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Double Star Sketching and Digitizing Tutorial

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#1 Jeremy Perez

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Posted 24 August 2006 - 07:37 PM

Introduction

This tutorial covers the methods I have been using to create some of my recent double star sketches. For the digital portion, I will discuss methods used in Adobe Photoshop CS2. These techniques should translate pretty well to earlier versions of Photoshop as well as The Gimp. The double star I use as an example is Struve 3053--a surprising and beautiful yellow and blue pair in Cassiopeia.

Step 1 - The Observation

Aside from soaking up and enjoying the view of the double, the key things I want to record as accurately as possible are the position angle of the stars, their separation, their relative magnitudes, any discernible color, and the context of any surrounding stars.

The first thing I make note of is color. For me, the impression of color is strongest during that first look through the eyepiece. (Even under conditions where I'm never strongly dark adapted.) So I'll immediately note what colors stand out. I start with looking for the basics: Red to Orange to Yellow to White to Blue and shades in between. Sometimes, as you may have noticed, the contrasting colors a double throws at you don't follow the rules. So from there I'll just throw a description right back at it that reminds me of something I'm familiar with, like 'key lime pie green'. I haven't brought colored pencils or pantone swatch books into the equation for this. I just try to be as descriptive as possible with any colors that don't fit the mold.

When it comes to estimating position angle, I have to lay some groundwork. I don't own a filar micrometer, so my estimates aren't of any scientific use, but I try to get as close as I can. To do this, I try to mark cardinal directions around the sketch as carefully as I can ahead of time. Since I have an equatorial telescope with a polar alignment scope, I'll perform a polar alignment before observing. Then all I need to do is nudge the declination axis back and forth a couple times to note North-South. If you are using an alt-azimuth scope or otherwise won't be doing polar alignment on an EQ scope, paying careful attention to East-West drift of stars through the field will also allow accurate placement of cardinal directions. With those directions noted around the sketch circle, I move on to estimating position angle of the stars based on an imaginary clock face.

Estimating the separation of the double isn't a precise science for me either. For widely separated stars, I just try to determine what fraction of the field of view separates them: one-tenth...one-twentieth & so on.

Step 2 - Sketching the View

Once I've spent some time observing the double, I start to sketch the double star and any stars I see in the view using a 2H pencil. The reason I like to use a harder lead at this point is that it allows me to lightly plot the stars so that the faint stars are nice and delicate right from the start. It also makes any need for erasing a lot easier at this stage if I decide I need to correct something. After the stars are plotted where they belong, I use that same 2H pencil with a little more pressure and swirling to bulk up the boldness of the brighter stars. If a particularly bright star demands it, I'll use an HB weight pencil. This is of course my personal preference. You may prefer to use a #2 or HB pencil exclusively, or even pen and ink. Use what you are comfortable with. (See Figure 1)

Figure 1

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#2 Jeremy Perez

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Posted 24 August 2006 - 07:38 PM

I prefer to attribute the colors of the primary and secondary in my written notes. If the stars are so similar in magnitude that it's hard to tell which is primary or secondary, I will use cardinal directions to describe them (e.g. the northeast star vs the southwest star). If you have a view that is fairly heavy with stars and you are seeing color in other places, you may need to resort to noting color directly on the sketch. In this case, I will lightly abbreviate a color notation next to the star, such as YO = yellow-orange, LB = light blue, etc. When you scan and clean up the sketch later, you will have an opportunity to clone out/erase that notation from the image.

Step 3 - Scanning, Inverting and Cleaning Up the Sketch

With the sketch finished, the next step is to scan it. For a few thoughts on scanning sketches, see the Scanner Peculiarities section of my Digitizing Tutorial. You can scan either as RGB color or as Grayscale. But if you intend to add color, you will need to convert a grayscale file to RGB somewhere before step 5. If you scanned as an RGB color file, be sure you don't have a color cast in your image--such as a yellowish tint from the paper. If you do have a color cast, a quick fix is to convert the image to grayscale, and then convert it back to RGB.

