Sky Brightness Continuous Measurement

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Sky brightness, or darkness as we here prefer, is probably one of the most important factor in visual or instrumental observation of celestial objects. The lower the sky brightness number, the better the contrast we can have for the target object.

Here is a simple chart that shows two weeks of continuous monitoring of the sky. The top numbers are when it was night time. These numbers are in calibrated magnitude scale.

The dips there are the presence of clouds which made the number go a bit higher momentarily during the nightime. Remember, magnistude scale is reversed.

Dark Noise vs Temperature

When we first step in astrophotography, among the first instructions we hear is you CCD needs to be cooled. There are many cameras for lunar, solar and planetary imaging which do fine without cooling because the targets are very bright. But for Deep Space Objects, which are very faint, we need cameras which can be cooled below 0 Celsius.

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C14 with SBIG ST9XE CCD Camera

My camera has a regular peltier cooling option and it has an advantage of water cooling as well. These days i am not using water cooling because pipes must be attached and it pulls the scope which in turns troubles the mount in making an accurate pointing model.

I did a test run of my camera to actually see how much Dark Noise it generates at different temperatures. I acquired 60 frames of 60 seconds subs and then BIAS subtracted them and then stacked them. Then i measured the average Pixel Value of the Dark Noise and plotted it with the temperature range from -20C to +35C with 5 degrees increments. The results are interesting.

Figure 1 starts with a lot of Dark Noise in the frame which was captured at +35C and then it ends with a frame at -20C. One can see the dramatic effect of lowering of the noise with lower temperature.

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Figure 1. Animation of 12 frames of CCD having temperature of +35C to -20C

The graph (Figure 2) shows it in more detail. Above 0C, the Dark Noise increases rapidly.. kind of exponential growth. At +35C, the average pixel is producing some 10K ADU value. Now this is extremely high and no Deep Space Object can ever be imaged at this temperature with this CCD. Now DSLRs have low noise generating CMOS chips and they do get to have some nice Deep Sky Objects. There are those who install cooling units in their DSLRs to get more faint signal coming from space.

Below 0C, the noise is comparatively very low. Yes cooling gets you to image very faint object but there is not a huge difference of dark electrons generation in sub zero CCD temperatures.

Yes Pros use liquid nitrogen etc.. but that’s quite a different world than our mare mortals’. So yes.. we should be all happy with our sub zero chips. We have bigger enemies than CCD temperature.. we need to kill the clouds first!

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Figure 2. Dark Current Vs CCD Temperature

Exposure vs Linearity

Astrophotographers 'hate' sky background at night time. This is worst in cities of course with all the street lights, Billboards and what not. We live on the ground and we light up the sky only because we are careless.

Here is a simple data from a series of images i took from my observatory. Starting from 1 sec and doubling the exposure with each image; these are seven in total.

Celestron C14, SBIG ST9XE CCD camera, Astrodon Luminance filter (and more) was used to acquire this data.

Notice a perfect linear relationship here (notice R^2 value). CCD cameras have excellent linear response to star light photons and here, sky background photons. This is the reason we like CCD's or CMOS for that matter (until they remain linear). CCDs are always better than CMOS.. there are many reasons for that.

I have attached a short clip of these seven images as well.. you will see very few stars in a single second exposure and many more with increasing exposure. Telescope was not being guided so you will see black bands in the images; this is the result of aligning unguided exposures.

AAVSO Data Submissions

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I have started submitting variable stars' data to AAVSO (American Association of Variable Star Observers), the largest group for variable star observers in the world; it has more than 20 million observations and counting.

This image is the screen shot of my submission report on AAVSO homepage. So happy to see 'PK' there at the website :)

The equipment and softwares i am using are as follows:

Telescope: Celestron C14

Mount: Losmandy Titan

CCD Camera: SBIG ST9XE

Filter Wheel: SBIG CFW-8

Focuser: TCF

Filter: Astrodon Photometric V Filter

Softwares: Maxim DL and Maxpoint