Okay, now that I have registered again for kindergarden, lol.
Allen
>From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>Reply-To: olympus@xxxxxxxxxx
>To: olympus@xxxxxxxxxx
>Subject: [OM] Astrophotography, was: Re: New Person to the group
>Date: Mon, 19 Feb 2007 07:05:54 -0500
>
>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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