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Imaging the Dumbell Nebula in Bortle 6 Skies

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#1 Clete

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Posted 23 October 2019 - 03:06 PM

My plan is to image the Dumbell Nebula, aka M27, tonight from the back yard.

 

I'll be using the following equipement:

 

William Optics 80mm triplet refractor with their reducer/flatener

Canon SL2 DSLR camera (unmodified)

Astronomics CLS clip filter

 

I will be autoguiding and so should be able to get nice long expsure times but that's where my question is. Just how long should I go?

 

The night before last I was taking 210 second exposures at ISO 800 and then had an issue that cut the power to the scope and ended the session early. Of course, I could just pick up where I left off and continue with the 210 sec ISO 800 images but I'm worried that they're over exposed. Even with the CLS filter, the unprocessed images coming right off the camera are almost white and I'm not sure that processing will fix it.

 

What do you guys recomend as far as exposure goes on M27 on a moonless night in the city with Bortle Class 6 light polution?


Edited by Clete, 23 October 2019 - 03:07 PM.


#2 mtc

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Posted 23 October 2019 - 03:22 PM

In my Bortle6 skies, with my unmodded Canon T5i attached to 127mm Mak with .63 Focal Reducer, I can comfortably capture 240sec at ISO1600.

Here's a single frame, for reference:

M27_240s_1600iso_+30c_20190921-mini.jpg


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#3 descott12

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Posted 23 October 2019 - 03:27 PM

Looks pretty good. The white halo is likely due to vignetting and could probably be almost eliminated with flat frames.

Have you tried that?


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#4 mtc

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Posted 23 October 2019 - 03:38 PM

Looks pretty good. The white halo is likely due to vignetting and could probably be almost eliminated with flat frames.

Have you tried that?

Not yet; I haven't applied Darks or Flats, which would largely mitigate the vignetting.. then I could remove the light pollution with a bit of processing... This is just a single, unmodified (pre-process) image.


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#5 Clete

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Posted 23 October 2019 - 04:51 PM

https://www.flickr.c...eposted-public/

 

That's one of my unprocesses images. I had to convert it to jpg format in order to post it to Flicker but it looks the same.

 

I feel like going to a full four minutes with double the ISO would just be a total white out.

 

How do you post images into your posts like that?


Edited by Clete, 23 October 2019 - 04:52 PM.


#6 mtc

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Posted 23 October 2019 - 05:40 PM

After you attach your image, there is another button that says 'add to post' or somesuch. It adds a url to you post and you place it where you want it to appear.

#7 Clete

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Posted 24 October 2019 - 07:32 AM

Well, I figured out why my 210 sec ISO 800 exposures were so washed out!

 

It's basically because I'm an idiot!

 

I thought I had my Astronomics CLS clip filter in my camera and didn't. UGH!

 

I can't believe I did that! Oh well! I guess I'm going to get some first hand experience on just how much difference a CLS filter makes with Bortle class 6 skies!


Edited by Clete, 24 October 2019 - 04:55 PM.

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#8 bobzeq25

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Posted 24 October 2019 - 10:05 AM

Figure out how to put the histogram on the back of the camera or (less conveniently) use a terrestrial photo program to see it.  Place it about 1/3 over from the left hand edge (precision gains you nothing).

 

Then, shoot more subs.  BIas, flats, darks, gradient reduction in processing, all go without saying.  <smile>


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#9 TareqPhoto

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Posted 24 October 2019 - 11:54 AM

I wish if this thread title was " Imaging ..... in Bortle 9-15 skies" grin.gif lol.gif



#10 Clete

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Posted 24 October 2019 - 04:55 PM

Figure out how to put the histogram on the back of the camera or (less conveniently) use a terrestrial photo program to see it.  Place it about 1/3 over from the left hand edge (precision gains you nothing).

 

Then, shoot more subs.  BIas, flats, darks, gradient reduction in processing, all go without saying.  <smile>

So, how much does it matter if that 1/3 over from the left histogram is achieved with the ISO setting versus the length of the exposure?

 

Is a lower ISO setting with a longer exposure time always better?

 

I've watched a lot of the Astrobackyard YouTube videos and know that Trevor usually uses ISO 800 but I don't know why.



#11 TareqPhoto

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Posted 24 October 2019 - 05:12 PM

So, how much does it matter if that 1/3 over from the left histogram is achieved with the ISO setting versus the length of the exposure?

 

Is a lower ISO setting with a longer exposure time always better?

 

I've watched a lot of the Astrobackyard YouTube videos and know that Trevor usually uses ISO 800 but I don't know why.

Stop watching those videos, i watched them and i felt like i know better now or i have the idea, turned out to be no true, most of them are just like me or you, experimenting and keep imaging as practice until they are happy with certain settings.



#12 mtc

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Posted 24 October 2019 - 05:41 PM

There is a tradeoff between these variables. You can increase exposure time up to the limits of your equipment and/or light pollution saturation. You might benefit from some images taken with a lower ISO and longer duration and some taken at a higher Iso and shorter duration. Just be sure not to saturate your pixels. Also, if your histogram is too close to the left edge, you're image is underexposed.


Edited by mtc, 25 October 2019 - 03:37 PM.


#13 nimitz69

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Posted 24 October 2019 - 06:15 PM

on a relative basis total integration time and proper cal frames are way more important than getting long exposure times.  I would just set ISO to 800 and determine an exposure time that gets the histogram about 1/3 from the left and start collecting data.  When I had a DSLR I rarely screwed around with ISO.  I just set it to 800 (sometimes 400), got the histogram where it should be and shot away (3-5 hrs per target minimum).  Subs were usually 2  minutes.

 

also, shorter subs are more forgiving on your mount unless you’re using something like a Mach 1.  While your learning keep things simple and variables to a minimum:  one exposure time & one ISO setting.  Once you are collecting great raw data and your processing skills have matured you can start experimenting w its different exposure times, ISO settings, etc



#14 bobzeq25

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Posted 24 October 2019 - 06:18 PM

So, how much does it matter if that 1/3 over from the left histogram is achieved with the ISO setting versus the length of the exposure?

 

Is a lower ISO setting with a longer exposure time always better?

 

I've watched a lot of the Astrobackyard YouTube videos and know that Trevor usually uses ISO 800 but I don't know why.

This get just a bit complicated, two things are going on.  The total read noise of multiple exposures is minimized by setting the histogram to 1/3.  For that, it doesn't matter how you get there, ISO, or exposure time.

 

The dynamic range of a single exposure is determined by ISO.  Lower ISO is better DR.  But, read noise in some cameras (like older Canons) goes up fast as you decrease ISO, so you sacrifice some dynamic range to get lower read noise.  For older Canons, that generally means 800 or 1600.

 

Bottom line.  Optimal ISO depends on the camera.  The two most important cases are older Canons (800 or 1600) or newer Nikons (200).  That's how you set ISO, and, as nimitz69 says, stick with it.  Then you set the histogram with the exposure.  That can vary night to night, or with target altitude.


Edited by bobzeq25, 24 October 2019 - 06:22 PM.

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#15 TareqPhoto

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Posted 24 October 2019 - 07:04 PM

Nice discussion here, so what is applied to DSLR is it applied to CCD/CMOS mono cooled cameras or even OSC cooled cameras?



#16 mtc

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Posted 25 October 2019 - 03:50 PM

So I took my stack of 38 240sec frames (one is seen above).

That's 2.5 hours.. Isn't that a reasonable time? or do I need to go longer? (I think processing is a bit of an art. wink.gif )

I used my darks & flats, stacked, cropped, adjusted background color/offset and a few bezier curves, I got this...

Looking past the dust in the image...I think there is more I can extract from my stacked result..

Here's my attempt...

M27-38-240sFrames-mini.jpg

 

Anyone care to take a shot at processing this stacked FIT file?

https://drive.google...QDuKhIUJedP-m3K


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#17 Clete

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Posted 28 October 2019 - 11:16 AM

So I took my stack of 38 240sec frames (one is seen above).

That's 2.5 hours.. Isn't that a reasonable time? or do I need to go longer? (I think processing is a bit of an art. wink.gif )

I used my darks & flats, stacked, cropped, adjusted background color/offset and a few bezier curves, I got this...

Looking past the dust in the image...I think there is more I can extract from my stacked result..

Here's my attempt...

attachicon.gif M27-38-240sFrames-mini.jpg

 

Anyone care to take a shot at processing this stacked FIT file?

https://drive.google...QDuKhIUJedP-m3K

I can't get anything to open that file. I've tried Photoshop CC, Fastone Image Viewer and Deep Sky Stacker.



#18 bobzeq25

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Posted 28 October 2019 - 12:17 PM

So I took my stack of 38 240sec frames (one is seen above).

That's 2.5 hours.. Isn't that a reasonable time? or do I need to go longer? (I think processing is a bit of an art. wink.gif )

I used my darks & flats, stacked, cropped, adjusted background color/offset and a few bezier curves, I got this...

Looking past the dust in the image...I think there is more I can extract from my stacked result..

Here's my attempt...

attachicon.gif M27-38-240sFrames-mini.jpg

 

Anyone care to take a shot at processing this stacked FIT file?

https://drive.google...QDuKhIUJedP-m3K

I took a look at it.  Your flats didn't work, at all.  The lack of bias may or may not have something to do with it.  As you noted there's more dust than I've ever seen, and it looks peculiarly sharp.

 

Sorry, there's not a lot to be done with it.  Getting flat correction to work should be your first priority.  It doesn't really matter what you image while you try to do that, a random starfield would be fine.

 

2.5 hours is reasonable.  Just 30 minutes would be fine for working on flat correction.  A major issue that needs to be fixed.

 

Again, sorry.


Edited by bobzeq25, 28 October 2019 - 12:18 PM.


#19 fewayne

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Posted 28 October 2019 - 12:33 PM

So I took my stack of 38 240sec frames (one is seen above).

That's 2.5 hours.. Isn't that a reasonable time? or do I need to go longer? (I think processing is a bit of an art. wink.gif )

An art? Oh, no no no, that's a typical newbie mistake. It's perfectly straightforward and simple, you just load the frames and push the button. (They're working on automating the button-push.)

 

Again, a look at your image histogram should tell the tale whether your exposure is sufficient or excessive. For most images, the "black" sky background makes up most of the pixel population, so that's where the peak is. If the left edge of that peak is clear of the left edge of the graph, no pixels are being clipped to black and so you have enough exposure.

From the right, same story: If no or only a very few pixels are hitting the right edge, the image's dynamic range is sufficient (no bright spots, i.e. stars, are being blown out.

More integration time reduces noise, but what I didn't understand when I started was that "noise" is not just "rice grain junk is sprinkled through my image". Noise also affects tonality, so that what should be a delicate range of adjacent shades becomes a glaring set of posterized steps, or smooshed into a single undifferentiated value.

That's one reason the peak-position guideline is "1/3 of the way", not "barely clearing the left edge": enhancing contrast also enhances noise, so you're better off not digging your nebulosity out of the very bottom of the well.

 

Just out of curiosity, what are you using for calibration and stacking? (Apologies if you mentioned that and I missed it in the thread.)



#20 mtc

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Posted 28 October 2019 - 07:17 PM

I took a look at it.  Your flats didn't work, at all.  The lack of bias may or may not have something to do with it.  As you noted there's more dust than I've ever seen, and it looks peculiarly sharp.

 

Sorry, there's not a lot to be done with it.  Getting flat correction to work should be your first priority.  It doesn't really matter what you image while you try to do that, a random starfield would be fine.

 

2.5 hours is reasonable.  Just 30 minutes would be fine for working on flat correction.  A major issue that needs to be fixed.

 

Again, sorry.

Thanks for the feedback. I can work on flats since it's been so cloudy lately. Then I'll retry.



#21 mtc

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Posted 28 October 2019 - 07:20 PM

An art? Oh, no no no, that's a typical newbie mistake. It's perfectly straightforward and simple, you just load the frames and push the button. (They're working on automating the button-push.)

 

Again, a look at your image histogram should tell the tale whether your exposure is sufficient or excessive. For most images, the "black" sky background makes up most of the pixel population, so that's where the peak is. If the left edge of that peak is clear of the left edge of the graph, no pixels are being clipped to black and so you have enough exposure.

From the right, same story: If no or only a very few pixels are hitting the right edge, the image's dynamic range is sufficient (no bright spots, i.e. stars, are being blown out.

More integration time reduces noise, but what I didn't understand when I started was that "noise" is not just "rice grain junk is sprinkled through my image". Noise also affects tonality, so that what should be a delicate range of adjacent shades becomes a glaring set of posterized steps, or smooshed into a single undifferentiated value.

That's one reason the peak-position guideline is "1/3 of the way", not "barely clearing the left edge": enhancing contrast also enhances noise, so you're better off not digging your nebulosity out of the very bottom of the well.

 

Just out of curiosity, what are you using for calibration and stacking? (Apologies if you mentioned that and I missed it in the thread.)

I've been using Nebulosity and recently starting trying DSS, though I don't recall which one I used for the file above. Should I be using  something else?



#22 bobzeq25

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Posted 28 October 2019 - 10:52 PM

I've been using Nebulosity and recently starting trying DSS, though I don't recall which one I used for the file above. Should I be using  something else?

I recommend Astro Pixel Processor.  It both stacks and processes, doing both in one program has advantages.  Does a good job, most beginners find it more intuitive than the alternatives.


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#23 Clete

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Posted 03 November 2019 - 09:55 AM

I decided to go ahead and process my data on the Dumbell Nebula.

 

This is the best I could do with it. It's not too bad really when you consider the fact that I had left off the light polution filter and the original images were over exposed to the point of being almost white.

 

Unprocessed frame

 

Full frame image

 

Cropped image


Edited by Clete, 03 November 2019 - 09:58 AM.


#24 Clete

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Posted 03 November 2019 - 10:21 AM

I played with it some more. This one's better. I got the background a little darker and bumped up the saturation of the nebula and minimized the stars some more....

 

Dumbell Final Version



#25 TareqPhoto

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Posted 03 November 2019 - 12:02 PM

Very nice, i wish to have nice results myself as well, keep up, you are doing great waytogo.gif


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