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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Wow, so how many months has it been since I said I'd post a tutorial about this? :: flashes procrastinator's badge :: I'm going to split it up into 7 entries so I don't break the bulletin board. I hope you find it helpful 
So you've sketched a few DSOs, and you would like to scan them and post them to the web on a forum or gallery or maybe your own site. You can definitely make it as straightforward as you want--scan, crop, resize, save, post. I know there are times when that will work, but what if you are having trouble getting that hint of nebulosity to show up on the monitor? What happens if you invert the sketch so the sky is black and the stars are white? How do you deal with the variability in people's monitors? What about introducing color? I've been playing around with these issues on my own sketches since I started posting them here, and I'd like to share what I've found out so far.
What you will need
- Access to a scanner. (Or a digital camera--more on that later.)
- Access to some form of image editing software, especially one that allows you adjust the tone of the image by means of a 'levels' or 'curves' tool.
- A certain level of familiarity with your software to access tools for the following tasks: Cropping, Image Resizing, Cloning (Rubber-stamp tool), Painting Tools. I use Adobe Photoshop, and software details will be based on this program. If you use another image editing program, you will need to cross-reference the Photoshop tools to their counterparts in your image program. Note that in some places I will mention that the option/alt key needs to be held down. This means hold down the option key if you use a Macintosh, or hold down the Alt key if you use a Windows PC.
A couple free image enhancement programs for Microsoft Windows are:
PixInsight (As far as I can tell, PixInsight wil not let you make selections, or selectively edit to clean up or colorize parts of your images.)
Gimp (An excellent program. Available for both Windows and Mac OSX, but requires developer tools for an OSX installation.)
At the end of the tutorial, I will include some basic information about what tools in PixInsight and Gimp correspond to Photoshop. Both programs do well with global image enhancement, but Gimp is better at selective editing, cleanup and colorization. Many of its tools operate in a very similar fashion to Photoshop, and so would be easier to compare with the tips in this tutorial. I'd call Gimp a most worthy free substitute for Photoshop.
Scanner peculiarities
There are a few things you can do when scanning to increase the quality of your sketch images.
- Be sure that you keep your sketch pressed as flat as possible against the scanner glass. Any uneveness can result in ripples showing up in your scan when you try to bring out very faint nebulosity.
- Scan to a size and resolution that will cover all your potential uses for the scan. It is better to scan to a higher resolution and scale down later than to try the other way around. For example, I typically scan my entire record sheet to keep as an archive in case anything ever happens to the paper original. To that end, I scan the whole sheet at 300 ppi (pixels per inch) in RGB mode. I then save the page as a "Very High" quality JPG which often ends up as a 2-3 Mb file. I extract my sketches from that later and reduce their size and resolution individually to work for the web.
- If your scanner has an auto-calibration tool, by all means use it...unless its broken, in which case don't use it. I like to scan my images in full color, even though they are pencil sketches. I feel it adds a little bit of personality to the sketch to have some slight color variation in it to start with--even though I mess with this later. However, I try to make sure the scanner gives me the scan as close to neutral as possible. If your scanner is consistently giving you off-color images that are overly yellow, green or otherwise, it may be a good idea to just scan in Grayscale mode to begin with. You can always convert that grayscale image to RGB later for any colorization you wish to do. This will give you a perfectly neutral image to work with, which is better than trying to get rid of a bad color cast if you aren't experienced with that sort of thing.
- After you scan an image, keep an eye out for clipped highlights or shadows. Thats a way of saying to watch out that you aren't losing subtle details in the lightest lights and darkest darks. If you sketch with pencil on white paper, the highlights are going to be your potential problem area. If you sketch with white chalk or pencil on black paper, it will likely be the opposite problem. Here is an example of what you want to avoid:
Figure 1: Image with no clipping problems (M 92)
Figure 2: Image with clipped highlights
Figure 3: Image with clipped shadows
If you are having these problems, you need to adjust your scanner's gamma/brightness/contrast settings to alleviate it. Here's where I have to refer you to your trusty (hated?) scanner manual. If your scanner is fussy and you can't fix the hilights without really darkening the image, then go ahead and scan darker (as long as the shadows don't get clipped  . This is something you can fix in an image editing program. You cannot fix clipped hilights or shadows.
