Category Archives: Camera

De Streaking

In a recent article I explained how you can bump up the brightness of stars using a simple layering trick in Photoshop.  In this article we will be going in the opposite direction. Instead of increasing the contrast of stars our goal will be to reduce the streaking that results from long exposures or exposures at a long focal length.  For background on streaking, please refer to the article The 600 Rule which as rules go is a pretty bad rule.

Streak Reduction Procedure

The original image, cropped.

For the streak reduction procedure we only need one image.

  1. Open the document in Photoshop.
  2. (Noise reduce as needed – method to be described in a column coming soon)
  3. Duplicate the background (Ctl-J).
  4. Change the new layer blend mode to Darken.
  5. Select the move tool to “nudge” the top layer in the direction of the predominate streak (in this case up one and right two).  It helps to zoom (Z) pretty far before making the nudge.
  6. Apply levels and curves to brighten the image and correct the color. The color correction process is described in this article.  The easiest way to apply an adjustment layer is to view the Layer tool (key F7) and click the adjustments icon at the bottom as shown here:

    To make it easier to see what you are doing, you can apply a curve to lighten the image. Make sure to drag any temporary adjustment layer to the top of the layer stack.

The nudge operation may result in foreground elements being noticeably duplicated like the tree tops below:

Background plus nudge foreground, unmasked.

Fortunately it is easy to undo that duplication by masking off the area by painting black on a layer mask.  Here is the same darkened layer combination, but I have created a mask and painted out the area around the duplicated tree tops. To create a mask click the icon that looks like a camera on the Layer Tool – it’s just to the left of the adjustment icon shown earlier.

Tree duplication has been masked off here.

This image looks better. Now we might want to apply the bump operation using the combined background and nudged darken layer.  There are three ways to get the layers combined: use flatten image, select the two layers and select layer-merge, or – and this is a really useful trick, use the keys: Ctl-Alt-Shift-E (that’s how I got Layer 2).  On the Mac you use Option rather than Alt.  Two ways to get to the flatten or merge operations include clicking on the tool bar under “Layer” or clicking the tiny little menu icon at the upper right of the layer tool window.

After a little bump:

Caveats

This destreak method works best if you are not using a wide or ultra wide view. With a really wide view, the star streaks are not uniform. The closer the stars are to the north or south poles, the more curved their streaks will be.

In another article I will walk through how you can composite together images taken at different times or exposures, as was done below.

DeStreaking [C_049387+406]

Dark Frames And Your Night Photography

In an upcoming webinar (Down with the Noise) I explain a lot about noise: causes, contravention and cures. This is a bit of a prelude and addresses the questions:

  1. What is a dark frame?
  2. What do I do with my dark frame?
  3. What do I do if I don’t have dark frame(s)?

What Is a Dark Frame?

A dark frame is one or more images taken at the same exposure length, ISO and ambient temperature as the light (normal) frames but with the lens or body cap on the camera to prevent any light from reaching the sensor. When doing many kinds of night and low light photography dark frames can be quite helpful. And when doing star trails or other night imagery dark frames may save your bacon. A dark frame is what your camera does after a long exposure when long exposure noise reduction is turned on. But you’ll be far more efficient if you take those frames yourself. If you’re taking 100 light frames, e.g. for a star trail, you can take 3 or four dark frames and waste 50% less time (and not have gaps!)

Contrary to popular belief dark frames and long exposure noise reduction do little to reduce the random noise that is present in every exposure. That random noise is most pernicious in dark photos and shadow areas.  Dark frames, however are good for the following things:

  1. Reducing or eliminating hot pixels and amp glow
  2. Removing any “bias” in your image – that is bringing the black back.
    Want me to translate that: an unexposed area on your sensor should read as “0,0,0”  for Red, Green and Blue but I will bet you you don’t get zero!

This would probably be a good place to show you what a dark frame looks like. But you’ll be disappointed. Dark frames are usually quite black.  So instead of showing you JUST the dark frame, here is the dark frame boosted to show the speckles from hell – though they may not be obvious. Here I have made the speckles more obvious by boosting the darks using Curves in Photoshop. At this level of detail there are not any obvious hot pixels.

