For Edmonton Centre Stardust newsletter, September 2019

This past July I got a chance to give it a spin. I was eager to see if such a unit would benefit my imaging of ice crystal halos precisely because it did not track equatorially. As a bonus, I could (partly) assess the field rotation problem on a starfield at night.

The cube is a beefy unit, a tad smaller and lighter than a car battery. The carrying case is 28.4 lbs (12.9kg), of which 10lbs (4.2kg) is the counterweight (important for balancing telescopes and camera lenses to prevent grinding the drive gears). The tripod is similarly robust at 17.2 lbs (7.8 kg). Having the 5 minute demo from Luca earlier in the day, I assembled it at a neighbourhood park, literally a few metres from a streetlight, which is not a bad idea for a first assembly so you can see what you're doing. The mount found the first star 40 minutes after starting, though I could have saved 11 minutes by turning on the GPS right away to register satellites while levelling the unit. As you can see in the photo, the easiest way is to use a construction bubble level in two directions rather than use the little circular bubble on the side of the mount.

Newer Alt-Az units (the model we have has been discontinued) undoubtedly take less time to acquire a GPS signal. Although you can manage without GPS by manually entering all the specifics, having the mount lock in makes everything easier. You point the arrow south, turn your scope straight up (use a level lying across the dew shield), press ENTER a few times to accept GPS values, and you're ready to "Select and slew". I chose Vega and it showed up less than 1/3 of a field of view from center of a 135mm telephoto lens. I then went to "SYNC to target", pressed the arrows to center it, then ENTER. Next object I went to was less than a 1/4 field from center, and so it was for the rest of the night. Pretty easy if you don't kick the tripod!

I could choose named star or Messier or NGC object with a couple of short jumps down the menus. At first I was put out that Deneb was 75 button pushes of 170, but then I realized you could type in "075 ENTER" and bam there it was. Similarly with Messier objects and NGCs: just move the cursor over to the 0 and input the number.

You can see the stars towards the edges are all short circular arcs around the center of the image. Quite disappointing (unacceptable?) for a beginner of 1 month experience I would think. Astronomers call this field rotation. It shows up when an AltAz (Altitude-Azimuth) mount track in a diagonal line but the sky rotates around the poles (see parallactic angle). All professional telescopes have "de-rotators" that turn the camera/sensor at just the right rate to cancel it out. Visually, who cares if things rotate 15 degrees in an observing session? Photographically it matters.

One shortcut to step around the problem is to take short exposures that do not reveal the trail, in my case 25 seconds at 200mm, then later in software, stack the images by compensating for the rotation from one frame to the next. I was able to do this with ImagesPlus software, but (with my older v5.5) I had to manually click on two stars for each image so the software could recognize the rotation. This works, but is quite tedious!

The concept of stacking is to add signal while the noise averages itself out. 25 seconds is a low signal - in Skynews many people take subframes of 5 minutes for a good signal. You actually need more than 10 times the half minute to get as good a signal to noise ratio than a single 5 minute one.

Obviously, the 8 frames stacked is much better. On the one hand, no one is going to be impressed by a pale gray blob. On the other hand, hey, this is a 67mm aperture from the Edmonton suburbs with forest fire smoke aloft! Pretty remarkable that one can see anything at all.

How deep can one go? With 24 frames of 25 seconds apiece stacked together pointing at the Owl cluster in Cassiopeia, I was able to reach magnitude 14.4, plus or minus a couple of tenths depending on the colour of the star and bias of the detector. Pretty impressive from the city suburbs.

Using only the iOptron's goto function to get to Pluto, it put me on the right field within 1/4 of the width, while I was at 200mm focal length. Pluto is way down low in Sagittarius, so it is not surprising that I did not quite reach its mag 14.4 glowing speck through the suburban glow and forest fire smoke.

So would I recommend the iOptron? Short answer: definitive NO. With all the success I managed here, you might be surprised with that. Sure, I can "get away" with it for some things, but for the same price you can purchase an equatorial unit. Yes, the equatorial means a rotating camera that starts and ends at different angles, and is more complicated than a simple up-down-left-right unit, but that learning curve is short and actually pretty easy. It's not much more than having to learn the buttons on a new photocopier, microwave, or TV remote when you change cable <=> fibre.

The benefit is much less work at the image processing side when you get back home and want to turn those frames into an image. But maybe the newer image processing software that I am not familiar with can handle it. Maybe it does not. My second set up took 20 minutes. The newer goto gets that under 5. Why would I want this delay?!

"Field rotation makes it completely useless" is wrong. At least for the parameters I used here. 25 seconds at 200mm means you can get away with 50 seconds at 100mm. Better signal. I have not tested whether this means that with a 50mm lens you can do 100 seconds. The geometry changes as one moves away from the celestial equator.

Back to the question that sat in my mind: do I need this for halo imaging? Well, it is nice, but I've found with the panorama software Hugin I can take any equatorial set-up and rotate it into "up-down" orientation and re-project it onto a dome or VR360 view. If I can already do that, do I *need* the other? Thankfully, there is no requirement to spend more money, just re-orient your head and run some software.

Acknowledgments:

I would like to thank the Edmonton RASC for loaning me the iOptron cube and Luca Vanzella for having shown me the basics of its use.