Star trails with slow (LF) lens?

[appletree]

I follow this guy on instagram. Shoots LF and a ton of night shots/star trails.
I have been very inspired to go out and shoot some star trails. Never had though, and hardly know the ins and outs of my 4x5 Chamonix yet.

[mijosc]

Does anybody have any suggestions to deal with condensation on the lens? I tried a star trail with my Hasselblad a few years ago, but the lens fogged up. I had planned on trying again with my large format in Glacier National Park or the Grand Tetons this summer.

I had thought about rubber banding a hand warmer to the lens to keep it above the dew point. Do you think that would work? Do those last long enough for a 4-6 hour exposure?

[John Layton]

To help prevent fogging...perhaps throw a couple of silica-gel dry packs into the camera then re-mount the lens, then mount a small, battery operated fan (one that does not vibrate) on a boom arm - and point this at the front of the lens? Have never tried either of these - but just a thought!

[Jac@stafford.net]

Does anybody have any suggestions to deal with condensation on the lens?

This might seem counter-intuitive but my 8x10 sky camera lens had an optional heated filter: a few thin wires through the glass to a 22.5V source, but a lower (18V) voltage worked. Contact www.surplusshed.com. Ask them if they still have Metrogon filters.
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[Corran]

Guys, since stars are pinpoint light source it is the real aperture dimension that is important not the f number. Nobody of you saw discussions of this point in the past of this forum? Check the archive.

Perhaps more specifics and links to relevant discussions or search terms would be more helpful, rather than snarky comments.

I am 100% certain that the "real" aperture (f/number) is quite important, assuming you want to shoot with a foreground.

[prendt]

Go ahead, serve it out.

[Jim Jones]

[QUOTE=mijosc;1243723]Does anybody have any suggestions to deal with condensation on the lens? . . . [QUOTE]

I made a large lens hood from a rectangular tin can and mounted a resistor inside it beneath the lens. I don't remember the amount of energy the resistor dissipated, but it was perhaps a few watts. A homemade example for a telescope is shown at http://www.skyandtelescope.com/astro...iy-dew-heater/. Since star trail photographs can be more effective with wide angle lenses, most designs intended for telescopes aren't so useful on cameras. You might find more information by googling for heated dew shield, but most links are for commercial units for telescopes.

[Jac@stafford.net]

I made a large lens hood from a rectangular tin can and mounted a resistor inside it beneath the lens. I don't remember the amount of energy the resistor dissipated, but it was perhaps a few watts. A homemade example for a telescope is shown at http://www.skyandtelescope.com/astro...iy-dew-heater/

That is a terrific tip! Thank you.
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[David Lobato]

Raising the object (lens in this discussion) temperature only a few degrees is enough to prevent condensation. Hence, hand warmers may work fine. A few watts of thermal heat is probably sufficient. Don't overheat the lens and thermally stress its parts with too much electrical power. It's been a long time since Thermodynamics class but there is also the heat flux out of the lens to account for when figuring the energy necessary during the night exposure. For a 5-6 figure cost telescope the thermodynamic calculations are worth it.

[Drew Bedo]

Ummm . . . .What about a portable hair dryer? A few short shots of a low-heat or no-heat setting during the exposure might do the trick. Although in-elegant,tis could be an effective solution if not deeply commuted to doing extensive star-trail imaging.

Alternatively: the Astro-photography world offers battery powered heat wraps and extended dew hoods. Perhaps something purposed designed and off-the-shelf could be adapted to LF photography.