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

Andromeda partly blurry @200mm

astrophotography
  • Please log in to reply
17 replies to this topic

#1 pocpoc47

pocpoc47

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 15 Sep 2020

Posted 17 September 2020 - 11:12 AM

Hello everyone,

 

I am new here on this forum, and I have tried a few times to take pictures of Andromeda untracked using DSLR and lens.

My first few trials went as expected (not great but understandable mistakes and limitations), but the last one, I got something I could not explain.

 

I used my trusty Canon 70D with the great Canon EF 70-200mm f/2.8 IS at 200mm and ISO 3200.

I used 775 subs 1 seconds long.

 

I recentered Andromeda once in the process before it would exit the frame.

 

This image is the result of stacking with DSS, and autostretching with Siril:

M31_200mm_775x1s_3200ISO_DSS_stretched.jpg

This one was stacked with Siril (and autostretched):

M31_200mm_775x1s_3200ISO_Siril_stretched.jpg

 

As you can see, the stars on the left of Andromeda are blurry, and there is a change in color in the middle and side of the frames.

 

What could have caused this? Could it be the mirror slap? Or the Image Stabilization (I believe it was off)? Could it be an issue when I recomposed the shot? Maybe I should recenter more often than once? The focus (I used a DIY Bahtinov Mask)? Or any other advice?

Thank you for your help! Clear skies to you all,

 

Pocpoc

 

 



#2 meansrt

meansrt

    Explorer 1

  • *****
  • Posts: 76
  • Joined: 05 May 2020

Posted 17 September 2020 - 12:25 PM

I don't know what is causing the blurriness that's very strange. Maybe you have some dust or a smudge or something on that side of the lens or dew formed ONLY there? Very strange. 

 

As for the color change you are seeing in the second picture, that is quite easy to explain. Your camera is standing completely still while the stars are moving in real time. Over the 775 seconds you took the photos in, the stars have shifted quite a bit. The stacker programs are trying to stack all of these 775 images so that each star is aligned. Since the stars are moving, the stacker program has to shift each image very slightly each time it stacks an image so that the stars are still aligned. This makes it so that the edges are darker because less images are stacked there due to the shifting that occurs and the purple parts are where all 775 images are stacked on top of each other. Think of it like a venn diagram, the purple parts are where all the images are stacked on top of each other and the dark parts are where some but not all images are aligned. 

 

Am i making sense?


  • Jschroedl likes this

#3 StarBurger

StarBurger

    Viking 1

  • -----
  • Posts: 597
  • Joined: 06 Mar 2018
  • Loc: North Country NY

Posted 17 September 2020 - 12:36 PM

+ 1 for meansrt. A good explanation. A result of stacking un-tracked frames including the blurriness due to lens distortions as new field edges are brought into play.

Tracking will eliminate this.


  • meansrt likes this

#4 SilverLitz

SilverLitz

    Apollo

  • -----
  • Posts: 1,177
  • Joined: 17 Feb 2018
  • Loc: Louisville, KY

Posted 17 September 2020 - 12:43 PM

I wonder if you have some sensor tilt, as the left edge is clearly off the focus plane.

 

Assuming you shot with a tripod (non-tracking) there will be some star movement blurring, but that will show across the frame.

 

Also, though not related to focus, for your 70D use ISO 1600, as that is the 70D's ~ISO Invariant level.  For a 80D and most Rebels, the best is ISO 800 .


  • Jschroedl likes this

#5 meansrt

meansrt

    Explorer 1

  • *****
  • Posts: 76
  • Joined: 05 May 2020

Posted 17 September 2020 - 12:44 PM

+ 1 for meansrt. A good explanation. A result of stacking un-tracked frames including the blurriness due to lens distortions as new field edges are brought into play.

Tracking will eliminate this.

To echo this, try re-centering Andromeda a little more often so that it doesnt too close the edge of the image. If it really is something to do with lens distortion, that should eliminate the problem. It does seem likely that it has to do with being too close to the edge of the lens as from your stack up we can see that andromeda is approaching the left side of the image and therefore getting close to that side of the lens. We can tell the direction it's moving in by looking at the direction of the darker to lighter frames. 

 

SilverLitz also brings up a good point about the sensor tilt. If recentering andromeda more often doesnt fix the issue perhaps it has something to do with that. 


Edited by meansrt, 17 September 2020 - 12:45 PM.


#6 spereira

spereira

    Soyuz

  • *****
  • Moderators
  • Posts: 3,915
  • Joined: 21 Apr 2017
  • Loc: Bedford, NH

Posted 17 September 2020 - 01:26 PM

Moving to DSLR ...

 

smp



#7 mmalik

mmalik

    DSLR camera modifications

  • *****
  • Vendors
  • Posts: 10,954
  • Joined: 13 Jan 2012
  • Loc: USA

Posted 17 September 2020 - 02:02 PM

I would start over by properly framing the object and not worrying too much about an off center acquisition. Regards



#8 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,091
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 17 September 2020 - 02:08 PM

I've only used DSS a handful of times, but I saw similar issues to your top image. I think it struggles with wide, dense star fields and lens distortion. Part of the image would be in focus, but part would be smeared like that. It looks like SIRIL is handling it better, so that might be the better choice. Crop away the discolored part, which is caused by misalignment of the frames. You can see the same thing in your DSS stack, although it appears to handle the background normalization better.

 

Nice job with a static tripod though!



#9 pocpoc47

pocpoc47

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 15 Sep 2020

Posted 17 September 2020 - 05:22 PM

Thank you for all your inputs! I will try again tomorrow night (if the heavens are favorable).

I'll try to reframe more often to keep her mostly in the center of the frame, this can only help remove artifacts.

 

About sensor tilt, I don't think it is possible. I have had this camera for a few years and have shot thousands of pictures and never noticed any focus shift.

 

Concerning the ISO invariance, I thought the ISO to use was the one where read-noise starts to increase linearly, but it seems to me that this happens at 3200.

 

Screenshot_5.jpg

 

Since ISO 2011 and 2546 have the same read noise, shouldn't the SNR be higher for 2546 than 2011? (and since they are the same analog gain than 1600 and 3200, I should use the "real" values)

 

Thank you again for all your help! I hope I'll be able to get better at this.

 

Pocpoc

 



#10 chanrobi

chanrobi

    Messenger

  • -----
  • Posts: 406
  • Joined: 02 Sep 2019

Posted 17 September 2020 - 06:05 PM

Hello everyone,

 

I am new here on this forum, and I have tried a few times to take pictures of Andromeda untracked using DSLR and lens.

My first few trials went as expected (not great but understandable mistakes and limitations), but the last one, I got something I could not explain.

 

I used my trusty Canon 70D with the great Canon EF 70-200mm f/2.8 IS at 200mm and ISO 3200.

I used 775 subs 1 seconds long.

 

I recentered Andromeda once in the process before it would exit the frame.

 

This image is the result of stacking with DSS, and autostretching with Siril:

attachicon.gifM31_200mm_775x1s_3200ISO_DSS_stretched.jpg

This one was stacked with Siril (and autostretched):

attachicon.gifM31_200mm_775x1s_3200ISO_Siril_stretched.jpg

 

As you can see, the stars on the left of Andromeda are blurry, and there is a change in color in the middle and side of the frames.

 

What could have caused this? Could it be the mirror slap? Or the Image Stabilization (I believe it was off)? Could it be an issue when I recomposed the shot? Maybe I should recenter more often than once? The focus (I used a DIY Bahtinov Mask)? Or any other advice?

Thank you for your help! Clear skies to you all,

 

Pocpoc

Need to find out if the blurriness is evident in single frames. If it is, probably is a lens aberration issue.



#11 SilverLitz

SilverLitz

    Apollo

  • -----
  • Posts: 1,177
  • Joined: 17 Feb 2018
  • Loc: Louisville, KY

Posted 18 September 2020 - 09:39 AM

Here is my summary of 70D analysis by DxO:

 

70D DxO.png

 

This site has ISO Invariant levels (plus more) for several cameras, for the 70D it shows ISO 1280, which almost exactly maps to Stated ISO of 1600 (which DxO is actually 1277)

 

https://www.photonst...anon EOS 80D_14



#12 pocpoc47

pocpoc47

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 15 Sep 2020

Posted 20 September 2020 - 04:44 AM

So I tried again the other night taking your advices into account and it turned out much better!

