Thursday, 18 September 2014

First Star Shots with ZWO ASI120MC camera

So, having had a bit of a play with some night-time all-sky imaging and daytime terrestrial targets, I finally had some reasonably clear skies last night (well, barely reasonable - it was hazy, with about 50% cloud cover), but I thought I'd see what my ZWO ASI120MC camera can do. By the time I got set up, there weren't any planets in the sky visible from my garden, and the Moon hadn't risen, so even though this is mainly a planetary camera, I thought I'd see what it could do with an easy DSO target.

I set up my Celestron SLT 130 (130 mm aperture / 650 mm FL Newtonian on a GoTo AltAz mount), because I thought the bigger aperture and shorter focal length would make finding some sort of target easier than trying to use my 90 mm Mak. I had already worked out in daylight hours that the Newt would reach focus on the sensor quite easily by taking the eyepiece adapter out of the focuser, and screwing the camera body straight onto the M42 T2 thread at the top of the focuser. (That's a great design feature by ZWO - providing the camera body with an integral T2 thread in the camera body AND providing a standard 1 1/2" eyepiece adapter!)

I set up and aligned the telescope, pointed it at the brightest star in the southern sky, took out the eyepiece adapter, and screwed on the camera, and wound the focuser to roughly the position where I knew focus could be found.

Then I plugged the camera into my Toshiba Encore Windows 8.1 tablet, started FireCapture , and was gob-smacked to see a slightly out-of-focus star image straight away. Wow! Surely it can't be that easy?! I focused to get the sharpest image I could, and fired a few frames - Yep!  I was clearly getting recognisable star shots, including a few background stars I hadn't noticed by eye.

I then slewed the telescoped to an old favourite - the 47 Tucanae globular cluster.

I had to manually adjust the telescope alignment a bit to get it centred, but once it was in frame, the SLT's tracking kept it nicely on the sensor. I then fired off a few short videos and BMP series using FireCapture, experimenting with different exposure settings. For each series of shots with new settings, I tried resetting FireCapture's automatic Dark Frame function and taking a new dark frame, to compensate for the changed exposure, gain,  sensor temperature, etc.

I then retired indoors to have a look at what I had caught, and what I could extract with stacking frames using AutoStakkert etc.

And here's what I found:

Some of my AVI videos don't seem to contain any useful data - specifically, any I took with frames with more than 1-second exposures. I'll have to do a bit more digging to see if this is a fundamental restriction with using AVI format captures, or just the settings I was using (or simple "user error"!)

However, the AVI videos with sub-1.0-second exposures seem to be quite usable. I still need to learn a lot about getting the best exposures, but I was pretty chuffed as a first effort! Here's a sample which is 71-seconds duration, at 1280x960 resolution, with 1.00 second shutter-speed and Gain set to 100:

(Hint: You probably don't need to watch the whole 70 seconds - there isn't much in the way of plot surprises or or character development!)

AutoStakkert is super-easy to use! Whether this simplicity limits its capability, I'm not sure, but I think I'll continue to "cut my teeth" on AutoStakkert before going back to RegiStax . Here's a quick shot made from the same video, using the best 50% of frames (straight from AutoStakkert with default settings; no other post-processing):

Again, I have a lot to learn, but there's no mistaking it - that's 47 Tuc alright!

Then I thought I'd try some much longer exposures, so I set it up to take a series of BMPs with 60 second exposures - here's a sample (again with Gain set to 100):

The image is straight off the camera, with the same Field of View as the previous AVI - no retouching or post-processing.

At first I was puzzled as to why the BMP seems to be zoomed in so much compared to the AVI and stack, but the more I think about it, I think it's really that the 60 second exposure captures so many more faint stars than a series of 1-second exposures (well, DUH!), so they're showing pretty much the same Field of View, but the AVI only captures the "hot core" of the cluster, while the 60-second frame captures much more of the full structure. I'll try again soon with a series of intermediate exposures - say 10-second, 20-, 30- and so on, and I'll expect to see the cluster "grow" as the exposure time increases.

I've also learnt a lot about my limitations, and the limitations of my equipment; e.g. there are clear signs of telescope vibration and "Field Rotation" on the 60-second shot, so 10- to 20-second exposures might be more sensible. It's also pretty obvious that even though I took a 60-second "dark", there's still quite a bit of sensor noise. (Stay tuned for my upcoming experiment with camera sensor cooling using a Peltier TEC cooler!)

And of course, the old mantra of astro-photography: 
Focus! Focus! Focus!
These shots were all focused by eye on my little 8" tablet screen; next time out, I will try for better focus using my Bahtinov Focussing Mask .

But overall - I'm pretty happy for my first effort!