Astrophotography Notes

Exposure guides



Lodriguss: Recommended Exposures for Deep-Sky Objects

Based on the darkness of the observing site, moderate temperatures (50F to 60F), reasonably high elevation of the object in the sky.

Light-polluted suburban observing site (Naked Eye Limiting Magnitude = 4)

ISO \ F# 2.8 4 5.6 8
400 30 sec 1 min 2 min 4 min
800 15 30 sec 1 min 2 min
1600 7.5 sec 15 sec 30 sec 1 min

Moderately Dark Observing Site (Naked Eye Limiting Magnitude = 6)

ISO \ F# 2.8 4 5.6 8
400 4 min 8 min 16 min 32 min NR
800 2 min 4 min 4 min 16 min
1600 1 min 2 min 4 min 8 min

Very Dark Observing Site (Naked Eye Limiting Magnitude > 7)

ISO \ F# 2.8 4 5.6 8
400 8 min 16 min 32 min NR 64 min NR
800 4 min 8 min 16 min 32 min NR
1600 2 min 4 min 8 min 16 min

If you can't get to a really dark location, have no fear! You can still shoot deep-sky objects from your driveway. Exposures will be limited in duration by the sky fog from any man-made light pollution at your observing site. To create the equivalent of a longer exposure needed to capture a faint deep-sky object from a dark location, you just need to add or average together many shorter exposures. This is made possible by the linear nature of the digital sensors in DSLR cameras. Of course, you don't get something for nothing, and the price you have to pay is that you need a whole lot more short exposures than you would think to equal a longer one under dark skies.

Under light polluted skies, 10 six-minute exposures do not equal 1 sixty-minute exposure under dark skies. You have to increase the total exposure by the same factor by which the sky is brighter. For example, you need 60 minutes of exposure at a relatively dark sky site to get a good signal-to-noise ratio of a particular object. Your naked-eye-limiting magnitude there is 6th magnitude.

A site, such as your driveway, where the naked-eye-limiting magnitude is 4th magnitude, is 6.3 times brighter (2 magnitudes = (2.51 * 2.51) = 6.3). That means that in your driveway, to achieve the same signal-to-noise ratio in a 1-hour image shot at a dark-sky site, you would need to expose 6.3 times as long, or a total of 6.3 hours.

Exposure equivalents

Tot No of Frames Individual Exposure ISO Total Exposure
1 16 minutes 200 16 min
2 8 minutes 400 16 min
4 4 minutes 800 16 min
8 2 minutes 1600 16 min

It is important to note that a single four-minute exposure at ISO 400 is not equal to a single two-minute exposure at ISO 800. It is the total exposure time that is critical for astronomical images. This is non-intuitive to daytime photographers because they are used to doubling the ISO and halving the exposure and thinking they are getting something for nothing. But in reality they are not because they are getting a lower signal-to-noise ratio. In film days they got more noise in the form of grain.

Recommended Number of Exposures

Exposure length for light frames is determined by a number of considerations, but on a good mount with guided exposures, a good starting point seems to be around 5 - 10 minutes at ISO 800 or 1600, at temperatures from 50 to 70F (10C to 21C). At colder temperatures, lower ISOs can be used with longer exposures.

Taking test exposures and examining the histogram are the easiest way to determine the correct exposure your your site on a particular night given the local atmospheric conditions as far as haze, humidity, light pollution, and sky glow.

When you preview the image on the display on the back of the camera, you will see that the sky looks fairly bright when the histogram mountain reaches about the 25 percent point. You may think that this is overexposed because when you look at the night sky with your unaided eye it seems black.


This is, however, the correct exposure. You do not want the sky completely black in your images. If it is truly black, you will not be able to capture the faint detail in the object you are photographing. In most cases, this faint object detail is barely brighter than the sky background. If it is underexposed, with most of the pixel brightness values on the far left side of the histogram, it will be located in the noise in the image and difficult, if not impossible, to extract.

Recommended ISOs
* Above 60F (15.5C) - ISO 1,600
* 40F to 60F (4.4C to 15.5C) - ISO 800
* Below 40F (4.4C) - ISO 400

Panorama notes

With 15 degree detente disk on pan head, 24 images result. At full cylindrical image output, final panorama dimensions (approx): 13048 x 5750 pixels.

  • This supports an 86" print at 155 dpi print output.

If ISO 1600, f5.6, and 4 minute exposure (single frame) is applied, total time to take panorama would be 96 minutes. plus, dark frame needed for additional 4 minutes.

Best sky perspective with least distortion when stitched is with pan head set to 30 degree positive on vertical axis.

Example: the backyard


Initial camera orientation becomes centered in stitched panorama image.

Kenko Skymemo tracking mount

Kenko home:

  • Time / focal length
    • FL (mm) / Max. exp (min)
      • 50 - 70 / 100 - 40 / 200 - 30 / 300 - 15

Polar alignment
* Rotate polar finder and match positions of Big Dipper
* Position mount to place Polaris in reticule line gap
* Position mount to place Delta Ursa Minor in secondary line gap

polar-1.jpg polar-2.jpg

* Reticule update reference


Canon camera settings

Long Exposure

  • Set dial to M
  • Set image size to large and RAW
  • Set ISO
  • Use small wheel to set shutter to Bulb
  • Set power switch to above On
  • Use big wheel to set aperture
  • Set lens to Manual Focus
  • Use viewfinder to set
  • Tape down focus and zoom

Live View

  • Set to Enable in Menu
  • Press big wheel button to start
  • Press again to stop
  • Press Magnify to focus
  • Use focus ring to focus

Mirror Lockup

  • Set to Enable in Menu/Custom Functions
  • Press shutter button to raise mirror
  • Press shutter button again to expose
  • Unnecessary for long duration imaging

Long Exposure Noise Reduction

  • Default set as Enabled under Menu

Timer Remote Controller

  • Self - time before first frame triggered
  • Interval - time between sequential frames- min 1sec
  • Long - time of open shutter
  • Frames - number of frames to capture

Astrophoto shooting agenda

  • Wide field
    • Compose area of interest
    • Single frame with varying exposure variables
      • Time
      • Aperture
      • ISO
    • Sequential frames with varying exposure variables for image stacking
      • Determine desired exposure, set remote controller
  • Panorama wide field
    • Mount panorama head to tracking mount
    • Configure as either single or multiple frame
  • DSO with 300mm lens
    • Configure as either single or multiple frame
  • Shoot dark frame

High Dynamic Range Settings

  • Determine baseline exposure - an average of the scene
  • Set camera to Manual and enter baseline shutter and aperture values
  • Set Menu/Auto Exposure Bracketing - set to +2 and -2
  • Press AF-Drive button
  • Use big wheel to set drive to continuous mode

Example images with exposure data

ZodiacalLight_KK_20071011_900.jpg Startrails_KK_20071008_900.jpg kharusi-1.jpg kharusi-2.jpg

Night Sky panorama



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