Re: [APML] M33 exposure results

[Jerry Lodriguss <jerryapml@astropix.com>]

Hi Greg,

In my experience ( http://www.astropix.com/HTML/I_ASTROP/I11/I11.HTM ) if 
you want to match the sky background density, you have to expose 3x with 
the IDAS LPS filter.

For example, at my site, I can expose for 15 minutes at f/2.8 with E200 
without the filter, and need 45min at f/2.8 with the filter.

45 min at f/2.8 equals 360 min at f/8...

Jerry


At 08:26 PM 10/24/2004, you wrote:

>Hi, Alan,
>
>I got the film back yesterday and looked at it. The damned thing showed some
>field rotation. Field rotation! I've never made that mistake before so you
>can imagine how peeved I was. I'll process it anyway for practice and to see
>what can be done with this general length of exposure. One exposure was 155
>minutes, the other 180 minutes. I'll try again as soon as the Moon permits;
>M33 is a priority with me.
>
>Quick comment: I took one look at the frames and was astounded by how thin
>they were, even with that length of exposure. Let's see... I was shooting at
>f/7, you were shooting at f/8. For a fixed focal ratio, the brightness
>density (at the film plane) of an extended object will be the same at the
>same exposure. The only thing that changes is the image scale as the focal
>length changes. That said, my image should have been (8/7)^2 brighter than
>yours, which is 1.31. That means my 155 minute exposure would be equivalent
>to a 202 minute exposure by your f/8 system. However, I had to use a Tokai
>LPS which introduced a filter factor. Assuming that factor to be 1.5, my 155
>minute exposure would thus be equivalent  to 135 minutes on your system with
>no filter. Hmm. Not so hot. Maybe that's why the image was so thin. Damn.
>Did you think your image was thin?
>
>Regards,
>
>Greg
>
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