|So I've spent the last few days down at my dark sky site in Okeechobee county Florida. SQM of 21.03 Tuesday night. Yeah, be jealous, just don't build a Walmart down the road...
I have a couple of educational projects I'm working on, such as explaining why you don't need a polar scope for people who didn't take all that vector math stuff in high school because it didn't apply to "real life" <vbg>, and improving on my wind-blown daytime polar alignment video. Editing video takes forever, so here's a blog with something useful right off the bat.
There are some things you just know will work, but you really should "try" them, and try them more than once (the essence of science is repeatability!).
I decided to do a rigorous review of the one star polar alignment technique, and evaluate it's performance quantitively for both photography and visual astronomy. For night #1, I setup a 150mm f/7 refractor on the MYT and put a DSLR on the back. The refractor weighed more than the mount (35 lb.), and is probably representative of what a portable imager might be willing to lug into the field. I setup the tripod, leveled it... and was careful to not be too careful.
Imagine it's dark, there are crying bored children, and a wife complaining about being bitten by bugs (this is my own experience... I'm not being sexist, I know there are plenty of women interested in astronomy...just not the one I married ;-).
I cleared the sync using the polar alignment routine on the iPad, and centered Capella after dark using only the altitude and azimuth adjustments on the mount. In order to simulate a visual experience I did this all by eye though the view finder of the DSLR, I did not do an Image Link (other than the iPad, I had no computer hooked up at the time!).
At this point for visual astronomy, I should be ready to go, and if I'm guiding I should be ready to go. "What's that?" Didn't you run TPoint first and dial in the polar alignment?
Yes and no. I did in fact run TPoint, because I wanted to know just how accurate my somewhat clumsy, potentially rushed attempt at polar alignment really was. I did not make any mount adjustments however. A recurring theme in the forum and in personal emails and conversations is that running TPoint "takes too long". I want to know if I can tell people, "Fine... just don't run one then!". The Paramount's have homing sensors, which makes the one star alignment technique possible (just how with all the vector and coordinate frame stuff will be the subject of a future blog... but in layman's layman's (sic) terms... look, 10 - 7 = 3, and 7 + 3 really does give you 10 back.
Sometimes I think we do a bunch of TPoint samples and adjust our mounts out of some perceived need that isn't really there (I have the power, I MUST use it!). I've even seen some of our users bragging about how many samples they ran before they started imaging...
I'm not sure that's helping either. I'm only shooting at 1050mm focal length, and I'm guiding. How close to the pole do I really need to be? Well, let's see how close I actually am, and see how well it does tonight. I did an automated 97 point TPoint run. After running Super Model, I pulled up the polar alignment report. With Super Model I had 21.2 arcsecond pointing, yet I was off a whopping 27.7 arc minutes in azimuth, and 20.7 arc minutes in altitude. (18.8 ticks and 11.3 ticks respectively on the calibrated adjustments).
This sounds like a lot (about half a degree), but remember we are guiding. Running TPoint is great, it's even fun, and there is a satisfaction to knowing how close you are to the pole. But it can easily become an unnecessary expectation and burden if you're in a hurry. Again, a common theme on the forums is "how long it takes"... well it took about 15 minutes to setup the mount with the camera, and maybe less than five to futz with my one star alignment. The 97 point TPoint run was not needed, it was just so I could have a metric for how off I could get away with. What I did next will surprise a lot of people.
I did NOT make the recommended adjustments to the mount. I left it be nearly 1/2 a degree off the pole. Is that close enough to guide? Of course it is! I shot 5 minute subs with an f/7 refractor on an uncooled DSLR. See for yourself the image of the Pleiades, including the close up crop, but not resampled image showing very small round tight stars. I did a single 10 minute sub, but decided for 5 as I was overexposing too much around the bright stars. I've also included a snapshot of the guider graph... the guider was not working that hard (to be fair, note I did have a PEC table loaded - thanks Tom Bisque for doing this for me ;-), and aside from a few spikes from the dither operations, it's pretty decent.