[Prev][Next][Index][Thread]

(meteorobs) Dewing problems.



R. L. Thompson wrote:
>
> 1) I lost some good shots because the camera objective dewed up.
>

    You can make a chain of resistors and wrap them around a lens shade.
Feed a voltage to this chain, increasing the voltage in incriments until the
resistors just start feeling warm, (not hot), to the touch.  This should do
the trick.  I use a power resistor fed with a/c voltage from a variac.  You
can also use a light dimmer switch available from home building suppliers.
We do this for the convex mirror on a Sandia All-sky camera. If you use a/c
power to do this make sure your connections are well insulated to prevent
shock. (BE CAREFUL)  Always start at the lowest voltage level, adjusting it
until the resisters just feel warm.  Dew cap heating devices are also sold
by atm telescope sellers.  Kendricks, comes to mind.  They make them for
camera lenses and up to 14 inch Schmidt Cassegrain telescopes.  Probably
larger, now!
    You can also increase the length of your exposures by using a
synchronous motor diven chopping shutter.  You can select different on/off
ratios for this purpose.  The direct cameras used at the now closed
Meanook/Newbrook Meteor Observatories in Alberta, Canada, used a ratio of 2
off and 1 one but 1 to 1 is okay also.  This not only provides you with
apparent angular velocity of the meteor, single station estimated height
information if you can identify the meteor with a particular shower, and
also lets you increase your exposures by 2 to 3 times before film fogging
becomes a problem.  Chopping ratios of 10 to 60 times per second have been
used.  For small format cameras 10 to 20 times per second is a good choise.
    Hope these suggestions help!

Ed Majden


To stop getting email from the 'meteorobs' list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html

Follow-Ups: References: