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04-27-2016, 03:08 PM   #1
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Over exposure in Astrophotography and O-GPS1

Hello everyone,

I am very new to astrophotography and I am running into some problem that I hope you may help me out with.

First, the gear I am using:
Camera: Pentax K-S2 with O-GPS1
Lens: Pentax F 50 mm f/1.7 with lens hood attached

I have followed the instructions as I had found on this website provided by Stephane Poirier.
Accueil / Albums | PhotosSteph

In general, this is my procedure:

1. Put GPS unit on camera, turn both on.
2. Wait for satellite connection until I am located in 3D.
3. Turn on RAW. Turn off automatic noise reduction for long exposures (Slow Shutter Speed NR – OFF).
4. Set camera switch to manual focus and the dial to bulb mode.
5. Select ISO 1600 and set aperture to 1.7
6. In GPS menu, select ASTROTRACER and switch ‘Action in B Mode’ to the star symbol.
7. Set the ‘Timed Exposure’ on, and ‘Exposure Time’ to 2’ 30”.
8. Perform a ‘Precise Calibration’.
9. Place camera on tripod at the location I performed the calibration.
10. Turn on Live View and manually focus on a star with 10X magnification.
11. Set the self-timer to 2 seconds.
12. Press the shutter and wait.

Now here is my problem:

1. All my images are white at 2.5 minutes (very over exposed). I have provided some images using different exposure times ranging from 1/17 of a second to 150 seconds.
2. I have no clue what I am doing wrong. I am in a city (Edmonton) making these shots, so it could be light pollution? I didn’t imagine that this pollution would be so dramatic on the image.
3. Is there some setting that I am not utilizing properly?
4. Could you suggest any website that goes through the basic steps of astrophotography and specifically using the O-GPS1 with detailed procedures?

Thanks for helping out this noob.

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04-27-2016, 03:34 PM   #2
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are you completely new to photography in general? lower your iso or use a shorter exposure.
04-27-2016, 03:38 PM   #3
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What maltfalc said. Based on the noise in the top shot, I'd say your ISO is way too high. That looks like ISO 8000+ by the standards of my K3. Try your ISO in the triple digit range with exposures that long.
04-27-2016, 03:51 PM   #4
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Since you're brand new to photography in the dark, I would actually suggest that you forget about any instructions and just start experimenting and have some fun doing it!
Start on a moonless night outside of town with iso 800, point your cam at a constellation and try out some series of exposures. You don't need a very long exposure time if you just want to capture the stars. At f/2.0 maybe about 3 or 6 secs is already enough. For a basic shot of the Milky Way, try 30 secs at iso 1600. Later on, since you have the astrotracer, you can cut back on the iso to reduce noise. The unit will help to compensate for the longer exposures.

Don't forget to read up on basic exposure theory, for instance here: The Ultimate Beginner's Introduction to Exposure

04-27-2016, 03:58 PM - 1 Like   #5
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light pollution is yer worst enemy here!!! also not so sure the o-gps-1 worked correctly on the 9sec exposure......that is just past the trailing time not even using it (o-gps) and trailing is occurring.......no telling on the blown out image......the o-gps is a mysterious device and I have my ups n downs with it......fresh/strong battery....large metal objects like cars can effect it.....with a 50mm I would try about 60 secs......stop the lens down 1 maybe 2 stops and lower my iso to 400 and adjust out from there.....if trailing occurs I would reduce my time to 45sec.....if too bright lower the iso more.....its all trial & error for me.....I start with that kinda base line with 14mm to 50mm and adjust out......but I really think light pollution is the biggest deal.....dark clear skies!
04-27-2016, 04:17 PM - 1 Like   #6
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Yes alot of that is light pollution and it definitely shows up when you use longer exposures.
As previously stated the KS-2 doesn't quite handle high ISO as well as K-3 so drop that first.
Try ISO800, 1:30s, and f2.8 to get a sharper shot. Change the exposure timer depending on those results, dark-add time or Light-less time.
There will be only so much you can do but after a few trys you'll get an idea of what you camera can do at that location.

