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New Skills for Better DeepSky Astrophotography

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Last Updated on February 25, 2022 by Practical Astrophotography Staff

 

 

 

 

 

 

 

 

 

 

Many DSLR Deep Sky Imagers can benefit from a few tips that are overlooked very easily for better Deepsky images.

Getting started in Astrophotography is easier than one might think but mastering it will take far longer and sometimes a lifetime. These tips are not only for beginners or people with a small amount of experience., but can be used by those who haven’t quite gotten those images they see on the Internet.

1.) Use a Modified Camera
Imaging with a modified camera can help with Deep sky images There are a number of individuals whom you can purchase modified cameras from(I recommend Gary Honis) or you can send your camera and have it modified. They modify the camera by replacing the infrared cutoff filter that sits in front of the sensor with another filter that still cuts off IR but lets through more of the deep red visible spectrum. This is important for picking up the photogenic red nebula’s that dot the Milky Way. With a stock camera (the image at far left here) most nebula’s will appear pale and dim. But a modified camera really snaps them up (as at right in the example here). Modded DSLRs are compromised for daytime use but can still be used by creating a Custom White Balance to counteract the pink caste these cameras lend to all images.

2.)  Find Dark Skies
Nightscape photography benefits from moonlight. Not deep-sky photography. We also need skies as free from light pollution as possible. While some light pollution filters can reduce the effect of sky-glow to some extent, there is no substitute for imaging under dark skies. Plan on getting away on New Moon weekends to a lakeside cottage, a favorite campsite, or the local astronomy club’s “dark site.” If you already live in a rural area free of light pollution, you’re all set to shoot from your backyard. The best accessory you can give your cameras and telescopes is a dark, moonless sky.

3.) Take your time to get Focus, Use a Bahtinov mask.
Stars are the most merciless subjects for revealing poor focus. As with nightscape shooting, it’s worth taking the time to precisely focus any lens or telescope. Live View makes it easy. Set the camera’s Live View options to “Exposure Simulation,” with the ISO set to high, the aperture set to wide open, and the shutter to set 30 seconds or Bulb. Aim at a bright star and manually focus until the star is as pinpoint and color-free as possible. With lenses, even the slightest turn of the focus ring will bloat stars or add colored halos. With telephotos and telescopes refocus through the night. As the air cools and tubes contract, the focus can shift. Clever image processing can cure a lot of imaging ills, but it cannot make up for poor focus.

4.) Take Dark Frames
For deep-sky images I always turn on Long Exposure Noise Reduction, so the camera takes a dark frame to reduce noise. Some photographers prefer to take dark frames separately at the end of a session (by capping the lens or telescope), then subtracting the dark frames in “post-production.” In A-B tests I’ve never found that procedure as effective as having the camera take a dark frame at the same time as the exposure. The reason is that only then is the sensor at the same temperature for both the dark and “light” frames, essential if noise levels are to match and subtract accurately. Yes, this increases the time it takes to acquire images, but the results are better. But here’s a tip: If you are using Canon full-frame cameras (5D and 6D models) and you control them through the shutter port, not the USB port, you can take 4 or 5 images in quick succession before the dark frame locks up the camera. One dark frame then gets applied to each light frame, shortening the total exposure time. however, most will argue that you should take your Dark frames at the end of your imaging session as they are not dependent on the weather. This way you can maximize your imaging time.

5.)  Autoguide!
When shooting with focal lengths over 200mm and exposures over 1 to 2 minutes, most mounts will require guiding. Yes, they have motors that follow the sky, but minor errors in the gears, among other sources, will trail stars slightly during long exposures. To eliminate tracking errors we employ an “auto-guider,” a little CCD camera looking through a small guide scope. It watches a guide star and sends a pulse to the motor to counteract every time the guide star wanders off. Auto-guiders vary, but most require a laptop to run guiding software (the free PHDGuider program for Windows and Mac is popular). The QHY5LII works very well – it is sensitive enough to find and reliably guide on stars looking through no more than a compact 50mm guide scope. Once set, an Auto-guider allows exposures of any length, all perfectly guided, while you nap.

6.) Shoot shorter sub-frames
There has been a lot of discussion on the optimum sub-exposure length here lately. There are multiple factors in setting sub exposure length beyond read noise impacts and maximizing S/N. Factors include seeing, wind, and tracking errors, along with having enough frames for stacking to reject unwanted things like airplane and satellite tracks. These factors limit the long exposure time side of the sub-exposure efficiency trade. The short side is impacted most by the time delay between exposures, especially in today’s very low read noise cameras. Keeping exposures short certainly has its benefits. For one, you get more images shot in a night, especially when you are limited to the amount of clear skies with good seeing. With shorter sub-frames, you avoid excessive noise from your Chip that is not cooled. You can stack 20-30 images or even hundreds and get the same results as taking fewer but longer sub-frames.