Canon Digital SLR

Users Section


    Canon EOS DSLR Users Section


Canon EOS cameras with CMOS chips are very popular with todays backyard astro-imagers.

With large CMOS chips, at 6 to 8mp, very good noise control, even at high ISO's, the Canon DSLR line up is hard to beat.

With the introduction of the first camera made for Astrophotographers, The 20DA, a new era in imaging was opened. The

20DA is a very powerful tool, in the hands of a dedicated user. With myried functions, controls, options, and overides, 

it takes some time and trouble to get the most from the Camera, Lens/Telescope, Mount System.

Special techniques should be used in both image capture, and processing. This section will cover image capture, or

the Building of the Exposure Set.   The CMOS based cameras take well to exposures at 800 and 1600 ISO

settings.   This allows for very short exposure times, wile still getting very faint detail. The trick to getting fantastic results,

is understanding that even though any one shot will show 'noise', and will have very little 'range' to it. 

By combining (stacking) several captures, we can 'render' the noise out of the image, and increase the 'Range' by using special

image processing techniques. 

With more signal and less noise with each added later in processing, the value of  a multi-capture Exposure Set, is clear.

Some of the questions we will try to resolve :

How does one go about capturing an exposure set?   What ISO setting is best, and how long should an image be exposed?

What if my mount will not tract for long without guiding?   How many captures will be needed for a good capture set? 

How does one know when the exposure set is complete? 


Below is an example of an exposure set, by Lapacca, with m42 as an example.  An exposure set like the one below can

be used on any bright nebula. If your Optical system is faster or slower, make a test of the long exposure and adjust;

Faster scopes will require less exposure time, wile slower systems may require longer exposures for the same detail to be

recorded. Trial and error are the name of the game, photons are free...... take some!

 Data collection by Fr. Manuel Baeza mccj, LA


Pre session set-up : Test your gear at home, before you make a long trek to dark sky.

Telescope, Mount, Camera Checklist Complete - ( If you make a check list you will have a better time out in the field )

Remember, in this sport is you will be doing a lot of things over and over, a missing piece back at home, can be a big bummer

It's like going fishing, you will find the big things like the boat and pole, are just as important as a little things like bate and hooks!! 

Location for Example : Frazier Park CA ( Aprox. 5500ft. nice dark Mag. 5 sky, and quite steady )

Hardware :

                Camera : Canon EOS 20D  -  Note: Instructions exclusive to the 20DA will be shown in Brackets {........}

                Telescope : Televue NP 101 

                Mount : Losmandy G-11

With Mount set-up and scope installed, Mount is polar aligned using the drift-align method. A process applied and refined until

the amount of drift Very small / none for up to 5 minutes. The exposure can be guided, manual or auto, but field rotation will become

a factor even if all of your exposure set images are well tracked. When combined, in later processing, poor alignment will make

stacking a chore, if not impossible.   For most, the drift align method will take approx. 30 minutes to an hour or more to complete.

Once aligned and tracking, the mount is camera ready,  Take care to select an objects that will be as close to the zenith as possible

during the Exposure Set, this will insure the object is well placed, high in the sky, where the atmospheric conditions will be best.

Now ready the camera, for the Exposure Set, make sure the camera's batteries are charged (2 or more piles are a must)

or the camera must be attached to the AC power source. {Included with the Kit},

If using the AC adaptor for the first time;

             Make sure you install the 'dummy battery' with the cord exiting the small notch in the inner

             base of cameras grip. 

        NOTE: If battery door is removed, or if it is not closed, the camera will not power up.

Set the camera according to manual for the following functions;

Image quality set to RAW (This will allow for 16bit files / non compressed format )

ISO 1600 

White Balance : Daylight (Sun Icon)

Under the Main Menu - Tools section - Custom Functions (C.Fn) Make sure the following are set :

Custom Function  #2 - 0: Off -  ' Long exposure noise reduction is off '  Sounds like a good thing, but not for our use here.

Custom Function  # 8 - 1: On - ' ISO Expansion - On : ISO is 'Pushed' to EF3200 ISO 

Custom Function #12 - 1: Enable  ' Mirror Lock up enabled'  - Will lock the mirror with the first push of the cable release : Second press

                                                    Starts the exposure : Try this a few times and get use to the function.

Camera set to Manual Exposure :

Shutter Speed set to : Bulb

Camera OFF

Add T-ring / T-mount to camera and set it up for prime focus photography.

Point the scope at a bright (2nd or 3rd Magnitude star) in close proximity to the object you will be shooting (in this case m42)

Focus the camera on the star through the view-finder, making course and then fine adjustments, try to get a 'feel' for where the

focus is. Use a Hartman mask, or other focus aid.

{ Set the cameras speed setting one step past 'Bulb' and you will find the 5x Live focus setting (Fn-1)

When ready to focus, trip shutter (via cable release) once to open Mirror/shutter, view star and focus.

(Fn-2) One more click past Fn-1 will give a view at 10X for fine focusing, TIP : If the star is too bright

when focusing, go to a slower ISO setting }

Once focused, Camera ON, set shutter speed to 'Bulb' and take one 30-Sec. capture.

Post view image on LCD screen, 

Check for focus,  Composition, and Tracking. 

Turn off camera for a few  minutes before starting the Exposure Set. (Allow chip to cool down after Focusing)

Take first Capture : 5 minutes

Allow Camera rest for a few minutes, between captures, so the chip can cool, shoot until the following set is complete;

2x   5 minutes

2x 2.5minutes

2x   2 minutes

2x   1 minute

        This gives a total of 8 exposures, at a combined or effective time of 14 minutes, and took about 20 minutes to complete:

        Upon checking the Exposure set in Photoshop, even the 1 minute exposures were too long for the bright detail, so two

        more captures were taken, one at 30sec. and the final at 15sec. These short exposures will hold the highlight detail for

        later processing. If the mount is tracking well, set the ISO to 400 and capture a 10 minute exposure.

When the captures are complete take a Dark Frame ( 1 exposure with the body cap on, at the longest exposure time from above )

This frame will be used in later processing : 

With the above set Exposure complete, the images are now ready to be processed.  


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