and now for something completely the same...

magic image Michael, you must be very happy with it.
So thats 1 hour 20 minutes worth of exposures when added up, would have using exposures of longer then 60 seconds have gained you anything, say if you took exposures of 5 minutes, what would you have got ??

Nice capture on the very faint edge of spiral in the top left corner.
Did you use a focal reducer ?

The 30sec exposures were with the focal reducer whilst the 60sec exposures were without the focal reducer. When combing the images, the software can compensate to develop the resultant image.
Yes, the FOV could be a fraction bigger alright but this was the price I paid for taking the images on two different nights with different camera positions - I am then limited to a common overlapping area in all exposures.
Thanks for the kind words and constructive feedback.
Cheers,

would have using exposures of longer then 60 seconds have gained you anything, say if you took exposures of 5 minutes, what would you have got ??

Good question Dave and I don't have a definitive answer. My guess is that it would depend on the target. With M51, I can capture detail across the majority of the galaxy in short exposures. However, when looking at the image histogram, the detail inn the spiral ares is near the lower end of the range, near the zone which has noise. To overcome the noise, I have taken loads of images. As all these images have the same detail of the galaxy, the detail of the galaxy gets combined whilst the noise gets cancelled. If I were imaging M51 again and I had a choice of taking 100 x 1 min exposures or 10 x 10 minute exposures, I would definitely go for 100 x 1min. Whilst the longer exposures would help detect the faintest extra detail in the very outer edges of the galaxy,the benefits gained would be lost many times over as I would only have 10 images and the resulting image would be noisy and not near as smooth looking.
However, all that could change if I were imaging a very faint object, say a mag 18 galaxy. In this case, I would have no choice buy to use long exposures in order to detect it in the first place.
Hope this answers your question.

would have using exposures of longer then 60 seconds have gained you anything, say if you took exposures of 5 minutes, what would you have got ??

Also a question I have considered. The way I look at is this. Consider imaging a mag 18 star. Image for 30 seconds you get virtually no signal hit on the detector. (the light falling is too low to cause any significant well charge on any pixel). Now image for 5 minutes and you get a reasonable signal hit on the detector. Now take 10 of those 5 minutes and you get a good signal plus low noise. Now think of what happens if you take the same exposure time in 30 second subs, i.e. (100 x 30secs) You still have a very low signal detection on each subframe and when you average these you still have a low signal and maybe don;t see the star at all on your 100 x 30secs whereas you do see it on your 10 x 5's.

In theory 100x30's should equal 10 x 5's but because a CCD pixel requires a certain level of photons to make any detectable signal on any given pixel, in practice you lose out on fainter detail on your shorter subs.

A example is the recent M13 image I posted recently. I have done M13 before, always with short subs and got nowhere near as deep even more overall exposure time. Time for some empirical study methinks!

WOW

One off the best of M51 i've ever seen when are you getting the colour filters
looking forward to seeing that one in colour

i'm well impressed

Michael,

BEAUTIFUL result.

The question of many quick exposures or few lengthy exposures is something that comes up often. I am not sure there is a definite answer and I concur with Dave about losing faint detail with the quick exposures. Otherwise, we would all be taking thousands of 1-sec exposures.

Part of the problem is the read noise and the sky background. We need an exposure which will be above both of these and sufficient to record the faint detail. However, if we go overboard and pursue exposures which are too lengthy, the sky background will certainly overwhelm the result.

I can understand and appreciate the desire for many exposures so as to get a smoother image due to the stacking of many images. However, on the side of the coin, we have the greater depth with the fewer but longer exposures (at the expense of some grain).

Here is an example of mine where I pigged out on the luminance (total time) for M101 but did not get the result I wanted: www.astrovox.gr/forum/album_pic.php?pic_id=3496 ... I suspect with many more exposures at five minutes (instead of fifteen minutes) the result would have been much better.

Ditto for the photo of M51 in the same mosaic.