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Old 06-09-2013, 08:53 AM
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Exposure Strategy M16?

I would appreciate some advice re exposure strategy for M16. The attached .jpg screen capture is a 2.5 min stretched 2x2 luminance subframe from my TEC140 with a 2X AP barlow. 2x2 resolution is 1.18 arcsec.

I notice that at 2.5 min many of the stars are already blown out (never mind the egg shapes! There was no guiding). I intend to capture 1x1 at .59 arcsec so I estimate that I would need to increase capture time 3-4X to achieve the same density as the attached image. However, I'm not at all sure if this is close to the "right" subframe exposure. I could go longer but risk even larger stars, or go shorter to try to avoid saturating the stars as much. Or, is it best to just increase exposure for the nebula and try to replace (or tame) the stars later?

Thanks for any feedback!

Peter
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Old 06-09-2013, 01:25 PM
jase (Jason)
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Measure a background ADU of a few frames, average it, then input the information into here - http://www.ccdware.com/resources/subexposure.cfm
This is give you a starting point but you'll probably fine you'll need longer.

Question however...are you sure the stars are really blown out? Is the ADU count of the bright stars already at 65k in only a 150s exposure. Unbinned will be less sensitive so I wouldn't judge 2x2 as the baseline. If you are using tools like MaximDL etc, the screen stretch function can give the appears that the stars bloated and plain ugly. The truth however is in the star centroid values. MaximDL's line profile tool can be very handy in diagnosing star related problems.
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Old 06-09-2013, 02:06 PM
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Jason,

Thanks very much for your reply.

I have been using CCDStack to measure the star centroid ADU. I picked two stars, one bright, one dimmer on the photo and received the following:

Image File Imager1120-2.5 min unguided 2x2
centroid {X=903.9657, Y=482.3339}
FWHM 2.3
background 1,451.43
maximum 34,510
flux within halfMax 73,604

Image File Imager1120-2.5 min unguided 2x2
centroid {X=950.3286, Y=534.4593}
FWHM 5
background 1,446.66
maximum 65,535
flux within halfMax

So, I would say, yes, the stars have reached 65K adu in 2.5 min at 2x2. Am I making an error?

Thanks for the link to the CCDware calculator! I missed that one.

Peter
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Old 06-09-2013, 02:37 PM
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You should saturate bright stars (>mag6)with only a few seconds of exposure so will essentially always have saturated stars in the image. Antiblooming will stop them bleeding but the centre will still remain saturated.
It is a trade off. You need much longer exposure for the nebula and have to accept the saturated bright stars.
You can do clever things like layer in very short exposures after removing the stars from the long exposures but this is beyond my limited processing skills. Try if you like.
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Old 06-09-2013, 02:48 PM
jase (Jason)
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For sure. The second star is hitting the 65k limit. Is this on the individual sub or the luminance master?

The 8300 has a full well depth of around 25k so depending on your optical configuration, you'll need to adjust the exposure time accordingly. You should evaluate unbinned as opposed to 2x2. Also star saturation doesn't hurt too much, at least not as much as a NABG chip. Letting your pixels bleed for the sake of improved counts in the dim areas is a good thing providing the brighter stars don't bloat.
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Old 06-09-2013, 03:34 PM
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Luminance is usually shot 1x1 although sometimes in a mosaic you could get away with 2x2 as you are getting very wide fields of view.

2.5 is way too short. At AAIC it became clear the top guys are doing really long exposures like 30 minutes or an hour. I switched to 15 minutes from 10 minutes recently (I used to use 15 minutes but my mount was not up to it so I went to 10mins). I see John Gleason the Ha imaging expert routinely uses 40 minute subexposures for his Ha and that is with a fast Tak FSQ106ED refractor.

I use 20 minutes for narrowband but if there was clear weather then I think I'll be extending that to 30 minutes to 40 minutes.

This is a common question that often leads to different points of view about stacking to reduce noise versus long exposure.

My take on it is you will never get certain detail from short exposures no matter how many you take as they don't get above the noise floor of your camera. This then does not get stacked out and is lost in the noise.

A very long exposure though gives the opportunity for faint data to be registered and accumulate above the noise of the camera.

I would put a caveat on this strategy and that would be often the longest exposure is limited by the accuracy of your mount, whether you are using self guiding, OAG and no differential flexure and also the stability of the weather.

There is no point in doing 30 minute subs if the weather is unstable. You will lose a lot of images.

Also some objects have a very bright core and a high dynamic range image is the right approach - ie a series of very short images for the core and longer ones for the dim outer nebulas etc.

Galaxies really need long exposures and long total exposure time.

A similar argument exists for binning colour exposures or using 1x1.

To me it would depend again on other factors. If you want maximum detail and have the time then 1x1 is the go for everything. If time is limited then 2x2 is the go. If seeing is poor then 2x2 may also be a good strategy for colour. Binning 2x2 also rounds out guiding errors so if your tracking is not so hot then 2x2 makes sense also. 2x2 also rounds camera lens aberrations so some less than perfect lenses respond to using 2x2 as well as stopping down the lens.

