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05-08-2017, 03:36 AM   #1
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Interval shooting oddity?

I am happy with my K-70, doing some good stuff and expanding my photographic horizons. I have been using Interval for Time lapse shots, and editing in LRTimelapse. Intervals are usually 5 or 10 seconds. some results below.
Nameless Valley Sunset 4K on Vimeo

Mt Nameless sunset in 4K on Vimeo

happy with these and quite a few more. But the other night I decided to try to capture some of the Eta Aquarids meteor shower. Interval shooting would seem to be the way to go, so, not having really done star shots before, I did 5 minutes of research , a couple of trial shots and settled on 25s @F3.8, ISO 800., focal length. I left the Interval setting on 5 secs, as it seemed to be working. As it turned out, ISO 800 was too low, end of the sequence was at ISO 1600--better, but still underex.
But my question is this--if I am shooting at , say 25 sec exposure, and the shutter seems to be open for 25 secs ( by ear) , how did I get the below picture? This is a layering of three consecutive shots in PS ( my first ever), showing the one tail, of, apparently, the one meteor. Is the camera internally recording the five second interval within the 25 sec exposure? Should I be setting the interval to something longer than the exposure time? Logic would say so?



05-08-2017, 04:48 AM   #2
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Are you sure its a meteor and not a satellite?
05-08-2017, 04:56 AM   #3
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Ah, now there's a thought! I was actually starting to notice some pretty good satellite flashes after that, as they caught the distant rising sun, forgot about that as I was processing. I was lying on the tray of my ute ( truck :-) ), rugged up, up a bush track away from the small coastal town I live in, very good for lack of light pollution. I had actually given up on the meteor shower by the time I started noticing them, and had an interval shoot going for the sunrise. But yes, that could be it.
05-08-2017, 06:59 AM   #4
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Check out Heavens-Above for lists of satellite appearances.

Put in your location first.

Try especially time lapse on some Iridium flashes and space station passes.


Last edited by AstroDave; 05-08-2017 at 07:01 AM. Reason: minor typo
05-08-2017, 04:19 PM   #5
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That's a typical skip trail of a "slow" (in angular speed) moving object in more than one exposure. Too slow to be a meteor. Doesn't look like a plane (missing the little strobe dot pattern) Most likely candidate is a satellite.

Or aliens


and before anyone asks.... no I'm not serious about the aliens!
05-12-2017, 06:14 PM   #6
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After some experimentation, it is obvious that the Interval starts as the shutter closes after the previous shot, and is independant of the actual exposure time. So I can use a 2 sec interval on a 25 sec exposure. I intend to try interval shooting the ISS on a very good pass next Saturday at home, and stack the resulting images. It will be one hour after sunset, and cloud permitting, could be a good blue hour shot. Fingers crossed for weather.
05-31-2017, 10:48 AM   #7
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I believe that the object in your image is an Iridium flare. Satellite flare - Wikipedia There is a website that predicts them and other night sky objects such as the ISS at Heavens Above. Heavens-Above Simply enter your viewing location and it will show when they're visible at your location. If there's nothing showing in the table it just means that there is nothing visible during that time period. The website will also provide magnitude (brightness) and show a map of the object's path overhead. For the ISS and Iridium flares a single frame will give a better image. Multiple frames produce gaps in the arc. The shutter closed between frames creates gaps in the trail. The ISS and Iridium flares are bright enough and the duration short enough that a single frame is preferable. For meteor showers use Manual mode and interval shooting. The interval you choose is the time from the beginning of one frame until the beginning of the next frame. It isn't the time from the end of one frame until the beginning of the next frame. The interval you select must be longer than the exposure. i.e. if you're are shooting 25 second exposures, the interval must be at least 26 seconds. If you set 10 second exposures, the interval you set must be at least 11 seconds. If you wanted 25 second exposures with 5 seconds between each image, you would set the exposure for 25 seconds and then add 5 seconds to get an interval setting of 30 seconds. You also need to turn off the high iso and long exposure noise reduction. They double your interval. In other words, if you are making 25 second exposures with the noise reduction on, your interval will have to be 51 seconds or more. (25 second exposure + 25 second noise reduction + 1 second or more because interval must be greater than exposure time.) Noise reduction does a dark frame subtraction for each image. You can do a dark frame subtraction with a single dark frame in PP. Just do a web search for dark frame subtraction.
05-31-2017, 03:32 PM   #8
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Thank you for all that, it clears matters up. I have another thread on this in Photo Critique. Layering of ISS pass - PentaxForums.com The interval thing bought me undone there. I was sure I was shooting the sequence in this thread with the interval set much less than the exposure time, but Subsequent attempts have seen me unable to replicate that. i can look at the originals and see exposure time, but the interval I was using is harder to pin down. Exposure time was 25 seconds, and i was getting 2 exposures per minute, according to the file info. I was perhaps using a 30 sec interval.
I knew I would get gaps, and the layering was going to give me stuttering stars, but I viewed it as a technical, learning exercise as much as anything. I tried a single exposure yesterday morning here ISS pass in Western Australia - PentaxForums.com with less than optimal results. I am now thinking I should invest in the accessory GPS unit so I can utilise the Star Tracking on the K-70--this should give me steady stars and a satellite trail, at least up to the max of 5 minutes it is meant to work to, which certainly will cover any ISS passes.