- Give some consideration to what file format you want to use to save your original resource document. This is the file you will come back to later to extract and optimize your sketches for the web. There are "lossy" file formats (e.g., JPG) and there are non-lossy formats (e.g., TIF, PSD, EPS). Lossy file formats sacrifice some level of image quality to make your file sizes much smaller. JPG will give you a choice: Smaller file size = Big loss in quality or Larger (but still manageable) file size = Minimal loss of quality. Non-lossy formats will not sacrifice any image quality, but you will have larger files to deal with. Since I keep 300 dpi, letter-sized scans for all my reports, I prefer some compression. Not that CD-ROMs aren't affordable, but big files can be a pain to transfer around. And frankly, the fewer CDs I need to keep track of, the better. So I like to save in JPG format using "Very High" quality (Level 10 in Photoshop). For these type of images, I haven't noticed any quality issues that concern me in the slightest. Try out some file formats and see what you think.
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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To invert or not to invert
Another question you will want to consider is whether or not to invert your scanned image if you are sketching on white paper. For the purposes of this discussion, the word "invert" refers to changing a positive image to a negative image and vice versa--not to mirror-image issues that arise from the use of a telescope that has an odd number of reflections, such as an SCT with a diagonal.
For reference, here is how I will refer to images:
Figure 4: Negative (M 34)
Figure 5: Positive
So for the purposes of this discussion, when you sketch with regular pencil on white paper, your a producing a negative image of what you see in the eyepiece. Inverting that image will produce a positive.
Many amateur astronomers prefer to view astronomical images as negatives to better see faint details. I have noticed the usefulness of this myself with very detailed astro photographs at my desk, as well as in the field under a red light. It is easier to see faint details on a negative sky image. You may prefer to present your sketches that way as well.
As you may have noticed, I present mine as positives. I do this for a few reasons (that I don't claim are better for everyone, just that they are my preference):
- The level of detail in my sketches is not currently ;-) of astro-photograph caliber. So I don't see as much value in presenting them as negatives. In other words, the signal to noise ratio of my sketches doesn't seem to warrant the level of scrutiny that a negative image will stand up to. Particularly considering the following--
- With some basic adjustments to a positive image in an image-editing program, fainter details can be brought out well enough to show up on a variety of monitors. If you don't take care with the shadow values in your image, you risk having some of your viewers miss dim details if their monitors aren't well calibrated. The odds are that if somebody's monitor is going to cut off details at either end of the luminosity scale, they are most likely to clip off the shadow values, leaving highlights relatively safe. This can be a very good reason to leave your sketches as negatives. But I like to take some extra steps to convert to positive format and work to give it a better chance of surviving overly dark/contrasty monitors because of this next item--
- A positive image can convey very nicely what you would expect to see through the eyepiece. This isn't to say I can't appreciate the view a negative sketch provides, but a positive sketch just draws me into the experience much more fully. I think a lot of it is an aesthetic value. There is knowledge to be gained in everything I see in the eyepiece, but I am also highly drawn to the subtle, glowing beauty of these deep sky wonders. I think that a positive sketch gives me the best opportunity to convey that feeling to others. I hope that I can eventually encourage people to take a gander themselves if they realize that they can drink up that sort of view their own eyes. Besides that, it reminds me most closely of what I saw myself when I go back later and browse through the images.
- One additional bonus of a positive sketch image is that it allows me to come back and represent any color I noticed in the eyepiece. (yes, there is color to be seen out there. It ain't Hubble-color, but it's there :-) Positive images can be very receptive to adding this extra bit of detail.
The tutorial I give here will focus on positive images. If you prefer to present your images in negative format, you should still be able to glean some information here. Most of the image enhancement information will work for negative images--just in reverse.
Sizing your image
A concept that needs to be nailed down is one of image size. For web presentation, the two numbers you want to be most concerned with are width and height. These numbers are stated in pixels. You will want to decide what size best conveys the details of your sketch online without going so large that you inconvenience your viewers. That inconvenience can come from files that are too large to display fully in their browser, requiring annoying scrolling to view all portions of the image. Overly-large images can also discourage users with dial-up connections who will have to wait an undesirable amount of time for your images to load. On the other hand, if you make your images too small, it may become difficult for viewers to appreciate some of the finer details of your sketches. I would suggest taking a look at sketches from different amateur astronomers online and getting an idea what might work best for your sketches. On my site, I've gravitated to images that are 300 pixels wide for DSOs, and images that are 250 pixels wide for double-stars (which don't require as much resolution to convey the view). Below are samples of the same image at different sizes to give you an idea of what you might expect to see:
Figure 6A: 300 Pixel Wide Sketch (M 35 and NGC 2158)
Figure 6B: 200 Pixel Wide Sketch
Figure 6C: 150 Pixel Wide Sketch
Figure 6D: 100 Pixel Wide Sketch
Figure 6E: 400 Pixel Wide Sketch
In order to get your image to the desired size, you will need to first crop it to get rid of anything unnecessary. In Photoshop, you use the crop tool to mark a rectangle around the area you want to keep and then type Enter/Return.
