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Old 05-06-2020, 06:47 AM
DaBris (Daniel)
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Red vignette and noisy DSLR

Hi all,
Just after a bit of advice, I have been using an old Canon 60d for Astrophotography recently, and I am experiencing quite a lot of noise and red vignetting.
I have attached single 600 sec sub ISO 800 of the running chicken for reference.
I am also using an astronomik CLS clip in filter.
Hoping one of you knowledgeable people could point me in the direction of improving these shots of mine and explain if I am doing something wrong.
Thanks in advance,
Daniel
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Old 05-06-2020, 05:49 PM
Mickoid (Michael)
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Hi Daniel, not knowing the type of scope you're using makes it a bit hard to solve your dilemma. How are you attaching your DSLR to the focuser? That is quite severe vignetting, and it makes me wonder if you're picking up some of the inner tubing of the draw tube or adapter. My DSLR goes just behind where the focusing knobs are on the draw tube. Are you using a field flattener or corrector of some kind? You don't need a t adapter, just a t ring and you should be able to achieve focus.

I also have one of these clip in cls ccd filters and from my experience using my 550d, they do seem to " create their own noise ". This is one reason why I seldom use it. You need a lot of subs with this filter to hide the noise it generates and due to their selective wavelength filtration, exposures can be very long, especially at lower iso's.

Flat calibration files will help remove the vignetting.
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Old 05-06-2020, 06:56 PM
DaBris (Daniel)
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Hi Michael,
Thanks for responding. I use a Skywatcher 72ED, a .85x reducer and the Canon is connected via t-ring. I achieve focus with the draw tube approximately 10mm out. From what I can see I should be well and truly connected properly, I feel as if the problem may be with the DSLR, perhaps an issue with the sensor?
I have attached a pic of my latest attempt using this setup. This is 46x600sec exposures, should be alot better than what I have ended up with. I have used master darks, flats, bias and a bad pixel map all correctly captured at the same temp, ISO and duration required.
Just cant seem to nail it.
It has been a while since I tried imaging without the filter, perhaps as you say it might be the cause of my problems.
I live about 7km from Brisbane CBD so light pollution is quite heavy.
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Old 05-06-2020, 07:37 PM
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Nikolas (Nik)
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looks like vignette caused by the filter
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Old 05-06-2020, 10:34 PM
DaBris (Daniel)
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Thanks for your opinion Nik, I think the filter may better suited to someone with a different setup and location than mine.
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Old 06-06-2020, 12:30 AM
Xeteth (David)
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This is a long shot and someone might be able to explain better than me, but - have you tried taking much shorter subs?

Whenever I tried taking long subs with my old ED102 and 650D I would get severe vignetting which was from the severe light pollution washing out the image (I noticed you mentioned you're near Brissy).

I live about 10-15km out from the Melbourne CBD and the most I could achieve before starting to get my images washed out was about 60 seconds. I could be wrong, but worth a shot!
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Old 06-06-2020, 08:23 AM
Startrek (Martin)
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A 600sec sub using a DSLR under a major Cities light pollution ( even with a CLS filter is going to be absolutely full of noise ( read noise, shot noise and dark current ) Was the moon up as well ?
I image in Sydney with my 600D @ ISO 800 and a 90 sec sub is my limit even during the new moon phase
DSLR’s have a poor quantum efficiency at around 40% so pushing 600sec subs is multiplying the noise and reducing your signal to noise ratio ( SNR ) Dedicated cooled CMOS Astrophotography cameras have a QE of 80% and higher
Try the following -
Choose a different easier target like a globular cluster
Take 60 sec subs, and lots of them ( 40 to 60 )
Take darks , at least 25
Stacking reduces noise so use a free staking software like Deep Sky stacker
Dither your sub frames, dithering helps reduces noise
Good processing software will remove any residual noise gradients from your final image
There’s a good YouTube clip on Astrophotography and Noise by Craig Stark , it’s worth looking at to understand the fundamentals of Noise in Astrophotography
Cheers
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Old 06-06-2020, 09:43 AM
JA
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Hi M,

You already had some good input on the possible cause of your noise and vignetting, but here is another perspective. You mention that you use ...

