A Low-Cost Path to Astrophotography

I’m occasionally asked to recommend a camera to get into astrophotography. Of course, my first answer is to use the camera and lenses you already have. Beyond that, it’s difficult to recommend a specific brand and model because I don’t have every camera at my disposal, and we are blessed (cursed?) with a continuous stream of new and improved cameras. Here are some thoughts.

Multiple Paths

First off, let me say that I’ve been taking astrophotos since the dinosaur age (back in the days of film). Second, while I have used a variety of brands of cameras (Sony, Panasonic, Canon, Pentax, Fuji) for travel and other purposes, my go-to cameras have been Nikons for “serious” shooting. I also have a few dedicated astrophotography cameras in use.

But a couple of years ago, I decided to get a “consumer” camera modified for astrophotography. When a camera is described as modified for astrophotography, it means that the response of the camera sensor has been extended further into the red end of the spectrum to pick up the astronomically important hydrogen-alpha (H-alpha) emission line. This is done by removing/replacing the IR blocking filter, which cuts a bit into the deep red end of the visual spectrum. While normal daylight photos taken with such a camera can be re-balanced to appear “normal,” if exact standard color reproduction is necessary, a modified camera should not be used.

Although special factory production versions of commercial cameras exist (e.g. Nikon D800a and Canon 60DA), They are built in limited production runs and are probably sold out as you read this. They are also “behind” the current models and more expensive than the standard models. These do, however, have the advantage of including built-in daylight color balance and additional astrophoto firmware features, such as longer exposure capability.

The other option is to have a third-party camera or astronomy equipment dealer modify a camera to remove the standard IR filter and replace it with one that passes more of the H-alpha line. In fact, having a third-party modify a camera gives you more options such as to completely remove the IR filter to allow the possibility of shooting IR photos (blocking visible wavelengths) or even UV photos. 

In addition, third-party mods allow the selection of a filter which allows more H-alpha light to pass through to the sensor than the factory-designed astro models, which use more aggressive IR cutoffs to keep daylight performance closer to the standard models. But note that whatever replacement filter option is selected, the filter must match the factory filter so that the focal point is not changed, or standard lenses for that camera may not come to focus.

My Path: The Canon RP

As a long-time Nikon shooter and having built a collection of F mount lenses, it would have been logical to select a modified Nikon body. However, I took this opportunity to also experiment with the mirrorless camera body experience. In fact, I did a complete 180-degree flip and went with a Canon RP body (26 megapixels, full frame, mirrorless). Why? One answer is that this camera wasn’t intended to replace any of my “normal” use cameras. 

I wanted to have a camera modified for red-sensitive astrophotography, and by virtue of selecting a mirrorless camera body, I could get an Nikon F mount lens adapter, making it fully compatible with my existing camera lenses and telescopes. In addition, the thickness of the lens adapter allowed me to get (imported from Japan), an adapter that allows for the insertion of filters (58mm, threaded) as well as a 3-point tapered ring which allows the camera to be freely rotated and locked into any framing orientation. Other brands of adapters have subsequently become available, which allow filters to be mounted in filter drawers and conveniently and quickly swapped out. For my use, I’ve found the need to swap filters quickly is not important.

Of course, an important concern was the cost of the camera body. Having anyone modify a camera body will certainly void the warranty and possibly make it unrepairable in the eyes of an authorized factory repair facility. The Canon RP currently sells for about $1000, which is a good value for a full frame mirrorless camera (in my opinion), and cheap enough for an “experiment.”

Astronomical Filters

Why are the swappable filters an important point? For most astrophotographers, light pollution is an issue, so better results can be obtained by using special astronomical filters in the optical path. These are not low-cost, but for most astrophotographers, should be considered “must-have” items. For my situation, my default astrophotography shooting setup includes a light pollution filter, which is designed to selectively block out bands where light pollution sources contribute a lot of interference. These are the prominent emission bands found in fluorescent lights, mercury vapor lights, and sodium lights commonly used in outdoor settings. Also filtered out by the IDAS filter I use is the natural "light pollution" source from high-altitude oxygen. Despite the complicated bandpass of this specialized filter, it manages to maintain a color balance that does not require a lot of post-processing adjustments.

Dual Narrowband Filter

Sadly (for astronomers), the easily blocked lights are being replaced by LED lights, which are broader band and harder to selectively block. In this case, for many astronomical objects, the problem can be attacked from a different direction: use filters which block everything except the light from astronomical targets. This works for many deep-sky nebulosity objects, targets which glow by the light of ionized hydrogen (H-alpha) and Oxygen (OIII). These are often called dual narrowband filters, as they have narrow bandpasses at H-alpha (deep red) and OIII (blue-green) and work well with mirrorless and DSLR cameras with color sensors to give a relatively natural color balance (important for stars) while improving the contrast of the target nebulosity.

Single Narrowband (H-alpha) Filter

Though it can be argued that a narrowband H-alpha filter is wasted on a color-matrix camera since only the red pixels in the sensor are used, I have also experimented with a (very) narrowband H-alpha filter. This isolates the H-alpha light of emission nebulosity, which is in the red end of the visible spectrum. This light is scattered less in our atmosphere than light from the blue end of the spectrum and is thus least affected by manmade light pollution and even moonlight. The net result is that even under the moonlight, even extremely dim targets can be imaged successfully.

In the image above (full moon superimposed for scale), the remnants of a supernova explosion that occurred 40,000 years ago can be seen despite being imaged while a half-illuminated Moon was 83 degrees away in the sky. The actual appearance of the nebulosity in the image is pure red (as are the filtered stars) as one would expect for H-alpha, but has been converted to a monochrome image for presentation so that it is more easily seen on a screen.

The image above shows the region around the Orion belt star Alnitak, including the Horse Head and Flame nebulae. This image was taken with the Moon up in the sky, 73 degrees away, five days before the full Moon phase.

Why Not Choose a 'Real' Astronomical Camera?

Experienced astrophotographers may ask why I didn’t simply go with a “real” astronomical camera. After all, they have the advantage of active cooling, which significantly reduces noise. In addition, true monochrome cameras are available, which have better resolution since pixels are not split between R,G, and B filters.

My reason is that astronomical cameras invariably need an external computer to operate them. Typically, this is a traditional laptop computer (usually Windows) and all the complications associated with a computer setup. While this may not be so important in an observatory setting, needing to bring along a computer adds a lot of additional weight, complication, and reduces reliability in portable setups. It’s true that very small computers (e.g. Raspberry Pi) are available, one still needs at least another tablet or phone to control the computer along with network wiring or a properly configured WiFi network. And while additional cooling and noise reduction would be nice, modern cameras have quite good performance as long as you don’t have to image in extremely warm locations.

A monochrome astronomy camera would indeed provide better resolution and sensitivity for the same size of sensor, but for color shots, additional filters (and exposures) are required along with the complication of a filter wheel for at least R, G, and B filters and additional software to control the filter sequencing. Add the frustrating possibility of getting only 2 of the 3 colors needed before being interrupted by clouds, and my enjoyment of astrophotography starts to wear thin.

Another factor that can’t be ignored is the fact that a full frame camera like the Canon RP is much less expensive (even after modification) than a full frame astronomy camera, especially if an additional computer and/or tablet isn’t already at hand to support the camera.

The Canon RP Experience

So, how has the Canon RP worked out for me? The short answer is that I’m very happy with it, especially as I just use it for astrophotography. As expected, the camera’s user interface took a little adjustment, but it was nothing I wasn’t expecting. I was able to rapidly get to the point of using it in the dark with the limited functionality I needed for just astrophotography.

I was easily able to find an AC adapter to replace the rather small battery, enabling me to run all night without worrying about losing frames due to a dead battery or worry about the camera overheating because the battery had warmed up with heavy use.

With the addition of an external programmable intervalometer, I’m able to set and forget for as long as my telescope mount will allow shooting. In addition, in Bulb mode, the camera’s display shows the elapsed exposure time on the rear display. Even without the intervalometer, the Canon RP can shoot continuous frames (up to 30 seconds long), making it suitable for night-time time-lapse movies (e.g. meteor sequences) too.

While I prefer the “real” optical viewfinder of a DSLR, the viewfinder of the camera is not as important for my use in astrophotography as long as the camera has a decent rear LCD live view. The view on the Canon RP's rear LCD is adequate for focusing, though I would like it even better if the magnified view provided at least twice as much magnification to ensure perfect focus.

I had heard some Canon camera users complain about fixed banding patterns in long exposures, but in my use (up to 10 minutes), I have not had any problems with banding or excessive random noise. The high-ISO range (up to 40,000) is great for taking quick framing shots. I typically back off to ISO 1,600 for actual image frames.

Conclusion

Using the Canon RP mirrorless camera has been a successful astrophotography “experiment” for me. It has been a flexible, low-cost, reliable, and (most important) low-frustration way to do deep-sky astrophotography. It would probably be the same experience with any modern mirrorless camera and should provide a great way to start if you don’t already have an astrophoto-optimized camera.