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Stacking jpg astro files. Strange results or not?

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Hi folks,

I am getting what seems to me to be strange results in stacking astro files. Attached is a single jpg taken with 55mm dslr, 20 sec, at iso 1600. Looks pretty good as a single file but does not stack well. Stacking used 11 files with and without blacks and whites. A tripod was used with no tracking involved over about a few minutes for the 11, 20 sec exposures. 

Sort of expected the stacking to be as good as the single frame.  

Thanks for any advice,



astro canon jpg stacked 8.11.21 ??.afphoto astro canon jpg stacked blacks and whites 8.11.21.afphoto

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Hi Pauls,

Regarding exposure time, I have read that 500/focal length= max sec exposure. In this case I was using a 55mm lens but only f3.5, and the iso was set at 1600. Would you say that if I took many untracked shots of 8 sec each that the stacking would be much better? I do have a simple EQ1 mount so will certainly try this too. 

Thanks and regards, Irving

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  • 2 weeks later...

@irandar: The results are typical - it cannot work this way.

Stacking expects pictures where the objects have an predictable, identical offset - example:

Photo 1: Star A at position 100 x 200, star B at position 150 x 170.
Photo 2: Star A at position 110 x 205, star B at position 160 x 175.

So we've got an offset of 10 in the X axis, an offset of 5 in the Y axis. The same offsets for star A and star B. And the same offsets for stars C, D, E, ...

Your main problem is not exposure times, it's looking towards the rotational axis of the sky, the celestial pole, while there's significant time differences between your photos. Your AP stack shows that the North Star, Polaris, is somewhere near the upper right corner of the picture, the pivot point of the sky. This causes objects near the upper right corner "moving" slower than objects far away. Thus the offsets more and more increase with the distance from the North Star, aren't identical anymore. You can see this by the different lengths of the star trails.
That's why the stacking algorithms produce that star trails, it just cannot work with stars of different offsets in the same picture.

Just imagine sitting inside a dome with stars painted on it's walls, the dome rotating around you. If you look up the stars near the top move slower than the stars at the sides. And if you look parallel to the ground, the stars seem to move with the same speed. If you take photos of the top and stack them, you'll get the same results as with your stacked photos. But if you take pictures parallel to the ground, the results will be much better as the offsets become almost identical.

Remedies: a) Very short exposure times, as already mentioned in this thread. PLUS: Photos need to be taken immediately after each other. b) Taking photos more parallel to the ground plane. c) Use of a less expensive motorized mount like Star Adventurer or AZ-GTi, or a "real" mount starting from the EQ5 class, not below.

I'd go for option c) if you wish to dive deeper into astrophotography as these mounts also allow mounting of smaller telescopes. You won't be able to achieve real long exposure times of several minutes, but this would solve your problem. If you wish to stick with your DSLR and shorter focal lengths for astrophotography, mounts like Star Adventurer and AZ-GTi are your friends. For everything else a "real" mount (EQ5 and up) is the way to go.

Additional info:
a) Your photos are out of focus, stars are discs, not points. Focusing on stars isn't easy, manual focusing with maximum magnification of the preview might help, also a Bahtinov mask.
With higher focal length/magnification, a Bahtinov mask would be the way to go. With lower focal length/magnification (like 55 mm), it's better to focus via the preview at maximum magnification, take a sample picture (short exposure time, higher ISO) and check if stars are points by watching the taken photos at maximum magnification, correct the focus if needed. Nothing's more frustrating than finding out all your pictures are out of focus - wasting the results of several hours of imaging.
b) Take some dark frames as Pauls already mentioned. You've got masses of defective pixels in your photos, that tiny red, blue, green or white dots just every digital camera produces with longer exposure times. Dark frames help eliminating these. Also playing with the Stacking Options (Threshold and Clipping iterations) might help.
c) Use RAW photos. JPGs usually look better out of the camera, but the camera's JPG algorithms apply a lot of changes, often in a more or less random and unpredictable way. But astrophotography and stacking need unmodified data to achieve best results. Just set your camera output to RAW + highest JPG quality, so you always get a pair of RAW and JPG, can't forget setting the output to RAW. :)

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