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Re: (meteorobs) Shooting the skies
VoyageMag wrote:
> Being an astrophotographer for many years, I have learned
> that the best f-stop for shooting the skies with print film is either F 4.0,
> or F-5.6, using ANY film. (much deleted about taking high quality photos).
I agree what you suggest will produce many fine astrophotographs.
Unfortunately, it will capture very few meteors.
Sensitivity is very important since the more sensitive your system is,
the far more likely you are to record a meteor, a rare event at the best
of times.
In theory, the camera is most sensitive when the aperture is wide open,
but often in practise the circle of confusion at that setting is
unusually large. Typically closing 1/2 to 1 stop tightens the circle of
confusion to more than offset the reduction in illumination. There
after, the circle of confusion will be continued to be reduced until
about f/5.6 or f/8, but the reduction in illumination is too great to be
offset and sensitivity suffers. After about f/8, diffraction effects
about the aperture start to increase the circle of confusion and
sensitivity decreases more disproportionately.
Finding the most sensitive aperture setting should also be done
inconjunction with finding the correct infinity setting.
On a very dark night, try timed exposures of the same interval of the
same region of the sky using widen open to 2 stops closed and about 5
positons about the infinity setting. The one that shows the most stars
is the most sensitive combination of settings. Don't be surprised if
you get three or four combinations showing about the same sensitivity.
The type of film is not important to the aperture setting, but can
potentially influence the focus setting so it is advisable to use a
representative film. When possible, photographing the polar region will
simplify comparisons. The camera need not track the stars for the test
or in actual use for that matter.
Thereafter, you just select the aperture and focus settings that give
you the best sensitivity for meteor work. They should also work well
for diffuse nebula etc. but will not image stars as well as say f/5.6.
Unlike most other forms of astrophotography, a film that exhibits a loss
of reciprocity with a very short exposure time is highly desireable to
record meteors. A typical meteor will illuminate a nominal film grain
for something like 1/10000 of a second to 1/1000 of a second. No
reciprocity failure will affect the meteor signature. Stars, nebula,
stray light etc will all suffer from reciprocity failure allowing a
longer exposure to the same level of fog than achievable with normal
astro film.
Of course, if you are sure you recorded a meteor, you terminate the
exposure!
I understand that Ilford HP5 has both fast onset of reciprocity and good
recording characteristics, but I can't comment from personal experience
yet! Now that I'm in Toronto, reciprocity characteristics are important
but mercifully, HP5 can be developed locally.
Any suggestions about other films to try are appreciated.
Regards,
John
--
John Ohrt *** Toronto, ON, Canada *** mailto:johrt@ultidot net
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