Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Can Someone Diagnose these Veil Nebula Miss-fires?

CMOS EAA dso equipment reflector
  • Please log in to reply
28 replies to this topic

#1 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 01:12 PM

Hi CN

 

I captured the two images below last night in decent skies...at least given the LP of my suburban Boston location.

 

I'm hoping some of the experienced and expert nebula imagers out there can help steer me back on course relative to getting the nebulas visible and color-correct while observing with Sharpcap 3.2 Pro. I thought perhaps comments specific to the two types of problems  I'm experiencing might be a reasonable way to get to the issues and help me improve my knowledge and ability to see and enjoy these beautiful and interesting objects.

 

1. The first image shows a vignetted view with the East Veil Nebula (the Fish-hook) faintly visible in the upper right. I had to over-expose this to even see the nebula shape. Obviously the color is wrong. Given my current scope and cameras, is there a way for me to eliminate the vignetting and bring out the nebulas and their honest colors?

 

2. The second image shows the same general view a bit later on (132, 15 second frames versus 28 in the first image). Here the vignetting is gone and the nebula is faintly visible but largely over-whelmed with stars and with a general reddish glow to the lower left which does not quite look like a vignette. I was not able to eliminate the glow or get the nebula to stand out in a normal way against the stars.

 

Although I own what I thought was a reasonable nebula filter, I'm not using it since it turns out it's for visual observing and I just learned that, for imaging you don't use filters intended for visual - an expensive learning experience. But, I've recall seeing numerous lovely nebula captures posted here in CN that specifically mentioned that no filters were used - I'm a bit puzzled and would really appreciate some help.

 

thanks very much for any/all input on the above problems and any other aspect of imaging !

Gary

 

Image 1: NGC 6995 Eastern Veil Nebula - Vignetted:

Raw16/Bin1/Exp15 sec/Gain 340/Frames 28/Brightness 7/Total 420 secs /Darks no Flats

 

Image 2: NGC 6995 Eastern Veil Nebula - Overwhelmed by Stars & Non-nebular glow

Raw16/Bin1/Exp 15 sec/Gain 299/Frames 132/Brightness 10/Total 1980 secs /Darks no Flats

 

 

 

Attached Thumbnails

  • Image1_Veil East_Vignette_Stack_28frames_420s_WithDisplayStretch.jpg
  • Image2_Veil_Stars_Stack_132frames_1980s_WithDisplayStretch.jpg


#2 sterec

sterec

    Lift Off

  • -----
  • Posts: 5
  • Joined: 23 Jan 2019

Posted 22 September 2019 - 01:43 PM

Although they are a pain to do, the calibration frames are really helpful.  I am pretty confident that some flats would make image 1 far more pleasing to the eye.



#3 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 04:16 PM

Although they are a pain to do, the calibration frames are really helpful.  I am pretty confident that some flats would make image 1 far more pleasing to the eye.

Hi Sterec

Perhaps they would help but they are not allowed as part of EAA observing. I occasionally use Darks and I plan to learn to do Flats, but calibration and bias frames, etc are part of AP and I don’t have the patience or interest for that route. I greatly admire those who do. Thank you chiming in!

Gary



#4 Barkingsteve

Barkingsteve

    Sputnik

  • -----
  • Posts: 33
  • Joined: 13 Aug 2018

Posted 22 September 2019 - 04:59 PM

Hi Sterec

Perhaps they would help but they are not allowed as part of EAA observing. I occasionally use Darks and I plan to learn to do Flats, but calibration and bias frames, etc are part of AP and I don’t have the patience or interest for that route. I greatly admire those who do. Thank you chiming in!

Gary

Hi Gary,

You should apply a master flat and dark before stacking in sharpcap pro, they really do help. Other than that, it looks like a histogram issue possibly, are you adjusting both histograms and adjusting colour balance while you live stack ? i have bortle 8-9 skies and keep my brightness on 0 and gain around 150-200, this usually serves me well. 

The red glow could just be a light pollution gradient, you can use visual filters but it may not have an ir/uv cut so you may get bloated stars. Here is the eastern veil  i took the other night, scope is a 130mm f/5 newtonian with a 294mc pro at -5c in sharpcap pro with a master flat and dark applied. This was using the tri band filter from altair asto and was 24 x 15 second stacks at 150 gain. I was adjusting both histograms and colour balance the whole session. Image saved as viewed, resized and saved as jpeg.

 

evnStack_24frames_360s_WithDisplayStretch.png.jpg

 

Filters really do help in light polluted areas, i have gone through about 6 filters now and i really only need 2, one for nebula and a general mild light pollution filter for broadband targets ( idas p2 ) they are pricey but will serve you well for years.


