Rule No.1 In Practice

This is partly a follow-on post from my previous image of the Heart Nebula, or IC 1805 for those of us who use the correct designations.

My previous nights image revealed most of the core of the nebula. This is what is most easily seen in a quick 60 second exposure, but the really faint details around the edge weren’t really visible, especially with only 2 hours of exposure.

At the end of my last post, I described how light pollution is a real killer for astrophotography, and how it can be overcome by hours on hours of exposure. It all depends though on your location, and where it is on the Bortle Scale. You can find out your location on this website.

TitleClassNaked Eye Limiting MagnitudeSky Quality Magnitude
Excellent Dark Sky Site17.6–8.021.7–22.0
Typical Truly Dark Site27.1–7.521.5–21.7
Rural Sky36.6–7.021.3–21.5
Rural/Suburban Transition46.1–6.520.4–21.3
Suburban Sky55.6–6.019.1–20.4
Bright Suburban Sky65.1–5.518.0–19.1
Suburban/Urban Transition74.6–5.018.0–19.1
City Sky84.1–4.5<18.0
Inner-City Sky94.0<18.0
“Gauging Light Pollution: The Bortle Dark-Sky Scale” by John E. Bortle © 2006

You can also find your Bortle Class via the Clear Outside App, along with the Sky Quality Magnitude.

As you can see, the weather’s turned lousy again. Typical UK weather.

If I was imaging at a Bortle 5 location, the theory states that I would need 2.5x less exposure compared with my Bortle 6 that is one magnitude brighter, to get a similar quality image.

So for example, if I did two hours imaging at Bortle 5 with a sky quality magnitude of 19.9, I would need 5 hours if I was photographing in my back garden with magnitude 18.9. As I currently don’t have a power tank or portable battery for my EQ5 Pro, I can’t easily travel to a darker location, but I can do more exposure to compensate. A lot more.

So as it was clear all night for a change, I decided to pile on the exposure on IC 1805. It was a warm night, hovering around 18ºC, which doesn’t help with the noise issue on an uncooled DSLR. However, watching Trevor’s videos on AstroBackyard.com with him using his full spectrum modified 600D (T3i), he suggested opening the screen up and turning the display off while imaging. Apparently it can keep the sensor upto 5ºC cooler, and doing so I found that the noise level to be a lot lower than usual. Also, it was a lot easier to see my individual images on the rotating screen of the 600D.

Once again, my tracking wasn’t as good as it was the previous evening, limited to 60 seconds before star trailing occurred. It’s getting better though, I’ve gone beyond the 30-second barrier that I was facing earlier this month. So with crystal clear skies all night, I collected four hours alone in one evening, with 60-second exposures at ISO800. I decided on the four hours to allow me to get more exposure if they were stacked by themselves, but would add considerably more total exposure if I could combine my two imaging sessions together in Deep Sky Stacker.

It felt like a long evening, but I started to capture data early at around 10:30PM, doing 30 minute batches as per my usual. With bias, flats and darks, I finished imaging around 3:40AM. At that time, M31 – Andromeda Galaxy – was overhead at the zenith and the Pleiades was well risen in the east. I decided to do some manual astronomy for half an hour with the 28mm LET eyepiece; the Pleiades filled the the field of view beautifully, but the Andromeda galaxy was just a mere smudge in the centre of view.

It’s a common misunderstanding that these objects taken by the Hubble telescope can be seen just as clearly through an amateur telescope and an eyepiece. This just isn’t the case. Not unless you have a much larger aperture and viewing at a dark sky location.

So then this afternoon in Deep Sky Stacker, it was the first time I had used the “Groups” function to separate the two imaging sessions. This allows the calibration frames to be associated with their respective lights, and not create artefacts in the final image.

The total integration time is just shy of 6 hours by a couple of minutes. And what a difference the extra exposure time makes! In my previous post, I said I wanted to see the complete heart shape of the nebula, and was expecting to not achieve it without going to a dark sky site. However, you can clearly see the whole of the nebula, and not just the core. So it just proves that rule number one of astrophotography is correct: There’s no substitution for exposure!

A side-by-side comparison between the two images. Notice the slight crop as I had to combine two stacks that weren’t quite perfectly aligned.

Now I’m really proud of this image! More so than the original, which I was still really proud of for my first image with my new camera.

However, it now highlights the importance of using a coma corrector, which is on my astrophotography wish list. It’s seemingly ever increasing though!

Anyway, it looks like imaging is off for another fortnight again, as the weather here in the UK is turning cloudy. We haven’t had more than two clear nights in a row since before I received my telescope, which is really annoying. Hopefully though, it turns clearer as we head into August and can photograph some more targets and fully utilise my new full spectrum camera.

However till then, wherever you are, clear skies!

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