Stars moving across the sky

This week I took 2 pictures, f/11 for 15 minutes, and 30 minutes on Delta 100. After processing at school (D76 1:1 for 9 minutes) I got pictures with barely visible trails. Printing the 30 minute picture for 15 seconds at (I think) f/11 at grade 5 produced a print with a grayish sky and star streaks, many bright, others faint.

What do I do to the exposure to increase the contrast?

reub2000 said:

This week I took 2 pictures, f/11 for 15 minutes, and 30 minutes on Delta 100. After processing at school (D76 1:1 for 9 minutes) I got pictures with barely visible trails. Printing the 30 minute picture for 15 seconds at (I think) f/11 at grade 5 produced a print with a grayish sky and star streaks, many bright, others faint.

What do I do to the exposure to increase the contrast?

Click to expand...

You could try processing the Delta 100 in Dektol (Print Developer) instead of D-76.

Better to switch from Delta 100 to Tri-X 400 and process it in undiluted D-76.

reub2000 said:

This week I took 2 pictures, f/11 for 15 minutes, and 30 minutes on Delta 100. After processing at school (D76 1:1 for 9 minutes) I got pictures with barely visible trails. Printing the 30 minute picture for 15 seconds at (I think) f/11 at grade 5 produced a print with a grayish sky and star streaks, many bright, others faint.

What do I do to the exposure to increase the contrast?

Click to expand...

f:11 is stopped down too far. The stars are moving relative to the film, so they're not producing enough exposure at any point along their path at f:11. Open up to at least f:4 or f:2.8. But if you have significant light pollution, the sky will brighten up as well and kill the contrast. Cure for that as discussed earlier is to find darker skies, which may be hard to find around Chicago. Was the moon set when you shot?

Visit here: http://cleardarksky.com/csk/prov/Illinois_clocks.html
to find when a good time to shoot is and poke around the site to find out about the closest dark skies to you. Note the light pollution index column on the linked page above. The web site will show you color coded sky pollution "contour" maps for your area. Here's one for an observing site about 15 miles west of Waukegan (sp?). http://cleardarksky.com/lp/smith01_ILlp.html?Mn=light pollution

Lee

Looks like I'm an 8 or 9 on the Bortle Scale. Shame. When I tried at f/4 and f/5.6 the sky was opaque on the film with no visable stars. Acording to the map the best place to shoot would be in the middle of lake michigan.

Follow an earlier suggestion and photograph moving car headlights and tail lights from a highway bridge. Use a tripod, a fast lens and a time exposure with the lens aperture wide open.

reub2000 said:

Acording to the map the best place to shoot would be in the middle of lake michigan.

Click to expand...

The middle of Superior would be even better. Arizona is a better place to set up a tripod. Some of the cities there even have rules about stupid lighting.

Lee

Lee L said:

The middle of Superior would be even better. Arizona is a better place to set up a tripod. Some of the cities there even have rules about stupid lighting.

Lee

Click to expand...

Lee,

My alternative self has a house in Tucson - itself is too close in to avoid city lights - but near enough to real dark that I want to try it there too.

But getting back to wintertime dark skies in the northern climes - I was thinking of a heater of some kind across the lens (e.g. hair dryer). But would this create "distortion" due to the warmed air in the immediate front of the lens on a sub-freezing night?

copake_ham said:

. . . I was thinking of a heater of some kind across the lens (e.g. hair dryer). But would this create "distortion" due to the warmed air in the immediate front of the lens on a sub-freezing night?

Click to expand...

That would be overkill. Resistors dissipating a Watt or two beneath the camera lens and within a large lens hood sufficed for northern Missouri. Convection put the warmth where it was needed.

copake_ham said:

Lee,
But getting back to wintertime dark skies in the northern climes - I was thinking of a heater of some kind across the lens (e.g. hair dryer). But would this create "distortion" due to the warmed air in the immediate front of the lens on a sub-freezing night?

Click to expand...

George,

You can use a hair dryer to warm up optics for visual observing, but it may not last long enough to get the exposure you need. It would likely create enough turbulence during the shot to ruin the image. Hair dryer elements can also glow if hot enough.

See my note about 9 postings before yours for "dew chaser" comments.

String enough resistors in series to draw about 2 Watts from a battery, make sure they are electrically insulated, and wrap the string of resistors around the lens barrel near the front element. Use a lens shade, and you can even wrap some closed cell foam around the outside of the resistors to hold in the heat. A couple of Watts will keep the lens just above the dewpoint without creating turbulence. Figure out how many hours you want it to run and use a lead acid, deep cycle, or power supply with sufficient amps or amp/hours for the task. Some people use gutter heating tape of the right length/resistance to generate a couple of Watts. I use one that was given to me that I can wrap around two lenses at once. You can use a cigarette lighter plug for 12V (well... 13.8V) from a car battery.

One of these http://www.sciplus.com/singleItem.cfm?terms=2945 will also work off 12V, giving you about a Watt. (DON"T USE IT ON MAINS VOLTAGE TO WRAP YOUR CAMERA!)

Since you're a ham, I know you don't need the math spelled out for you, or that last warning. You can google for more options or ideas.

Hope this helps.

Lee

Here's a link with DIY instructions. For most camera lenses one Watt is likely sufficient, but two won't hurt unless you need to stretch battery life.
http://skytonight.com/howto/diy/3304231.html

You need to remember that the earth takes 24 hours to revolve 360 degrees. Therefore it takes 1 hour for the stars to intercept an arc of 15 degrees. A one hour exposure then would give you a very short streak.

To learn how long it takes for a streak the wength you would like, dran one and then measure it with a simple protractor. You probably are thinking of streaks which require 3+ hours of exposure.

Not quite on the exposure issue, but if you use neg film you'll want to make even more sure than usual that the negs are absolutely clean when you print them, because the smallest dust spot will stand out like the proverbial sore thumb against a black sky. It's less of an issue with slide film where dust prints black, but there, of course, exposure becomes much more critical.

David.

p.s. Helen, that was a lovely shot.

Success in long star trails requires dark skies. f/11 on T-Max 100 film works fairly well for 8 hour exposures on a good midwestern night. This was 8 hours at f/6.6 on Kodak Tech Pan developed 6 minutes in print developer for enhanced speed and contrast. A dew cap somewhat like Lee described was used.

Hi Jim,

Nicely done.

Rich