Edited by Barkingsteve, 22 September 2019 - 05:19 PM.

  • Rickster, eros312 and GoFish like this

#5 OleCuss

OleCuss

    Vanguard

  • *****
  • Posts: 2446
  • Joined: 22 Nov 2010

Posted 22 September 2019 - 06:14 PM

As best I can tell, calibration frames are welcomed on this sub-forum.  The key is that you do effectively nothing beyond automated processing - as in try to show us what you saw on your screen while observing rather than what you could do with extensive post-processing.

 

Yeah, we're also supposed to post our data regarding the equipment used and the information about the sub-exposures.

 

Calibration frames should be fine if you choose to use them.



#6 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 06:48 PM

As best I can tell, calibration frames are welcomed on this sub-forum.  The key is that you do effectively nothing beyond automated processing - as in try to show us what you saw on your screen while observing rather than what you could do with extensive post-processing.

 

Yeah, we're also supposed to post our data regarding the equipment used and the information about the sub-exposures.

 

Calibration frames should be fine if you choose to use them.

I looked into this online a bit and it seems that 'calibration frames' is a term applied to Darks, Flats, and Bias frames. I know folks here use the first two of these routinely and that Sharpcap 3.2 facilitates the subtraction of both. I cannot find anything in the Sharpcap manual that addresses Bias frames and I know nothing about them...yet another learning opportunity.

 

So, assuming Bias frames are able to be applied 'on the fly' during live stacking, all the above 'calibration' frames should be fine for EAA....others may chime in.



#7 OleCuss

OleCuss

    Vanguard

  • *****
  • Posts: 2446
  • Joined: 22 Nov 2010

Posted 22 September 2019 - 07:50 PM

The SharpCap author sees/saw little benefit to bias frames.  The idea is to match your darks to your lights (same exposure and gain) and the bias frames don't actually add anything to your imaging.

 

In this context the bias frames are thought to mostly be beneficial for adjusting your darks to match your sub-exposures.

 

I don't claim to have the definitive answer as to whether this is correct or off-base.  But FWIW I think SharpCap works pretty nicely with a set of flats and darks matched to your lights.



#8 Rickster

Rickster

    Viking 1

  • *****
  • Posts: 831
  • Joined: 09 Jun 2008
  • Loc: NC Kansas Bortle 3 SQM 21.8+

Posted 22 September 2019 - 08:32 PM

The veil nebula can be tough depending on your sky conditions and level of light pollution.  I happened to get a decent shot of the Eastern veil on 9/1 from my dark sky location.  To give you an idea of how much gain and stretch I had to use, I have attached a single 30sec exposure straight out of the camera and the final stacked/stretched image.  The Central America portion of the nearby North America nebula is considerably brighter and might be a better choice.

 

I don't have any experience shooting under light polluted skies, but I would think it would be tough digging the fainter areas of the nebula out of light pollution sky fog.  Your fellow city dwellers are no doubt correct that a good filter will be needed at your location.  It appears to me that your light pollution has forced you to set your black level quite high and therefore you are clipping the fainter stars and the fainter areas of the nebula. 

 

The following images were taken with a 6in Newt (AT6IN), a GSO coma corrector, and an astromodded Canon T3 DLSR on a night of average seeing at my usual dark site.  Note that the background is black, even straight out of the camera.   

 

First image is straight out of the camera; 1x30sec, ISO 6400, small fine jpg 2456x1504, no flats or darks or filter (other than the OEM IR/UV cut filter).

 

Second image has been stacked and processed on the fly using Astrotoaster:  10x30sec, ISO 6400 (max gain), small fine jpg 2456x1504, no flats or darks or filter (other than the OEM IR/UV cut filter). 

Attached Thumbnails

  • IMG_0033 (Custom).JPG
  • veil2 (Custom).jpg

Edited by Rickster, 23 September 2019 - 12:58 PM.

  • eros312 likes this

#9 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 09:01 PM

Rick, are you referring to the images in Post 1 or 4? When you say the black level is high, is that meaning that the black line is too far to the right?

