Now that I’ve purchased a full spectrum camera, I can now photograph any target that I want in the night sky, and not be hindered by the infrared filter cutoff in my DSLR camera. You can read about it while getting a quick science lesson on emission nebulas, here.
IC 1805 is a Hydrogen-emission nebula that’s located just south-west of Cassiopeia, around 7,500 light-years from Earth. It gets its name from the easily identifiable shape when photographed with a hydrogen-alpha filter. Unfortunately, I don’t have a narrowband filter yet but I can imagine it will be on the wish list, along with everything else that I want! This really is the most expensive hobby that anyone can take up!
After a pretty cloudy weekend, I wasn’t holding out any hope for a clear night on Monday. However, the ClearOutside app once again showed its real value to any astronomer, as it suddenly cleared up around 9:30PM.
I was determined to get better tracking compared with my previous photograph of Andromeda, so I returned to my usual position on the patio. I made sure that the tripod was perfectly level by using the standard bubble level going around in a circle on each leg. I repeated this process iteratively until there was no adjustments needed on any of the legs.
I continued the setup of the mount and polar aligned the whole mount within 5 minutes. After rebalancing the telescope, I star aligned with Arcturus, Deneb and Caph, giving me errors on the DEC and RA axis’s similar to what I have been used to, which is good.
When I entered the designation IC 1805 into the SynScan hand controller, I initially couldn’t tell if it was on its target or not. This nebula is VERY large in my telescopes field of view, almost filling my entire image. To make matters harder, it’s so faint it’s hardly visible in my Bortle 6 skies.
I ended up booting up Stellarium on my MacBook Pro, and manually plate solving the stars to line up the whole nebula in the frame. To make the most of the size, I turned my camera 90º to get a portrait image, but unfortunately I had to cut off IC 1795 – Fish Head Nebula. Maybe next time?
I knew that the hydrogen-alpha emissions would be faint when shooting in broadband RGB. The camera sensor hasn’t got the quantum efficiency of that of a mono CCD, so again, rule number one of Astrophotography comes to the front. There’s no substitute for exposure.
I started out testing a number of exposure lengths as the twilight faded into darkness. 30-seconds, 45-seconds, 60-seconds, and still no star trails! I pushed it further upto 90 second exposures and there was still no star trails. I was astonished that the system could do over 60-second exposures unguided! I decided to not push my luck and kept it at 90-second exposures, which would increase my signal to noise ratio in the final image, when compared to my usual 30-second exposures.
Once again I did my photographs in batches, giving 45 minutes of exposure per batch. I left a 5 second delay between images just to allow the sensor to cool down between shots and the exposure to be saved to the card.
Everything was plain sailing, until it got to around 1:30AM. The clouds slowly started to roll in from the northwest, completely covering all of Cassiopeia. I had to stop my fourth batch with 19 sub-exposures still to go. The cloud was caught in the last exposure and had to be deleted. However, it turns out I had exactly 2 hours of exposure. What luck!
At this point, with the entire sky now covered in cloud, I proceeded with the usual darks, bias and finally ending in flat frames taken on the same evening. I usually take my flat frames the following morning after a decent night sleep, but I had my iPad with me from watching YouTube Astrophotography videos all evening, and it means that everything is taken at the same temperature, which reduces thermal noise.
Processing a full spectrum image is a little bit more in-depth than a standard DSLR image. For one, the colour balance is completely off, with star colour being overly red and needs to be corrected without affecting the rest of the image.
To really bring out the red hydrogen emission, I removed the red channel of the image and processed it in a separate document as a grayscale image. Using similar techniques with levels and curves to reduce the black level, while increasing the detail in lighter areas. When this was copied back into the original image, the red really popped through.
Following some photoshop tutorials, I learned how to reduce the amount of stars and reduce the halos around stars using the ‘Minimum’ filter. ‘Colour Range’ and ‘Select & Mask’ are a really useful combination to create star masks and to aid with colour correction.
Overall, I’m really happy with my first image and the new full spectrum camera. It opens up a whole new world of imaging to me, and I can’t wait to run with it! If only I could go to a darker location with less light pollution.
It’s actually surprising how much light pollution can make a difference with Astrophotography. For every difference in ‘magnitude per square arcsecond’ of light pollution, it takes 2.5x more exposure to get the same amount of detail in the image.
So for example, at my location, my sky quality magnitude is 19. If I go to a location that is Bortle 5 with magnitude of 20, it’s 2.5x less exposure needed. So to work that out:
- 2 hours = 120 minutes
- 120 ÷ 2.5 = 48 minutes
If I took two hours exposure at this Bortle 5 location, I would need 5 hours of exposure at my back garden location to get the same amount of detail!
So that’s something to take into consideration when imaging. If you have the opportunity to drive 30 minutes down the road to a darker location, it could save you hours of exposure in the long run if the image was taken back at home.
But first, I still need to get a portable battery to power my mount. I need to sort it out sooner rather than later as well, as my astronomical society is starting its meetings again in September. Hopefully I’ll get the chance to photograph some southern objects at a Bortle 5 location! 🤞
Hopefully it’ll be clear again this Thursday, so I’ll need to decide on another target by then!
Till then, clear skies!