Does only exposure determine sub times

[Jeffkop]

Didnt quite know how to title this thread, so forgive me if its confusing.

What I am asking is:

Assuming you can take a 30 min image before some part of it becomes over exposed, is it better to take
1. 1 30min image
2. 2 15min images
3. 3 10 min images
And so on

You get my drift here I hope.

Thanks again

Jeff

[bluescope]

Hi Jeff

Are you talking about imaging with your DSLR or your ST2000XM.

Usually the longer the sub the better. I would say the accuracy of your polar alignment and guiding would determine the length of exposure before any obvious start trailing/field rotation etc may occur. As far as burn out is concerned most people do a variety of exposures to compensate for bright areas in an image.

I'm sure someone like Jase or Mike ( strongman ) or Fred will give you more detailed info on this topic.

:thumbsup:

[jase]

Jeff, good question. A CCD responds to light in a linear fashion, thus the longer the exposure time, the fainter the details you’ll acquire and theoretically, better the signal to noise ratio. Depending on the target, the difference between a 30min sub and a 10min sub can be significant especially when working with filters that have a small bandpass window such as Ha filter.

However, to specifically address your question, you need to determine the quality of one 30min sub compared to three 10min subs. If the one sub contains satellite trails, cosmic ray hits and other anomalies, it can only be corrected through combining more data with a form of an outlier pixel rejection algorithm (sure you can chase them in PS, but why bother). Thus it’s difficult for one 30min sub to equal the quality of three 10min subs. Having stated this, you’ll find that the three 10min subs will not be as deep as the one 30min sub because you’ve only allowed 10mins of light collection compared to 30mins - remember CCD's respond in a linear fashion. Combining more and more 10min subs will improve the signal to noise ratio, but it doesn’t make the image any more deeper…you are still limited to 10mins of light collection on the sensor. You can try using the sum combine function in an attempt to make the image deeper and bring out those faint wisps of nebulosity, but from my experience it’s not the same as collecting 30min subs – certainly when comparing background ADU counts vs object in question.

As you combine more subs you combine, the data will be become smoother and you’ll be able to stretch it harder without introducing noise. Technically, two subs data is barely workable. You’ll find that data combine algorithms usually require a minimum of three to function, and all work significantly better with much more.

So, now that I’ve told you going deep with 30min subs is the way to go assuming you are willing to also collect three or more 30min subs, there are some other things you should know.

Firstly, your ST2k has an ABG chip, thus a gate bleeds the pixel wells when they reach a saturation level as designed by the manufacturer. This prevents blooms, which is the full saturation of a pixel well that subsequently spills charge into surrounding pixels. Clearly, the exposure times for an NABG chip will be shorter than that of an ABG to counteract blooming of bright stars in the FOV. For now, this is something you don’t need to worry about and is what makes an ABG chip somewhat easier to use. What is important to note however is as there is no bleeding with NABG chips, they are typically more sensitive than their ABG relatives. The signal to noise ratio of a 3min sub taken with a NABG chip will yield a greater signal to noise ratio than that of an ABG of the same exposure time. Hence, you need to use long subs with an ABG chip to obtain a similar signal to noise ratio. i.e. make up of the lack of sensitivity.

Secondly, going deep with long subs is great for nebulosity and faint details such as dim structure in spiral arms, however watch out for the stars. Stars are bright and don’t need much exposure time to reach saturation. Saturated stars aren’t too much of an issue with ABG camera as there is no blooming as mentioned above, however you’ll still find they’ll loose colour the deeper you go. This is more pronounced with LRGB blends as the strong luminance data de-saturates the colour data. You can specifically process the stars in an attempt to bring them back, but sometimes it’s difficult to achieve. Star management i.e. managing the stellar profiles in processing is a different topic in which I’m sure others can share their experiences. To counter act this, one method is to take some shorter RGB subs and blending them into the deeper data. Russ Croman did a presentation at AIC2004 on preserving star colours and profiles, this maybe of interest - http://www.rc-astro.com/resources/talks/PreservingStarColors.htm

Finally, as Steve mentions, the longer the subs, the more demanding it can be on your equipment. Good polar alignment is a must, but equally good tracking/guiding. It’s frustrating if you keep loosing 30min subs due to guiding errors and the like. Its probably just a frustrating as collecting three hours of data to only realise your focus is slightly out or shifted due to temp. changes. The duration can also depend on your environmental conditions and equipment. Light pollution never helps. Also imaging at F/5 compared to F/9 can make a difference to exposure times.