You will then need to scale the image. In Photoshop, go to the Image menu and select "Image Size...". When the Image Size dialog window (Fig. 7) appears, be sure that the "Resample Image" item is checked (Fig. 7, D). This checkbox allows you to change the pixel dimensions of the image. If your version of Photoshop gives you the option of choosing the resampling algorithm (Fig. 7, E), I would suggest "Bicubic Sharper"--it better maintains the integrity of fainter stars. Next, be sure that the "Constrain Proportions" item is checked (Fig. 7, C). This makes sure your image scales proportionally--that it doesn't get overly stretched or compressed in one dimension compared to the other. Now enter a number in the width or height field (Fig. 7 A & B) that you have decided will be your desired image size. The other dimension field will change value to match proportionally. Click OK and see the results.
Figure 7: Image Size dialog
Your image is now at the correct size, and you are ready to optimize it.
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Dealing with a color cast
Take a look at your sketch and be sure you don't have a bad color cast to it. If it has a color cast and you aren't comfortable doing color balance correction, the easiest thing to do is convert it to grayscale. Then convert it back to RGB. This will give you a purely neutral image, that you can now edit for color to your taste.
Figure 8: Image with color cast
Figure 9: Image with color cast that has been converted to grayscale and then back to RGB color mode
Inverting the image
If you have decided you want to present your images as positives, your next step is pretty simple. Go to the "Image" menu, then to the "Adjustments" sub menu, and then select "Invert". This will give convert your sketch to a positive that might already bring back more vivid memories of your eyepiece view. But wait. It gets better. We will now do some cleanup, contrast and brightness adjustments that will bring out the subtle details and glow of your sketch. If you already sketch white on black, or if you prefer to leave your images as negatives, then you will skip this step.
Figure 10: Original negative sketch (M42, M43)
Figure 11: Sketch inverted to be positive
Adjusting luminosity and overall color
The next thing we want to do is lighten the black background of your sketch a bit. This will allow people to better view the softer, subtler details of your sketch. There is a trade-off that happens here. The stark contrast of a pure black background is very striking, and if you are dealing with open clusters, this can work very well. But if you have any nebulosity or unresolved starlight you are trying to convey, the last transition to black can get lost. By lightening the entire background, we raise the threshold of that visual contrast and we accommodate some of the darker monitors out there.
Figure 12: Open cluster sketch, unlightened (M 37)
Figure 13: Open cluster sketch, with lightened background
Figure 12: Nebula sketch, unlightened (M 16)
Figure 13: Nebula sketch, with lightened background
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Adjusting luminosity and overall color (Continued)
Now, I'm going to make a suggestion that you don't have to follow. You can either lighten the background of the entire image, or you can choose to lighten just the area inside your sketch circle. I prefer to lighten inside the sketch circle only. The reason is that as long as I'm doing this, why not take advantage of the fact that the view through the eyepiece usually shows some glow to the background sky compared to the non-visual part of the eyepiece. (I don't own a Nagler, so I don't know what it means to not have a non-visual area in my eyepiece ;-) I leave it up to you. I think it makes up a little bit for the fact that you no longer have that striking pure black background. I like consolations.
Selecting just the circle can take a little bit of fiddling. First, grab the oval selection tool. Then move your cursor as close to the center of your sketch circle as possible and hold the option/alt and shift keys and start dragging out a circle. (The shift key constrains the oval to a perfect circle. The option or alt key causes the circle to originate from the center rather than diagonally from one side.) Your circle probably isn't perfectly centered, but size it as close to the size of your sketch circle as possible and then drag it around until it is centered. If it is too big or small, you can redraw it, or you can go to the "Select" menu, then to the "Modify" submenu and choose either "Expand..." or "Contract..." and enter 1 or more pixels in the box to enlarge or reduce your circle. Once the selection circle encompasses your sketch circle, you are ready to work some magic on the image. If you aren't sold on the idea of adjusting only the circle, then you just saved yourself a little bit of time. 
Figure 16: Dragging the selection circle from the center outward
At this point, we have a choice of two prominent means of adjusting the tonal range of your image: Levels or Curves. I prefer to use curves, because they provide an incredible amount of flexibility in how you adjust the values of your image. But because the learning curve (eheh) is heavier, we'll keep the discussion to the use of Levels. In a later tutorial, I'll address curves. They are incredible.
Before I forget--now would be a good time to grab your sketch pad or observation record sheet so you can compare the tonality of the digital image to what you tried to convey with pencil and paper at the telescope. Refer to it as you adjust the image to decide what enhancements you need to focus on.