 

200mm f/2.8, 717x1s, 1600ISO

 

Here is the stretched image from Siril:

 

M31_200mm_717x1_1600_Siril_Stretched.jpg

 

And the fully processed image with Siril and Photoshop:

 

M31_200mm_717x1_1600_Siril_Processed-Edit.jpg

 

Much better than my previous attempt.

I believe the blurriness was caused by the not so sharp performances at the edges of the frame, being wide open at f/2.8. Recentering much more frequently helped a lot.

Stopping it down could probably help but I don't think I can live with the loss of light with no tracking..

 

Thank you all for your help!
Clear skies,

 

Pocpoc


Edited by pocpoc47, 20 September 2020 - 04:45 AM.

  • meansrt and limeyx like this

#13 meansrt

meansrt

    Explorer 1

  • *****
  • Posts: 76
  • Joined: 05 May 2020

Posted 20 September 2020 - 10:44 AM

Very nice! if you do end up stopping it down maybe you could just do 2 second exposures to make up for the loss of light I'm sure it would make a radical difference in star shape



#14 pocpoc47

pocpoc47

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 15 Sep 2020

Posted 21 September 2020 - 03:54 PM

But at 200mm exposing for 2 seconds would create star trails no? I will try a few exposures to see if it trails and my try a full imaging session if the result seems good.

 

Also, I have a question about calibration frames:

I take darks, biases(offset) and flats (no dark flats since I have short exposures for my flats).

How should I process them?
As I understand it, there are two possibilities:

 

1. substract the master_bias from the darks and from the flats, and then substract the master_bias and the master_dark from the lights, divided by the master_flat

2. substract the master_bias only from the flats, and then substracting the master_dark (which also contains the sensor bias) from the lights, divided by the master_flat

 

Is there any advantage using one method or the other? Or am totally misunderstanding something? I don't need to scale the darks.

 

Or maybe it's better that I start another topic for this question?



#15 meansrt

meansrt

    Explorer 1

  • *****
  • Posts: 76
  • Joined: 05 May 2020

Posted 21 September 2020 - 04:09 PM

But at 200mm exposing for 2 seconds would create star trails no? I will try a few exposures to see if it trails and my try a full imaging session if the result seems good.

 

Also, I have a question about calibration frames:

I take darks, biases(offset) and flats (no dark flats since I have short exposures for my flats).

How should I process them?
As I understand it, there are two possibilities:

 

1. substract the master_bias from the darks and from the flats, and then substract the master_bias and the master_dark from the lights, divided by the master_flat

2. substract the master_bias only from the flats, and then substracting the master_dark (which also contains the sensor bias) from the lights, divided by the master_flat

 

Is there any advantage using one method or the other? Or am totally misunderstanding something? I don't need to scale the darks.

 

Or maybe it's better that I start another topic for this question?

It's only 2 seconds I dont think it'll be a big deal. 

 

As for the calibrations frames, I'm not sure on the specifics I just let AstroPixelProcessor or DeepSkyStacker handle the processing of them automatically. 



#16 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,091
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 21 September 2020 - 04:17 PM

Subtract your master bias from your flats to make a calibrated master flat. Then you subtract darks from your lights and divide by master flat. Unless you’re doing something fancy like dark scaling, think of your dark frames as thermal noise plus bias. You just need to subtract it once.

A flat dark is the thermal signal plus bias from your flat exposure. But since flats are usually so short, you can take a short cut and ignore the thermal signal part of the frame.
  • limeyx likes this

#17 pocpoc47

pocpoc47

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 15 Sep 2020

Posted 21 September 2020 - 04:41 PM

So I only need to calibrate the darks for fancy stuff, I get it!

 

Thank you!



#18 DubbelDerp

DubbelDerp

    Apollo

  • *****
  • Posts: 1,091
  • Joined: 14 Sep 2018
  • Loc: Upper Peninsula of Michigan

Posted 21 September 2020 - 04:53 PM

Not at all... I only use lights, darks, flats, and flarks.


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: astrophotography



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics