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02-18-2019, 08:59 PM   #1
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Two astro Q's

I'm trying to get back into star trails and star stacking but... it seems I've forgotten some of the fine points!

I did a test shot tonight, with the wrong lens, the DA 35mm, of orion. The stacking worked well, but I subtly missed focus on the sky, the stars were all a bit out of focus. Any good tricks on making sure you have the focus right?

Second I'm trying to remember best practices for aperture for astro stacking. I had stopped the lens down to f4.5, and shot each shot for 5 seconds. As this was all just a test I'm not too worried. Generally does stopping down your lens a bit help for astro? I would think it does, and yet I vaguely remember advice to the contrary.

I'll probably be doing some milky way photos around the 6th of March with either a Zenit 16mm or a Rokinon 8mm, hoping to remember how to do this a bit better by then.

02-18-2019, 09:25 PM   #2
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Live view with zoom is helpful for focusing- if the moon is around, you can cheat off of that.

Generally, you want lenses that are sharp wide-open for astro. To get as much light as possible, shoot close to wide open. F2.8 or F3.2 should do the trick with the 35mm.

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02-18-2019, 11:42 PM   #3
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For star trails take a series of pictures of night sky. Maximum exposer time without tracking it depends on focal length of the lens. Longer the lens shorter the max time. I use 14mm Samyang wide open and exposer time 20 to 30 seconds.
Then stack photos with Startrails:
Best regards, Andrej
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02-19-2019, 10:36 AM   #4
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Here are my thoughts...
  • Focusing - I cheat. I pre-focus my lens in the afternoon and then tape the focus ring on the lens down and put the body in manual focus. I focus on something a couple of miles away and all is good. I usually shoot wide at 15mm to 18mm and I have found that it's extremely difficult to focus on anything small even with 15x zoom and a little 10x stand magnifier. It's a bit easier at 30mm, but I would just rather not do it.
  • Aperture - With wide angle, wide open. With longer focal lengths you can afford to stop down a bit, because the physical clear aperture - the aperture's physical area, is substantially larger with the longer focal lengths.
  • ISO - This is a balancing act. I've found with my K5IIs, ISO 1600. Using my K1 ISO 800, provides much better dynamic range and the colors are better.
  • Exposure Time -
  • With the Astrotracer - 60 seconds is a good number across all focal lengths. At 35mm or less, you can push it to 70 seconds. At 90 seconds, I see trailing in the corners from the wide angle lens rectilinear distortion.
  • StarTrails - Stack with 30second frames
Hope that helps....

02-19-2019, 10:57 AM   #5

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QuoteOriginally posted by bobbotron Quote
Any good tricks on making sure you have the focus right?
I focus by using live view on a bright star. Put the star 1/3 of the way between center and a corner to allow for field curvature. Manually focus until the star shows up on live view, then activate live view zoom to fine tune focus.

Some autofocus lenses offer an imprecise manual focus ring that has too much play. I find that focusing too far in one direction, then slowly backing up the other direction, helps to get things into focus.

Periodically recheck focus by looking at your last photo. This is especially important when pointed near zenith since gravity can cause zoom/focus creep. An especially loose lens might need gaffers tape to prevent movement.

QuoteOriginally posted by bobbotron Quote
Generally does stopping down your lens a bit help for astro?
"It depends." Stopping down offers more forgiveness in focusing, and can compensate for some lens limitations such as vignetting and chromatic aberration. You lose light, though, forcing more images to stack or using a higher ISO.

I recommend not stopping down to start. That simplifies everything else. If you find that you always gets soft images or other defects, then evaluate stopping down 1/2 stop and a full stop with careful comparisons of photos taken on the *same night*.
02-19-2019, 09:20 PM   #6
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Went back for round 2 tonight as it was nice and clear. Full moon behind the camera, so obviously challenging conditions. None the less, better results than the other night, progress!

The zenitar 16mm is good fun to use for this kind of thing.

Dusting off my demo copy of Nebulosity 4.


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