 

Also, no one has commented yet on the round over-exposed area in the first image in post 1. I called it vignetting but I’m not clear on what causes it or why Flats would cure it. The entire image field is receiving photons so why is the round area in the center more exposed than elsewhere?

thank you,

G



#10 GoFish

GoFish

    Surveyor 1

  • *****
  • Posts: 1545
  • Joined: 30 Nov 2016
  • Loc: Kentucky / Colorado

Posted 22 September 2019 - 09:09 PM

I am still learning, too, but this is my present understanding of calibration frames using SharpCap for EAA:

  • Using darks and flats applied during real-time stacking is acceptable practice for the images posted to the EAA forum
  • I saw a post in the SharpCap forum where Robin suggests applying bias (or flat darks) calibration to flats. This is consistent with my limited understanding of applying calibration frames in general AP. There is a checkbox for applying bias frames during flats capture in the dialog. 
  • For the 294, presumably because of its large sensor, I think that flats are essential or vignetting can be overwhelming
  • I think flats are more important than darks with this camera when cooled to -5C, but I think it’s best to use both

 


  • Barkingsteve likes this

#11 Rickster

Rickster

    Viking 1

  • *****
  • Posts: 831
  • Joined: 09 Jun 2008
  • Loc: NC Kansas Bortle 3 SQM 21.8+

Posted 22 September 2019 - 09:55 PM

Rick, are you referring to the images in Post 1 or 4? When you say the black level is high, is that meaning that the black line is too far to the right?

 

Also, no one has commented yet on the round over-exposed area in the first image in post 1. I called it vignetting but I’m not clear on what causes it or why Flats would cure it. The entire image field is receiving photons so why is the round area in the center more exposed than elsewhere?

thank you,

G

Gary,  Sorry.  I was only referring to your second image.  I am not sure what to say about the first one.  For starters, I am not familiar with your scope.  Is it also known as a Comet Catcher?  Does it have a 1.25" or 2" focuser?  If it is 1.25", that would explain the vignetting.



#12 Barkingsteve

Barkingsteve

    Sputnik

  • -----
  • Posts: 33
  • Joined: 13 Aug 2018

Posted 22 September 2019 - 10:37 PM

Your first image looks like vignetting, this can be avoided by using flats. The imaging circle is brighter in the middle, a master flat made and applied before the session will even out the bright and dark areas and eliminate 'dust bunnies'


Edited by Barkingsteve, 22 September 2019 - 10:40 PM.


#13 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 10:50 PM

Gary,  Sorry.  I was only referring to your second image.  I am not sure what to say about the first one.  For starters, I am not familiar with your scope.  Is it also known as a Comet Catcher?  Does it have a 1.25" or 2" focuser?  If it is 1.25", that would explain the vignetting.

Rickster

it is 1.25. Why does that cause part of the frame to be brighter? The entire frame has data. I’ve never heard optically what caused this issue. 

G



#14 Barkingsteve

Barkingsteve

    Sputnik

  • -----
  • Posts: 33
  • Joined: 13 Aug 2018

Posted 22 September 2019 - 11:11 PM

Rickster

it is 1.25. Why does that cause part of the frame to be brighter? The entire frame has data. I’ve never heard optically what caused this issue. 

G

It doesn't cause part of the frame to be brighter, it causes part of the frame to be darker, this is vignetting, think of it as tunnel vision. Taken from wiki:

In photography and optics, vignetting (/vɪnˈjɛtɪŋ, viːnˈ-/; French: vignette) is a reduction of an image's brightness or saturation toward the periphery compared to the image center


  • GoFish likes this

#15 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 22 September 2019 - 11:19 PM

Ok, I get what you’re saying but I don’t get it optically. I guess I’ll try finding a diagram that shows what’s happening optically. I suspect this issue closely relates to over/under sampling - another area I’d like to have a better grasp of. 

Thanks for your patience with my questions. 

G



#16 GoFish

GoFish

    Surveyor 1

  • *****
  • Posts: 1545
  • Joined: 30 Nov 2016
  • Loc: Kentucky / Colorado

Posted 22 September 2019 - 11:28 PM

Ok, I get what you’re saying but I don’t get it optically. I guess I’ll try finding a diagram that shows what’s happening optically. I suspect this issue closely relates to over/under sampling - another area I’d like to have a better grasp of. 

Thanks for your patience with my questions. 

G

Purely an analog optical effect. Has nothing to do with sampling. Happened in the days of film, too. 



#17 Barkingsteve

Barkingsteve

    Sputnik

  • -----
  • Posts: 33
  • Joined: 13 Aug 2018

Posted 22 September 2019 - 11:32 PM

Optical vignetting
This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease in light intensity towards the image periphery.

the outside tube ( focuser tube or telescope tube or both ) is shading the edges making them darker, nothing to do with sampling. flats will cure this.



#18 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 23 September 2019 - 08:43 AM

Optical vignetting
This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease in light intensity towards the image periphery.

the outside tube ( focuser tube or telescope tube or both ) is shading the edges making them darker, nothing to do with sampling. flats will cure this.

 

 

Purely an analog optical effect. Has nothing to do with sampling. Happened in the days of film, too. 

