ASI183 - First light

As mentioned in an earlier update, I now own a ZWO ASI183MC-Pro astro camera. The recent clear nights here (an unprecedented 4 nights in a row!) gave me the opportunity to get the telescope outside and to begin scaling the learning hill that is the next step in my progression.

It was not an easy first night. Having made two changes to my gear (auto focuser and new camera) I was not aware of how they would interact, nor what would be required to get things working.

Let’s start with the EAF. I had calibrated it inside and measured the backlash inherent to it, but I was not aware of how to drive it and achieve excellent focus. Having reviewed numerous Internet forum posts and a few YouTube videos, I now know that I need to make 2 focussing passes - one with a large step size to achieve the initial focus, and a second one with much smaller step sizes to get the focus dialled in correctly.

One quick word about Crayford focusers. They’re great when they work, and not so great when they start slipping. I spent a couple of hours getting mine tuned as much as possible, to the point of installing rubber washers (small rings of heat-shrink insulation) so the axle would properly grip the focus tube. It’s not perfect, as the tube still slips slightly, but I can cope with it. It’s not economical to install a ‘proper’ rack-and-pinion focuser with the OTA that I have. IF (or when) I upgrade the OTA, I will look very carefully at those that have a R-and-P focuser.

When it comes to imaging with a DSLR, it’s pretty simple. You have two controls to deal with - ISO and exposure length. Dedicated astro cams have a lot, lot more knobs and levers to play with. Thankfully I can ignore most of them and concentrate on the big two - gain (roughly equivalent to ISO) and offset (which I have no idea about).

As near as I can work out, increasing the gain will increase the sensitivity of the camera sensor, but at the cost of colour depth and the risk of quickly over-saturating bright objects like stars. With the ‘183, reducing the gain below about 50 will apparently increase the risk of introducing vertical colour bands in the images.

So, anyway, I managed to get everything working on that first night (for different values of “working”) and collected some images of NGC290, as mentioned earlier.

One thing that became apparent very early in the night was how quickly the images transferred from the ‘183 to my laptop (about a second, compared to a 6 second delay when using the DSLR) and the increased size of them (40 MBytes vs 25 MBytes).

The other thing about asto cameras is their need for calibration images. With a DSLR you can get away with only capturing light images (if you’re lucky and your sensor does not generate hot pixels). Dedicated cameras require lights, darks, flats and bias images. (Although I’ve read that ZWO do not recommend the use of bias frames…) All these calibration images work to remove unwanted noise, amp glow (an artefact of the camera’s electronics) and any vignetting in your image train. Yes, it is a lot of work to collect them all, but you can create libraries of them (different gain values, different temperatures and focal lengths) to save time in the future.

The first image from the ‘183 is below. To my eyes, it looks like an explosion in a confetti factory. I’m not at all happy with it, but I must be honest - my expectations were far ahead of my abilities and I have a long way to go before I can be happy with an image I capture.

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The second and third nights out, I tried my hand at NGC104 (aka “47 Tucanae”), a large cluster of stars close to the SMC. This time, I had the focus routine mostly sorted out and I was able to get a tighter focus on the stars. I was still experimenting with what settings to use for gain and offset.

This image used a gain of 40, as I was wanting to get a greater colour depth in the image. Which may not have been terribly useful as there aren’t exactly a wide range of colours in that area of space.

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The final image was captured using a gain of 100 (almost at the ‘183’s Unity gain of 120) and an offset of 8. I also have the focus pretty much sorted out, as the stars in this image are a lot sharper than before.

The different colours between the two images are down to my post-processing and it’s inconsistent trajectory. I aim to produce an image that looks good. Whether is it totally accurate in the colours it is presenting, well, that is a whole argument I am going to keep well away from.

• Astro