Greg.
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Old 07-09-2013, 10:37 AM
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Quote:
Originally Posted by Terry B View Post
You should saturate bright stars (>mag6)with only a few seconds of exposure so will essentially always have saturated stars in the image. Antiblooming will stop them bleeding but the centre will still remain saturated.
It is a trade off. You need much longer exposure for the nebula and have to accept the saturated bright stars.
You can do clever things like layer in very short exposures after removing the stars from the long exposures but this is beyond my limited processing skills. Try if you like.
Thanks Terry. Yes, it seems one must accept over saturated stars and just deal with them later.

Peter
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Old 07-09-2013, 10:41 AM
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Quote:
Originally Posted by jase View Post
For sure. The second star is hitting the 65k limit. Is this on the individual sub or the luminance master?

The 8300 has a full well depth of around 25k so depending on your optical configuration, you'll need to adjust the exposure time accordingly. You should evaluate unbinned as opposed to 2x2. Also star saturation doesn't hurt too much, at least not as much as a NABG chip. Letting your pixels bleed for the sake of improved counts in the dim areas is a good thing providing the brighter stars don't bloat.
Hi Jason,

The photo I posted is just a single subframe. In fact, it is so far the only photo I've taken of M16 after a night of re-installing the barlow and running FocusMax V curves, etc. After I took the 2.5 min shot I thought to myself that it would take some sorting out deciding on the best subframe lengths. I thought I'd start with LRGB and then add Ha. It will be a project when the weather clears and will take a while I suspect.

Thanks foe your assistance.
Peter
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Old 07-09-2013, 10:51 AM
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Quote:
Originally Posted by gregbradley View Post
Luminance is usually shot 1x1 although sometimes in a mosaic you could get away with 2x2 as you are getting very wide fields of view.

2.5 is way too short. At AAIC it became clear the top guys are doing really long exposures like 30 minutes or an hour. I switched to 15 minutes from 10 minutes recently (I used to use 15 minutes but my mount was not up to it so I went to 10mins). I see John Gleason the Ha imaging expert routinely uses 40 minute subexposures for his Ha and that is with a fast Tak FSQ106ED refractor.

I use 20 minutes for narrowband but if there was clear weather then I think I'll be extending that to 30 minutes to 40 minutes.

This is a common question that often leads to different points of view about stacking to reduce noise versus long exposure.

My take on it is you will never get certain detail from short exposures no matter how many you take as they don't get above the noise floor of your camera. This then does not get stacked out and is lost in the noise.

A very long exposure though gives the opportunity for faint data to be registered and accumulate above the noise of the camera.

I would put a caveat on this strategy and that would be often the longest exposure is limited by the accuracy of your mount, whether you are using self guiding, OAG and no differential flexure and also the stability of the weather.

There is no point in doing 30 minute subs if the weather is unstable. You will lose a lot of images.

Also some objects have a very bright core and a high dynamic range image is the right approach - ie a series of very short images for the core and longer ones for the dim outer nebulas etc.

Galaxies really need long exposures and long total exposure time.

A similar argument exists for binning colour exposures or using 1x1.

To me it would depend again on other factors. If you want maximum detail and have the time then 1x1 is the go for everything. If time is limited then 2x2 is the go. If seeing is poor then 2x2 may also be a good strategy for colour. Binning 2x2 also rounds out guiding errors so if your tracking is not so hot then 2x2 makes sense also. 2x2 also rounds camera lens aberrations so some less than perfect lenses respond to using 2x2 as well as stopping down the lens.

Greg.

Hi Greg,

I appreciate your voluminous reply! You make some great points. My experience so far has been mostly in photographing galaxies so it has been fairly straight forward having an approach to capture as much light as I can before the light pollution gets in the way. With M16 the problem seems somewhat different with stars very bright and a lot of nebulosity showing even in shortish exposures. I might try the CCDWare exposure calculator based on the backgrounds of some test images.

As I mentioned earlier I intend to shoot 1x1. The "test" shot was just a quick grab at 2x2 to set up the camera framing, etc. I intend to do LRGB first then add Ha. For sure my Ha exposures would be quite long without worries about light pollution! For LRGB though I think LP will be the limiting factor, but your point about getting faint details out of the noise through long exposure is well taken. I think my guiding problem is finally resolved so I hope it won't become a limiting factor.

Peter
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Old 08-09-2013, 08:18 AM
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Yes I forgot you have light pollution to contend with. There is a difference in exposure strategy between light polluted areas and dark sky areas. Basically you can do practically as long as you like at dark skies. But in light polluted areas sky glow becomes a problem at some point.

Using those gradient removal techniques at AAIC is a key processing skill for light polluted images. Ultimately narrowband is the go if light pollution is too bad.

Greg.
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