Next, you will need to 'invert' the sketch which will give you a black background with white stars (see Figure 2). In Adobe Photoshop CS, this command is found under the following menu: Image > Adjustments > Invert.

Figure 2

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#3 Jeremy Perez

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Posted 24 August 2006 - 07:40 PM

Next, I like to do a little clean-up on the double star members, and perhaps on some of the brighter field stars. I try to find a balance between cleaning up any obvious imperfections in the stars I sketched (see Figures 3 and 4), and making them too perfect. Right now I'm not trying to produce purely digital sketches. I still like to keep some of the hand drawn appearance wherever I can. To clean up, I use the 'clone' tool (a.k.a. 'rubber stamp tool'). For a description on how to use this tool, see the Image Cleanup section of that same Digitizing Tutorial.

Figure 3

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#4 Jeremy Perez

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Posted 24 August 2006 - 07:41 PM

Figure 4

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#5 Jeremy Perez

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Posted 24 August 2006 - 07:42 PM

Step 4 - Softening Star Edges

To lay the groundwork for adding color to the stars, I like to soften their edges a bit. The coloring method I use causes color to adhere to varying shades of gray, but leaves the white areas alone. If the star has a pretty hard edge from pure white to pure black, there won't be much area for the color to get a foothold. So I'll grab the 'blur' tool and give it a light setting of around 10% pressure. Then I zoom in fairly close to the star and run the blur tool around its perimeter a few times (see Figure 5) until it takes on a softness something like you see in the Figure 6.

Figure 5

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#6 Jeremy Perez

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Posted 24 August 2006 - 07:42 PM

Figure 6

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#7 Jeremy Perez

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Posted 24 August 2006 - 07:48 PM

Step 5 - Adjusting contrast

I usually like to adjust contrast and brightness issues at this point. That's not to say you shouldn't care for it earlier or later if you would like. But at this stage, you are far enough along to see what will happen to the soft edges you've created, and it will give you a good start for the next steps.

To make this adjustment, I suggest avoiding the "brightness/contrast" command, since it doesn't offer as much control. I personally prefer to use "Curves" to make these adjustments, but this may be more power than you need, particularly if you find the interface intimidating. A good middle of the road tool is "Levels". See The Levels Dialog in the Digitizing Tutorial for pointers on using this tool.

Bring up the "Info" palette to see how light or dark different portions of your image are getting. Hover the cursor over areas of interest to see what the info palette reads. If each of the RGB values reads '0', that equates to pure black. If each of the RGB values reads '255', that equates to pure white. Values in between note varying shades of gray. If the values are not equal, then there is a color cast to the image. Once you add color later, this will be evident near any stars you have colored.

The goal I have when doing this is to brighten up the core of the boldest stars to pure white (this will give a value of "255" in each of the R, G, and B fields). (See Figures 7 and 8.) The cores of smaller/fainter stars don't have to reach pure white though. I also try to adjust the darkness of the background to just a bit lighter than absolute black. For these double star sketches, I aimed for values of around 5 or 6 in the info palette. (When I'm working on sketches with nebulosity, I tend to brighten that value up to 12 or so...even up to 20...since that can help show the fainter reaches of nebulosity on a variety of computer monitors.)

Figure 7

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#8 Jeremy Perez

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Posted 24 August 2006 - 07:48 PM

Figure 8

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#9 Jeremy Perez

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Posted 24 August 2006 - 07:49 PM

Step 6 - Adjusting White Star Cores

While shaded gray areas on the fringes of stars are important for picking up the color you are going to add, pure white cores allow you to convey brightness. In the previous step, you made the cores of the bolder stars pure white. The bolder and brighter the star, the more pure white I like to have visible in the center. As you examine the image, if you need to expand that white area a bit, get a soft paint brush and adjust its size to allow you to softly paint white into the middle of the star (see Figure 9).

Figure 9

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#10 Jeremy Perez

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Posted 24 August 2006 - 07:50 PM

On the other hand, if the core of the star needs to have that white area constricted, you can use the blur tool to blur the inner edges of gray until they start to bleed further and further into the center (see Figure 10). You can also use the 'Burn Tool' to very softly darken the area.