If there is a single piece of advice to give, then it would be to monitor your subs closely. After you’ve calibrated them, review the background ADU count and the target in question. Check the overall quality of stars, if they are bloated due to poor seeing; remove them from the combine function. Extremely faint details on short subs will be difficult to measure their intensity in relation to the background. This is an indication that you could go longer. Longer subs will show a marked improvement, unless of course hindered by sky glow such as pollution. In short, go long/deep, but work within your boundaries. You don’t want to reduce your acquisition efficiency by throwing away every second sub due issues associated with long sub exposures.

[Jeffkop]

Thanks Jase, Im sure there is a wealth of info in all that, sitting here trying to apply it now. The link to the Preserving Stars is VERY HANDY to.

Hope others get to read it as well, for whomever hasnt is missing out on some vital info I think.

Gee you have to be an expert at a LOT of things to pull this imaging off dont you.

Thanks also Steve hope this helps you too.

Oh well off to take all the gear outside n setup now .. looks like the cloud is clearing.

Thanks again

Jeff

[gregbradley]

Hi Jeff,

There is a free ccd exposure length calculator on the net. Try googling it.

It may be by Ron Wodaski. As I recall 10 minutes was ideal for the 2000XM with the KAI2020 chip.

I think practical considerations come into it.

A lot go 1x1 binning for luminance and 2x2 binning for colour. One set is to do 15 mins at 1x1 for luminance and 10 minutes at 2x2 for colour.

This way you only need 2 darks - a 10 minute one at 2x2 and a 15 minute one at 1x1.

Planes, meteorites (more at a dark site), clouds, etc make long exposures more likely to be unusable. But more likely unless you have a very expensive and high quality mount 30 minutes is not going to be easy.

So 10 minutes 1x1 for luminance and 10 minutes 2x2 RGB or even 15 mins for luminance would be my suggestion.

It can be quite frustrating getting LRG and then clouds come in and no blue yet.

So another tip is to do one LRGB at a time rather than 6 L then 3 R, 3 G, 3B as you can't get that colour image at all until you have all of them.

30 min subs really requires excellent polar alignment (highly accurate) and a highly accurate mount and autoguider and stable clear skies (nothing worse than up to 25 minutes and you can see some clouds sailing over!).

Greg.

[Jeffkop]

Well it seems intuition has served me well Greg. I had been imaging at 20 mins lum and 15 min RGB, but since reading some bits and pieces lately, decided that I could cut a lot of the risk out and still achieve the same result by doing lots of 3 or 5 minute sub then stack them. Well my results doing that did not end up as good as I would have hoped ... the reason for my question today in fact.

Last night I got 54 mins of LRGB of M17 (again) and I really havent achieved much in the way of more signal over the last time when I did one sub each of LRGB at 20mins.

I have realised that imaging clusters for 20 mins is BAD and so like you suggest, its different strategies for different objects. Im yet to try masking areas of nebula' that have saturated stars washing it out and then mixing in a very short exposure of that area to fill it back in again ... knowing how to drive photoshop better will see me through in that regard.

I dont have any problems guiding for 20mins or even longer for that matter and whilst I havent taken a huge amount of shots I have only ever been inconvenienced once by a falling star.

Point taken about the order in which to do the subs, MaximDL's sequencer does it identically to how you suggested ... although I got caught a little last nite because I added a dark in at the ... what I thought would be the end of the sequence, but it snuck it in after the first LRGB shots and so I didnt know if the guide star was still going to be OK, if not how that may effect the position of the subsequent images ... but I must have been lucky and got a reasonable G11 ... because after the 3min dark ... there was less than a .2 error in x and y so it just kept on guiding happily on the same star.

And on a guiding note, there has been another variable ive learnt about of late thrown into the image alignment mix .. field rotation ... like I keep saying ... the answers to one question almost without fail, lead to more questions and so it feels like the information that I need to know to successfully do this imaging is like the universe itself ... expanding !!!

I really do appreciate the feedback I get here from all you folk in the know, thanks very much.

