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clarification about imaging Acquisition


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hi all

i have some inquiries to clarify the missing understanding of some points in astro imaging Acquisition as follow:

  •   How long the exposure or minimum exposure i must take in each LRGB channels in target ???,
  •   whats should be the average for the background ADU in sub-exposure ???
  •   What is background level in each LRGB channels
  •   when i  take the LRGB ??

is there any points that i should take into consideration when i Acquisition an image??

regards,

sam

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Hi Sam, There's a lot of things there that are going to be dependant on a number of variables.

1) What camera do you have? - Some chips are more sensitive than others for example. I assume that as you've mentioned LRGB that you are using a mono camera with filters?

2) What is the speed of your scope? This will greatly affect the exposure time of an image.

I have learnt largely from trial and error. In narrowband images I always take 30 minute subs. In luminance I go for 10 minute subs and for RGB I am going for 5 minutes. Of course this is also largely target dependant. M42 for example requires very short subs for the core to retain any colour.

I don't bother with assessing background ADU levels. I also know that while I take 30 minute subs, the detail I get in an image is massively different between my f/8 scope and my other scope at f/3.9.

I guess what I am saying is that one size doesn't fit all, not only between individual imagers and their kit, but between targets on a nightly basis, how much LP you have, whether there's a moon about for example.

Experimentation would be the order of the day I think. You want to get an ADU in the RGB subs that try not to blow out the core of the stars too much.

Hope that helps, sorry I can't give you a definitive answer. Someone will probably be along in a minute who can and pours cold water on what I've just said .... in which case...... ignore me :D

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Hi Sam - I'd reiterate what Sara's said... The bit about when to take LRGB is probably more to do with the moon phase - Depending on how close the target is to the moon, the moon can totally mess up what otherwise would have been a perfect night if it's more than 3/4.  That's the time to reach for the NB filters... and Ha in particular.  OIII is particularly effected, even if using a VERY narrowband version.

There's also a consideration regarding imaging the Blue channel - due to the atmosphere scattering blue light more than the other channels, it's best to shoot the blue exposures as high up as possible...  (I frequently take L in one session, Red and half blue the next and Green and the other half blue the following)

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Hi Sam, There's a lot of things there that are going to be dependant on a number of variables.

1) What camera do you have? - Some chips are more sensitive than others for example. I assume that as you've mentioned LRGB that you are using a mono camera with filters?

2) What is the speed of your scope? This will greatly affect the exposure time of an image.

I have learnt largely from trial and error. In narrowband images I always take 30 minute subs. In luminance I go for 10 minute subs and for RGB I am going for 5 minutes. Of course this is also largely target dependant. M42 for example requires very short subs for the core to retain any colour.

I don't bother with assessing background ADU levels. I also know that while I take 30 minute subs, the detail I get in an image is massively different between my f/8 scope and my other scope at f/3.9.

I guess what I am saying is that one size doesn't fit all, not only between individual imagers and their kit, but between targets on a nightly basis, how much LP you have, whether there's a moon about for example.

Experimentation would be the order of the day I think. You want to get an ADU in the RGB subs that try not to blow out the core of the stars too much.

Hope that helps, sorry I can't give you a definitive answer. Someone will probably be along in a minute who can and pours cold water on what I've just said .... in which case...... ignore me :D

hi

i have the qhy9m ccd, with  newt GSO200mm @f4.555  . i am new in ccd i buy the qhy9m lass than 2 month , i upgrade from canon dslr , i was testing  thee ccd  and i think that my image have too high sky background level , so  i need to  know ??? what is background level mast by in each sub exposure for each filters  of  LRGB

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Hi Sam - As Sara said, I'm not sure there really is any specific background ADU level to aim for, as exposure length really will vary from object to object and be dependent on what detail you want to capture / how "deep" you want to go.  For example, with (say) a dark nebula, or maybe if you're trying to capture the integrated flux around M81/M82, you'll need longer luminance exposures... but to maintain colour, you don't want to expose so long that you hit the max pixel ADU (63,555).

However, for some DSO's, you may find yourself having to take maybe 2, or sometimes even 3, sets of exposures and then combining them.  For example, with something like M42, if you want to retain detail in the Trapezium, you might take a separate set of (say) 30s exposures. but then you'd get very little detail (colour) in the outer nebulosity, so for that you might look at perhaps taking 10 min luminance and maybe 5 mins RGB(?)... But in order to get better star colour, you may then decide that you want to take some separate (say) 2 min exposures for a star layer only...

As an exercise, I once tried some 30 minute luminance exposures (using a Hutech IDAS LP filter) using an f4.5 OTA on NGC1333 - The attached was a stack of 6x30 min exposures... (Unfortunately I had to abort this image as I had a tilt issue :sad:)...

20131005NGC1333L18001-6P1lr_zpsa139e30f.

But if I'd tried the same length luminance exposures on (say) M42 or M31, it would almost certainly have completely burnt out most of the image!

I also once tried a single 60 min exposure with an Ha filter... but on this particular object that was obviously far too long for the brighter areas, but the detail in the less bright areas was beginning to come out quite nicely ( :smiley:).  With this example, I may well have got away with processing it twice - Once as here (for the less bright areas) and then again not quite so stretched to maintain detail in the lighter areas (but again, due to that same tilt issue, I didn't bother taking this further)

AutosaveImage-002IC1848_Ha_3600lr_zps7e9

What I tend to do now, having chosen a new DSO to image, is do a Google Image search for the object and then have a look at the exposures that others have used, bearing in mind the scope and f ratio...  That usually gives me a good starting point, but if I want to go deeper then I just up the luminance exposure length.  Similarly, as my own personal rule of thumb (and this is only me!), If I take (say) 6 hours of exposures on Luminance (maybe 36x10 mins), I'll then try and get 2 hours each of RGB (maybe 24x5 mins each) - I'm sure that some (many?) would call that overkill on the RGB, and I tend not to use binning (again, a personal thing), but it's a "formula" that I'm happy with :smiley:.

Alternatively, or indeed in addition, you could also post a specific query in here, asking what exposures others might suggest for imaging Messier / IC / NGC "X" as and when you choose to have a go at it and then use those as a starting point.  Personally, I don't believe that background ADU is actually that important as long as you don't hit the pixel saturation point... or if you want to go deep to get that extra detail in the faint areas, then you're accepting that you may have some areas burnt out which in some cases you may then need to take a separate set of exposures for...

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The advise I was given (and I have seen in a number of other places) is: take the longest sub you can without saturating the highlights. The rule of thumb to achieve this is to ensure that the peak of the histogram is about 1/3 of the way from dark to light.

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Difficult question. If you make it your rule not to saturate the hightlights, especially in luminance, you will never capture the faint stuff. The great thing about LRGB is that, in an RGB-only layer, you have the highlights safe and sound and can learn to layer them in without using the highlight-saturated luminance. But the luminance will let you haul the faint stuff into play. The whole charm of this game is its subtlety.

Olly

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I agree with above - I am still struggling with the representation of rgb with luminance as there is a big choice of how much of each you represent - for instance some would say no luminance for stars - but then your rgb data has to be tight and good and not binned - experience and experimentation but you make the ultimate choice - best wishes Tony

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