Archive for category gadgets
When we were debating which telescope to buy initially, one of the considerations was whether to get something with a guidance system or not. I voted nay, at least partly for money reasons, but also because I felt like a guidance system was basically cheating. I wanted to learn my way around the sky, and find things all by myself thank you very much.
About a year later, we went to the 2010 Ice in Space camp. It was the first time I’d ever been around lots of fellow amateur astronomers, and it turns out that actually a lot of them have guidance systems of one sort or another. And most of them still know something about the sky and finding their way around it. By this stage I had started getting interested in searching out double stars, and was finding it increasingly frustrating: I was often having trouble just finding the point-of-reference star, let alone the faintish double that was meant to be close by.
You can tell where this is leading. I soon sucked it up and agreed to getting a guidance computer. It helped that James had had his Dobsonian blinged out with a ServoCat and a computer and that I was just a wee bit jealous.
We both got an Argo Navis. The first thing I asked on seeing the product was “Why is their symbol an Egyptian hieroglyph when their name is Greek?”, but that’s neither here nor there.* The Argo is of Australian design and manufacture, which is very cool, and it works an absolute treat. (I also had to get a new mount to go with the Argo, and we went with the Losmandy GM8. But that’s another post.)
The Argo is very simple to use. It has two buttons and a scroll wheel, and a screen that can show two lines of text. The Losmandy is a GEM (German Equatorial Mount), so setting it up is easy: having aligned the mount with celestial south (so that it tracks properly… and let’s be honest, it’s close to south but probably usually “southish”), wait for two bright stars to be obvious, line the telescope up with your finder/telrad, let the Argo know which stars they are, and you’re away.
There are several ways you can use the Argo, and you can of course change your method over the night. If you don’t have specific plans for your night’s viewing, you can use the Argo’s Tour mode, and choose what you’d like to have a look at: Popular Deep Sky objects? Planets? Messier objects? Planetary nebula? Choose your category, state whether you want to look within a small area (if, say, Sagittarius is high and you want to know what’s within it) or over the entire sky, and what magnitude limit you want, and then… go where the Argo tells you. What sort of mount you have will determine how you actually find things, of course, but for me it’s just a matter of pushing the scope around, following the arrows, and watching the numbers for RA and Dec count down to zero (and then the arrow reverse, and the numbers rise, when I overshoot…). The Dob has a ServoCat, so it automatically goes to your object when you hit ‘enter’. The whine of the motors is amusing in the middle of the night…
Alignment is an important part of how the Argo works. The Argo gets a sense of where it is, what the sky looks like, and hence where other stars and objects should be according to Right Ascension (RA) and Declination (Dec). When you dial up M42, for example (who doesn’t love the Orion Nebula?), the display will indicate in which directions (up/down, left/right) the telescope needs to be moved in order to centre on it. The numbers reduce as you get closer, and become more precise as the object gets closer. I found it quite intuitive, and easy to use. And I’m pretty hopeless spatially.
As I said, there are other ways of using the Argo while observing. There are some clever flow-on effects of the Argo knowing where it is pointing; for example, you can identify an object you’re looking at, by going to the appropriate mode setting (Identify). Because it also tells you the coordinates of the area in the sky you’re pointing at, you can also use the Argo to find something – say from an atlas – that isn’t otherwise listed in the Argo’s menus. This is particularly useful when looking, for example, for double stars.
The Argo comes with a cradle, and with the Dob it was simply a matter of constructing a little tray for it and the Servo controls. The ingenious solution for using it on my tripod was velcro. I have velcro on all three legs of the tripod and on the back of the Argo (and the hand control, and the dew heater…). So I can move the Argo around to whatever side I’m on, and I can just slap it onto the tripod when I want to have my hands free. Velcro: best invention ever.
In all, the Argo is very easy to use, even for someone like me who is neither particularly tech-savvy nor very good with directions.
*Turns out Argo Navis is the name of the constellation that today is Carina, Puppis and Vela. There you go.
If you own a scope 8 inches or larger then filters can make a big difference to the fine details you see in a range of non-stellar objects. They work by blocking out some wavelengths of light, while allowing full transmission of others. Depending on their type and composition, different nebulae emit light at different wavelengths and by using a filter matched to these the overall background and star brightness is reduced significantly more than the nebulae itself. While all filters cause some overall reduction in light transmission, the differential reduction of the background (especially under light polluted skys) and the transmission of the nebulae’s specific emission lines increases the relative contrast allowing the eye to see more detail than is possible without the filter.
Filters are not magic; their effect is minimal or negative for many objects because of the reduction in total light transmission. However, for some objects – especially marginal ones – the contrast increase can make it significantly easier to make out the details of an object. The Tarantula Nebula in the LMC is one of my favourites to view through a UHC or OIII filter: it reveals different and fascinating details through both. The OIII filter is also ideal for making planetary nebulae stand out from the background stars. NGC2438 in M46 is a excellent example for this.
David Knisely has done an extensive and detailed comparison of the UHC, OIII and Hb filters on almost 100 objects using a 10 inch scope. I’m not going to try and replicate this excellent analysis; instead I recommend you read the descriptions for objects you’re familiar with to get an idea of the improvement you can expect with the different filters.
Many companies make filters. Personally I have the UHC, OIII and Hb 2″ Astronomik filters mounted on a moonlight filter slide with a heater strip and temperature sensor set up to keep them dew free. The filters don’t appear to introduce a colour cast, or any other undesirable artefacts when viewing. They do however require a slight refocusing when changing between a filtered and non-filtered view.
Like many astronomy accessories filters are not cheap, so here’s my buying advice. If you’re only going to buy one filter, get a UHC. Next buy an OIII, especially if you enjoy planetary nebulae. An Hb filter is honestly an excessive luxury, but if you have a big Dob (15+ inches) and you want to be able to enjoy some very faint nebulae objects like the Horsehead it makes a noticeable improvement.