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Histogram Question?


My2Stars

Question

I have just started AP and have BYEOS. I'm sure this is a basic question that someone can help a beginner with.

 

I was taking subs of M31 at .30". I know that this really isn't long enough but the histogram on BYEOS was showing between 1/3-1/2 from the left. The capture on the laptop screen was very good. Could see a lot of detail. I know that the screen is not the real indicator but am just giving some info.

 

Yet when stacked the results are showing way under exposed. Wasn't the histogram reading correct?

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The Histogram from a DSLR is produced by evaluating the Embedded JPG (which is Preprocessed and Stretched In-Camera), so the Histogram is not 100% representative of the RAW Data.

But it is still quite useful.

 

The Screen Image is also Stretched, whether you capture in RAW or JPG.

But if the "Default Stretch" can bring out such Detail, then you can figure that there is "Enough Detail" for Stacking and Stretching.

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Yet when stacked the results are showing way under exposed. Wasn't the histogram reading correct?

 

It is not showing under exposed, it has not been stretched yet.  If the details are seen in BYE (embedded jpeg) then the details are there and good processing/stretching will bring out the details.

 

Regards,

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The Histogram from a DSLR is produced by evaluating the Embedded JPG (which is Preprocessed and Stretched In-Camera), so the Histogram is not 100% representative of the RAW Data.

But it is still quite useful.

 

The Screen Image is also Stretched, whether you capture in RAW or JPG.

But if the "Default Stretch" can bring out such Detail, then you can figure that there is "Enough Detail" for Stacking and Stretching.

 

 

It is not showing under exposed, it has not been stretched yet.  If the details are seen in BYE (embedded jpeg) then the details are there and good processing/stretching will bring out the details.

 

Regards,

 

Thats where my confusion comes in. Many imagers are saying that my stacked image is way under exposed and can only be stretched so much. Whereas your reply states that "if the details are seeing in BYE..." it should be okay.

I'll have to try again and pay more attention to exactly how MUCH detail is showing.

Here is a link to the stacked, non stretched sub.

 

https://www.dropbox.com/s/o67qpiblsmxxwtq/M31%20for%20ST%20Embed.TIF?dl=0

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Question #1:

The TIFF file in the Dropbox is obviously a "Processed File", since Canon DSLRs don't natively produce TIFF output.  This TIFF File REALLY IS rather Under-Exposed (or perhaps Data has been lost).

What Processing / Stacking has been done to the File??  That Processing may have cost you some of the "Details" that you claimed above.

 

Question #2:

What "Imagers are saying" the Image is really underexposed??  

Are they looking at only this TIFF File??  Or also at the original .CR2 file which you indicated above "could see a lot of detail"??

 

Question #3:

What equipment and Exposure details were you using??  All you indicate is "30 second exposure".

This appears to be a DSLR and Lens (not Scope) setup, given the FOV (100-200mm FL maybe?). And Tracking was used, per the lack of significant Star Trailing.

What ISO was used??  It appears to have been "reasonable" given the lack of strong ISO/Thermal Noise.

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Question #1:

The TIFF file in the Dropbox is obviously a "Processed File", since Canon DSLRs don't natively produce TIFF output.  This TIFF File REALLY IS rather Under-Exposed (or perhaps Data has been lost).

What Processing / Stacking has been done to the File??  That Processing may have cost you some of the "Details" that you claimed above.

 

Question #2:

What "Imagers are saying" the Image is really underexposed??  

Are they looking at only this TIFF File??  Or also at the original .CR2 file which you indicated above "could see a lot of detail"??

 

Question #3:

What equipment and Exposure details were you using??  All you indicate is "30 second exposure".

This appears to be a DSLR and Lens (not Scope) setup, given the FOV (100-200mm FL maybe?). And Tracking was used, per the lack of significant Star Trailing.

What ISO was used??  It appears to have been "reasonable" given the lack of strong ISO/Thermal Noise.

Thanks for the help.

Here is all the info:

Canon T2i hooked to Sky Watcher 80mm APO on SkyWatcher EQ6 Mount. Captured with BYEOS at 50 subs @ .30" ISO 1600. 

Stacked (all 50 had high scores) with DSS with no WB or RGB Calibration. Am trying Star Tools and they want the stack as virgin as possible.

Have been told that Stacked Image Tiff is under expose. Way under.. Yet the histogram on BYEOS was a little over 1/3 from left. The CR2 file looked good as did the captured image on BYEOS screen. Thats where I'm confused. sounds as if the histogram needs to be more towards the right end (according to some). Am trying that right now with M15. Shooting at .90" ISO 1600. Image is over blown on capture screen so can't even tell if they are keepers or not. Histogram is near right end. I know that sensor temp is very high (hot here) and have heavy LP.

 

Heres a screen shot of M15 now.

post-13959-0-52012100-1473472504_thumb.jpg

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Just to confirm. Your exposure of M31 is 0.3 seconds?

 

I shot M31 with my 5 inch refractor and a 0.8x focal reducer using a cooled CCD camera nearly a year ago. My exposures were 7.5 minutes each for Luminance and 6 minutes each for red, green, and blue filters. Here is a link to my final result --> https://photos.google.com/photo/AF1QipNomU3ig4qbY5txnLX4Lg3YEaKi_93BKx4uGlbH

 

I provided this so you can understand. My 127mm refractor gathers more than twice the light of your 80mm scope. My camera is also more sensitive, with a deeper well, and 16-bit data. And still my exposure durations were 7.5 and 6 minutes respectively.

