Sketching close double stars
Posted 20 September 2005 - 11:42 PM
Eric G. (Cildarith) contacted me recently and raised some excellent questions about a few sketches that I posted in the double star forum 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.
I'm attaching a pdf (260K) of my double star scale sheet in case you're interested in zooming in at 1600% or printing it out.
Thanks for your thoughts.
Posted 21 September 2005 - 05:27 AM
Excellant posting. I just downloaded your PDF file to see if I can use it with some of my double star sketches.
Posted 21 September 2005 - 06:03 AM
I don't know how useful that pdf would really be for sketching purposes, other than to demonstrate how exceedingly tiny an accurate double-star sketch would have to be for something in the 2" separation category. Let me know what you think.
Posted 21 September 2005 - 07:12 AM
That's really cool! Interesting, the method you've figured out for getting a more accurate way to get DS's rendered.
Posted 21 September 2005 - 10:15 AM
Posted 21 September 2005 - 11:40 AM
Ideally, I would like to sketch double stars at a scale of about 1mm/1"; of course this would require a 792mm circle (over 2.5 feet(!) in diameter) with a star like Zeta Aqr a pair of tiny dots separated by 1.8mm at the center.
No, I don't think so, either!
Here is my recent attempt at Zeta Sagittae (separation a healthy 8.6")...
Posted 07 October 2005 - 10:12 AM
Here is my attempt to have my cake and eat it, too!
The main drawing shows the field of view, while the inset represents a circle about 1 arcminute in diameter, centered on the primary.
Posted 07 October 2005 - 10:14 AM
Posted 07 October 2005 - 10:43 AM
Frankly, I think the discussion of scale is not as important as should be a discussion of rendering the accuracy of the image. I sketch double stars in my observing log books. They are grossly oversized. I might draw a 2" double with it's companion in a drawing that ends up measuring 1/2 inch across, just to represent the two stars accurately. But what makes the sketch accurate is the position of the secondary and the distance placement of the secondary in relation to the first diffraction ring. Or it could be the components are so close that they overlap and in this case accuracy is determined by precisely sketching the percentage overlapping disks. What's important here I think is that the accuracy of any sketch can be determined by the mathematics used in the physics of optics. There are some very precise measurements that can be determined about the view of stars in any scope and this comes in handy when viewing doubles. And magnification or field of view have nothing to do with it.
First would be the skecth has got to have NESW so an estimate of the position angle can be made. It's helpful if the observer makes that estimate and shows it in the data. Sometimes we read reports from people that attempt to observe really relly close doubles and they make an estimate of the PA that is off by 30-40-50 degrees. That raises the question did they really see the secondary or not.
Second is along with a larger bright spot for the primary and a fainter bright spot for the secondary, the sketcher should at least make an attempt to sketch the first diffraction rings. This single piece of information tells you so much about the scope used and the separation of the double that it is equivalent to having a measuring scale inside your scope view.
For instance, for a sketch that shows a 2 arcsecond double as viewed in a 100mm scope, if the observer shows the secondary outside the diffraction ring, we know either the sketch is inaccurate or the scope was larger than 100mm or the double was wider than 2" and maybe the observer was looking at the wrong star.
How do we know all that from a simple skecth? Because based on diffraction formulas, for a 100mm scope, the center of the first diffraction ring is located at almost exactly 2 arcseconds and the secondary should be dead center on that diffraction ring.
Of course, since diffraction is dependant on wavelength, any information the observer could list on his sketch about the spectra of the components will help veryify the accuracy of the sketch.
For doubles with separations on the order of 3" 4" or more, the sketch may be showing the secondary at some distance outside the first diffraction ring, and it should be fairly easy to determine the distance of the double in the sketch based on a knowledge of the aperture and difraction. In fact, this holds true for any scope size and any double separation, although its usefulness soon drops off after separation get larger, ie. 8", 10".
Of course, sketches should include all the data like scope, magnification, star mags and sep, etc.
Just a few thoughts, after having seen lots of sketches of doubles stars posted, but many sketches that really don't give you much information. I'm only trying to provide suggestions that would help confirm the accuracy of any sketch. There may be many observers whose concern is simply rendering an artistic skecth of the view. But if accuracy is what you're after, these are considerations you probably need to keep in mind to make double star sketches more accurate.
FWIW, I've been able to make some estimates of separtions of sub arcsecond doubles based on my sketches. Also, I've been able to accurely determine vigneting in some small scopes and determine effective aperture was much smaller than the primary lens, all based on the placement of the secondary in relation to the difraction ring. And those skecthes show two stars about a half inch across.
Posted 07 October 2005 - 11:08 AM
Ed, those are all excellent points, particularly about including diffraction rings to lock in the scale of the view to the separation of the stars and the diameter of the scope's primary. On my observing sheets, the sketch is not placed conveniently with regard to the data section to scan the whole thing and encumber folks with an unnecessarily large jpg. So I've been including that data as html text along with the sketch. Do you think (from the standpoint of people browsing the web) that this could be a potential handicap to the value of a sketch to have the sketch as a smaller jpg and the data as html text? I'm already considering revamping my sketch template to work with the inset style demoed by Eric, so maybe some other considerations are in order.
Posted 07 October 2005 - 01:07 PM
Those are awesome, so realistic sketches, complete with diffraction spikes and all !
Posted 07 October 2005 - 01:08 PM
Posted 24 October 2005 - 02:26 PM
Sketches: STF 2445, STF 2455, STF 2457, Albireo, STF 2540, Zeta Sagittae, Theta Sagittae, STF 2653, STF 2769, 1 Pegasi, STF 2841, STF 2877, STF 3007, STF 24, 55 Piscium, 65 Piscium, 36 Andromedae, Psi-1 Piscium.
Comments and suggestions are most welcome.
Posted 24 October 2005 - 03:26 PM
This is like walking in to the party just as the piñata gets busted open. Those are beautiful. That 36-And looks like it was a real stinker. Very nice work capturing the 'impression' of those doubles.
Posted 24 October 2005 - 04:43 PM
Posted 25 October 2005 - 05:20 AM
I havn't looked at all your sketches but from what I've seen they are absolutely beautiful. Even the dark gray background looks quite realistic. You should post them on the Double Star Forum.
Posted 09 November 2005 - 08:45 AM
What method are you using that allows such a realistic rendition of the doubles (color, halo etc.)?
Thanks for any information you can provide.
Posted 10 November 2005 - 11:34 AM
At the eyepiece I sketch the field and label the stars in order of decreasing brightness, estimating their magnitude and their color. Generally I limit myself to the brighter field stars, the ones I use to anchor the sketch. From this field sketch I create a finished sketch indoors after the session:
Posted 10 November 2005 - 11:35 AM
Posted 10 November 2005 - 11:38 AM
Posted 10 November 2005 - 11:39 AM
Posted 10 November 2005 - 11:40 AM
Posted 10 November 2005 - 11:41 AM
Posted 10 November 2005 - 11:42 AM
Posted 10 November 2005 - 11:43 AM