OK, back to the computer. Go to the "Image" menu to the "Adjustments" submenu, and select "Levels..." You will now see a composite histogram that shows where the tonal values of your image reside. At the bottom of the menu is a shaded bar that goes from dark to light (Fig. 17, A) that represents which side of the histogram represents which shades. In the large window above you will see what looks like a black mountainous profile (Fig. 17, B). If your image is a positive DSO sketch, that mountain should be piled up above the dark portion of that shaded gray bar. The reverse will be true of a negative DSO image. This is telling you what proportion of the pixels in your image are black, white and whatever shade of gray in between. The taller the black mountain, the more pixels there are at that part of the tonal range.
Figure 17: Levels dialog window
The first thing we want to do is change the low value in the "Output Levels" section (Fig. 17, C). Right now it says "0". This means that the darkest dark possible is a perfect black of zero. I would suggest raising it to a value between 20 and 30. If you are working on an image with nebulosity, you may be pleased to see an immediate improvement in the visibility of dimmer portions of the image. You have just brightened the darkest darks a small amount, allowing any slight increase in brightness from absolute black to be easier to see. Depending on how light or dark your overall scan was, you may need to lower these numbers. You don't want your sketch to look like it is plagued with light pollution.
Figure 18A: Shadow output level set to 10
Figure 18B: Shadow output level set to 20
Figure 18C: Shadow output level set to 30
The next step is to play with the three arrows beneath the histogram. We will treat them separately below.
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Adjusting luminosity and overall color (Continued)
The black slider
The black slider (Fig. 17, D) is the touchiest one, and has a very limited use in my experience. Its job is to mash all the almost-black values down to black--or whatever not-quite-black value you just entered in the "Output levels" section above. Go ahead. Move it around and see what happens. The only use I would have for this is if your scanner has given an uneven shade to your background that you want to even out. Sometimes I will see a very slight ripple pattern in my background caused by dew wrinkles in the paper showing up in the scan (remember, holding your paper flat on the scanner will make you happy later). If I adjust this value up a tiny amount, I can mash the lighter portion of those ripples down to the level of the rest of the background. But there is a sacrifice to be made. You will lose some of your subtle nebulosity in this action. You will need to make a judgement here as to whether the sacrifice is worth it. In some cases I keep the ripple because I don't want to lose some very soft shading that is important to the sketch. If you have the time and you really want to craft the image, you can use some advanced techniques to deselect your nebulosity and only flatten out the rippled background. I will not be diving into that here. (You may now roll your eyes in mock gratitude
Figure 19A: Image demonstrating background wrinkle problems. Wrinkles have been emphasized to demonstrate the problem. Normally they wouldn't be this pronounced, although still noticeable. (M 79)
Figure 19B: Image demonstrating removal of background wrinkles by moving black slider slightly to the right.
The white slider
On the opposite end of the histogram, the white slider (Fig. 17, F) is a little safer to use. Its job is to pump all the almost-white values up to pure white. Go ahead and move it around and see what happens. This is where you get to use some craftsmanship to decide how brilliant you want your stars, since they are what will be most heavily affected. This will cause slightly grayish stars to become white, and it will also begin to lighten up nebulosity--although you will fine tune that next with the gray slider. I can't give you a formula for this. You will need to decide, based on your sketch and your memory, how bright the stars should be.
Figure 20: Image before adjustment with white slider
Figure 21: Image after adjustment with white slider to brighten whites of stars. The difference is subtle here. I didn't have to move it much, but you may notice a slight brightening overall in the nebula.
The gray slider
In the middle of the histogram is the gray slider (Fig. 17, E). It's job is to determine what range of values in your image will become 50% midtones. If you move it toward the white end, the pixels become darker. If you move it toward the black end, the pixels overall become lighter. You will use this primarily to adjust the brightness of nebulosity. If the nebulosity appears too bright and blunt, move the slider toward the light end to darken it (Fig. 22). If it is too dim, move the slider toward the dark end to brighten it (Fig. 23). Pay close attention to your sketch and try to achieve the effect that best represents what you drew.
Figure 22: Image after moving gray slider to the right to darken middle values, thus making the nebula less blunt--more subtle
Figure 23: Image after moving gray slider to the left to lighten the middle values, thus making the nebula more prominent. Overall I don't like this move for this sketch, although it does bring out the comma shape of M43 better.