Here is a link with some dark sky sites if want to experiment a little more in your area.
Clear Sky Charts in Alberta (List)
04-27-2016, 04:29 PM - 1 Like   #7
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The F 1.7, ISO 1600, and 2.5 minutes may all be (combined) contributing to over exposure. I like depth so I usually use F8 - F13, 100-400 ISO, and test with time for exposure length to satisfy those approximate settings. It is a matter of the F stop, ISO, and length of exposure complimenting each other. Too much of an ISO is going to cause grainy images snd sometimes a lower number F stop may not provide enough depth of field for reasonable sharpness throughout the image.
04-27-2016, 05:38 PM - 1 Like   #8
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That overexposure is most definitely light pollution. Against truly black skies, increasing the exposure will only increase:

1. number of stars visible
2. noise (in the case of ISO)
3. star trails (in the case of exposure time)

A general increase in the brightness over the whole image can only be caused by the ambient light level (i.e. light pollution). Don't be fooled by what your eyes tell you is "black" as they will dynamically adjust. The best measure of how dark the skies are is how many stars you can see with your naked eye. Under very dark skies you can see many thousands, versus maybe a few hundred at most under most suburban skies. All but maybe a few dozen of the very brightest stars will be invisible in a city.

Also, DOF is irrelevant for wide angle astrophotography unless you have foreground objects within a few feet (and totally irrelevant with astrotracer as all earthbound objects will be blurred by the star tracking). I'm fairly certain everything at distances measured in lightyears qualifies as "infinity" no matter the aperture.


Last edited by Cannikin; 04-27-2016 at 05:53 PM.
04-27-2016, 06:32 PM - 1 Like   #9
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Yup. Waaay overexposed! In an urban or even suburban setting light pollution will always be a problem. The only thing you can do about it is get away from lights. For a city the size of Edmonton, about 50 km is good. If you're shooting stars/constellations you can still stay in the city, but you will have to deal with light pollution. If you plan on taking images of the Milky Way you have to get away from any lights. For meteor showers you want the high iso and long exposure noise reduction off but if you're just shooting stars/constellations turning the noise reduction on will help with the noise (At the cost of doubling the exposure time.) Look into doing dark frame subtraction instead of in-camera noise reduction. The F50 1.7 is a very good lens for night work. Stop it down a click or two to sharpen the images. With the OGPS-1 you don't have to use high iso. The longer exposure available from the OGPS-1 allows for lower iso and smaller aperture. You have the right gear and the correct idea. You just need to experiment a little.
This shot of Orion was taken with an F50 1.7 (on a K5ii) in a rather large city The iso was 400 the aperture was 2.5 and the exposure was only 10 seconds. That should be a good starting point for your location.
04-27-2016, 07:28 PM - 1 Like   #10
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Evening!!

