Messier 13 - Long exposures?

[Tim]

Conventional wisdom is to take a whole bunch of shorter subs for M13 to avoid burning out the core. In practice, at least with a OSC camera, you can probably go a lot longer and still resolve the detail in the core. The longer exposures help to bring out the very faint outer stars in this DSO, and is the only way to see them. M13 is amazing in an eyepiece, but cameras reveal so much more of it. Of course, there is a price to pay in star colour when you expose longer.

This M13 image consists of 14 x 900 and 3 x 600 seconds with Skywatcher Esprit 150ED. Not as long a total exposure as I really like so the background is a little noisy.

I will post a 2 x 1800secs of M3 for comparison later.

Thanks for looking.

Tim

[AlistairHowie]

Thanks for posting, Tim.

I'm really interested in this discussion because I took a pic of M13 on Saturday with an Atik 4000 OSC and an 85mm f/5.3 scope. I only managed 6x300s - so eight times less exposure than you managed, and my result is below (excuse the colour balance!). I can make out IC4617 (apparent magnitude 15.2) a little more clearly in your pic, but it's still present (just) in mine ... that's the really faint galaxy in approximately the same direction as the more obvious one.

I'd have expected a much larger difference between our pics. Is this the difference of f/7 versus f/5.3, or a cooled CCD versus a DSLR?

[martin_h]

I found this image.............. http://www.spiegelteam.de/m13.htm

[Komet]

Yeah ... compare a 16" to small scopes.

There is more colour in there than you think Tim. That said I would have expected more with those exposure times. I got most of the star colour from my recent attempt at M13 (http://stargazerslounge.com/topic/182322-m13-rgb/) by using some 600second exposures but even so I would have expected more from yours. The background noise is really grid like on your image also. Is this normal with a OSC (I don't use one - mines mono with less resolution)?

[Tim]

Make sure you view the full resolution image Mick? The progressive saving of the jpeg gives the grid and if your browser doesn't load the whole image you get left with it I think.

[Komet]

Done that - downloaded it and looked. This is a crop at 100%.

If you push for more colour then this becomes extremely apparent.

As I said I don't use OSC cameras so have no idea about their foibles. Just wondering really.

[Tim]

It's odd how those blue stars always show up so well, the really small ones. On longer exposures with the OSC I find the colour washes out of even the most vivid stars Mick, especially the blues, better to capture it with filters I should think. However the OSC and DSLRs do seem to give a natural look to the colour scheme, which can sometimes look forced with LRGB images.

I've just stacked a bunch of LUM data from my C11 of M13, will be interesting to see how that looks with this colour overlaid.

Alastair, as you see, DSLRs are great for this target. With so many factors involved, ie, focal length/ratio, seeing & transparency, light pollution, position of target at time of capture, aperture, QE of camera etc etc it is almost impossible to do a like for like and come up with anything like meaningful information

[Komet]

it is almost impossible to do a like for like and come up with anything like meaningful information

Amen to that

[Tim]

It's one of the reasons I prefer the mono camera Mick. I don't use noise reduction methods at all if I can help it. More subs would have made this smoother, it's only four hours/14 subs, would normally get 25- 35 subs. Might also be some artifacts from the DBE routine in PI perhaps.

It doesn't help that jpeg compression makes it worse.

[AlistairHowie]

With so many factors involved, ie, focal length/ratio, seeing & transparency, light pollution, position of target at time of capture, aperture, QE of camera etc etc it is almost impossible to do a like for like and come up with anything like meaningful information

Good point ... but I'm still intrigued!! Did you take yours with your DSLR?

[LongJohn54]

This is my M13 taken shortly after I bought my Atik 314L and before I had a filterwheel and filters and pre-guiding. I think I cooled the camera to -10c.

It's experimental more than anything else. Pretty sure all I did was point Atik's Dawn at the files and stack and combine.

So, 12 x 60 seconds plus 1 dark (not needed really) got me this -

M13 12 at 60s 2012-08-08 by LongJohn54, on Flickr

I'm too much of a beginner to know what's best but was quite pleased with this image.

John

[AlistairHowie]

Wow - well that shows just how sensitive the 314L+ is ... which of course everyone knows.

I know it depends on set-up, seeing, etc, but in general, can it be deduced that bright star clusters like M13 don't enormously benefit from long exposure times, versus other DSOs?

[Tim]

Clusters are unusual insomuch as they are stars, inherently bright, and even the tiny point sources tend to show up. You don't need long exposures to get very nice results.

However I think you still get more signal on the fainter stuff if you can expose longer, giving more definition to the faint stars.

With my skies, If I take 5 minutes of M13, with the LP filter and OSC camera, the results are poor. Much better with the mono CCD though, see my M13 Hybrid and LUM only version on astrobin.

[ollypenrice]

I'm not much persuaded by short exposures to control bright objects. The Trapezium is the obvious exception.15 minute subs in a sensitive CCD at F3.9 can split Alnitak into a double. (http://ollypenrice.s...ODKHORSE-X3.jpg) You just need to do multiple stretches for the dynamic range. ( On a cluster, though, I would tend to ditch the luminance layer. I think its main role will be to burn out stellar cores. So I'd vote for longish RGB only.

My impression of QHY colour cameras (and their devotees will doubtless belabour me about the head with wooden mallets for saying it) is that muddy browns tend to predominate.

This is a very crisp M13, though.

Olly

[Tim]

My impression of QHY colour cameras (and their devotees will doubtless belabour me about the head with wooden mallets for saying it) is that muddy browns tend to predominate.

Olly, please try and answer this question before the pack gets hold of you

Are they the same in dark locations have you noticed? I just assumed mine was picking up the ambient sky colour which is indeed muddy brown :s It was a bit better from Kelling Heath, but still needed a LP filter to avoid bad gradients.

The preview of the .fits file off the camera in nebulosity is always yellow/brown :/ and general blue response seems to be lacking a certain something.

[Coco]

Conventional wisdom is to take a whole bunch of shorter subs for M13 to avoid burning out the core. In practice, at least with a OSC camera, you can probably go a lot longer and still resolve the detail in the core. The longer exposures help to bring out the very faint outer stars in this DSO, and is the only way to see them. M13 is amazing in an eyepiece, but cameras reveal so much more of it. Of course, there is a price to pay in star colour when you expose longer.

This M13 image consists of 14 x 900 and 3 x 600 seconds with Skywatcher Esprit 150ED. Not as long a total exposure as I really like so the background is a little noisy.

I will post a 2 x 1800secs of M3 for comparison later.

Thanks for looking.

Tim

Wonderful flat field.. stars do melt into each other on over exposures.. On M13 I take 2 sets of images like I would for M42..

[Tim]

Wonderful flat field.. stars do melt into each other on over exposures.. On M13 I take 2 sets of images like I would for M42..

That's kind of what the thread is about Guy, whether you need to take two sets? Layered cores always look a bit weird to me, same as M42's if the layering isn't cognitive of the fact that the core IS a lot brighter than the outer regions.

With my mono camera 90 seconds is ample to get every star in the cluster, anything over that and the core starts to weld together. But with the OSC it can go up to 1800seconds and still resolve individually. I'm surprised there is such a massive difference in the two cameras.

[sologuitarist61]

Nice!

[andyo]

nice image