Dark Frame Overview – Boosted to show details. Note where the markers are – they are shown in the next frames.

And next is the same dark frame zoomed to 3200% unaltered. Hover your cursor over the image to see the same area boosted using curves.

Single Dark Frame (Linear Mode) Unmodified – or cursor over to see boosted version.

When inspected carefully, and with the dark level significantly increased it is possible to notice the hot pixels and possibly banding in a dark frame.  While there were plenty of red speckles and obviously green and blue as well in my stable of dark frames, the “hot pixels” didn’t leap out at me.  If you look carefully at the image you’ll notice I also used the color sampler tool to provide RGB values for 3 different locations on the image.  Of note is location 1 where the R (red) value is 14.  What is particularly worrisome about that value is that even after the entire frame has been boosted to the equivalent of 1.6 stops, you’ll notice that a value of 14 is still larger than all three colors at spot 3. After boosting, that red pixel really stands out with a value of 44.  Our first take away is that boosting the brightness boosts the noise.  The second thing to notice is that the red spot is NOT a hot pixel.  How do we know? Compare 4 dark frames (all boosted)

Boosted Dark Frame                             Click these–>   Frame 1 ~ Frame 2 ~ Frame 3 ~ Frame 4

Takeaway 2: There really is randomness!

Now that we have noticed the randomness, we realize that if we average enough of these frames together we can get the average “bias” – that is the amount of offset above zero in the image.  And if there are hot pixels, the good news is they will be in there too.

But How Do I Use a Dark Frame?

The simplest answer is to feed your dark frame(s) to a program that already knows what to do with them like StarStax, StarTrails or Image Stacker.  But you can do it yourself, and perhaps more elegantly using Photoshop.  How?  Place the dark frame as a layer over the image you want to correct and change the blend mode to Subtract (or difference). Adjust the opacity of the blend until it looks just right.

But I Did Not Take a Dark Frame, Now What?

All is not lost. If you have enough frames you can create a unique kind of dark frame. I took over 300 28-second exposure for a star trail along Lake Gaston in North Carolina.

In the image below I created the top frame using the Brighten mode in StarStax. I could just as easily have created the top frame using the StarCircleAcademy Stacking Action.

I used Darken mode to create the middle frame by feeding it my 100 darkest images. Using Darken mode as the stacking option means that hot or stuck pixels that are in every image as well as the lowest value of sky glow will be collected into a single result.

I then loaded the light (Brighten Mode) and the dark (Darken mode) frames into Photoshop. I placed the dark image over the brighten stack and changed the blend mode to Subtract.

What Happened Here?
Dark Frame Substitute process

Several interesting things happened:

  1. The hot pixels were almost completely annihilated
  2. The sky gradient caused by lights glowing in the distance was also almost eliminated.
  3. The contrast in the sky and elsewhere was improved
  4. The red bias on the railing was mostly removed.

A few less desirable things happened, too. The bright red glow on the railing once subtracted caused some of the railing to turn green. And the subtraction created some “holes” and “halos” in the image – especially where the brightest lights are found.  With some minor touch up, most of those issues can easily be fixed.

Is this the end? By no means! There are a LOT more interesting techniques to follow. Stay tuned.

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Astrophotography Equipment Recommendations – Beginner to Intermediate

For basic astrophotography I recommend starting with a wide angle lens and a sturdy tripod.  That’s it. Go out there and get some Milky Way or starry sky shots. Take plenty and average stack them (after aligning them). More on this later.

To image things like the moon, planets, galaxies and nebula you’ll want to move up to a decent telephoto lens (200-800 mm effective focal length) and an Equatorial Mount.

Mounts

To my thinking there are 4 categories of mounts with their approximate prices and assembled total weight (excluding telescope or camera):

  • Light, single drive (e.g. the AstroTrac, $900, 15 lbs, the Polarie or the SkyTracker)
  • Cheap ($189) and probably useless to decent but limited AstroView Equatorial $350, 26 lbs.
  • Mid-range, accurate with features like autoguide ports, and GoTo: Celestron CG-5GT, $690, 42 lbs; Orion Sirius, $1150, 43 lbs; Orion Atlas, $1400, 76 lbs. All are heavy!
  • High end: A hefty hunk of metal with a hefty price point: e.g. Celestron CGE Pro, $4,400, 154 lbs.

