Eric Graff from Cloudy Nights contacted me recently and raised some excellent questions about a few double star sketches that I posted last week. They touched on concerns that I've been mulling over since I posted them. You see, the Astronomical League double stars I've been working on are filled with a lot of pretty wide doubles that can be scaled easily into the sketch circle. But when I got around to a particularly interesting double, Zeta Aquarii with a separation of 1.8", I realized that my attempt to sketch the reality of a tight double star wasn't as straightforward as I had deluded myself into thinking. To quote Eric regarding his own tight double star struggles, they "look like two little balls squashed into one another". So from an aesthetic standpoint, it doesn't represent the eyepiece view very well. But what about overall accuracy?
Well I was pretty much already convinced that the Zeta Aq. scale was way off. The text I included with the report was "...The sketch is deceptive. To indicate brightness, I drew the star circles bold. This made me have to push the stars apart to keep them separated. So they were actually tighter and closer in the 240X FOV than they appear in the sketch." It lead me to take a look at some other reasonably close doubles I had sketched. 94-Aquarii was one of those. The separation for this double is supposed to be 12.7", but after measuring my sketch, I drew it as 28". I'm not saying I want to bring micrometer accuracy to my sketches, but to the extent I can either avoid blunt inaccuracies, or at least be aware of them, I want to know what I'm up against.
So I decided to quantify the situation. I fired up Adobe Illustrator and drew a 1.75 inch sketch circle. I then added a hatched scale divided into 720 units across the circle to represent 1" divisions in the 720" diameter of my 240X view. I don't suppose I should have been surprised at how tiny those divisions are, but I was still a bit overwhelmed. I next used the scale to draw a .9" diameter circle to represent the size of the airy disc for stars in my 6 inch scope. Then I drew a second star and positioned it 1.8" from the first to represent the view of Zeta Aquarii. When I printed the sketch circle, I was treated to a slightly elongated speck. Hah. Nice. So I doubled the size of the sketch to a 3.5 inch diameter circle and printed again. This time, there were two tiny specks touching each other. So I moved up to a 5.25 inch diameter circle and printed again. Now there were two distinct specks separated by a tiny speck of white paper. In all cases, the size of these specks was tinier than I could reliably make with even a very sharp pencil, mechanical or otherwise. And forget about scaling the star weights to indicate brightness.
For the heck of it, I imported my original Zeta Aquarii sketch and measured it against the scale I created. In that sketch, the diameter of the stars themselves works out to 14" each, and they are separated by 21". Heheh. So I've pretty much come to the conclusion that if I don't want to sketch on a foot-wide sketch circle, I'm not going to be able to produce "true" field-of-view double star sketches for close companions. Not to mention how indecipherable they would look when uploaded to the web.
So far, I've come up with a couple options. 1) I can shake on the delightful salt of impressionism and just acknowledge in the observation notes that the close double is "not to scale" compared to other stars in the field of view. Or 2), I can imagine a one-twentieth scale circle in the middle of my eyepiece when making an observation and sketch to that. The thing is, those tight doubles sometimes work nicely with other widely spread stars in the field, and option 2 would exclude those. I think I'll work with option one when necessary and see how it goes. But I'm interested in other thoughts on the approach to sketches that run past the limits of pencil and paper.
One thing is for sure, I'm going to be a lot more conscious of how fat I scale the width of bright stars in a close double.
Here is a link to a pdf (260K) of my double star scale sheet in case you're interested in zooming in at 1600% or printing it out.