OSC vs Mono from a city

So you’re an astrophotographer in a city and are looking to get a new camera to image deep sky objects like galaxies and nebulae. Perhaps you’ve used a DSLR for a while and want a dedicated astrocam, or maybe you’re diving in at the deep end. It’s not long before you realise you have to make a decision between the two main camera types: OSC and Mono. This is a topic that comes up a lot in places like astronomy forums, and sometimes the loudest voices insist that Mono is the only real choice from a city. Until recently I would have agreed with them. In fact, I used to shoot Mono from my city location — but have recently switched to OSC. This is an unusual move in the world of astroimaging.

It’s a complex topic and in trying to distill it down I’ll inevitably leave out a lot of info, but I hope to give a fair appraisal of the relative benefits and drawbacks of OSC and Mono cameras, particularly for city use. This is all based on my own experiences, having used both options. We’ll begin with a bit of background info just so we all know what we’re talking about.

OSC stands for One Shot Colour. This type of camera allows you to capture full colour images in single shots — hence the name. They’re convenient and give good colour images with a minimum of hassle. Note than when using an OSC camera for astrophotography you should take lots of photos and integrate (also called stacking) them to get a good final result. The “one shot” aspect of the camera is to get a simple colour image, not an awesome final picture.

Mono cameras work differently. Instead of making colour images, they see in monochrome (i.e. black and white). To get colour images from them you need to put filters in front of the camera sensor. To make an image like an OSC manages in one shot, you’d put a red filter in front of your camera sensor and take a photo; then put a green filter in front of your camera sensor and take a photo; then put a blue filter in front of your camera sensor and take a photo; then digitally combine them all to make a final Red Green Blue (RGB) image. As with OSC cameras, you need to take lots of images and then integrate them to get a good image that you can then go onto edit.

As a side-note, objects like galaxies and star clusters give out most of their light in RGB rather than SHO. Getting good images of them from a city is a real challenge, regardless of whether you’re using an OSC or Mono camera. Your best bet is to aim for very long integration times to help combat light pollution.

So actually Mono is better?

Now we’ve gone from thinking that OSC is the obvious choice to being convinced that Mono is clearly best, and actually essential if you’ve got light-polluted city skies. This was the case until recently, and is what a lot of astroimagers still think. But now OSC, our urban astrophotography underdog, has a secret weapon: dual-band filters.

Just as Mono imagers have access to loads of great narrowband filters, recently-released dual-band filters offer similar for OSC imagers. An example of a dual-band filter is the Optolong L-eXtreme. This is the one I use. (Other dual-band, and even tri-band filters are available too, but let’s use the L-eXtreme for this example). You place your L-eXtreme filter in front of your OSC camera, and it only lets through two narrowband wavelengths of light: H and O. That’s two of the three Holy Trinity narrowband wavelengths used by Mono imagers. It’s certainly enough to make great bi-colour images. During processing, you can even merge the data you’ve captured to create a synthetic (fancy way of saying fake) “S” image to throw into the mix. The result isn’t as scientifically accurate as you’d get with Mono plus SHO filters, but it can look great — and for most of us that’s what counts.

At this point it’s worth mentioning a drawback to the L-eXtreme that not many people are aware of: it loses effectiveness at fast focal ratios. See the graph on page 11 of this test report by Jim Thompson for details. If your telescope is faster than around f/5, you may want to scout out other options.

Still, I think that these dual-band filters are real game-changers for many OSC astrophotographers, especially those of us in cities. They allow us to keep the benefits of OSC and mitigate the main drawback, i.e. not being able to take narrowband images. To be clear, Mono plus filters has advantages over OSC plus dual-band in many cases, and dual-band filters aren’t effective for every telescope or every target. But for the first time OSC can compete from a city.

OSC and Mono compared

OK, let’s try to break it down into different criteria so we can compare the advantages and disadvantages of each camera type.

Cost | Winner: OSC
Here are some example costs if you wanted an OSC or Mono camera set-up. These are same-generation cameras and suitable for use from a city.

OSC
ZWO ASI 2600MC-PRO	The OSC camera. Allows you to take RGB images.	£1899
L-eXtreme 2″	This filter lets your OSC camera record Hydrogen-alpha and Oxygen-III (HO) narrowband wavelengths.	£239
ZWO 2″ Filter Drawer	This lets you easily swap out your L-eXtreme (or other) filter. It’s not essential, but is convenient.	£79
		£2217

Mono
ZWO ASI 2600MM-PRO	The mono camera.	£2389
LRGB filters	Lets you record RGB data, like an OSC camera does, but with the added bonus of Luminance (LRGB).	£269
Narrowband filters	Lets you record Sulphur-II, Hydrogen-alpha, Oxygen-III (SHO) narrowband wavelengths.	£559
Motorised filter wheel	A gadget that lets you easily switch between filters.	£399
		£3616

You could buy cheaper (or more expensive) versions of the kit listed, and of course prices will vary, but this at least gives an idea of the kind of price differences you’re looking at between OSC and Mono. The Mono kit is superior, outlined in more detail below — after all, you’re getting Luminance and Sulphur-II filters, as well as other advantages — but expect to pay about 150% the price of an equivalent OSC set-up for the privilege.

Image acquisition | Winner: OSC, but not by much if you’ve automated your Mono set-up
Taking images with an OSC camera is relatively straight-forward (well, as much as anything in this hobby ever is). With Mono you’ve got extra kit to think about and you need to put a bit more thought into planning your imaging session. And no-one enjoys taking Flats (a type of calibration frame) for lots of filters. You can use software and buy gadgets like motorised filter wheels that automate a lot of the leg-work. That closes the gap, but OSC is still simpler by its very nature. Plus, your local sky conditions are a consideration. If you often have cloudy skies, acquisition with Mono is a bit trickier. Maybe you’ve gathered all your Red and Green data, then clouds roll in and stop you getting any Blue. Awkward. There are ways you can mitigate this, such as switching through your filters regularly throughout a night, but it’s never going to be as robust as OSC. In principle you can get equivalent RGB data with a Mono camera actually faster than you can with OSC for a few reasons, such as OSC wasting pixels by having so much green. In my experience the difference isn’t that great though, and is often offset by other factors, such as filter changes and clouds disrupting imaging runs.

Image processing | Winner: Draw
You would naturally assume that processing OSC images is a lot simpler. After all, you’re getting a colour image to work with right away — none of the combining images taken with different filters that’s necessary with a Mono camera. In my opinion it’s a common misconception that editing OSC data is easier. While OSC does give you the option of very simple processing, in reality there’s more to it than that. For a start, OSC images have an over-abundance of green, so you need to do some processing to reduce it. No such issues with Mono. And if you’re doing serious editing with OSC data then you’ll be splitting your channels to edit them separately anyway, and then recombining them later. Which is basically what Mono imagers have to do. In the end, the amount and complexity of image processing is fairly similar for OSC and Mono.

Versatility | Winner: Mono
No doubt about it, Mono is more versatile than OSC. You’ve got access to more filters and can combine their data however you like. Want to keep things simple and just record Hydrogen-alpha? Easy. Keen to combine SHO data to make images with the same colour scheme that the Hubble Space Telescope uses? Sure, you can do this. (That’s also possible with an OSC camera plus dual-band filter too, but you need to fake a Sulphur-II channel). You can also shoot a greater variety of targets, and be better equipped to try photometry and spectroscopy. With Mono, the world — or rather, Universe — is your oyster.

Image quality | Winner: Mono but only just
This is a tough criterion to judge. In theory, Mono is the clear winner. It’s very sensitive, and you get lots of detail with a Luminance filter. You also often hear that Mono images achieve higher resolutions. In theory this is true, but how noticeable is it? Can you look at an image and tell whether it was taken with an OSC or Mono camera by its resolution? Personally, I can’t. I’d like to do a good side-by-side comparison sometime; if the difference if evident then I’d accept the resolution argument. But for now, I think that your image’s total integration time and your skill in processing has far more impact on the final image quality than whether you used OSC or Mono. So, I’m going to give this one to Mono, but only just.

Fun factor | Winner: depends on what you want
When deciding whether to buy an OSC or Mono camera, we have a tendency to get our heads stuck in the fine print of camera specifications and theoretical limits. Here’s the thing though: most of us are into this hobby for fun, so don’t lose sight of that. Which camera type would you find more fun to use? That’s the one that you’ll use the most.

What should I choose?!

Sorry, there’s not an easy answer here. Given the theme of this website you were probably expecting me to say that you should get an OSC. If only it were that simple.

If you’ve got a big budget, lots of clear skies, plenty of free time, and want the greatest versatility, then Mono is probably the right choice for you. But if you fall down in any of those categories it’s worth at least considering OSC. This is thanks to the development of those wonderful dual-band filters. Before they came along, I’d be saying that from a city, Mono was the only real choice. Now, OSC with a dual-band filter can work really well (which I hope this website’s gallery proves).

The price difference alone may push you into the OSC camp. It’s worth noting that the second-hand market for astro equipment is very good. The people that buy this kind of kit tend to take good care of it, often with a mind to sell it on to fund upgrades in the future. So if you’re set on Mono, you could likely get an older-generation set-up second-hand for around the price of a brand new latest generation OSC. Or, you could find a bargain OSC camera and get imaging for less than you think. Maybe the money saved could be used to buy some useful upgrades?

If you’re still stuck with analysis paralysis choosing an astrocamera, here’s a tip that might help you decide. We tend to think we’re putting ourselves in the OSC or Mono camp for life. But it doesn’t work that way; you won’t be using your new camera forever. Consider how long you expect to use it before you’ll want to sell it and get an upgrade. Technology is always moving on, after all.

Let’s say that figure is five years. Which camera type will you get the most use out of in that timeframe? If you’re short on time, cash, or just want something (relatively) simple to produce pretty pictures, maybe OSC is right for you. If you want to get really deep into the hobby and can dedicate all the necessary resources to it, perhaps Mono is the answer. I used to shoot Mono, and am now OSC. Maybe I’ll go back to Mono in the future.

In the end, whatever you choose, the most important thing is to get your camera out there collecting photons.

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