Have seen R. Freeze's M51 image - impressive. As I'm relatively new to 416
imaging, I have some questions...

If I understand correctly, the image of M51 is a composite of 4, 1 minute
exposures. If taken with a 416 and PView 6.4 "shift & combine," there
wasn't any actual "shifting" of the images when being combined (either by
actual scope guiding since the 416 doesn't do that or by software image
aligning as PView 6.4 doesn't automatically do that). So, taking image
download time into consideration, the actual time from the start of the 1st
exposure to the end of the 4th exposure was probably close to 7 minutes.
Obviously, the scope has an excellent tracking setup.

Now, my questions:

1. I don't think I fully understand the signal/noise issue. My working
assumption is that to maximize the signal/minimize the noise you should take
the longest exposure possible, while still avoiding blooming and/or tracking
problems. So, if that's correct, would it have been better to take 1, 4
minute exposure rather than adding 4, 1 minute exposures together? Or is my
base assumption wrong?

2. If the tracking was actually accurate over 7 minutes, would 1, 7 minute
exposure be even better yet?

3. If not using PView 6.4 "shift & combine," is the proper procedure for
taking multiple images to take a series of separate images, then add them
together manually and aligning the pixels with each add? Further, if that's
right, should the dark frame be subtracted from each image before they are
added together?

4. Since PView 6.4 doesn't actually "shift & combine" with a 416, I use
Pictorview 2.0 where "shift & combine," actually aligns each image via
software as it combines. Using "shift & combine" with Pictorview 2.0, is
the proper procedure to use the automatic dark frame subtract? If not, I
don't think I know just how to subtract a dark frame. (If taking 4, 1
minute exposures and adding them, do you subtract 1, 4 minute dark frame or
subtract 4, 1 minute dark frames?)

5. I understand that the 416 has some chip heat-related over-exposure in
the upper left corner of the image which is normal. I also end up with
about 20 white pixels in any exposure over about 1 minute at -20 degrees C.
I find that taking an automatic dark frame (same length exposure) doesn't
fully eliminate the upper left corner over-exposure nor the 20 white pixels.
The intensity is reduced, but not totally eliminated. I have found that a
dark frame subtract of about twice the image exposure length (same
temperature) eliminates the problem. Using PictorView 2.0 "shift & combine"
(#4 above), if the automatic dark frame subtract doesn't completely
eliminate the white pixels and left corner over-exposure, the final image is
not as good as it should be. Anyone with a similar experience?

Thanks to all for any help.

Jim McMillan

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