Deep Space Imaging Part 3 – Connecting your DSLR

Connecting your DSLR to a telescope

To connect your DSLR to a telescope you simply need a T-ring and T-adapter. The T-ring attaches to the DSLR and the T-adapter to the telescope. The T-adapter 42mm thread is a worldwide standard to connect cameras to other devices, but T-rings are brand specific, like lenses. Once the T-ring and adapter are fitted the DSLR simply slides into the eyepiece draw tube of the telescope.

Have you tried to connect your DSLR to a telescope but couldn’t bring your image to focus? This is because there is not enough back focuser in focus between your telescope’s focal plane and the camera sensor (the focal point is either inside or outside the range of the focuser).

Non-EQ telescope mounts

Your telescope acts as a big lens, and a tracking mount may not be needed for very short exposures of the sun, moon and planets. These other telescope mounts utilise altitude azimuth movement and, while many are motorised go-to mounts, they are suitable for short duration exposures, not lengthy exposures as required in deep space imaging.

Image acquisition

Tethered shooting via astronomical software is available to help automate the acquisition process. It helps greatly to automate an exact timed sequence and shoot straight to hard drive. This is, however, not a necessity and using a simple shutter release with the camera set to the bulb function, shooting to memory card also works well.

Note: Many astrophotographers see shooting tethered as a drawback, as you need more power out in the field to operate more devices.

For deep space photography modified DSLR cameras are generally used.

These are more sensitive to Hydrogen Alpha wavelengths of light, the reddish hues in diffuse nebula. Canon manufactures the 60Da which is tailor made for astrophotography, and Nikon launched the counterpart D810A earlier in 2015.

Currently stock, from the factory, DSLR cameras have CMOS sensors with anywhere from 1-3 filters in front of them, meant to block everything but the visible light spectrum to create realistic digital photos. For astrophotography, we want the sensor to record more of the spectrum, the frequencies invisible to our eyes. Many people remove the filters to allow the sensor to record more infrared or sometimes even full spectrum. The Canon 60Da and Nikon D810A is however shipped ready for astrophotography use.

Accessories

As with any photographic hobby, accessories can aid in overall improvement of image quality, but are not always necessary. However, they do go a long way to ensuring your stars remain round and rich in colour. Here are the most common accessories used in DSLR Astrophotography.

Coma correctors and field flatteners

A coma corrector is a lens accessory that reduces the elongation of stars at the edges of the photographic field ensuring the image stays sharp and flat across the the entire field. We recommend investing in this accessory as you’ll find coma in your photos can ruin the overall appearance. 

Filters

Various filters are available that cut out unwanted light wavelengths, the most popular being light pollution filters that reduce the unwanted light produced by populated areas.

Guiding

Although auto guiding can be a whole article in itself, it’s worth a mention here as it’s not a necessity. The principle of guiding is simple – it aids in keeping your telescope tracking an object precisely. A secondary telescope and camera is fitted to the main imaging telescope and is locked on to a star. This camera sends corrections to the computerised telescope mount to correct inaccuracies in alignment that will show up in long exposure photos at longer focal lengths.

Barlow lenses

A Barlow lens multiplies the effective focal length of the telescope (magnifies the image). There are multiple sizes ranging from 1,1x to 5x or more.

Read part 4: Solar system imaging