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Rule of Thumb to Avoid Star Trails
Oct 15, 2016 06:53:43 #
The answer is simpler than the mathematical exercise I went through to find it.
I will not bore you with the details but it involves calculating how far a star's image travels across your sensor during the time the shutter is open in relation to the diameter of the circle of confusion (COC) assumed as [sensor diagonal]/1500. I did the math mainly to confirm that the COC is the right basis for making the calculation.
The short answer is to keep the shutter open no longer than 400/[focal length], for example:
Lens focal length(mm) 25 50 100 200 400 500
Shutter speed (sec) 16 8 4 2 1 0.8
For a crop sensor use the full frame equivalent focal length, e.g. for a 1.5 crop sensor and a 50 mm lens, use 75 mm.
I confirmed my results by searching the internet for the conventional recommendation and found the Rule of 500 which recommends using instead 500/[focal length]. I also found recommendation of 600/[focal length]. My formula is just a little more conservative.
Of course you can always attach the camera to an equatorial mount and expose for much longer or simply record the start trails.
I will not bore you with the details but it involves calculating how far a star's image travels across your sensor during the time the shutter is open in relation to the diameter of the circle of confusion (COC) assumed as [sensor diagonal]/1500. I did the math mainly to confirm that the COC is the right basis for making the calculation.
The short answer is to keep the shutter open no longer than 400/[focal length], for example:
Lens focal length(mm) 25 50 100 200 400 500
Shutter speed (sec) 16 8 4 2 1 0.8
For a crop sensor use the full frame equivalent focal length, e.g. for a 1.5 crop sensor and a 50 mm lens, use 75 mm.
I confirmed my results by searching the internet for the conventional recommendation and found the Rule of 500 which recommends using instead 500/[focal length]. I also found recommendation of 600/[focal length]. My formula is just a little more conservative.
Of course you can always attach the camera to an equatorial mount and expose for much longer or simply record the start trails.
Oct 15, 2016 09:41:11 #
selmslie wrote:
Of course you can always attach the camera to an equatorial mount and expose for much longer or simply record the start trails.
Of course you can always attach the camera to an equatorial mount and expose for much longer or simply record the start trails.
Or simply use a Pentax K1 and expose for 5 minutes using any lens you choose.
Oct 15, 2016 09:52:19 #
selmslie wrote:
... or simply record the start trails.
That should be "star trails". Some typo's are immune to proofreading.
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Oct 15, 2016 12:59:20 #
Interesting. One of the millions of things I never thought of! Thanks for your work.
Oct 15, 2016 13:43:53 #
SonyA580 wrote:
Interesting. One of the millions of things I never thought of! Thanks for your work.
You are welcome. It was a fun exercise.
Oct 16, 2016 09:38:38 #
MT Shooter wrote:
Or simply use a Pentax K1 and expose for 5 minutes using any lens you choose.
Please explain...
Oct 16, 2016 09:45:33 #
The Pentax K-1 uses a method of vibration reduction that can be used to reduce the effect of the earth's rotation on pictures of heavenly bodies- see http://www.pentax.com/en/k-1/impressions/impressions01.html
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Oct 16, 2016 09:52:37 #
Pentax K1 has GPS and something called "astrotracer" which means that the sensor moves, follows the stars. How much it moves depends of the lens you are using and where on earth you are.
Oct 16, 2016 10:17:52 #
Donwitz wrote:
The Pentax K-1 uses a method of vibration reduction that can be used to reduce the effect of the earth's rotation on pictures of heavenly bodies- see http://www.pentax.com/en/k-1/impressions/impressions01.html
I thought MT Shooter was kidding. I'm glad I kept my mouth shut.
If you already have a Pentax system, the ASTROTRACER feature might make it worth considering the K-1 as an upgrade - about $1,950 for the body. It's not reason enough to switch brands.
I would be skeptical of reviews from anyone who has already invested in the K-1. It would be more interesting to see a review that compares it to an equatorial drive and mentions its limitations. I suspect there is a practical time limit for the ASTROTRACER feature.
For a fraction of that price you can get an equatorial drive to mount on your own tripod so you can use it with any camera. It takes a little more effort to set up but it won't break the bank. An equatorial drive can track stars for hours.
Oct 16, 2016 10:28:23 #
The limitation is about 5 min, it depends a little of the lens, time of year and your place.
Oct 16, 2016 10:41:42 #
Narog50 wrote:
The limitation is about 5 min, it depends a little of the lens, time of year and your place.
That's does not surprise me.
You could expose for hours with an equatorial mount if you lined it up perfectly but that's easier said than done. You might also have to revert to film since a digital sensor starts to accumulate noise after a while.
The Astronomical Photography Forum has more ingformation.
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Oct 16, 2016 11:02:39 #
For me, five min is more than enough, but I can understand that there are people that want longer exposuretime
Oct 16, 2016 20:44:09 #
Oct 16, 2016 23:31:49 #
Donwitz wrote:
The AstroTracer software, which makes use of the ShakeResistance hardware built into every modern Pentax body and the GPS built into the recent high-end Pentax bodies, was first introduced on the APS-C flagship K-3ii {currently $900 at B&H}The Pentax K-1 uses a method of vibration reduction that can be used to reduce the effect of the earth's rotation on pictures of heavenly bodies- see http://www.pentax.com/en/k-1/impressions/impressions01.html
Oct 17, 2016 09:14:19 #
I confess that I'm still a dinosaur. I only use film. I haven't gone over to the dark side, yet.
Star trails are no problem with my Canon AE-1. Simply place the camera on the tripod, screw in the cable release, focus on your object, e.g. moon, big dipper, etc., push plunger of cable release and lock down. Set alarm, go to sleep and wake up 4-6 hours later, release cable release, then wait for a week or so for the developer to have my photos ready. I'd love to post my results but back to the dinosaur bit, I don't have a scanner. Sorry.
Star trails are no problem with my Canon AE-1. Simply place the camera on the tripod, screw in the cable release, focus on your object, e.g. moon, big dipper, etc., push plunger of cable release and lock down. Set alarm, go to sleep and wake up 4-6 hours later, release cable release, then wait for a week or so for the developer to have my photos ready. I'd love to post my results but back to the dinosaur bit, I don't have a scanner. Sorry.
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