Figure 10

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#11 Jeremy Perez

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Posted 24 August 2006 - 07:51 PM

Step 7 - Adding Color

Now comes the fun part--aw heck, it's all fun, right? Anyway it's time to colorize the stars as you described them in your sketch or notes. Grab a soft paint brush that is slightly larger than the star, set the paint brush to "color" mode, and give it an opacity of 20% or so (see Figure 11). Next, whip up the color you want to apply to the star. I've found that I like to set up a swatch palette of the most frequently used colors that I can just pick from when I need it (see Figure 12). With the color ready, move the brush over the star and click one or more times to dab the color on. More clicks will give you stronger color (see Figures 13 through 15). Lower the opacity on the brush further if you need to get a very subtle color. Since the paint brush is in 'Color' mode, it won't brighten or darken anything. It will just colorize the existing values.

Figure 11

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#12 Jeremy Perez

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Posted 24 August 2006 - 07:51 PM

Figure 12

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#13 Jeremy Perez

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Posted 24 August 2006 - 07:52 PM

Figure 13

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#14 Jeremy Perez

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Posted 24 August 2006 - 07:52 PM

Figure 14

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#15 Jeremy Perez

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Posted 24 August 2006 - 07:53 PM

Figure 15

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#16 Jeremy Perez

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Posted 24 August 2006 - 07:53 PM

Step 8 - Adding Additional Glow

This last step is a bit of a personal preference. I like to add an additional soft glow around brighter stars to enhance the sense of brilliance. To do this, select a soft brush that is 2 to 3 times the diameter of the star. Set it to 'Screen' mode at a very low opacity of around 5 or 6%, and click over the star a few times. You should notice a large but subtle brightening begin to form around the star (see Figure 16). As you continue to click, this glow will get brighter (see Figure 17). Be careful not to take it too far. The fainter the star is, the smaller you will want the paint brush to be, and the fewer times you will need to click the color in (see Figure 18).

Figure 16

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#17 Jeremy Perez

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Posted 24 August 2006 - 07:54 PM

Figure 17

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#18 Jeremy Perez

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Posted 24 August 2006 - 07:54 PM

Figure 18

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#19 Jeremy Perez

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Posted 24 August 2006 - 07:55 PM

Once you are done with this step, you can back up to a 100% view and see how it looks (see Figure 19).

Figure 19

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#20 Jeremy Perez

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Posted 24 August 2006 - 07:55 PM

FInally, you can add any additional details to the sketch, such as cleaner cardinal direction markers and other details about the sketch. With that taken care of, your sketch is finished (see Figure 20).

Figure 20

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#21 Jeremy Perez

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Posted 24 August 2006 - 07:57 PM

Be sure to check out this post. It discusses double star sketching and contains an excellent, illustrated tutorial by Eric Graff showing the methods he uses to digitally prepare colored double star sketches. There is also a continuously updated treasure trove of excellent, digitally colored sketches by asteroid7 in the double star observing forum that needs to be seen to be believed.

#22 stevecoe

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Posted 24 August 2006 - 11:42 PM

Jimmy;

Great stuff, thank your very much for sharing your expertise and putting this tutorial together. I am a Paint Shop Pro 9 user, but I am certain that much of this will be useful.

Like you, I give myself a written description at the eyepiece and I use a "quick peek" method so that I don't stare at the star for too long. I will look at a constellation or just close my eyes for a few seconds and then look at the star again for a few seconds. Once I am confident, I will write some notes.

Thanks again;
Steve Coe

#23 desertstars

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Posted 25 August 2006 - 02:06 PM

This is a great addition to our 'how to' material in the Sketching forum!

For the time being I've rendered this thread "sticky" so it will remain in sight for a while, regardless of the traffic it receives. In time, I will add it to the Best Of 'How To' thread that is permanently stuck, where newcomers will have ready access to it. :grin:






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