I now know a little bit more about a whole lot more

Jeff

[tornado33]

with a DSLR I did an experiment, by taking 30 minute subs of the Helix, and also some 10 minute ones. adding two 30 minute subs returned a noisier result than six 10 minute subs, no doubt the un cooled sensor was picking up too much dak current over 30 minutes. For cooled cameras that wouyldnt apply and one cound go longer. It also does depend on the dynamic range of the object. eg M83 has a bright core that might saturate in too long an exposure
Scott

[jase]

Jeff,
I believe Greg is referring to the following calculator
http://www.ccdware.com/resources/subexposure.cfm

I would suggest using it as a guide, but experiment with what works best. As mentioned in my previous post (in which I apologise for all the typos) you’ll need to review your subs to check your background ADU count for the calculator to work or at least have a greater form of accuracy.
Greg’s recommendation on subs is a good start. It is also what I commonly use 15min lum 1x1 and 10min RGB 2x2. I do also take 20min Ha subs. I’m not worried about having a large calibration library. That is the least of your worries as you can easily refresh the darks in an automated fashion.

MaximDL’s sequencer is indeed a good tool as it rotates through the image sequence ensuring that you build subs across the different filters evenly. This is fine to build experience; however in the quest of improving image quality, the process is not optimal. The reason why I say this is that while the object is rising, you may not want to collect blue filtered or luminance data due to atmospheric extinction scattering light or low resolution. Hence, while the object is rising, you may only want to collect red filtered data that is not compounded by atmospheric extinction. A common method I use in data acquisition is a stepped approach. As the object rises, I collect red filtered data. Once it hits approximately 70 degrees start collecting green, then followed by blue. This minimises the scattering of blue light. As the object approaches meridian or is as high as it will get, collect luminance to improve resolution. So the sequence is RRGGBBLLLLBBGGRR as it crosses the meridian. This is somewhat off topic considering we are not talking exposure times, but it re-emphasises the point that you’ll go to extremes to ensure you collect quality data, even if that includes a few long exposures.

Indeed, Scott. You nailed it... you need cooling for long subs to be effective. Some good info for DSLR users can be review here - http://www.samirkharusi.net/sub-exposures.html

[bluescope]

There's always something to learn Jeff, however it takes a while to decipher some of the things Jase talks about, he speaks a different language to some of us mere mortals sometimes :lol:

:thumbsup:

[Bassnut]

Some excellent advice there wow, so detailed, Jases effort is especially usefull, a mine of precisely targeted info, I havent seen it so well explained before, thanks Jase :thumbsup:.

A small 2c from me (based entirely on experience). Although CCDs are linear, and signal always benfits over background glow on long exposures, it gets to be diminishing returns in urban skies. Processing (such as deconvolute and sharpen in PS) is limited by data density. If your prepared to sacrifice dim outer neb on nebulae, and depth generally, but get very detailed brighter regions (with the extra processing more stacking allows), use 15min rather than 30min subs. As Greg and others say, this also reduces tracking error/alingment/dud subs effects for a given imaging session time. You also didnt say if you were talking about the ST or DSLR. With the DSLR, over 10mins/sub wouldnt be usefull.

[leon]

Now that was a very interesting and informative read, great stuff, and much to ponder over.

Leon

[gregbradley]

The reason why I say this is that while the object is rising, you may not want to collect blue filtered or luminance data due to atmospheric extinction scattering light or low resolution. Hence, while the object is rising, you may only want to collect red filtered data that is not compounded by atmospheric extinction. A common method I use in data acquisition is a stepped approach. As the object rises, I collect red filtered data. Once it hits approximately 70 degrees start collecting green, then followed by blue. This minimises the scattering of blue light. As the object approaches meridian or is as high as it will get, collect luminance to improve resolution. So the sequence is RRGGBBLLLLBBGGRR as it crosses the meridian. This is somewhat off topic considering we are not talking exposure times, but it re-emphasises the point that you’ll go to extremes to ensure you collect quality data, even if that includes a few long exposures.

That's a good tip Jase. I haven't come across that before but I have obviously noticed how much nicer images are when imaging at or near the zenith. I strive to image an object during that period and time when I will image it to catch it there if at all possible.

Greg.

[Jeffkop]

Well thanx a million jase and greg, looks like there are more than myself getting some grand advise here.

Its easy at this stage to go into information overload. The more perfect your images the more perfect you have to be next time and so its like the drag racer. Its comparatively easy to get close to your target ... then its very small gains that come from big effort, where you have to have every facet of the operation honed and peaking.

So ... off to check on the guiding at the moment, make sure I havent lost the guide star behind a tree.