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Photographing Comet Hale-Bopp
(Thursday). Since this relates to the above question, and might be of general interest, I'm posting my answer here.
Here's the problem: the moon is really, really bright -- thousands of times brighter than the comet. So the simple answer is that
there is no single proper exposure for both the moon and the comet. The moon is an object brightly lit by the sun - the
approximate exposure for it when it's full or nearly full is 1/film ISO at f/11. When it's a crescent the proper exposure is 1/film
ISO at f/8. As you already know, the proper exposure for the comet is around 20 seconds at f/2 with an 80mm lens (or longer, if
you have a tracking mount for your camera).
The complex answer is this: you can do a double exposure. It won't be technically correct, but it will look cool. Here's how:
First, using the longest lens you have, expose the crescent moon at 1/film ISO at f/8. For example, if you're using Fuji 800, the
proper exposure would be 1/800 at f/8. Since most cameras won't do 1/800, you'd pick the closest thing, which might be 1/750 at
f/8, or 1/500 at f/11 if your camera can't do 1/750. Place the moon on one side of the frame - be sure that your camera is set for a
double exposure.
For the second exposure, change lenses, put the comet on the other side of the frame, and expose for 15 or 20 seconds at approx.
f/2. Now you have a picture with both the moon and the comet properly exposed in the same frame.
If you really want to do a single exposure which shows the moon and the comet on the same frame, you have to simply let the
moon overexpose.
-- Russ Arcuri, April 7, 1997
I also should have noted that the ECLIPSED moon calls for a longer exposure than the normal, uneclipsed moon. I'm not sure of
the proper exposure, but it would probably be along the lines of 2-3 seconds at f/5.6. Perhaps someone who knows the correct
answer to this question could comment? I'm really just guessing here.
-- Russ Arcuri, April 7, 1997
I think that comet shots are a great opportunity to "go digital". When I developed my Hale-Bopp shots, I found that I had a
reasonable set of photos, but with one common flaw: grain. This was particularly in the sky and in the silhouette of the treeline
against the sky. I've seen the same in a lot of other peoples' Hale-Bopp shots, which is why I'm bothering to post this.
Here are two ways to fix such grain in an image-editor, once you have a scan of your image: 1. Slam all of the grainy stuff to
black. This is easy to do (by adjusting the input range in the Image:Adjust:Levels dialog in PhotoShop, for example), but very
heavy-handed. For example, it would obliterated the treeline mentioned above. 2. Use an agressive noise-reducing filter. I like
median filters for this purpose. The problem is that if applied to the whole image, any filter agressive enough to suppress grain
would also act upon the stars. The trick here it to create a mask including only the stuff you want filtered (should be everything
but stars), and use that mask to control which pixels the filter can touch. The photoshop "Select Color Range" dialog is a good
place to start for creating such a mask, though there are a vast number of other approaches. I hope this helps someone!
-- Patrick
-- Patrick Chase, July 7, 1997
Two excellent photoshop ways of killing grain are as follows
A. Either in the scanning software(Nikonscan) or in the Adjust curves, use the set black point tool on a dark area of the image.
This will make it a smooth black instead of the grainy black.
B. Convert to CMYK, change to the black channel by hitting CTRL+4. On this channel, use the dust and scratches tool, usually
with a 4 to 5 pixel radius, and a threshold of 0 levels. If you find you lose too much image sharpness, hit CTRL+`, to get back to
http://www.photo.net/astro/intro (4 of 6)7/3/2005 2:14:43 AM