Bump up Those Stars

After publishing the article on the 600 Rule (and why it’s a lousy rule), I was heavily prodded by a reader who insisted that a 4, 5 or six pixel star streak is not noticeable and not to worry. My counter argument is: “it all depends!”  A larger print (greater magnification) will reveal the streak.  But that got me to thinking – and dangerous things happen when I start thinking.

How Do I Boost the Visibility of My Stars?

The obvious answer is to bump up the brightness and the contrast – but that creates other problems – including magnifying the noise. But there is a simple way to boost those stars – at a relatively low image quality cost.  And this method may very well remove some of the dash like appearance of a star in a longish exposure.

Are you ready? In Photoshop duplicate the layer. Select the duplicate layer. Activate the Move tool. Hold down the control (windows) or option (Mac) key and click the up (or down) arrow key exactly once. Photoshop calls this maneuver a “nudge”  Now change the blend mode to Lighten, 100%.

What do you get? Here is the original.



Now watch as we perform the “bump” (cursor over the image to see what was bumped)

[acx_slideshow name=”BumpUp”]

Why Does this Trick Work?

Actually, your first question might be: why doesn’t my image get blurrier? The answer is that it does but you probably won’t notice even if you print the image LARGE. You can counteract blurry by masking off everything that is not sky in the nudged layer.  The reason that the stars seem to be brighter is that you’ve added more bright pixels in nearly the same location. The eye sees more light where every star is. If you look closely you may also notice that some other features get brighter too – the snag (dead tree) at the far right and the waterfall. To see the change in foreground details compare the Original with the Up One images.  Then compare the Up One with the Down One. In the Down One image I masked off any area that is not in the sky and as a result the tree and waterfall brightness do not change.

Perhaps you’re asking a different question like:

PS My thanks to Tiberiu Tesileanu whose questions and comments lead me to experiment with the “bump” strategy.

17 thoughts on “Bump up Those Stars

  1. Steven Christenson

    Boosting stars within the Milky Way is not something I recommend, by the way. It makes more sense to pull off the technique when there is strong light pollution that reduces the contrast as in this example:

    Radiometric Alignment [C_043302]

  2. Marsha

    Steven – thanks for sharing this great tip. I am going to reprocess a few of my shots taken during the meteor shower to see if I can get those stars a little brighter through the light pollution. Why do you not recommend doing this w/ the MW?

    1. Steven Christenson

      I don’t recommend the bump up technique with the Milky Way because it won’t make the “milky” part more prominent, just some of the brighter stars in the Milky Way and to me that visually disrupts the allure of the Milky Way.

  3. Cynthia Farr-Weinfeld

    I just tried this with a shot from this past weekend and I hate to say it, but it really made my image look blurry. I blew it up just to 50% and it looked horrible. I’m not sure why it worked for your images. Weird! I love your tutorials and your astrophotography as well! Cindy

    1. Steven Christenson

      Dear Cynthia:
      Thanks for likin’ us!

      I’d love to see the image. If got really blurry, it’s likely that the “bump” was too far. It’s possible that for some reason the “nudge” operation works differently on your version of Photoshop. I notice sometimes that my one or two pixel move goes about 8 pixels which will make a mess. I zoom down to the pixel level and watch as I move the top layer. I probably should have shown an extreme close up in my article.

  4. Cindy Farr-Weinfeld

    Steven, where would I send you a copy of the picture? Do you have an email address I could send it to? I’d love to be able to try this technique. And I have really been enjoying your blog and tutorials. You are a fabulous photographer–I love that you aren’t just doing astrophotography but astrolabes apes–much harder I think! You’ve got to think composition as well as lighting and exposing for the stars! Cindy

    1. Steven Christenson

      You can drop the images somewhere and post the link here. For example you could upload it to your website and not feature it in a gallery so that its not visible except to someone whom you provide a link to. You have some stunning images yourself on your web!

      Thanks for your praise!

  5. Robert McCadden

    For star trails, when your pointed north, it may be worthwhile to experiment with the transform command in lieu of nudging.

    When you pull up transform command (ctrl+t), there is a little bulls eye that appears in the middle of the image. The bulls eye specifies the center of the transformation. If you were to place the bulls eye on Polaris and then specify a small positive or negative percentage for the horizontal and vertical amount, I believe that the result would be that you would get a concentric offset vs. the purely horizontal or vertical offset you get while nudging. This may work better for circular trails.

    I may experiment with this myself this evening.

    1. Steven Christenson

      But if you rotate you’re going to cause many other problems including interpolated pixels… and you won’t be using lighten mode since overlapping pixels won’t add anything to the result. You’re also assuming that the rotation will be exactly circular which it probably won’t be due to lens distortion. What you’re describing is a reasonable approach to filling gaps, however, and it is also why “bumping” works pretty well for small gaps.

      Finally, there is no rule that says you can’t bump in EVERY direction – as I showed in the article above. The soon to be released Advanced Stacking Action has an automatic “bumper” tool.

  6. Pingback: Advanced Stacker Plus – It's HERE! | THIS SITE IS NOT LIVE!

  7. Juan

    hello, and thanks for the tips.

    However, I have been unsuccessful to achieve it. I am stuck at the ‘click the up/down arrow’. Which arrow is that?

    I am using PS6. I moved the clone layer onto the lower one, saw no arrow anywhere applicable, changed blend mode… but no effect whatsoever.

    What am I missing? Thanks in advance,

    1. Steven Christenson Post author

      We amended the article to say “up (or down) arrow KEY“… hopefully that makes clear. You are literally MOVING the layer. Please also take a look at the note from Cynthia above. The behavior of the arrow key (how far the nudge nudges) depends on how much you’ve zoomed in on the image.

    1. Steven Christenson Post author

      This technique does not produce banding. If you have “banding” something else is going on. To give an example photo, cut and paste a link to a place you’ve saved your image (e.g. Flickr or SmugMug, etc.). If you want to get really fancy, you can use <img src=”http://yourpathhere”/> In your comment (but better to give a link!) For example if on flicker the “download arrow” produces the desired “text” to insert in your comment. For example:

      <img src=”https://farm6.staticflickr.com/5507/12624059343_1b56735bcd.jpg”>

      Shining Star

  8. Ryan

    What zoom level are you at in photoshop when you do this?

    It seems that one arrow click in photoshop moves the layer a different amount depending on how zoomed in you are. For instance when fit to screen (12.5% zoom) one arrow click moves the layer by quite a bit, blurring the image. When zoomed to 100% it moves a significantly smaller amount that does not blur the image, but has little noticeable effect on brightening the stars unless you are zoomed in (even when doing both a layer for “up” and a layer for “down” combined).

    1. Steven Christenson Post author

      Good question. Unfortunately the “amount of zoom” depends on the image size and the size of the screen you’re using to view the image, so the best we can say is: zoom in enough that the shift is from one to three pixels.


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