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Astrophotography Lesson - Finding Opportunity in Adversity

Article and Photos by Randall Light, M.D.
Email: randylightmd@yahoo.com
Website: www.randalllight.com

The summer and fall of 2018 have been a difficult time to make images of the night sky. We have had an extraordinary number of rainy days, cloudy nights, and both high and low temperatures. Recently, when two consecutive clear nights were forecast, I was excited at the prospect of being able to do some astrophotography. Daylight savings time had ended. Sunset was around 5:40 p.m., and I would be able to start imaging about 7:10 p.m. On closer inspection of the forecast, the first night was going to be near transparent, but the seeing was going to be below average. This was almost certainly due in large part to recent heavy rain resulting in significant moisture in the atmosphere. Although a nearly transparent night would mean an opportunity to photograph any object of interest, poor seeing would cause stars to be bloated and might cause problems with guiding. Dew formation was also of concern. It was time to think about what could best be photographed under these challenging conditions.

Reviewing the Options

The two telescopes in the Regina Caelorum Observatory are 1000mm and 2000mm focal lengths with a field of view of 120" and 60" respectively. Making images with either of them would certainly show the effect of the poor seeing. This would have to be an opportunity to make images of objects that occupy a large area in the sky. Stars would then be much smaller and would appear to be part of the background. Using Sky Safari to review the options for the next evening, I was pleased to find that the Heart and Soul Nebulae (300" x 180") would be overhead and available at a reasonable hour. In addition, the Veil Nebula Complex, the whole Elephant Trunk Nebula, and the North American and Pelican Nebulae combination would all be available with each one requiring a field of view of about 240" x 180". This was going to be an opportunity to make images with a standard photographic lens to obtain the required field of view. A portion of the spreadsheet that I use to calculate the field of view for various focal lengths is shown below. It also calculates the image scale for images made with the Nikon D810A, a full framed camera for astrophotography.

A 600mm lens or telescope would work for three out of four targets. A 300mm lens or telescope would work for all four targets. The size and weight of the available 300mm (f/2.8) lens would allow it to be mounted on top of the ten inch telescope in the observatory without having to setup a portable mount and risk being shut down by dew.

Imaging with a 300mm Lens

The next night the guide camera and guide scope attached to the Losmandy plate on the top of the telescope were removed, and the 300mm lens was clamped to the plate (Fig 1). An IDAS V4 nebula filter was inserted into the lens. Focus was obtained with the live view mode of the camera at maximum magnification. The targets were framed using the "closed loop slew" feature of TheSkyX. This feature uses plate solving to center the target. Because the 300mm lens is f/2.8 as compared to the telescope which is f/8, what would have been an 8 minute guided exposure became a 1 minute unguided exposure. The polar alignment of the mount had recently been checked and adjusted. The imaging sequences were setup in Backyard Nikon. Sixteen, 1 minute exposures of each target were made, and the equality of the images was confirmed.

Figure 1

Continued Imaging in Cold Temperatures

As the evening progressed, the nebulae being imaged were easily visible in the photos, the stars were small, and the observatory temperature dropped into the upper 30's. The cold temperature helped keep the camera sensor cool. During imaging sequences, I retreated to the warming room and monitored the progress of the imaging session remotely. By the time the imaging was completed a heavy dew had formed outside but not inside the observatory.

After some sleep and coffee, the images were graded, aligned, and stacked with PixInsight. Final editing was completed with Photoshop. Stacking the images improves the signal to noise ratio. The amount of signal increases linearly in direct proportion to the total exposure time. The noise increases in direct proportion to the square root of the total exposure time. Thus, 16 images stacked, will have a signal to noise ratio four times greater than an individual image. Figure 2 shows a cropped portion of a single image of the Western Veil Nebula. Mouse over the image to see the result of stacking sixteen of these images. There is much less noise seen as graininess and much more detail in the image.

Thanks to Mark Spearman, PhD for his support, encouragement, and the use of the Regina Caelorum Observatory.

Figure 2

The image above contains a single image of the Western Veil Nebula.

Scroll your mouse over the image and compare it to sixteen stacked images of the same area.

Figure 3.    Heart (IC 1805) and Soul (IC 1848) Nebulae

Figure 4.    Veil Nebulae Complex, NGC 6960, NGC 6995

Figure 5.    North American Nebula, NGC 7000, and Pelican Nebula, IC 5070

Figure 6.    Elephant Trunk Nebula, IC1396