How to image globular clusters with DSLR without burning stars colors?

[rofus]

Hi,

I always struggle imaging globular clusters. I have a DSLR Nikon D90 to a 750mm refractor (non apo), I can track up to 8/10 minutes without problems.

Problem is that I struggle not burning globular clusters stars. I'd like to be able to get the faint stars around and still keep the color of the brightest ones in the core.

I think probably multiple exposures is the solution? I need to find a good compromise between low ISO/longer exposure so that the brightest stars are not burned out (so I can keep colors and not make them just all white and bigger and reduce as well halos).

Any suggestion? Thinking about M13, I never get a satisfying image out of it.

[Moonshane]

I am not an imager but know that many objects require different exposures for the whole to be aesthetically pleasing. Eg the solar disk and prominences, galaxies with bright cores and I'd expect globs to be the same.

[rofus]

Thanks.

Yes I usually do quite well on other DSO like galaxies and nebulae, but globular clusters are never as good for me.

Anyone has any tips about globular clusters? Like if it's better to use very low iso (like 100) and longer exposures, and if you also use multiple exposures at multiple times to get a good/colored core and all the faint stars around?

[dph1nm]

Like if it's better to use very low iso (like 100) and longer exposures

ISO shouldn't affect the required exposure time, so you can use low ISO and the same exposure time.

NIgelM

[rofus]

ISO shouldn't affect the required exposure time, so you can use low ISO and the same exposure time.

NIgelM

But if expose 5 minutes at 800 iso or 100 iso result is very different obviously, I'm just not sure if anyone has some specific experience with globular clusters and have some tips about how to not burn white the stars and keep some of their original colours?

[pipnina]

But if expose 5 minutes at 800 iso or 100 iso result is very different obviously, I'm just not sure if anyone has some specific experience with globular clusters and have some tips about how to not burn white the stars and keep some of their original colours?

Reduce your exposure time so none of the stars are clipping and take more exposures instead. More exposures = less noise and fainter stars become visible once you stretch the histogram.

[Steve Ward]

Agreed , shorter exposures but loads more of them ... 

[rofus]

Reduce your exposure time so none of the stars are clipping and take more exposures instead. More exposures = less noise and fainter stars become visible once you stretch the histogram.

Thanks!

That's what I imagined..any idea about how much? I'll have some tests using very low iso to reduce noise, I anyway use drizzle as well so that helps as well with noise, and will do some tests to see what's the point of clipping.

[pipnina]

Thanks!

That's what I imagined..any idea about how much? I'll have some tests using very low iso to reduce noise, I anyway use drizzle as well so that helps as well with noise, and will do some tests to see what's the point of clipping.

Use your camera's unity gain or higher. I looked your camera up on http://www.sensorgen.info/NikonD90.htmland your unity gain is somewhere between 3200ISO and 6400ISO (Personally I'd use 6400 since 3200 is slightly below unity gain)

As far as how many shots... Until the image isn't grainy anymore!

[ollypenrice]

I take a renegade view. I do use cameras with great well depth (and low quantum efficiency) so this may affect my experience. However, I don't find any need for multiple exposure lengths other than on one image in maybe a hundred. M42 certainly needs them. Andromada maybe, but only maybe. Apart from that? I'm getting stuck.

Firstly have a look at your capture in linear form (before stretching.) Whatever you see there is there in your data. If the core of a globular is not a big white blob in that linear data you do not, absolutely not, need shorter subs. You have what you need. If you do see a saturated core then, yes, you need shorter subs.

The rest is the proper manipulation of your stretch. Photoshop allows layers, masks and (on the QT my favourite) the eraser. Make multiple stretches in multiple layers and keep what you want from each stretch/layer. Multiple exposure addiction is a red herring. (I do use it on solar imaging for disk/prom, certainly.)

I really do think that I've used multiple exposure lengths only twice since I started to understand processing.