Setting overall color
This next touch is optional, but it's something that I feel gives a little life to my sketches. As I make my sketches, the glow from most deep sky objects is colorless, but when I step away, I inevitably am left with a feeling a blueness to the light. I'm sure this is a figment of my tormented imagination, but I find that it brings a luminous glow to my sketches if I tinge them slightly with blue. This is a purely aesthetic call, and here's how you can do it if you are interested. At the top of your levels dialog box, there is a "Channel" menu (Fig. 17, G) that reads "RGB" if you've been working on a color image the whole time--you have been doing that right? If not, click OK in the dialog, change your image to RGB mode, and then re-open the Levels dialog. Now, go to that Channel menu and switch from "RGB" to "Blue". Next, grab the middle gray slider and move it a bit to the left. You should see the image take on a blueish hue (Fig. 24). Don't go overboard though, or you'll blow it for us all. You can try this on other channels if you'd like. If, for whatever reason, you want to convey what effect a particular eyepiece filter has on the color of the view, you can convey that using the various channels of the level dialog box. Just be sure you note what you're up to below your bright cyan sketch of M8, so people don't think you're trying to fake them out.
Figure 24: Image after moving gray slider of blue channel slightly to the left to increase blue in the image
Now it's time to click OK and move on. You may also go to the "Select" menu and click "Deselect" to get rid of those annoying marching ants.
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Image cleanup
If you are like me, your sketches will have some unintentional flaws. I leave it up to you whether you want to "Go Commando" and leave those rough edges in for a more raw expression of your work. I respect that. (You buy organic don't you ;-) Now, if you do want to tidy up a bit, there are some things to look for: not-quite-round stars, accidental pencil drag marks, stars you have crossed out for later deletion, unintentional specks on the scanner glass (really watch for these--compare to your sketch--you don't want specks in there that aren't actual stars you drew), etc. When correcting these problems, I like to use the clone tool (rubber stamp). It offers the best chance of painting out problems without leaving a trace. Using a regular paint brush can work, but you risk leaving brush marks if not all areas of your background are perfectly smooth and the same color and shade.
Grab your clone tool, and select a brush size for it that is appropriate for the size of the corrections you want to make. Usually, the smaller the better. Give it a soft edge if you can. Now go to all those areas that need to be removed. Hold down the option/alt key and click a spot near your correction (Fig. 25) that has the same shade as you want to appear in place of the bad spot (this is called "sampling"). Release option/alt, and click on the schmutz (Fig. 26). Be sparing in your clicking. Do only as much as you need to remove all traces of it. For stars that aren't quite round, zoom in nice and close and carefully clone your way into it to get rid of any oblongness or streakiness. Pay attention to how soft or hard your clone brush is here, so that the correction doesn't make one half of the star fuzzy and the other end solid. Try to keep it consistent. Work your way around the sketch, and when you are done, zoom out to a magnification of 100% and assess the situation.
Figure 25: Clone Sampling. Holding down the option/alt key activates the sampling target. Click the mouse button to target that area.
Figure 26: Cloning out stray marks. The crosshair shows where the clone data is being retrieved from.
Adding spot color
At this point, you are ready to add any color you noted during your observation. I'll cover this in two parts. Nebulosity color, and star color.
Nebulosity color
If you noticed any nebulosity color, start with this first, since it will wipe out any overlapping star color. I have seen greenish hues in M42, blue-green in M57, and cyan color in Comet Machholz. If you are fortunate enough to have a large scope, you will probably have even more opportunity to note and employ color in nebulous objects if you so desire.
First, grab the paint brush tool, and select a very soft brush that is smaller in size than the nebula you sketched. You want to allow some level of detail as you paint. Try a size that is 1/4 the diameter of your nebula to start with, and see how it works. Next, click on your foreground color square in the toolbox and select the color that best matches what you observed. When you look at the color square before you, I would advise keeping the color pure and choosing from an area at the top of the square. Click OK for the color selection. Now change the mode of your brush to "Color" and the Opacity somewhere from 1-10%. Now start to brush in color on the nebula (Fig. 27). Each time you release the mouse button and overlap brush strokes, you will make the color more intense. Be careful with this. I like to hold the mouse down for my entire first brush over the whole nebula, then I come back and intensify some of the brighter areas (Fig. 28), leaving the dimmer areas with less color intensity. The undo key is your friend here. Don't be afraid to use it. It can be easy to overdo the color.
Figure 27: Applying overall light color with a large brush.
Figure 28: Applying more overlapping color with a smaller brush in the brighter areas.
Star color
When you are ready to apply color to any notable stars, grab a brush that is slightly larger than the star. Make sure the brush is in "color" mode, and set the opacity from 50-100%. Now go to the foreground color selector and choose the appropriate color. I now keep star colors in my swatch palette for simplicity and consistency (Fig. 29). Add them to your swatch palette as you make them. Next, move your brush directly over the star and click once or twice to dab the color on (Fig. 30). If it is too intense, undo the action, change the opacity to a lower number and try again. If you drew diffraction spikes on your brighter stars (you are my hero), use a low opacity and brush along the spikes (Fig. 31). Overlap some strokes along the spikes closer to the star so the color gets more intense there.