In the examples I’ve shown mostly equipment from Orion for three reasons:

  1. I have Orion equipment and they have a local store.
  2. They have a good reputation for being helpful and consumer friendly
  3. Their website makes comparisons easy!

The Portable Solution

The best portable solution is clearly the well made AstroTrac with the power cable, finder scope (upper right) and the drive at the bottom.

MaierAstrotrac

To use this you need several other bits and pieces shown here excluding a standard camera tripod.

MaierAstroTracKit

It’s a well engineered, portable system. All the gear together (including tripod, drive, camera, telephoto lens, batteries, etc) is about 16 pounds – meaning you can carry it with you. The next closest equatorial drive solution is about twice that heavy.

The cost is a minimum of $680 for the drive, polar scope and power cable. But you’ll need some additional head components (about $210), a power supply of some kind ($30) and perhaps a sturdier tripod. The total outlay will be under a thousand making it comparable to the low end of the mid-range mounts.

PROS: The AstroTrac is easy to set up, and relatively easy to align if you use the geared heads and the polar scope. You can pack it in a suitcase or a backpack and take it on an airplane!

CONS: More expensive than a single drive equatorial mount. Only drives one axis (all that is generally needed). Maximum tracking time is about 2 hours. Repointing the camera may misalign the drive. Need to build or buy a 12V battery pack (though this is easy to do). Need to learn your sky to find things.

The Equatorial Mount

Go cheap, go big, go fancy… but you’re not going light.

OrionEquipmentRec

The AstroView – which I have – requires drive motor(s) for another $130 or so bringing the total outlay to about $380. It’s carry weight is about 35 pounds if you include the camera, and all accessories including counter weights.

PROS: Inexpensive, includes polar scope, lighter of the many mount options, can support modest refractor or small reflector. Tracks well.

CONS: No guide port, limited to about 12 pounds of capacity, no “GoTo” option so you have to learn your skies to use it well. Tripod is thin aluminum. It’s sturdy but may not hold up to extended use.

A step up from the entry level mount would be something like the SkyView Pro ($850) It includes a “GoTo” computerized control which is a great help to the novice and helps you with alignment routines. I’d probably opt for the Orion Sirius ($1150) however as it supports 10 more pounds (30 total) and for that extra $300 bucks you also get a polar scope, the ability to use a decently large telescope and fancier drive options. A highly recommend mount is the Celestron CG-5GT at about $690 add $50 for a polar scope. All of the GoTo mounts will “slew” (move rapidly and accurately) from one object to the next and you can enter the object into a keypad to get there. Save even more money by using your computer instead of the “GoTo” unit.

Attaching A Camera to A Mount

If you opt for a telescope mount, you will want to consider using a ball head for maximum ease of pointing the camera. However you CAN attach the camera directly to the dovetail bar and use it just like a telescope (with limitations on the field orientation). Here I have used a ring collar that couples my telephoto lens to the ball head. This allows me to rotate the camera to change the frame without having to repoint. It’s also better balanced.  There is enough room on the front of the dovetail to put another head and another camera.

CanonAstroAttach

BallHeadAstro

I even “cheated” and am using a camera as a counter weight – see it hanging there in front of me?

SLC_scopeAstro

Telescopes

If you decide to up the ante, here are a few commendable small, light refractors. None are “top of the line”, but I’ve had some pretty good success with the ED80. It’s biggest weakness is that it comes with no mounting bracket, and the focus mechanism is not the “dual speed” (fine focus) option that seems to help fine tune things. I did find that I could mount the ED80 on my scope without mounting rings by attaching it to a Vixen-style dovetail bar and a 1/4″ 20 cap screw. A hex bolt would work fine, too.  I drilled out one of the threaded holes in the dovetail bar.

OrionScopesRec

If you are thinking of going in all at once, various vendors offer bundles that might interest you.  Here are some examples from Orion (www.telescope.com)

OrionKits