For what you tried, you actually captured some images that did as well as could be expected. Your process with the camera/GPS unit appears just fine. I use a K5IIs, so the menus are a bit different, but what you are doing rings correct. I do have some suggestions, that I think will help capture some better images.
  • First, the location. Your back yard has too much light pollution. I pulled 53°30' 7"N 113°28'30.4"W from the coordinates from the first image (from the EXIF data). Then I dropped (I cut and pasted it) it in to the search box of the following web site - Dark Site Finder.
  • You are right in the heart of the downtown Edmonton light pollution area. You are in a "White" area, which is the absolutely worse place to be. Even though it looks dark to you, it is not dark for your camera. You want to move out to at least a "Brown" area. For the sake of comparison, drop in these coordinates 33° 27' 34.58" N 111° 28' 36.19" W This location is within a couple of feet of where I shot this set of stitched images (an hour before sunrise at astro twilight) and 50 miles from the light pollution of downtown Phoenix (to the right). The shots were at 18mm, f1.8, 10 sec, ISO 3200, RAW, no GPS, stitched with Microsoft ICE (free download). So, the moral of the story is location, location, location. I might suggest looking for a place south east of Edmonton so that the worst light pollution will be to your back as you shoot in the SE direction for the Milky Way.
  • Next - the Moon is not your friend with star shooting. You want to shoot when the Moon is not shining in the sky. The best time is during the new moon, or before it rises or after it sets. So you need a tool to help you with this. So, last night 4/26 at 10.58pm was the time stamp on the image. The moon rose at 11pm. Probably a bit too close. So, get the desk top version of this ap - it's a free download. The Photographer's Ephemeris.
  • When and where is the Milky Way - For this you want to down load the free application Stellarium. Just google it, put in your general lat and long (location) and start playing around with it. You can set the time and move time around and then see the Milky Way rise and where.
  • The GPS unit - The GPS unit is excellent. Now, having said that, there are some things that you need to keep in mind. When you calibrate, be at least 10 to 20 feet away from large metal items (cars). Sometimes you need to calibrate it twice. Carry some spare AAA batteries. Shooting stars elevated above about 45 degrees becomes a problem. The exposure time depends on the focal length. The longer the focal length the shorter the time. On wide angle - 18mm, the stars starts to travel along the extreme edges and corners of the frame when the exposure time is over about a minute. Here is another image using GPS 4 minute exposure, 18mm, f1.8, ISO 1600.
  • Focusing - for wider lenses, trying to focus on a star becomes very difficult. So, during the day focus on something a mile or so away, then tape down the focus ring on the lens, and make sure the camera is in manual focus mode.
  • There is an astro photography group here on the forum...
______________________

Also, I see that you are processing with Picassa. Picassa tends to overdrive the noise, so the images come out much more nosier than you will want. I too found out the hard way. Use something like Lightroom.

Just by looking at the map, you are going to want to go east of highway 21 out by South Cooking Lake to get a reasonable dark sky. However, in that location you will get some light domes from Camrose.


Last edited by interested_observer; 04-27-2016 at 07:45 PM.
04-28-2016, 02:05 AM - 1 Like   #11
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The ISO 1600 advice for astrophoto that are floating around many of the astrophoto web pages are based on Canons poor dynamic range at lower ISOs. On other camera brands you should go with a lower ISO usually in the 64-200 range. For K-S2 I think ISO 100 is a good choice. If the images get too dark you can lift the exposure in post processing with about the same quality result as lifting the ISO in the camera. This effectively enables you to choose ISO in post processing with good results.

You could also try to take several images and stack them in post processing. That will increase the contrast further.
04-28-2016, 03:20 AM - 1 Like   #12
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QuoteOriginally posted by Simen1 Quote
The ISO 1600 advice for astrophoto that are floating around many of the astrophoto web pages are based on Canons poor dynamic range at lower ISOs. On other camera brands you should go with a lower ISO usually in the 64-200 range. For K-S2 I think ISO 100 is a good choice. If the images get too dark you can lift the exposure in post processing with about the same quality result as lifting the ISO in the camera. This effectively enables you to choose ISO in post processing with good results.

You could also try to take several images and stack them in post processing. That will increase the contrast further.
Morning - I actually got up for a drink of water and wound up sitting down for a couple of minutes before heading back to bed. Over the last few months, there has been a wonderful discussion on just this topic - based on the "Unity Gain" concept, up in the astrophotography group. Your right, that it in general the topic started with Canon's poor performance, but the ISO 1600 is based on the actual observed/measured performance of the Sony sensor.Also, the type of astro shot you are going after - be it wide field (Milky Way), or deep space (specific nebula, etc.), will help determine how you to capture your shots. Stacking is a bit more difficult with a landscape element in the foreground. There is some absolutely fantastic work and images up in the astrophotography group.

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