Figure 29: Photoshop swatch palette with commonly used celestial colors.
Figure 30: Adding color to a star with a soft paintbrush set to color mode at 50% opacity.
Figure 31: Adding color to diffraction spikes with a soft paintbrush set to color mode at 10% opacity.
What about sharpening?
I used to sharpen my sketches to help pop the stars out better. But I began to notice that this took some of the life out of the sketches. They felt overly-flat to me. That certain amount of natural softness is what gives the stars a glowing quality. So I no longer use the Unsharp Mask filter. That's not to say it can't have it's uses, but I would be a bit wary that it doesn't do more harm than good.
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So that's it. Or is it? You may have other things to care for with your sketch, such as labels or rollover animations or who knows what. But for the scope of this tutorial, your image is now ready to save and upload to the web.
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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A note on saving
Photoshop will allow you to save JPG files through the normal save menu, but using the "Save for Web" menu can substantially decrease the file size of some images. I would encourage you to use this option to treat your visitors to shorter load times, and help save disc space and bandwidth on your web server.
Further note on using a digital camera to import your sketch
If you don't have access to a scanner, but you do have a digital camera, you can use it to photograph your sketches. I've done this on a couple occasions, and there are some things to watch out for. Uneven illumination, focus, color cast, and clipped highlights or shadows.
Uneven illumination
This can be the most frustrating issue with digital camera shots. If you shoot indoors under artificial illumination, I doubt very much you will get an evenly illuminated sketch, unless you have some great studio lighting. I would try taking your photograph outdoors. Ideally, try it on a cloudy day. This will produce soft, even, mostly neutral illumination. But who has time to wait for the clouds--oh wait. Just plan to observe that evening. It should cloud up pretty quickly But, ok, if you don't have clouds, try shooting it in the shade and in full sunlight. Compare the images and see which is more agreeable to your camera and your image software. Be sure that you attach your sketch flatly and securely to a board. Avoid wrinkles, especially in direct light. Also, make sure the board is angled as perpendicular to your camera as possible and that the center of your sketch is as perpendicular as possible to an imaginary ray coming from your lens. This will help you avoid perspective problems and uneven illumination.
Figure 32: Aligning camera perpendicular to surface of sketch.
If despite your best efforts, you still have uneven illumination, there are artificial steps that can be taken to remove light-to-dark gradients that run across your sketch. You may find some helpful pointers in astro-photography forums and websites. See discussions about generating "flats"...particularly "artificial flats". It's not a simple process, but it can be done.
Focus
A working scanner is always going to be nicely focused on anything pressed against the glass. Cameras are obviously more touchy. Take special care that your camera is focused squarely on your sketch. If you have control over the aperture of your camera, bring it up to the highest f-stop you can. This will increase the depth-of-field and give you a more forgiving range of focus. The biggest help though will be the use of a tripod. This will prevent any camera shake from blurring your sketch. (It will also help you keep the camera perpendicular to the center of the sketch.) If you don't have a tripod, try to rest yourself and your camera on something solid when taking the picture. Try to work hard at keeping the camera perpendicular to the center of your sketch.
Color cast
When shooting this way, it is very easy to get a color cast to your sketch. It can either the blue of the sky, or of any predominant nearby color that is reflecting sunlight on your outdoor 'studio'. If you aren't comfortable correcting this in your image editing software, then convert it to grayscale to neutralize it completely before converting back to RGB.
Clipped highlights or shadows
Until you get used to how your camera performs in this situation, shoot a few different exposures for each sketch, to be sure you don't have to settle with one where the highlights are blown out and you can't see any subtle nebulosity, or where your shadows are plugged up, and you can't fix the flat blotches in the center of your galaxies. See Figures 1-3 for examples of what to look for.
I hope you have found this tutorial helpful. If anything seems unclear or you have any questions, please let me know.
Jeremy
Reference info
PixInsight controls
Photoshop Crop Tool Counterpart: Works pretty much the same as in Photoshop
Photoshop Resizing Counterpart: Go to the "Image" menu, to the "Geometry" submenu and select "Resample". In this dialog, make sure the "Preserve aspect ratio" option is checked. Enter your new pixel dimension in either the "width" or "height" field beneath the "pixels" heading. After typing "enter" or "tab", the other dimension field will change to the correct value.
Photoshop Color Spaces Counterpart: To change from RGB to Grayscale and vice versa, go to the "Image" menu, and go to the "Color Spaces" sub menu.
