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From: ajs@hpfcla.UUCP
Newsgroups: net.space
Subject: Re: Orphaned Response
Message-ID: <22000014@hpfcla.UUCP>
Date: Sat, 18-Jan-86 23:01:00 EST
Article-I.D.: hpfcla.22000014
Posted: Sat Jan 18 23:01:00 1986
Date-Received: Thu, 23-Jan-86 08:53:47 EST
References: <8601151738.UUCP>
Organization: 18 Jan 86 21:01:00 MST
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> If you cannot track the stars, I would not recommend a picture longer
> than about 5 minutes with the telephoto as the image will begin to blur.
> If you use the 50mm lens, you could maybe get by with a good 5-min
> exposure.

The rule of thumb I recently heard, which applied post-facto to some
streaked pictures I took, is:  maximum exposure (seconds) to avoid
streaking = 600 / focal length (mm).  For a 50mm lens, you get 12
seconds.  For 200mm, 3 seconds.

You'll also need high-ASA film.  In dark skies, 60sec on 100ASA at 50mm,
f/1.7, produced negatives about 1/2 as dark as needed for good results,
with noticeable streaking.  If things are linear (and they are probably
not), you'd need something like 30sec (way too long) with 1600ASA,
200mm.

Alan Silverstein