M42 yes: http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-kNjFmJW/0/X3/M42%20TEC140%20LRGB%20V3-X3.jpg

M31 maybe: http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-3D2Hw7s/0/X3/M31%20Outer%20Halo-X3.jpg

The rest, no.

Olly

[Moonshane]

On that basis I retract my original statement based on what I thought I knew!

[ollypenrice]

On that basis I retract my original statement based on what I thought I knew!

Well, with a DSLR and its shallow well depth, what I find with my CCD rigs might not apply. I don't know.

Olly

[rofus]

On a dslr the lowesr the iso the better the result (less noise already in the raw files that makes drizzling even more efficient, less burnt highlights etc.). Last night I spent few hours on M13 and found a good balance being 640iso (almost no noise with a d90) and 2min subs produced nice contrast, full colours and no burnt out stars, so I got an hour or so that way.

As I usually do I also got another hour at 640iso but longer exposure, 5mins, for the fainter stars around the core (that usually requires more work but produces much better results than stretching shorter exposures).

I'll then elaborate both exposure times in Pixinsight independently, concentrating on the core for the 2min and the periphery for the 5min and will combine them later.

Will post result once done, previews show a quite good raw material to work on and get decent colours

[kalasinman]

Use your camera's unity gain or higher. I looked your camera up on http://www.sensorgen.info/NikonD90.htmland your unity gain is somewhere between 3200ISO and 6400ISO (Personally I'd use 6400 since 3200 is slightly below unity gain)

As far as how many shots... Until the image isn't grainy anymore!

I don't know how you came to that conclusion. Unity gain is the lowest ISO which produces one count from one photon received. The data on the link you shared isn't sufficient to determine unity gain, although it may be inferred to be the point where increasing ISO no longer produces an increase in Dynamic Range. In this case somewhere near ISO 800.

[rofus]

I don't know how you came to that conclusion. Unity gain is the lowest ISO which produces one count from one photon received. The data on the link you shared isn't sufficient to determine unity gain, although it may be inferred to be the point where increasing ISO no longer produces an increase in Dynamic Range. In this case somewhere near ISO 800.

Yes that's correct, not sure where that data came from but is obviously wrong (seems read opposite way) that high iso is unusable

On a D90 from direct experience iso 800 is the maximum to use for astro, more doesn't produce better results and adds visible noise.

[rofus]

That's the result of about 2 hours between 2m/5m/8m exposures (640iso, darks, drizzled). Resolution could be better with a longer focal telescope (my one is a wide field 750m) or bigger CCD sensor (D90 is just ok) to get more at 1:1 crop, but overall I'm happy of the result as it gets the colours right and get the 'style' I like 

Will try again once I'll have a longer focal length..for now I'm happy with this m13 and my actual setup (Skywatcher Startravel 150 + D90).

Thanks for help to everyone!

[kalasinman]

Nice job!

[dph1nm]

But if expose 5 minutes at 800 iso or 100 iso result is very different obviously

Seems to be some confusion about different ISOs here. The only real difference is well-depth (high ISO saturates quicker, which is relevant to the original question), and any possible difference in read noise (e.g. for Canon's the read noise is lower at higher ISO - not sure about Nikon's though). You still collect the same number of photons in the same time, so apart from the read-noise issue the signal-to-noise is the same. So you don't need a longer total exposure time if you use a lower ISO, nor are higher ISOs intrinsically "noisier".

NigelM

[rofus]

Not sure what camera you consider, but for any digital camera, for obvious reasons I'm not explaining here, a 10s pic at 6400 ISO will be always more noisy than the same 10s pic at 100 ISO.

That is not 'intrinsic', is just real life simple example of two same length pics (same exposure at higher ISO = more noisy data). Then obviously depending on the quality of the camera, sensor, temperature etc. this difference can be more or less obvious etc. but here I'm talking from a practical point of view

There are obviously many factors to consider when deciding iso/time factoring, not only the noise of higher iso, but also the time the sensor gets 'warm' and so noise becomes more obvious also at lower iso etc. Depending on the object, higher/lower iso and/or exposure times might change. I know for my Nikon that 800 iso is the maximum I can get to avoid 'too much' noise, then my mount can track up to 20m or 30m sometimes so I know I have lots of space to judge the best combination (considering also the point where light pollution becomes a problem etc.).

