From: sc02492 (sc02492_at_antispamdomain.not)
Date: Wed Nov 26 2003 - 02:57:59 EST
Hi Forrest, I agree with Greg and others- nice shot. Needs more
exposure time to capture the faint stuff though and to enhance the
red signal. Once you've captured as many faint details as possible,
it should be possible to reprocess without clipping too much of the
shadows end.
Greg, I think what you are noticing regarding the lack of red in
digital M42s is, in the end, all about exposure time. If you expose
long enough, which means cool temperatures and a good filter to
avoid sky fog, you can get enough red signal to compose an M42
equivalent to that seen with film or CCD work. I think we are
seeing many nice shots that are more dominant in blue and green, but
which stop short of acquiring enough red, to be balanced. If you
look at my recent M42, there's plenty of red in there, and I didn't
have to coax it out too much at all. But I used a filter, and for
some of the exposures I went 8 minutes at ISO800....
Steve
--- In digital_astro_.at._yahoogroups.com, "Greg Harp" <gregharp_.at._s...>
wrote:
> Forrest,
>
> That's a very nice M42!
>
> You did a great job combining exposures to render the majority of
> the detail within the dynamic range of the display. I don't know
> the actual photometric values, but in "real life" this object
seems
> to go beyond the nine or ten stop latitude of the Canon cameras,
so
> most people tend to lose one end of the dynamic range or the other.
>
> Since you're shooting at f/6.3 you might try some longer exposures
> to catch the dimmest parts of the nebulosity. I am thinking that
> four to five minutes at f/6.3 should do it.
>
> Or you might be able to add that up using your ten 2-minute
> exposures. In fact, I would try that before re-shooting. If you
> take pairs of these images (after calibration and alignment, of
> course) and do simple addition you should get (effectively) five 4-
> minute exposures. That should be enough to work with.
>
> At least I know that I have managed to capture pretty much all of
it
> at ISO 800 in one minute with f/2.8, and that's about a 5X times
> faster aperture. (2 ^ 2.333333...)
>
> As for focusing, even using the zoom in playback mode (after an
> exposure) on the LCD doesn't seem to help me much. It's just not
> enough.
>
> I have found that DSLRfocus is indespensible, which pretty much
> means you have to have a laptop. As it progresses to v2, a lot
more
> capability to analyze focus is being added. However, even v1
helps
> a lot by simply allowing you to magnify a star by some large
amount
> (certainly more than 100% -- individual pixels are easy to see in
> the zoom view)
>
> I am noticing that a lot of M42s shot with Canon DSLRs are a lot
> more magenta and gray than the reddish film images we've all seen
in
> the past. Those that aren't have had the red bumped up in
> processing, usually. Are we seeing the H-alpha sensitivity of the
> Canon sensor again? Or is that just color shift on the film due
to
> reciprocity failure?
>
> Unfortunately, I've never managed to locate a source of
information
> showing what the actual spectography of certain objects is, so I
> don't know what it's *supposed* to look like. Photos of this
object
> in particular vary quite a lot.
>
> --Greg
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