Photoshop Invert Command Counterpart: Go to the "Image" menu and select "Invert"
Photoshop Oval Selection Tool Counterpart: None that I'm aware of
Photoshop Levels Counterpart: Use the "Histogram" palette. Here, you can still use the black, gray and white sliders as in Photoshop Levels. The low value of the "Output Levels" in Photoshop corresponds to the "Low" field in PixInsight's Histrogram palette. The number you enter here however will be a negative value that you will need to test to see what is the best number. To adjust individual color channels, click the appropriate "R", "G", or "B" button. To get back to adjusting the composite channels, click the "RGB/K" button.
Photoshop Clone or Paint Tool Counterparts: None that I'm aware of
Gimp controls
Photoshop Crop Tool Counterpart: Go to the main Gimp tool palette and select the "Crop or resize the image" tool. (looks like an X-acto knife)
Photoshop Resizing Counterpart: Right click in the image, go to the "Image" menu item, then click on the "Scale Image" submenu item. Make sure the chain-link icon is linked together beside the Ratio X/Y fields. Enter your new pixel dimension in either the "New Width" or "New Height" field. After you click enter or tab, the other dimension field will change to the correct value.
Photoshop Color Spaces Counterpart: To change from RGB to Grayscale and vice versa, right click on the image, then go to the "Image" menu item, and then go to the "Mode" submenu item and select the desired color space.
Photoshop Invert Command Counterpart: Right click in the image, go to the "Image" menu item, then go to the "Colors" sub menu item, then select the "Invert" option.
Photoshop Oval Selection Tool Counterpart: Go to the main Gimp tool palette and select the "Select Elliptical Regions" tool. (looks like a dotted circle) The shift key performs the same here as in Photoshop, but instead of Alt, use Ctrl to center the circle on the origin of the mouse click.
Photoshop Levels Counterpart: Right click in the image, go to the "Image" menu item, then go to the "Colors" menu item, then select "Levels". This dialog works exactly the same as the Photoshop Levels dialog from this point forward.
Photoshop Clone Tool Counterpart: Go to the main Gimp tool palette and select the "Paint using patterns or image regions tool". (looks like a rubber stamp). Brush sizes may be selected from the "Brush Selection" palette. Opacity is controlled by the "Tool Options" palette. Use the Ctrl key to take your cloning sample.
Photoshop Paint Tool Counterpart: Go to the main Gimp tool palette and select the "Paint fuzzy brush strokes" tool. (looks like a paint brush) Select colors from the foreground color box in the main Gimp tool palette (pure colors will be on the right side of the color box, instead of the top). To change the mode of the paint brush to color, go to the "Tool Options" palette and select from the "Mode" menu. To change opacity of the brush, go to the "Tool Options" palette and change the "Opacity" slider. I would advise changing the brush cursor style to a crosshair by going to the main Gimp tool palette, going to the file menu and selecting "Preferences". In the Preferences dialog, go to the Interface section and click the "Image Windows" item. At the bottom of this dialog, go to the "Cursor Mode" item and change it to "Crosshair only".
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Orion SVP 6LT (6" f/8 Newt) || Orion XT8 (8" f/6 Newt) || 15x70 Oberwerk Binoculars
The Belt Of Venus || Astro-Sketch Gallery || Astro-Sketching Resources || Astro-Photo Gallery
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oldsalt
Astro Philosopher
Reged: 02/12/05
Posts: 7960
Loc: Pa - between starparties
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Jeremy,
Great tutorial. It hits all the high lights of getting your images into the computer and onto the web. I've been using Photshop for quite awhile, and hadn't really given much thought to how i was processing my sketches. Its one of those things like riding a bike or driving a car. you know what to do and how to do it, but try explaining the process. It gives one pause to stop and think about the process, and perhaps ways it can be improved.
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Special Ed
Post Laureate
Reged: 05/18/03
Posts: 3545
Loc: Greenbrier Co., WV 38N, 80W
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Quote:
Wow, so how many months has it been since I said I'd post a tutorial about this? :: flashes procrastinator's badge :: I'm going to split it up into 7 entries so I don't break the bulletin board. I hope you find it helpful
Jeremy, you've done such a thorough and excellent job that all procrastination is forgiven.
Thanks,
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Michael Rosolina
8" f/10 Orange Tube SCT
4.25" f/4.2 Astroscan Reflector
SVP 3.6" f/13.6 CA Reflector
40mm PST f/10
APM Germany HD 15x70 binoculars
Canon 12x36 IS II binoculars
Mark I Eyeball
My CN Gallery
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desertstars
Deja moo
Reged: 11/05/03
Posts: 30056
Loc: Tucson, AZ
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It was worth waiting for. Oh, that doesn't do it justice by a long shot. Okay, how about this: The post is now sticky, so it can be found, enjoyed, and used as easily as it deserves!