 

Hi Guys

Thank you for your explanations. Glad to hear that Flats will help. I’m still going to find a diagram that graphically depicts what is happening when vignetting occurs. I’m sure one exists somewhere out there......

cheers

G



#19 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 23 September 2019 - 09:15 AM

Hi Guys

Thank you for your explanations. Glad to hear that Flats will help. I’m still going to find a diagram that graphically depicts what is happening when vignetting occurs. I’m sure one exists somewhere out there......

cheers

G

Anyone interested in what the optical effect is that causes vignetting can check out the article at the end of this link - beginning in section 3.3 at the bottom of. Page 32. A decent diagram is included and the discussion on vignetting starts there and goes on for a few pages.

 

http://imarek.free.f...d/newton-en.pdf

 

I do think this effect, caused by focuser physical size and optics, has implications for ‘sampling’ but that’s something for later when I have some idea what sampling means relative to the CMOS cameras.

 

Enjoy the math!

G



#20 Rickster

Rickster

    Viking 1

  • *****
  • Posts: 831
  • Joined: 09 Jun 2008
  • Loc: NC Kansas Bortle 3 SQM 21.8+

Posted 23 September 2019 - 09:18 AM

Gary,  There is a website that has a free calculator that will calculate the vignetting and IIRC provides a ray trace drawing.  All you have to do is plug in the dimensions of your scope.  It does all the math for you.  I have to run right now but when I get back I will  dig it up for you.


Edited by Rickster, 23 September 2019 - 09:19 AM.


#21 biomedchad

biomedchad

    Viking 1

  • -----
  • Posts: 809
  • Joined: 13 Apr 2009
  • Loc: Galion, Ohio

Posted 23 September 2019 - 09:31 AM

yes with that amount of vignette you will not be able to achieve a nice EAA images in sharpcap..when you apply a stretch or color balance it will always be off.  just do a good master flat and dark and turn it on and youre good.  there is a great thread in the sharp cap forums on the various methods to produce and apply a flat specifically to sharpcap.

 

you must also decide how you will color balance.  you can either balance with the regular histogram and save that as a profile or you can elave red and blue at 50/50 and do all your balancing in the live histogram with the sliders.  just make sure those settings match your dark and flat creation (even though the flat will end up being monochrome).

 

a few small steps and you can really enjoy EAA to its max with sharp cap.


  • GoFish likes this

#22 Rickster

Rickster

    Viking 1

  • *****
  • Posts: 831
  • Joined: 09 Jun 2008
  • Loc: NC Kansas Bortle 3 SQM 21.8+

Posted 23 September 2019 - 01:07 PM

Gary,  Here is a link to the calculator that I mentioned.  https://stellafane.o...b/newt-web.html



#23 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 23 September 2019 - 01:55 PM

yes with that amount of vignette you will not be able to achieve a nice EAA images in sharpcap..when you apply a stretch or color balance it will always be off.  just do a good master flat and dark and turn it on and youre good.  there is a great thread in the sharp cap forums on the various methods to produce and apply a flat specifically to sharpcap.

 

you must also decide how you will color balance.  you can either balance with the regular histogram and save that as a profile or you can elave red and blue at 50/50 and do all your balancing in the live histogram with the sliders.  just make sure those settings match your dark and flat creation (even though the flat will end up being monochrome).

 

a few small steps and you can really enjoy EAA to its max with sharp cap.

Thank you BMC. I generally color balance with Wr and Wb at 50 and use sliders in live stack window to align peaks in the Display histo window. 



#24 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 23 September 2019 - 01:57 PM

Gary,  Here is a link to the calculator that I mentioned.  https://stellafane.o...b/newt-web.html

Thanks Rick. I’ll check it out. 

Vignetting hasn’t been an issue until I started viewing nebulas. 

G. 



#25 GaryShaw

GaryShaw

    Ranger 4

  • *****
  • topic starter
  • Posts: 300
  • Joined: 28 Nov 2017
  • Loc: Boston

Posted 23 September 2019 - 10:22 PM

Gary,  Here is a link to the calculator that I mentioned.  https://stellafane.o...b/newt-web.html

Rickster, this system doesn’t appear to deal with cameras and imaging vignettes and seems to be focused towards folks who are designing and building reflectors. Probably a pretty  nice resource for that. 

 

It it might be a good project for the folks who run Skytools to create a tool that would predict vignetting based on readily available scope and camera data.

 

I’m about to buy a new 8” F4 newtonian and would love to be able to determine ahead of time whether it will have vignetting in the images with the cameras I own and one I plan to buy. Do you have any idea if there’s a way to predict that ?

best,

G




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: CMOS, EAA, dso, equipment, reflector



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics