ZWO ASI 533mm Pro Review

A couple of years ago I reviewed one of my favourite cameras. The ASI 533mc Pro. Since then, ZWO have released a mono version of the same camera, and they sent me one for review.

With this article I’m going to focus on my thoughts and feelings after using this dedicated monochromatic camera.

Sensor

With all cameras we want to know about the business end. That is the sensor. So I’m going to start there. The ZWO ASI 533mm Pro is of course a mono camera, which means it doesn’t have a Colour Filter Array on it which the ZWO ASI 533mc Pro does. This means that all the light hitting the sensor can be used for the same filter that we place in front of the sensor.

The sensor within the ZWO ASI 533mm Pro is a 11.3mm x 11.3mm square format sensor. An aspect ratio of 1×1 and has pixels that are 3.76µm in size. That means the actual pixel resolution is 3008 x 3008 pixels, 9 megapixel.

9 megapixels may not sound like many, especially on a fairly recent camera. However the sensor itself has other things going for it. Also, higher megapixel counts will test the quality of both your seeing conditions, as well as the optical quality of whatever telescope you’ve put the camera on to. Sometimes less is more.

Crop Factor

One thing to look out for is the crop factor of this camera. At 11.3mm x 11.3mm in size, the IMX533 CLK-D sensor placed within the ZWO ASI 533mm Pro has a 2.7x crop factor to the focal length.

What this means is that the field of view you’ll get will be equivalent to a telescope 2.7x longer in focal length. A 1000mm telescope will effectively perform like a 2700mm telescope.

• Green: 1000mm focal length using a Canon 6D DSLR as the imaging camera. The Canon 6D is a Full Frame camera, so it has a 1x crop factor. 1000mm is 1000mm
• Purple: 2700mm focal length using a Canon 6D DSLR as the imaging camera. The Canon 6D is a Full Frame camera, so the crop factor is 1x. 2700mm is 2700mm
• Blue: 1000mm focal length using the ZWO ASI 533mm Pro as the imaging camera. The ASI 533mm has a 2.7x crop factor. So the 1000mm focal length is effectively 2700mm

Full Well, Read Noise & Quantum Efficiency

Digging deeper into the mechanics behind the ZWO ASI 533mm Pro sensor, I’m going to poke at the full well, read noise and quantum efficiency that’s boasted by this camera. I can’t do any proper testing myself, so I’m going to scrutinize the charts – just like most of us do.

Full Well

The full well of a camera can easily be visualised as a bucket that gets filled with water. The point in which the bucket is totally filled is the full well. In a camera sensor we use full well to describe how many photons a pixel can take before it goes totally white and loses all meaningful data, or oversaturates.

The full well with the ZWO ASI 533mm Pro is a whopping 50,000 electrons. This is a sizeable well, and you’d be hard pressed to fill this well up. However before we get too excited, we need to look at the chart more.

When we look at the chart and scrutinize it, we can actually see that if we shoot at Unity Gain which I imagine most of us will be. The full well actually drops down to around 18000 electrons. At 0 gain, we get the full 50000 electrons however the exposures will have to be a lot longer as this camera has a 3:1 conversion at 0 gain (3 electrons to make 1 ADU). Down at 450 gain, our wells are very small at about 300 electrons.

Read Noise

Read noise is an artefact with digital sensors. The easiest way to think about this is read noise is a fixed amount of noise that will be in every photo. Doesn’t matter if it’s 1/100 of a second of 10 minutes. The image will have the same read noise. The ZWO ASI 533mm Pro has an advertised read noise of 1.0e – 3.8 electrons. Lower numbers are better here.

Similar to full well, we have to look at the chart. Within we can see that the lowest read noise is at the highest gain. This is very beneficial to lucky imagers. As they often take shorter, but more, exposures. So having less noise to begin with is better. If you’re shooting at low gains, then you’re probably shooting longer exposures which will help offset the higher read noise.

At unity gain, we have a read noise of 1.5 electrons. This is absolutely fine for astrophotography exposures of any reasonable fine. So for me the read noise is a non-issue.

Quantum Efficiency

So quantum efficiency is such a fancy way of saying how well the ZWO ASI 533mm Pro uses the light falling on it. An easier way to think of this is the camera’s sensitivity.

Instead of the ZWO ASI 533mc Pro camera’s quantum efficiency which peaked at about 99% in green, but only 80% in blue. The ZWO ASI 533mm Pro can benefit from that a peak efficiency over 91%. So that’s 91% at very best. Of course high quality filters are preferable here as a cheap filter can reduce the amount of light getting through to the sensor.

Again, though, we have to look at the chart and along the bottom is the wavelength and along the left axis is the sensitivity.

• Red is about 620nm
• Green is about 500nm
• Blue is about450nm
• Oxygen-III is about 500nm
• Hydrogen Alpha is about 650nm

So the camera can use blue/green light the best, and drops in efficiency towards the red end. In practical terms you’d need to use longer, or more, exposures with these filters.

Using The ASI 533mm Pro

So all these figures and specifications might be good. But what is this camera actually like to work with and take photos with?

Well I found that the camera was quite sensitive. Using a reasonable f6 telescope (the StellaMira 66ED) I was only using 2 minute exposures in general on broadband targets and 5 minute exposures with the Optolong L-eXtreme dual narrowband filter.

Fitting reducers and flatteners is also a breeze. The ZWO ASI 533mm Pro comes with two adaptors already that help give you 37.5mm back focus. Add in the 17.5mm back focus built into the camera and you have 55mm. 55mm is a very popular and almost standard back focus range. Though you will have to add the filter wheel into this 37.5mm distance.

For example, if your filter wheel is 20mm thick then you’ll only need to use the 16.5mm extension to get very close to your required spacing. It also comes with a M42-M48 internal non-extending adaptor in case you need to step down the size difference with a reducer.

Then once everything was fitted it was just the case of plugging accessories like the auto-focuser and the filter wheel into the USB hub on the back of the camera and connecting the camera to your imaging computer. Oh, and powering it up of course.

This camera requires power. Even if you don’t want to use cooling, it has to be powered before it will transfer images. This can be a drag as it will reduce some of the portability on the device. However the other downside is the camera – as with all ZWO cameras – doesn’t come with a power adaptor.

Whilst I can gripe about this, and I do. From my understanding there are at least two reasons for this:

• Localisation: If ZWO supplied a power plug with the ASI 533mm Pro then they would have to add in the correct adaptors for that country’s plug socket. By leaving it out not only can they sell a lead if its required, but the customer will choose the correct lead.
• Packaging: ZWO like posting their cameras in nifty cube boxes which is a really compact and space efficient design. Adding a power brick and cable in there also will mean it won’t fit.

Going back to the actual use of the camera and I loved using it. I came into this camera with high expectations already due to the fact I enjoyed the ZWO ASI 53mc so much. At the end, the ZWO ASI 533mm didn’t disappoint. I wasn’t left wanting.

With the slightly higher quantum efficiency, and the nature of a mono camera in that all the light is useful per filter. The data that was coming through was very clean with good clarity and signal-to-noise. Unfortunately our skies could’ve been more accommodating to me testing a mono camera out, however.

The biggest difference however was of course the lack of the colour filter array. It cannot be overstated the difference 4 blue filtered pixels make over 1 blue pixel per 4 as it would be with the colour RGGB sensor.

Another benefit that I enjoy greatly with the ZWO ASI 53mm Pro is the absolute lack of amp glow. It really doesn’t matter if you’re taking a 30 second image or a 30 minute exposure. There isn’t any amp glow in your images. Whilst it is pretty straight-forward to calibrate out with dark frames. It not being there in the first place just makes life easier. Prevention is better than cure after all.

Cooling

As with all the ASI Pro cameras that ZWO have, the ZWO ASI 53mm Pro features the typical 2-stage cooling that you find pretty much in any cooled camera within the industry. This camera is capable of going -35°c below ambient temperature. So if your imaging night is 20°c, you can cool down to -15°c and benefit from the incredibly lower thermal noise.

