Hi, Fernando and all.
>BTW what do you mean with *piggyback* astrophotography?
I'll answer your last question first...
The most basic astrophotography is done with a simple camera with standard
lens in "B" setting, mounted on tripod. With fast films and lenses (say ASA
1600 and f/1.4), about 20-30 sec. exposures can record a surprising number
of faint objects. However, if you go beyond that limit, the Earth's motion
will show up on the pics -- the famous 'star trails', which sometimes are
desired. With longer lenses, the exposure limit is much shorter.
If you are looking for pin-point stars and higher resolution, you need a
tracking device to compensate for Earth's motion. Apart from the simplest
devices of this kind ('barn door tracker'), the usual way is to use a
telescope with a suitable mount, capable of tracking this motion -- usually
with a small electric motor. In "piggyback" astrophotography, the camera
*with its own lens* is mounted over the scope, but not "looking" thru it --
the scope (most appropiately, its mount) is just used as tracking device.
With not too long lenses (say 50-150mm), this method allows much longer
exposures (for slower lenses an/or films). However, tracking is seldom
perfect: set-up misalignments, gearing eccentricities, power supply
fluctuations, poorly balanced system, etc. Higher resolution pics need
almost perfect tracking, and this is achieved by the 'guiding' action. With
piggyback astropics, this simply means to keep looking *thru the scope* to
a reference star *while* doing the exposure. In case this star drifts, a
small correction on the motor's speed is needed. Keeping the star's
movement to a minimum will improve tracking, and thus resolution with a
longer lens.
The mot advanced astrophotography does *not* use the camera's lens, but the
scope itself is working as the lens for the body mounted on it, more or
less where the eye is supossed to be. Guiding this setup is much more
complicated, needing different techniques -- not worth explaining them
here!.
>Saw you used the 200/4 often. I know about its reputation
>regarding vibration, but it seems you use it frequently. I do, too. Would
>like to read your comments on this lens... you stopped your welcome to Kathy
>on the 135mm !!
Well, if I stopped my comments by the 135mm, is because I have almost no
experience on "terrestrial" pics with the 200/4, which I've used a lot for
astropics... The good thing of this lens for astro is the *easy tracking*
-- it's so well balanced that the scope moves smoothly with it piggybacked,
making the aforementioned guiding action almost unnecesary.
The 200/4 mounted on a body seems to concentrate most of the weight near
the gravity centre, thus making the system *easy to move* (just like a
mid-mount engine sports car!). That's a *good* thing for tracking with a
scope, but very *bad* for hand-held (or eve tripod) photography... I
believe that's the reason for its reputation.
BTW, the astrophotography section on my web page still has *a lot* of work
to be done... I've got many astropics over there, but I need to classify
them, scan them and upload them -- quite a chore!
Best regards,
...
Carlos J. Santisteban
<cjss@xxxxxxxxxxxxx>
<http://cjss.galeon.com>
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|