 

So, regardless of the histogram, your exposure is way too short to pick up the detail of the faint spiral arms.

 

Can you provide a raw, straight from the camera, image?

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Just to confirm. Your exposure of M31 is 0.3 seconds?

 

I shot M31 with my 5 inch refractor and a 0.8x focal reducer using a cooled CCD camera nearly a year ago. My exposures were 7.5 minutes each for Luminance and 6 minutes each for red, green, and blue filters. Here is a link to my final result --> https://photos.google.com/photo/AF1QipNomU3ig4qbY5txnLX4Lg3YEaKi_93BKx4uGlbH

 

I provided this so you can understand. My 127mm refractor gathers more than twice the light of your 80mm scope. My camera is also more sensitive, with a deeper well, and 16-bit data. And still my exposure durations were 7.5 and 6 minutes respectively.

 

So, regardless of the histogram, your exposure is way too short to pick up the detail of the faint spiral arms.

 

Can you provide a raw, straight from the camera, image?

Yes 30" exposures.

Here is a dropbox link to 1 CR2 Sub:

 

https://www.dropbox.com/s/1jnnmg9ev5r5dsl/M31_LIGHT_Tv30s_1600iso_%2B95f_20160814-00h22m56s840ms.CR2?dl=0

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OK, so 30 seconds not 3/10ths of a second. Still very short compared to my 7.5 minutes.

 

I converted the CR2 file to FITS and stretched it. The image has a lot of noise which can contribute to pushing the histogram to the right. My guess is that the noise in the individual subs is leading to a false interpretation of the histogram. Are you also shooting dark frames and flat frames? If so it would help to make the dark and flat masters available via DropBox.

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Your M15 images above are over exposed at 90 seconds.  The histogram is almost to the far right, not good.  You should aim at about 1/3 in from the left.  If this was a 90 seconds exposure then 45 seconds should get you to about 1/3 in from the left.

 

However, this level of overexposure in just 90 seconds is not normal.  

 

What is your light pollution like?  Are these taken in a city or in a suburban area?  You said you had heavy LP.... so my best advice here is to use a LP filter and see a huge difference.

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OK, so 30 seconds not 3/10ths of a second. Still very short compared to my 7.5 minutes.

 

I converted the CR2 file to FITS and stretched it. The image has a lot of noise which can contribute to pushing the histogram to the right. My guess is that the noise in the individual subs is leading to a false interpretation of the histogram. Are you also shooting dark frames and flat frames? If so it would help to make the dark and flat masters available via DropBox.

Sorry it took so long to get back to you but was out for the day. Here is the link to the DARKS & FLATS:

https://www.dropbox.com/s/tppnvstf47hap05/MasterDark_ISO1600_89s.tif?dl=0

https://www.dropbox.com/s/g0hb4p0v6eaityh/MasterFlat_ISO1600.tif?dl=0

 

Greatly appreciate all the help!

 

You M15 images above are over exposed at 90 seconds.  The histogram is almost to the far right, not good.  You should aim at about 1/3 in from the left.  If this was a 90 seconds exposure then 45 seconds should get you to about 1/3 in from the left.

 

However, this level of overexposure in just 90 seconds is not normal.  

 

What is your light pollution like?  Are these taken in a city or in a suburban area?  You said you had heavy LP.... so my best advice here is to use a LP filter and see a huge difference.

Guy -  Yes, Highest LP Zone. Depends on which map you look at. One map shows White Zone had highest. I am on the edge of that and red zone.

Very high air temps also (90*F) running a sensor of about 108F. 

 

Your comment is what I was referring to - The night before I took 30" exposures and the histogram was perfect, yet everyone saying underexposed. Which I would imagine since a 30" exposure on M31 seems way to short. The 90" exposures (60) are almost all the way to right and can barley see anything on capture screen since almost white. Doesn't make sense when others can take 3-5 minute exposures and histogram is still good.

 

Guess will have to try LP filter or dark site. Am sure that's what it is and the high temps.

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You really WILL NEED to seek Darker Skies (Philly's Light Dome is probably too much for a Light Pollution like the CLS).

You might have to seek a Dark Site such as Cherry Springs State Park in the Hills of North-Central PA (it's a favorite of DSLR Modding Expert Gary Honis).

You could also seek the advice of one of the Philly-area Astronomy Clubs such as DVAA about the local Dark(er) Sites.

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You really WILL NEED to seek Darker Skies (Philly's Light Dome is probably too much for a Light Pollution like the CLS).

You might have to seek a Dark Site such as Cherry Springs State Park in the Hills of North-Central PA (it's a favorite of DSLR Modding Expert Gary Honis).

You could also seek the advice of one of the Philly-area Astronomy Clubs such as DVAA about the local Dark(er) Sites.

Thanks for the info. The filter that was recommended for me was a IDAS LPS-D1 by Jon Rista. He also felt the CLS would not be beneficial. I will probably try that.

Yes, I do go to Cherry Springs when I can. Only 4 hours away. And I do belong to a club in S. Jersey that has dark skies and is only 1 1/2 hours away. Jerry Lodgruss belongs to that club also. I have just had a tough time getting anywhere off site but am anxious to image from a dark site.

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