This camera really benefits from some good cooling. There are others in the ZWO range which hit diminishing returns on your cooling quite quickly. But looking at the chart cooling down to -20°c puts you in a really good spot for thermal noise with the IMX533 sensor used in this camera.

On a warm summer’s night in the UK, the sensor could easily reach somewhere between 20°c and 25°c. If we calculate that for a 2 minute exposure we get about 3.75 electrons of noise per pixel. That’s a total of 33,930,240 electrons over all 9 megapixels of the camera. Okay that number is extreme for the sake of being extreme and I wouldn’t worry yourself about the ‘total amount of heat noise’ like 33.9 million electrons.

Cooling that sensor down to -15°c on the same night however would net us 0.0288 electrons of noise per pixel. Or 3.7212 electrons less. That’s a total noise of 260,584 electrons collected over all 9 megapixels. An extreme difference to be sure.

Just be mindful that the deeper you chill, the higher your risk of fogging the sensor window up. This camera does have a window heater on it, but you could overwhelm it if it’s a particularly humid night. Also, the deeper you chill the slower you’ll want to warm the camera up also to prevent thermal shock to the sensor. Just go slow and steady in your cooling and warming.

Suitability

That sort of telescope focal length does the ZWO ASI 533mm Pro suit? Well using the great free resource astronomy.tools we can find out that in order to correctly sample. This camera wants to be paired with optics between 388mm and 1163mm. That’s quite a focal range to choose from and contains many popular images.

400mm

At 400mm, you can fit targets such as the Rosette Nebula or M33 Triangulum Galaxy within the frame

600mm

At 6mm we’re looking at targets such as IC410 the Tadpoles Nebula, or M16 the Eagle Nebula

750mm

At 750mm you can fit in the Helix Nebula. Also, you can fit the entirety of the Moon within the frame giving you a beautiful full disc image.

1000mm

At 1000mm we’re looking at targets such as the Trifid Nebula or the Crescent Nebula.

Conclusions

So the ZWO ASI 533mm Pro has a wonderfully sensitive sensor that doesn’t produce amp glow, has 14-bits and a realistic full well of about 18000 electrons at unity gain. With the square format sensor framing targets isn’t hard, and you can then crop further in post to whatever aspect ratio you want. The pixel size of 3.75µm means it’s suitable for a variety of popular focal lengths also. The relatively small chip, paired with a large, deep focal length telescope and a barlow means you could also turn this into a planetary camera.

At relatively quick focal lengths the exposures on bright/moderately-bright nebulae etc can be 2 minutes for broadband or 5 minutes for 7nm narrowband imaging in my testing. Cooling plays a major factor to the cleanliness of the images produced and it being a mono, you can choose what light you want hitting all the pixels by the filter chosen.

It doesn’t come with a power plug and power is required in order to use the cooling and to even download any images from the camera. Though devices like the ASI Air Pro/Plus can power the camera. Also you need to double check the published charts to make sure you know what read noise and full well you’re truly getting. Also the small sensor has a 2.7x crop factor, which you really have to take notice of and check before you purchase this camera.

At the time of writing, the camera costs £1129. Though factor in this is a monochrome camera which means filters, a filter wheel and ideally an auto-focuser will be required. Using all ZWO products like I did means we’re pushing £2069 for the total package.

• ZWO ASI 533mm Pro
• ZWO Electronic Auto-Focuser
• ZWO 8 Position 1.25″ Filter Wheel
• ZWO 1.25″ LRGB Filters
• ZWO 1.25″ SHO 7nm Filters

Whilst a considerable investment, pair this with quality optics at a field of view you’re happy with and I’m confident this package will see you through many night’s enjoyable imaging. The ZWO ASI 533mm Pro is a joy to use, and in general fun. With a hobby that’s as demanding as astrophotography at times. Using equipment which is enjoyable to use will only make you enjoy it that much more. Which in turn means you’ll image more, and get more value out of your camera.