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Tom W.
SVP8 'She turned me into a 3-legged Newt' EQ
Ralph, the All-Purpose 102mm Refractor
Under the Desert Stars
Alcohol and calculus do not mix. Please don't drink and derive.
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kraterkid
Post Laureate
Reged: 03/07/05
Posts: 3881
Loc: Poway, California
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Hi Jeremy,
You're a well spring of information! Thanks so much for this level of care, those Photoshop tutorials are just wonderful, I had no idea I could make many of the adjustments you mentioned! Everyone here benefits from your experience, and BTW that Orion Nebula image is a great sketch and a choice as a subject in your tutorials, given the wide range and subtle variation in tones this object presents to the eye. It was very interesting to see how the M42 image could be enhanced to bring out a wider range of nebulosity. I've only scanned through this fine post, but I'm going to print it out and use your technique for my final image adjustments. Wow! 
Thanks!
Rich 
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Rich
[image]http://www.cloudynights.com/stars/CNS0606.jpg" border="0[/image]
My CN Gallery
[image]http://cleardarksky.com/c/JBObCAcs0.gif" border="0[/image]
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half meter
Postmaster
Reged: 05/05/04
Posts: 12518
Loc: Great Lakes
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Wow! There's a professional fee due you somewhere, somehow for that set of posts, Jeremy!
Thank you very much!
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope DS 90 <0.5A w/BF30
152 mm f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
20" Obsession/OMI Mirror/Servocat/Argo Navis
First Light for the 30" Obsession at BEOTS!
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gparkerson
Vendor (Astronomy Technology Today)
Reged: 01/21/05
Posts: 962
Loc: Louisiana
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Jeremy,
Thanks very much for the comprehensive tutorial. I've only recently developed the courage to try sketching and hadn't a clue where to go and what to do to get images from paper to computer. With your help, I hope to disprove what they say about old dogs.
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Gary
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Erix
Toad Lily
Reged: 12/25/04
Posts: 20482
Loc: Ohio, USA
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Jeremy, thank you so much for that tutorial...it is very much appreciated. I can't get over how thorough and professional it is and never dreamed so much could be done after a sketch to add that extra pizzazz. I sometimes have so much trouble getting the scanned image to accurately capture what I've sketched...this will certainly help out not only in that regard, but in the overall outcome of my sketches.
Thanks again!
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Erika
10" LX200 Classic, ETX70-AT, DS Maxscope 60mm, 12" Truss Dob, Orion ED80, WO Binoviewers, 10x50's and 7x50's Binoculars, Rebel XT 350
Having Fun in the Sun!
More solar fun: 2007 July - tracking NOAA10963
Support bacteria. It's the only culture some people have.
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Carol L
Reged: 07/05/04
Posts: 5880
Loc: Tomahawk, WI 45N//89W
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Thanks Jeremy, excellent work!!!
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 *Step-by-Step Lunar Sketching*
CN Gallery
Photo Gallery
8"SCT ~ 120achro ~ 90Mak ~ 80ST ~ 11x70s ~ 22x100s
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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Hello all,
Thanks for the comments. Obviously, there are a number of ways to skin a cat (possum), and this is just one. My current project is trying to figure out the best way to deal with planetary sketches. Particularly regarding the addition of color as seen at the eyepiece. I've been trying, and only had limited success. I can see the advantage of leaving those as black and white :: runs off to drool over Sol's sketches ::
Anyway, there sure is a lot of inspiring work to gander at here, and I appreciate every bit of it.
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Orion SVP 6LT (6" f/8 Newt) || Orion XT8 (8" f/6 Newt) || 15x70 Oberwerk Binoculars
The Belt Of Venus || Astro-Sketch Gallery || Astro-Sketching Resources || Astro-Photo Gallery
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cildarith
Carpal Tunnel
Reged: 08/26/04
Posts: 2122
Loc: San Diego, CA
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Thanks, Jeremy! Now I need to get some "real" software (and learn how to use it, LOL).
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Eric
6" f/6 Parks Newtonian
10x50 Bushnell Binocs
CN Sketch Gallery
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Jeremy Perez
Carpal Tunnel
Reged: 08/12/04
Posts: 1677
Loc: Flagstaff, Arizona, USA
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heheh. woah, I thought you were serious there for a second Eric Whatever you're doing, don't stop doing it!
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Orion SVP 6LT (6" f/8 Newt) || Orion XT8 (8" f/6 Newt) || 15x70 Oberwerk Binoculars
The Belt Of Venus || Astro-Sketch Gallery || Astro-Sketching Resources || Astro-Photo Gallery
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Anonymous
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