For prediction, I am using an excellent app on my Android phone, ISS Detector , which also gives me Iridium flash timing. All information is there, including a visual Radar screen which works off your internal phone compass, and also an actual elevation dot which you match against the predicted track. I'll try some captures of that.
Again, thanks for your help.

05-31-2017, 06:22 PM   #9
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You're quite welcome. There are many of us here who are glad to show off help out. I have an OGPS-1 and I like it for certain applications but it does have some limitations you need to know about. First, it needs to be calibrated whenever you use it and for long photo sessions it may need several re calibrations throughout the night. How well you do the calibration will affect how well the gps works. There are a number of calibration tutorials online. Second, it is also affected by large or nearby metal objects. (too close to a car, large metal buildings, etc) Third, is that "max of 5 minutes" thing. That usable duration will vary with the focal length of the lens. The shorter the focal length, the longer the usable exposure time. You won't get 5 minutes exposure time with a 300mm lens. From my experience, you should cut the recommended exposure time in half. Finally, star trails are caused by the star's motion relative to the earth (That includes the camera and it's sensor.) The OGPS-1 works by calculating and moving the camera's sensor in relation to the motion of the stars. If the camera sensor remains fixed relative to the stars , the sensor will be moving relative to the earth. You will get a blurred foreground. You can get around that by taking a second identical exposure without using the gps and then cut and paste that foreground over the blurred foreground. The OGPS-1 was one of the reasons I chose Pentax but if you aren't aware of the limitations and work within them, you'd wind up frustrated.
05-31-2017, 07:25 PM   #10
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Thanks for that. I had read some reviews on the OGPS-1 and most of those issues had been raised, but not the one about the different focal lengths and recommended exposure times being halved. Although one reviewer did note that he couldn't get his stars really sharp, and put it down to, perhaps, calibration. The focal length thing is only photographic common sense, really.
06-21-2017, 04:19 AM   #11
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Just a hypothetical about the OGPS-1. If it is shifting pixels to compensate for star movement, would the satellite track still be rendered the same as if tracking was not used? or would the curve of the track shown be greater ?
06-21-2017, 03:35 PM   #12
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QuoteOriginally posted by ranmar850 Quote
Just a hypothetical about the OGPS-1. If it is shifting pixels to compensate for star movement, would the satellite track still be rendered the same as if tracking was not used? or would the curve of the track shown be greater ?
First part of the answer:
The curve can be caused by Perspective Distortion, Geometric Distortion, Satellite orbital parameters, or because the camera's sensor is not moving planar with the subject. The last case can happen when using astrotracer with a wide angle lens and the scene contains stars both near the poles and the equator. (it doesn't happen when using a tracking equatorial mount)

Finally:
If the curve is being cased by the sensor not moving planar to the subject, then the shape will change depending on if astrotracer is on or not. In the other cases it will make no difference.
07-10-2017, 06:53 PM   #13
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QuoteOriginally posted by ranmar850 Quote
Just a hypothetical about the OGPS-1. If it is shifting pixels to compensate for star movement, would the satellite track still be rendered the same as if tracking was not used? or would the curve of the track shown be greater ?
I think you'll find the track of the satellite will be broken lines, as the sensor is shifted minutely. As would an airplane trail.
07-31-2017, 02:06 AM   #14
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Ah, pity. But I may invest in one anyway, when I have the spare cash.
08-23-2017, 04:34 PM   #15
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I found the answer to my main question in the camera menu in Interval Shooting Options. Default is 1-Interval , where the exposure begins at set intervals the other option is 2-Standby Interval, where the interval selected begins after the finish of the previous exposure. This is very relevant where longer exposures on timelapses are close to the interval you desire.
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