Re: [APML] 'challenge'

["John Boudreau" <jeboud@attbi.com>]

----- Original Message -----
From: "Matt BenDaniel" <matt@starmatt.com>
To: <astro-photo@seds.org>
Sent: Thursday, December 19, 2002 4:54 PM
Subject: Re: [APML] 'challenge'


>
> John Boudreau said:
> > A double exposure would be the way to go, *if* the
> > scope/camera/autoguider setup could be left _untouched_ for a day or
>
> Why the "untouched" requirement?  If the camera is always oriented
> orthogonally to the celestial axes, and if the images are centered in
> more-or-less the same place, what's the problem?  Your untouched
> requirement is ethical not technical, right?

This event has been discussed briefly here and in other web forums recently.
Some people feel that a composite would hurt the " authenticity" of the
image. I certainly don't have a problem with that as long as it's stated up
front, and of course a double exposure is a form of composite also.

But it's also the only way to exactly frame the two exposures.

>From your earlier post:
>
> It depends on the "rules".  You could take a long exposure of the Crab
> before Saturn crosses it.  Then you could take a short exposure of Saturn
> as it crosses.  Assuming you register the images on the stars, would that
> be "fair"?  It can even be accomplished without using any digital
> technologies (although I would).
>
> Matt

How long were you expecting to expose for Saturn, and still get stars in the
field to allow registration to the long exposure? ;o)

For my C11 for instance, the Saturn exposure would be about 1/30 sec at f/10
using a film like Provia 400F. Not many alignment stars to play with <g>.
And even allowing for longer exposures and thus allowing Saturn to be
recorded as a "blob", well, I think that Saturn would be _ridiculously_
overexposed in any exposure that could possibly give you even one or two
alignment stars.



>
> BTW if Saturn is in or near the FOV, its glow may "pollute" the image.

I don't know how far away from M1 it will be a day or two before the
transit. But as long as it's clearly out of the FOV during the long
exposure, all should be fine, especially if M1 is somewhat underexposed to
simulate a "visual view".
Of course, the wider the FOV the more of a problem Saturn's brightness
becomes for the M1 exposure. Those with long focal length and thus a narrow
FOV have an advantage here, as it allows them a shorter schedule between the
two exposures. Weather of course is another concern.


>
> Also, we're assuming that none of us have the resolution to register the
> motion of the Crab during the intervening period.
>
> Matt

I'm not quite sure what you mean by this, but I'll take a stab at it.. If
your talking focal lengths, there should be plenty of APML members that have
enough to resolve detail in M1 and it's surrounding field. With a fine grain
film your AP130 should do OK, but you'll have to shoot your long exposure
days earlier or later.
BTW--- At 2800mm, with my C11, Saturn's rings would still be only about
.75mm wide on the film. The most interesting composites will still be from
high-rez CCD imagers.

If you want to shoot separate frames of Saturn during the transit, perhaps
the best way to try to get registration is to take a quick exposure of
Saturn while autoguiding the scope. With the autoguider still tracking,
immediately take several exposures of several seconds to a minute to also
capture the brightest stars. Assuming good in-camera alignment
frame-to-frame, use the longer Saturn exposures as guides to register your
quick Saturn exposure to a long M1 shot taken at another date. In this case,
a wider FOV helps with acquiring registration stars as opposed to a much
longer f.l. setup. But it's not going to be easy to get *perfect* alignment,
as the frame edge of the quicker exposures will be difficult to discern from
the background with any real accuracy. And trying to figure the exact center
of the bright blob of Saturn in the overexposed frames will probably be a
best guess.


I still think that a double exposure with a long f.l.scope (2000mm or more)
has a _big_ advantage in accuracy here.

Regardless, anybody getting a successful, accurately aligned image of this
will have the astrophotography gods on their side <g>

John Boudreau
http://home.attbi.com/~jeboud/astro.htm


































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