Quote:
Originally Posted by Wavytone
Morton,
I tried both Sky Safari and the Theodolite app on my iPhone 4 and iPad 3 and while its a nice idea, it has a few problems.
1. To be any use as a finder you need accuracy of the order of ~ 1 degree, or better. Otherwise you're wasting your time.
2. Using the earths magnetic field to measure azimuth is not good enough for a finder. It's good enough for a compass as a crude navigation instrument, but not good enough for astronomy. The magnetic field has an offset to true north, this has to be compensated for. Unfortunately this offset is not constant, it varies according to your location and it varies with time, and it is also screwed with right royally if there is an aurora - this will cause a noticeable shift in the earths magnetic field. This variability is both unpredictable and sufficient to make software-based finders unpredictably unreliable...
3. Altitude. The iPhone/iPad sensors are good enough for altitude measurements, the resolution seems to be about 0.1 degree, however you must calibrate whatever software you're using.
4. It is extremely inconvenient, ergonomically. I attached my iPad to the front of my scope to see if it would figure out where the scope is pointing, and whether it was useable as a finder. Unfortunately it suffers from a very obvious problem - near the horizon its OK but at high elevation I rapidly got a bad crick in my neck from trying to get under the damn thing and look up at it.
|
SkEye is an Android-only app, so won't work for you, but for Android users, it is vastly superior as a telescope-mounted finder tool compared to SkySafari etc, because of how it works. I am not aware of any comparable iOS app. (But SkySafari is my pick of the "planetarium apps" for all other functions - and is available on both iOS and Android.)
1. I get better than 1 degree accuracy on both axes with SkEye. It has the ability to do a local "re-calibrate", so that you use SkEye to navigate to an easily identifiable target star in the general vicinity of where you are planning to observe, then "align" it to that star, and then you get better than 1-degree precision over that whole field of sky (60-degree range, say). When you swing to a new quadrant of the sky, if the alignment isn't good enough (it generally is!), just realign it to a new local star and carry on.
2. SkEye includes the local magnetic deviation to automatically know True North from Magnetic North, based on your location. (In fact, any decent smartphone compass should be able to do this. Compass deviation variation with time occurs over a scale of years , not minutes or hours, so a simple look-up table is all that is required to get better than 1-degree precision.)
3. Calibrate the compass by waving the smartphone in a figure-of-8 a couple of times before relying on it for accurate Azimuth control, but then it works just fine.
4. Ergonomics - this is where SkEye is really clever. Most "planetarium" apps in "compass" mode show you the sky view "straight through" the smartphone, perpendicular to the back of the phone. This means that to use the smartphone as a finder, you need to mount it EXACTLY perpendicular to the OTA. With SkEye, you mount the phone / tablet firmly onto your telescope in any convenient position / orientation - e.g. strapped straight onto the tube (as long as it isn't made of steel), or mount it at a convenient offset using a bracket of some sort. Then you tell SkEye what you are actually looking at through the telescope, and it works out the offset angles between the phone's orientation and the telescope's orientation. When you swing the telescope in azimuth and / or altitude, you swing the phone through the same differential angles, so SkEye can work out how much you have moved on both axes, and what you are now pointed out. Simple, but very clever, and as I said, I think it is still a unique feature.