Double Star Sketching and Digitizing – Page 1 of 4

Table of Contents

1. Introduction
2. Step 1 – The Observation
3. Step 2 – Sketching The View
4. Step 3 – Scanning, Inverting and Cleaning Up the Sketch
5. Step 4 – Softening Star Edges
6. Step 5 – Adjusting contrast
7. Step 6 – Adjusting White Star Cores
8. Step 7 – Adding Color
9. Step 8 – Adding Additional Glow

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.
[Edit – March 01, 2008: Note that in late 2007, I began using a Meade Astrometric Eyepiece with a few modifications to bring some control to my PA and separation measurements. A step-by-step article describing my current process with this eyepiece can be found here: Measuring Double Stars.]

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
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
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

Figure 4

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