Quote:
Originally Posted by DaBris View Post
... a Skywatcher 72ED, a .85x reducer and the Canon is connected via t-ring.
Now I can't find the official Skywatcher imaging circle specification for the Skywatcher 72ED, from an official source, see...https://skywatcheraustralia.com.au/p.../evostar-72ed/


but, I'm willing to venture that given its size, it won't support a full frame imaging circle without vignetting and more likely has something around a 33mm image circle like the larger Skywatcher Espirit 80ED, and similar spoces, per specs here...https://www.bintel.com.au/product/sk...v=322b26af01d5

If we take it that it has a 33mm image circle, then a .85x reducer, will reduce that to an imaging circle of ~28mm, which is close enough to the diagonal of the Canon 60D sensor (26.8mm, based on Horiz22.3mm & Vert14.9mm) to have vignetting starting to show, especially as the degree of stretch on the image is increased.

Also as already mentioned the filter may be an issue. Try with and without under the same conditions to rule it out. Sometimes the pass band of filters can change slightly with reducing focal ratio and although that usually occurs at much faster f/ratios, who knows? Maybe the effect can been seen earlier in your application due to a combination of factors.

Best
JA

Last edited by JA; 06-06-2020 at 09:58 AM.
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Old 06-06-2020, 06:47 PM
DaBris (Daniel)
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Thank you all very much for the replies.

Startrek...I did present an image earlier in this thread showing that I use Astro pixel processor, I stacked 46 exposures at ISO 800, all with correctly calibrated flats, darks, bias, and a bad pixel map the image shows what the result of that was. I don't use free software like deep sky stacker, is that better than Astro pixel processor? I use ASI Air to capture my images utilizing a 30mm F4 guide scope and ASI120mm mini camera for guiding/dithering. From your advice, I should try more exposures at shorter durations. Thank you, I will try that and report back.

JA...
At the end of the day, my setup gives quite a wide field of view, and I can crop out the vignetting on most images, my concern is really the noise, and if I can go by what others have mentioned here, I am expecting too much from long exposures, regardless of how good the guiding/tracking is, and I should focus more on getting a better SNR ratio by capturing more shorter exposures.

Many thanks to all.

Regards,

Daniel
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Old 06-06-2020, 09:42 PM
Startrek (Martin)
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Daniel,
There have been many comparisons done between various stacking software and Deep Sky Stacker ranks fairly high ( refer Dylan O’Donnell Star stuff channel on Stacking software ) plus it’s free . I’ve used it for nearly 4 years and has never produced a bad stacked image

In regards to Noise , remember DSLR’s are uncooled and the electronics controlling the sensor can get up to 35degC or more in summer and only as low as only 15degC in Winter. DSLR’s were never originally designed for long exposure Astrophotography but we have through trial and error learned how to use them even with their deficiencies. The thermal noise or dark current created by a DSLR cannot be reduced inside the camera unless you cool the camera down to a temperature which off sets the thermal rise during long exposures
Below is some technical data on Noise and SNR Etc....

Noise and SNR

Photons are the fundamental particles of light

Signal is photons from the DSO together with nuisance Skyglow and Dark current ( noise )

Noise comprises of Read noise and Shot noise

Quality of images are dependent on the Signal to Noise ratio expressed as
SNR = Signal / Noise

Signal is made up of photons directly from your image target
Noise is target signal + Skyglow signal + Dark signal + Read noise square root

“Skyglow” is basically a build up of luminance in the night sky caused by natural and human sources ( light pollution, reflected light off the moon , star light etc.. )

“Dark current” ( Dark Noise ) is a thermal signal produced by the cameras electronics which subtracts the good signal ( photons ) The higher the temperature of the cameras sensor the higher the thermal signal the higher the dark current or dark noise

“Read noise” is the noise produced by your cameras sensor electronics. A lower read noise has the ability to pick up weaker signals and differences in signal levels

“Shot noise” is the fluctuations in the number of photons detected by the sensor which includes both photons from your object and skyglow. This variation in the number of good and bad photons from each exposure reduces your SNR


Ways to increase Signal
Stack more images ( more Target Signal )
Longer exposures
Lower f Ratio ( more photons per pixel for same time exposure )Eg: 8” f6 newt vs 4” f4 newt using same camera
25% less photons are captured by the 4” scope but 3 times the photons hit each pixel on the sensor
Quantum Efficiency ( QE ) of Camera ( higher the better ) Low is around 40% and high is around 80%
Cool your sensor
Use filters in urban areas

Ways to reduce Noise
Stacking ( increases signal )
Dark frames ( removes thermal signal eg hot pixels etc.. )
Dithering ( removes fixed pattern noise or dark current noise )

Hope the above is helpful

Cheers
Martin
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Old 11-06-2020, 06:05 AM
DaBris (Daniel)
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Martin, thank you for the in depth reply. A lot of helpful information there, I think I will get rid of the filter and start fresh.
Cheers
Daniel
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