I know what you mean is not 'intrinsic' but I like 'simple' real life talking, and higher iso means for everyone more noise. One of the countless examples is at this Canon review (same pic, different iso):

http://www.the-digital-picture.com/Help/ISO-Noise.aspx

[kalasinman]

Some Nikons are "isoless" read noise is constant, for example the D5100. Higher ISOs become noisier as the read noise and count amplification causes the skyfog to be boosted as it rises and read noise drops. Once unity gain is reached, I see no advantage to amplifying the count without adding real data. That is why for faint objects, long exposures bring out more detail than short ones at high ISO. In a short exposure, the faint details may simply not provide sufficient photons to trigger a count.

[rofus]

Some Nikons are "isoless" read noise is constant, for example the D5100. Higher ISOs become noisier as the read noise and count amplification causes the skyfog to be boosted as it rises and read noise drops. Once unity gain is reached, I see no advantage to amplifying the count without adding real data. That is why for faint objects, long exposures bring out more detail than short ones at high ISO. In a short exposure, the faint details may simply not provide sufficient photons to trigger a count.

Exactly my point and better explanation by a practical astrophotography point of view and example

I do mostly DSO and faint large objects, and yes always long exposures/low ISO bring much more detail (and less noise on the fainter details) than shorter ones (or even worse equally long ones) at higher ISO.

Finding perfect balance is just a matter of trial and error getting used to equipment 'limits', ideal range, the sky pollution in that area etc.

I was struggling with M13 as it's by far the brightest object I imagined till now, but using lower iso and a combination of short and long exposures (depending on the region) did the trick to keep the core not burnt, some colours details and some peripheral information intact.

Most of all noise was negligible at 640 iso (on a Nikon D90) and many drizzled exposures to get rid of the remaining one. Since I use drizzling I have to say is the by far the best way to deal with noise  

[faulksy]

That's the result of about 2 hours between 2m/5m/8m exposures (640iso, darks, drizzled). Resolution could be better with a longer focal telescope (my one is a wide field 750m) or bigger CCD sensor (D90 is just ok) to get more at 1:1 crop, but overall I'm happy of the result as it gets the colours right and get the 'style' I like 

Will try again once I'll have a longer focal length..for now I'm happy with this m13 and my actual setup (Skywatcher Startravel 150 + D90).

Thanks for help to everyone!

thats just how i see it through my dob, its lovely isnt it , but only takes 10 mins

[rofus]

thats just how i see it through my dob, its lovely isnt it , but only takes 10 mins

Ahahaha that's the best comment by far! 

True indeed..but I like photography so I actually would anyway want to do all the work to produce a final pic to hang somewhere But I envy your Dob surely

[kalasinman]

Exactly my point and better explanation by a practical astrophotography point of view and example

I do mostly DSO and faint large objects, and yes always long exposures/low ISO bring much more detail (and less noise on the fainter details) than shorter ones (or even worse equally long ones) at higher ISO.

Finding perfect balance is just a matter of trial and error getting used to equipment 'limits', ideal range, the sky pollution in that area etc.

I was struggling with M13 as it's by far the brightest object I imagined till now, but using lower iso and a combination of short and long exposures (depending on the region) did the trick to keep the core not burnt, some colours details and some peripheral information intact.

Most of all noise was negligible at 640 iso (on a Nikon D90) and many drizzled exposures to get rid of the remaining one. Since I use drizzling I have to say is the by far the best way to deal with noise

Do you dither?

[rofus]

Do you dither?

Yes, I use Digicamcontrol to control my D90 and program the shooting session with dithering with PHD v1 and EQMOD, and then I do all the post in PI (apart final touches if needed in PS).