I don't know how familiar you are with astrophotography but your
selection of a lens is only the beginning. At 500mm you must give very
serious consideration as to how you're going to mount and guide the
camera and lens. The only exceptions are lunar photography and star trails.
A 500mm lens on a 35mm camera has a horizontal field of view of 4.12
degrees. The angular velocity of the earth is 15 degrees per hour.
Therefore, with a stationary camera, the image will move completely
across the 36mm long field in only 16.48 minutes or, looked at another
way, during a 1 minute exposure (short by astrophoto standards) the
image will have moved over 2mm on the film plane between shutter open
and close.
Since the moon is reflected sunlight, moon exposures are akin to
daylight photography with fast shutter speeds. Star trails are done
over long periods of time with a stationary camera but are typically
done with a wide angle lens. For anything else you will have to guide
the camera.
For an inexpensive means of driving a camera for astrophotography you
can consider what is called a "scotch mount" or "barn door mount" such
as illustrated here: <http://www.philharrington.net/scotch.htm>
Even this, however is limited to a focal length of about 200mm and a
maximum exposure of 10 minutes.
A tracking accuracy refinement of the barn door mount is called the
"double arm barn door mount" and can be seen here:
<http://www.astunit.com/tonkinsastro/atm/projects/scotch.htm>
But even this is still fairly limited for focal length and exposure time.
If you really want to shoot with a 500mm lens then I would suggest you
use what is called a "piggyback" mounting where the camera/lens is
attached to a telescope which can be electronically driven for short
exposures. But it will also require manual guiding and correction for
longer exposures. To do this cheaply I would suggest a Meade 4-1/4"
(120mm) Newtonian reflector and motor driven equatorial mount which can
be purchased at discount chain stores (but often only around Christmas
time) for about $100-125. This will give you an accurate guiding and
tracking mechanism at low cost. These scopes typically come with cheap
.967" (microscope size) eyepieces and adapters but can be easily changed
with a standard 1-1/4" telescope eyepiece adapter available from
Meade (mine was free with a phone call) and a good quality 1-1/4" wider
field eyepiece obtained for $25-35. Meade provides good quality for the
price. Other brands found in discount stores may be very risky.
The camera can be attached to the tube opposite the eyepiece and then a
counterweight of similar weight attached at the opposite end of the tube
and on the same side as the eyepiece. The counterweight must not only
balance the camera around the declination axis of the mount but also
must counteract the varying torque around the axis of the tube as its
position is changed.
If the scope is a Newtonian reflector make sure the tube is allowed to
rotate in its mounting saddle. The cheapest construction simply bolts
the tube to the mount head and the tube and eyepiece can not be rotated
to convenient viewing positions. Do not buy such an arrangement or you
will be eternally frustrated. Look for a mounting saddle with split
rings that can be loosened to allow rotation of the tube.
Mounting hardware can be found for this sort of stuff by perusing the
ads in any issue of Sky & Telescope magazine.
Chuck Norcutt
Allen Coltrin wrote:
> So you used a Oly 500mm lense? I've been thinking about this quite a bit.
> And I have to admit, I'm pulled between a mirror lens and a standard long
> fixed lens. Part of it is to do some astrophotography.
>
> Allen
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