it sits there flashing low signal you object drifts too far. Rather than moving it 1 or 2 pixels it just move a few hundred.
So Instead I just use the EOS Utility for capturing my image data and dither manually, but like you I have to remember which way I did it each time.

Since then however, I have upgraded my Laptop from Vista to Windows 7 professional. I haven't tried using Nebulosity and PHD together to see if the Dithering will work. Have had very few opportunities to get out this past year. But there is still the issue of the green color cast from Nebulosity that I don't like.
I prefer capturing RAW and Stacking RAW, Can't do that with Nebulosity.


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DonR
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Apr 02, 2010 17:34 |  #9

That's what I heard about Nebulosity and PHD - problems recapturing the guide star after dithering, and other communication problems.

EOS Utility only supports exposure times up to 30 seconds with my 350D, so I use a long exposure adapter with DSLR Focus software (great software IMHO, too bad it's no longer supported). Also, I use a battery grip with NiMH batteries, which makes EOS Utility think the battery charge is low, so it refuses to acquire images. DSLR Focus has an option to ignore the battery level indication and keep on acquiring, and the camera will go for hours and scores of long exposures with a set of freshly charged NiMH batteries, even though the voltage reads low from the start.

I thought Nebulosity supported acquisition and calibration of RAW images now - if not, that is a show stopper.




  
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tkerr
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Apr 03, 2010 07:14 |  #10

DonR wrote in post #9921969 (external link)
That's what I heard about Nebulosity and PHD - problems recapturing the guide star after dithering, and other communication problems.

EOS Utility only supports exposure times up to 30 seconds with my 350D,

As long as you have one of the Shutter control modules such as the Shoestrings astronomy DSUSB you can use DSLR Shutter from Stark Labs to work along with the EOS Utility. Then you can shoot timed exposures in Bulb longer than 30 seconds.
If you ever decide to get a new Canon EOS camera and get one of the newer models you will find you will no longer need 3rd party devices or software to do anything. E.g. the XSi and newer use the same usb interface cable to also control the shutter in any mode for as long as you want. I went from a 350D to a 50D and can tell you that is nice. That's one less usb cable to worry about.

As for Nebulosity, unless something has changed since I last used it, it does support acquisition with a Canon Camera. But it converts the files to Fits, and when calibrating and stacking there is a residual effect that turns everything such as the stars green.
It's so much easier just to capture using the EOS Utility then stack your RAW files with DSS.


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NovaTJ
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Apr 04, 2010 08:51 as a reply to  @ tkerr's post |  #11

DonR...I just ordered an Orion SSAG to begin autoguiding. I would like to learn more about this technique called "dithering". Can you point to any sources where I may go to learn more?

I use the EOS Utility with my 40D and can somewhat do exposures to 120 seconds with minor star trailing, but the amount of noise and color blotching bothers me. Noise Ninja helps, but doesn't eliminate it totally.

Thanks, Greg


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tkerr
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Apr 04, 2010 09:23 |  #12

NovaTJ wrote in post #9930009 (external link)
DonR...I just ordered an Orion SSAG to begin autoguiding. I would like to learn more about this technique called "dithering". Can you point to any sources where I may go to learn more?

I use the EOS Utility with my 40D and can somewhat do exposures to 120 seconds with minor star trailing, but the amount of noise and color blotching bothers me. Noise Ninja helps, but doesn't eliminate it totally.

Thanks, Greg

This might help

http://darkhorizons.em​issionline.com/AutoDit​her.htm#how (external link)

http://www.hiddenloft.​com/ccdap4/help/hs270.​htm (external link)

Some autoguiding and image capture software allows you to set up for dithering. i.e. MaxIm DL


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DonR
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Apr 04, 2010 16:08 |  #13

tkerr wrote in post #9924556 (external link)
As long as you have one of the Shutter control modules such as the Shoestrings astronomy DSUSB you can use DSLR Shutter from Stark Labs to work along with the EOS Utility. Then you can shoot timed exposures in Bulb longer than 30 seconds.

I have a home made parallel port shutter control module that I use with DSLR Focus. It works with DSLR Shutter too, but DSLR Focus is a much more full featured application. And as I mentioned EOS Utility doesn't like the NiMH batteries, while DSLR Focus works fine with them.

tkerr wrote in post #9924556 (external link)
As for Nebulosity, unless something has changed since I last used it, it does support acquisition with a Canon Camera. But it converts the files to Fits, and when calibrating and stacking there is a residual effect that turns everything such as the stars green.
It's so much easier just to capture using the EOS Utility then stack your RAW files with DSS.

Sounds like Nebulosity has some problems with the debayering process. I use Iris, which requires that you convert the RAW files to either FITS or the proprietary Iris PIC format, but they remain as non-debayered RAW files either way, until after the flats, darks, and bias frames are applied. Then they are debayered, and by default no color balance adjustments are made, so they come out very green. This is due to the fact that there are twice as many green pixels in the bayer matrix as red or blue, and also due to the higher sensitivity of the CMOS sensor to green wavelengths.

Iris has some very powerful commands for adjusting color balance, and I use them after the calibration and stacking on the final image. Performing the same color balance in Photoshop, or in any other software I've used, is much more difficult than doing it in Iris.

I tried Nebulosity, and really wanted to make it work as I'm a big fan of Craig Stark's software (especially PHD). I just couldn't make it do the right thing. I don't remember the color balance issue you speak of, but probably I've just forgotten it.

I tried demos of Maxim DL and several other packages too, but finally found Iris and haven't looked back. I do have DSS and use it sometimes due to the speed and simplicity, but I just can't get the same quality of results from it as I get from Iris.

Don




  
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DonR
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Apr 04, 2010 16:35 |  #14

NovaTJ wrote in post #9930009 (external link)
DonR...I just ordered an Orion SSAG to begin autoguiding. I would like to learn more about this technique called "dithering". Can you point to any sources where I may go to learn more?

I use the EOS Utility with my 40D and can somewhat do exposures to 120 seconds with minor star trailing, but the amount of noise and color blotching bothers me. Noise Ninja helps, but doesn't eliminate it totally.

Thanks, Greg

Hi Greg,

The idea of dithering is that each image produced by the DSLR contains some noise that is generated by the camera electronics (read noise), and a portion of that noise is reproducible (fixed pattern noise). The most obvious component of the fixed pattern noise manifests as vertical banding. While every frame, including very short bias frames made with the camera body capped, contains the fixed pattern noise, it isn't completely uniform from exposure to exposure. Acquiring a number of bias frames, averaging them, and subtracting the result from each exposure, gets rid of that part of the fixed pattern noise that is constant, but the noise that remains, although it is not noticeable in normal photos, can become a problem when contrast is stretched drastically, as sometimes is necessary in astrophotography images. I have attached a bias frame made with my 350D. It's a 1/4000 second exposure made with the camera body capped, and I stretched it severely in Photoshop to reveal the fixed pattern noise. There's also a little amp glow apparent in the lower right corner, even at 1/4000 second!

Dithering helps reduce the effect of fixed pattern noise by moving the subject on the sensor between exposures so that the banding doesn't accumulate in the same position relative to the subject on each frame, and therefore can be minimized instead of reinforced when the images are stacked.

I dither the old fashioned way. Between exposures I stop guiding and use the hand controller (actually a joystick since I use EQMOD to control my mount) to move the guide star a few pixels. Then I engage guiding again, let it settle down, and take the next frame. If I know I will be taking lots of frames, I will go up to three frames between dithering.

As has been mentioned in this thread, some software allows for automated dithering. Nebulosity together with PHD advertises the capability, but there are apparently still some problems with it. Tim provided some links to other automated dithering processses. The way I do it means I have to stay around and alert during the imaging session rather than going in, going out or going to sleep, but that's OK with me.

Don


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NovaTJ
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Apr 04, 2010 19:47 as a reply to  @ DonR's post |  #15

Thanks TKerr...those links helped alot! But like they said in the first article, it is for someone who has mastered autoguiding already. I have a ways to go still so I'll get back to "dithering" later on.:oops:

Greg


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