Useful formula for shooting stars

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

06/24/2009 05:00:14 PM · #1

I'm headed out to the coast for the weekend and I would like to do some night photography. I ran across a formula here that will calculate how many pixels a star will trail given your focal length, camera sensor, and the declination of the star. That's pretty cool. While I like star trails, I want to do some shots with as little trailing as possible. I'd also like to stack shots to get a crack at the milky way, although I'm not going to have any sort of tracking mount. The formula, for example, tells me that with a 16-35mm at 16mm on my 5D I can expect about 2 pixels of trailing on a 15 second exposure.

Now I've got a few question to ponder:

Concerning both noise and having to realign the stars manually in PS (remember, no tracking mount), who thinks it would be better to do 4 15 second shots with auto noise reduction on which will take 120 seconds (18 pixels of total movement) or who thinks it would be better to do 4 15 second shots without auto noise reduction on which will take 60 seconds (9 pixels of total movement)? The issue with manual alignment in PS is that stars move in a gentle arc so you can't just align them in a purely linear fashion. After a while you'd have to align them by somehow rotating the picture around the north pole which is probably impossible.

I love working stuff like this out.

Other things on my mind will be light pollution (Dark Sky Finder) which will be on a Bortle Scale of 3-4 which isn't too bad, but could be better. The moon sets on Friday night fairly early (11:00 PM) but the Milky Way then will be almost directly south which would be shooting straight into the little town of Newport from where I'll be.

Anyway, tips would be welcome. I like to have as much clear in my head before I'm actually shooting as possible.

Apple iPhone 5

06/24/2009 05:15:13 PM · #2

With wide-field astrophotography there are so many variables that I've found trial-and-error has given me some of my best results.

Things I've struggled with in the past include; random clouds coming over (uh oh, I said the 'c' word and just jinxed it for you) planes flying across the shot, gusts of wind shaking the tripod, me shaking the tripod, me not being able to see my watch for timing the exposure, a car passing by and ruining the shot with their headlights, not being able to get focus spot-on, random passers-by or police arriving on the scene asking what I'm doing, neighbours turning on their bathroom light, wild animals or cats or dogs creeping up and giving me a heart-attack, me getting generally freaked out by being crouched down in a field at 2am, condensation on the lenses and viewfinder, me forgetting I'm in the middle of an exposure and walking in front of it, the impossible task of trying to frame a pitch-black shot through the viewfinder with your head at an impossible angle, the sun rising unannounced...

All-in-all, you're going to enjoy yourself. :-)

jdannels

Nikon D300s

Sigma 30mm f/1.4 EX DC HSM for Nikon

06/24/2009 05:15:53 PM · #3

I would go without the Auto Noise Reduction. I rarely use it anymore, even on minutes long exposures. Here is an image without Noise Reduction jpeg vs. the raw conversion and no processing. Just converting from raw removed hot pixels and most of the heat sensor issues(magenta color).

If you have an extra two minutes, you could give both a shot. :P
I know you can rotate and image around any point you select in your image. When using free transform, I believe you can move the center spot where you usually rotate around, so you could put that on the North Star. But this will be tough since you are shooting South.
You could just shoot at IS0 1600 wide open and get it all in one shot. :)

GeneralE

Canon PowerShot S3 IS

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

06/24/2009 05:30:54 PM · #4

An Astrophotography Primer

RegiStax

Five_Seat

Canon EOS-6D

06/24/2009 05:33:09 PM · #5

I too love the waiting game of star-trail photography. I have found that the stars show up pretty well with a middle if the road aperture - something like f/8, which is good beacuse that tends to be the aperture that maximizes the performace of the lens.

There is the issue of the foreground as well. The last time I was doind any kind of star trail photography I had to worry about lights blowing the foreground. So to deal with that I bumped the iso to 1600 (on my 50D) and opened up the aperture as wide to f/4 and took a few test shots with different shutter speeds to see what exposed the foreground the best. Once I had the shutter speed, I lowered the ISO to 100 and the shutter to f/16 (which I found later to be too narrow) by using the rule that halfing the ISO doubles the shutter speed and doubling the f/number doubles the shutter speed.

For example: it was 5s f/4 ISO 1600 = 1250s f/16 ISO 100. It just so happened, I guess because of light pollution, that foreground was exposed perfectly but the stars were pretty faint.

When I get home, I can post the pic...

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

06/24/2009 05:43:39 PM · #6

Thanks for the tips guys. Paul, I was looking around at the RegiStax site and I couldn't tell if it takes into account the arc of stars when it does alignments. That would be cool if it does.

The good news of digital is I can do lots of shots and see what works, but I like to know at least a bit of what I'm doing ahead of time. The biggest decision now is where to shoot from. I'd like to shoot from Yaquina Head where I took the picture of the lighthouse, but a) shooting south may get light pollution from Newport and b) it would require a bit of sneaking around since the park technically closes at dusk. I'm looking about for another possible vantage. Google Earth is good for this.

I've also been using Stellarium to see when and where things are. All this stuff is helpful in trying to increase your odds of getting a good picture.

GeneralE

Canon PowerShot S3 IS

06/24/2009 05:52:44 PM · #7

Originally posted by DrAchoo:

I'm looking about for another possible vantage. Google Earth is good for this.

Yes, choosing the proper location is critical to obtaining that unique perspective ....

Apple iPhone 5

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

06/24/2009 06:36:03 PM · #8

Also, it's the shortest nights of the year around about now, so the sun never really sets properly. i.e. watch out for blue light leaking through on the horizon.

GeneralE

Canon PowerShot S3 IS

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

06/24/2009 09:42:26 PM · #9

Originally posted by JH:

Also, it's the shortest nights of the year around about now, so the sun never really sets properly. i.e. watch out for blue light leaking through on the horizon.

On the plus side, it's only about a day and a half past New Moon, so at least there's a minimum of competing light from our satellite ... I pulled over by the side of the road last night to take this before it sank into the Summer haze.

MelonMusketeer

Nikon D7000

Nikon MF 50mm f/1.4

06/24/2009 10:28:25 PM · #10

You may also find this site useful.
NASA Flyover info
Maybe you can get a space station trail like this one if you know when and where it is going to be coming over. You enter your zip code or location, and get the upcoming sighting opportunities to see the ISS.
Here is one of my shots of it, taken in town using a 15mm fisheye and 75 sec exposures.

I didn't use incandescent/tungsten WB for that shot, but do often do so for night shots. It takes the orange out of the sky when shooting anywhere near civilization with the HP sodium orange street lighting. Here is one shot with tungsten WB. The glow on the horizon is the orange light over a city about 25 miles away.

My experience with crescent moon shots is to keep exposure time down to less than 1sec at 100 mm and less than 4 sec at 25mm so that it don't smear too much. 2 sec at 50 mm would be about as long as the exposure can be to keep detail in the features on the moon without using a tripod drive system.
Enjoy your trip, and don't stress about getting the shots. You have the right gear, and the eye for composition to make it all great.

06/26/2009 12:58:29 PM · #11

OK, new question. If I am intending to stack shots, am I looking to properly expose each shot or can/should I underexpose it? I may be somewhat limited anyway because I'm looking to not go over perhaps 15 second exposures with a f/2.8, but I guess I can control the ISO to help exposure. So my question would be whether I should go for a "properly" exposed shot at ISO 1600 or an underexposed shot at ISO 400 if I could choose between them.

I'll probably try both, but having other people's experience is always great.