Saturday, August 25, 2012

Kuopio snapshottery

Kuopion kauppahalli (Market hall Kuopio) in Kuopio

Sunday, August 19, 2012

Dark side of ETTR

Three weeks back I was photographing a show of noctilucent clouds. Since there was enough light, I exposed the images to the right (or ETTR for short) to gain better signal-to-noise ratio. As exposure increases, the read noise of the camera can be regarded as a constant when exposures are short, hence better SNR. Images will have less noise, gradations will be smoother and colors better. ETTR works best when dealing with raw image files.

However, there were some unexpected consequences because of ETTR: as I brought the exposure down in Raw Therapee, I noted that there was visible posterization in the gradients on the sky. Paradoxically this artifact is caused by too good image quality, the steps between the tones in 8 bit image are so far apart that they become visible.

ETTR banding

As mentioned the problem only exists for 8 bit images displayed on 8 bit displays. Cheapest TN panels are usually 6 bit and they dither everything displayed. Dithering is the act of adding a small amount of noise to salient parts of the image to mask the posterization. Human vision likes details even if they're not real.

Above image has taken an adjustment of three stops in exposure, the posterization/banding is clearly visible in the sky part of the image. Below is an untouched exposure, here the noise from the camera acts as a dither, helping to hide the posterization. However, the image quality is clearly inferior due to the worse signal to noise ratio.

ETTR banding, alivalotettu kuva

From further testing I was able to rule out lossy NEF compression as the reason to this artifact, as both losslessly and lossy compressed raw files showed the same amount of banding (the lossy compression in the NEF files works by cutting some tones from the highlights as shown here).

Hopefully Raw Therapee implements some form of dithering for 8 bit images. I was unable to test other raw converters, but I assume the results will be similar - there would be too many people complaining about small amount of added noise because they wouldn't recognize it as a good thing.

This post was originally posted in Pallopanoraamablogi as "ETTR:n varjopuolet" at 2012-07-12.

Saturday, August 18, 2012

Astrotrac, first light

I recently acquired a tracking device for my camera, specifically the Astrotrac TT320X-AG with Tele-Optic wedge. AG in the product code means that it has auto guiding port, but I don't (yet) have such appliances.

This is how it sits on my tripod; first the wedge is on the tripod, and on the wedge is the Astrotrac and on the on the Astrotrac is the ball head and on the ball head sits the camera or lens. The wedge is used to align the axis of rotation to the celestial pole, as the sky rotates around that point and the camera must follow the stars while they move to avoid streaking of the stars.

Astrotrac TT320X-AG ja wedge Astrotrac TT320X-AG

The tracking device is basically built around the 20cm long screw and a motor which runs the screw and rotates the upper aluminium arm. As the arm moves, so does the camera mounted atop. Motor is powered by 12 volt power source, which can be a battery pack, transformer if the mains are close or accumulator such as lead-acid battery. Personally I built the power source from latter, a cable with 5mm round plug was needed to connect the two. If using rechargeable AA batteries, one must use ten of them because their voltage is lower than regular batteries (1.5V versus 1.2V from rechargeables).

Astrotrac TT320X-AG Operating the device is quite straight-forward: after aligning the axis of rotation to celestial pole and and the camera to chosen subject, the play button is pressed in the control panel. The star-like symbol controls the brightness of the indicator LEDs and and the volume button the... volume of the beeps. There's also button to rewind the screw. Volume and brightness buttons also choose the tracking rate if pressed while starting the apparatus.

After two hours of tracking the system must be rewound to continue. Depending on the batteries, there's enough juice to power the device for several nights of photography.

Anyway, here's the first light from the device. Polar alignment wasn't perfect as I did it by eye rather than drift-align or even polar scope.

Tähtitaivasta ja valosaastetta venerannalta
(Two and half minute exposure, it's not the tracking accuracy that's the problem but the sensor heating up as you can see in the corners.)

Nights aren't yet dark enough to capture the Milky Way very well, but in a week or so the night should be dark enough. The faint green glow is from northern lights that were present earlier, even though it was invisible to the naked eye, long exposure captured the glow in the sky.

Illan ja syksyn ensimmäiset reposet

Linnunrata ja revontulten kajo pohjoistaivaalla