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Pinwheel Galaxy first attempt, need advice

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#1 HowardSD

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Posted 09 April 2020 - 04:49 AM

My first go at M101 with my new ZenithStar 103, feel i left something on the table though! After stretching the image to bring out as much detail as possible it appeared very grainy, had to use a few PS masks to make it reasonable.

 

I had near 4 hours of data captured 78 x 180s, 20 darks, 15 flats @ ISO800. Does including more darks & flats decrease the grain or do i just need more exposure time or both (can't do bias as my stacking software doesn't accept them)?

 

Next question is: If i shoot the same image on different nights does each session need its own set of calibration frames and if so is it best to stack one session then add the next sessions images & calibrations to the previously stacked image?

 

 

 

 

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#2 einarin

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Posted 09 April 2020 - 05:20 AM

Strange, I think you should see more with 4 hour exposure.

Here's one with about same time and scope:
https://www.astrobin...8533/B/?nc=user

 

Maybe you could share your stacked image somewhere so we could try if there's more to pull up.

 

If you decide to add more light (recommended) you can use same darks but must take new flats.

 

What software are you using for stacking ?

In DSS there's file groups where you can add lights and calibration files and it makes a single stack of them.



#3 Huangdi

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Posted 09 April 2020 - 05:44 AM

Strange, I think you should see more with 4 hour exposure.

Here's one with about same time and scope:
https://www.astrobin...8533/B/?nc=user

 

Maybe you could share your stacked image somewhere so we could try if there's more to pull up.

 

If you decide to add more light (recommended) you can use same darks but must take new flats.

 

What software are you using for stacking ?

In DSS there's file groups where you can add lights and calibration files and it makes a single stack of them.

The image you linked doesn't really show fainter stuff tho, doesn't it? It just has better details

 

 

To OP:

 

I agree that you need more exposure time. I collected 14 hours at F7.5 for this to get a decent Snr, should have made it 20 though. 

 

Darks and flats won't really do anything against yoir grain, that's mainly processing and not enough data.

 

What software are you using for stacking? If you don't use bias, your flats won't work properly, so it's important that you use them. 

 

As has been said, bias and dark can be reused, in my opinion flats can be reused as well, but not for too long.

 

If you take a close look at your image, you can see dust moted next to the galaxy, so it looks like your flats didn't work. There is also a gradient from top to bottom, you might wanna work on getting rid of that.

 

All in all its a nice image though, double the exposure time and work a little on your processing and it'll be great! 



#4 BQ Octantis

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Posted 09 April 2020 - 06:16 AM

You need more lights. You can assess how much by stretching the image to the level of detail you want and then scaling down to where the shot noise level is acceptable. My assessment was, I could get a reasonable image from your data (in a JPEG) at 25% scaling:

 

test.jpg

 

If you want a good result at 100%, you'll need 42=16× the exposure time. At f/6.9, the Zenithstar 103 isn't a particularly fast optic…

 

Cheers,

 

BQ


Edited by BQ Octantis, 09 April 2020 - 06:18 AM.


#5 HowardSD

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Posted 09 April 2020 - 06:30 AM

The image you linked doesn't really show fainter stuff tho, doesn't it? It just has better details

 

 

To OP:

 

I agree that you need more exposure time. I collected 14 hours at F7.5 for this to get a decent Snr, should have made it 20 though. 

 

Darks and flats won't really do anything against yoir grain, that's mainly processing and not enough data.

 

What software are you using for stacking? If you don't use bias, your flats won't work properly, so it's important that you use them. 

 

As has been said, bias and dark can be reused, in my opinion flats can be reused as well, but not for too long.

 

If you take a close look at your image, you can see dust moted next to the galaxy, so it looks like your flats didn't work. There is also a gradient from top to bottom, you might wanna work on getting rid of that.

 

All in all its a nice image though, double the exposure time and work a little on your processing and it'll be great! 

HI there, thanks for your reply.

 

I use "starry sky stacker", one of a handful if that of Mac programs that can be used for stacking and it's simple however lacks an ability to add bias frames and also mosiac, this image was taken with three separate sessions and i had to manually align about 35% of the frames in photoshop before i could stack them! lol

 

For some reason i've been having trouble attaining good flats with the new scope (started off in December with a 72mm scope and my flats were ok), not sure why i'm having problems, ordered a light box and hopefully that will solve the problem, I've been pointing the scope at a clear sky with a cloth over the dew shield, worked for the smaller scope but problematic with this new one.

 

When the weather clears this weekend i'll get some more time put in on this target.

 

One question: Can i just add the new frames to this stacked image or would it be best to stack them all over again from scratch?

 



#6 Madratter

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Posted 09 April 2020 - 08:30 AM

Howard. You didn't indicate what camera you used, although it is clear it is a DSLR from the ISO 800 comment. That may or may not be optimum for you camera. Likewise your exposure times may or may not be correct given the camera and your sky conditions. How much light pollution did you have?

 

In general M101 is surprisingly faint for such a large object and is rather notorious among visual observers for having low surface brightness.

 

Whether you left much on the table will depend a lot on your sky conditions.


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#7 Huangdi

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Posted 09 April 2020 - 09:41 AM

One question: Can i just add the new frames to this stacked image or would it be best to stack them all over again from scratch?

You have to stack them all over again. Keep in mind that the whole purpose of stacking is to average out the pixel values and thus improve detail. It won't work if you have one image with an extremely high SNR (because its already stacked) and then let's say twenty more with extremely low SNR. So yeah, sadly you'll have to restack them.


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#8 salamander

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Posted 09 April 2020 - 10:14 AM

Speaking from a perspective somewhere below 'amateur' and from someone who also took a first try at the Pinwheel this past Sunday, the background speckling you're seeing may be a result of trying to process the stacked image beyond what the data you collected actually supports.  This has been one of my problems for a while ... I want to get as much detail as I can from the image, but the software can't distinguish between DSO data and background, and I tend to over-process the image.  If you try to bring the DSO details out too much, you can't help but also bring out the background.

 

When I took my subs on Sunday, the moon was at about 95% full and was not all that far from the Pinwheel, so I had a lot of background signal.  I may be wrong, but I think this is going to wash out some of the detail in your target, and you're just not going to be able to get it back without creating background artifacts like speckling.

 

By the way, I really like your photo of the Pinwheel.  I'm just hoping mine turns out half as good.


Edited by salamander, 09 April 2020 - 10:15 AM.

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#9 idclimber

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Posted 09 April 2020 - 10:46 AM

Pixinsight has a Mac version. They also have a trial version if you would like to try it out. 



#10 Huangdi

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Posted 09 April 2020 - 11:21 AM

Speaking from a perspective somewhere below 'amateur' and from someone who also took a first try at the Pinwheel this past Sunday, the background speckling you're seeing may be a result of trying to process the stacked image beyond what the data you collected actually supports. This has been one of my problems for a while ... I want to get as much detail as I can from the image, but the software can't distinguish between DSO data and background, and I tend to over-process the image. If you try to bring the DSO details out too much, you can't help but also bring out the background.

When I took my subs on Sunday, the moon was at about 95% full and was not all that far from the Pinwheel, so I had a lot of background signal. I may be wrong, but I think this is going to wash out some of the detail in your target, and you're just not going to be able to get it back without creating background artifacts like speckling.

By the way, I really like your photo of the Pinwheel. I'm just hoping mine turns out half as good.

Your post is basically an outcry to use masks. If there's one thing I've learned in three years of Astrophotography, then it is to mask. Literally everything.

Sharpening? Mask it!
Noise reduction? Mask it!
Stretching? You bet you'll mask it!
Saturatio? Mask it!
Contrast enhancement? Please, mask it!

Just think about it, do you really want to apply the same adjustments to the brightest part of the image and the darkest? Can there even be a decent outcome? I doubt it...

You'll be surprised how a) easy masks are to make in photoshop and b) clean the result will be. No ugly artifacts, no speckling, nothing. Just smooth details... At least in theory 😁

Edited by Huangdi, 09 April 2020 - 11:22 AM.


#11 F.Meiresonne

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Posted 09 April 2020 - 02:28 PM

 

In general M101 is surprisingly faint for such a large object and is rather notorious among visual observers for having low surface brightness.

 

Indeed it is. In my skies i can not see it in my refractors. Let alone binoculars.

 

In the Provence of France south, in good skies ,i can detect with 10x50 binoculars would you believe it...

 

It is a tough guy, thisone.  better try to image it,you will see alot more...



#12 HowardSD

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Posted 09 April 2020 - 02:49 PM

Howard. You didn't indicate what camera you used, although it is clear it is a DSLR from the ISO 800 comment. That may or may not be optimum for you camera. Likewise your exposure times may or may not be correct given the camera and your sky conditions. How much light pollution did you have?

 

In general M101 is surprisingly faint for such a large object and is rather notorious among visual observers for having low surface brightness.

 

Whether you left much on the table will depend a lot on your sky conditions.

Hi Madratter,

 

My camera's a Nikon D5500 I was also using a Baader Moon & skyglow filter with an AT2FF (already had that FF and works pretty good with the ZS 103), my area is a Bortle 8 zone, so there is a fair amount of light pollution. When i shot the majority of the frames (last weekend), the moon was pretty high in the sky, not ideal i know. 35-40% of the frames were from earlier sessions.

 

I think exposure time seems to be the general comment, so i'll focus on getting hopefully at least another 4 hours this weekend, more if i'm lucky... position in the sky for me is an issue. I have basically a 180 degree shooting area (due to street lights) NE - SW, so my imaging where I am doesn't really start now until around midnight, so that coupled with the indifferent weather atm!



#13 HowardSD

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Posted 09 April 2020 - 03:03 PM

You need more lights. You can assess how much by stretching the image to the level of detail you want and then scaling down to where the shot noise level is acceptable. My assessment was, I could get a reasonable image from your data (in a JPEG) at 25% scaling:

 

attachicon.giftest.jpg

 

If you want a good result at 100%, you'll need 42=16× the exposure time. At f/6.9, the Zenithstar 103 isn't a particularly fast optic…

 

Cheers,

 

BQ

Wish I could... 64 hours!!!!! Going to try again at 8 hours then 12 hours, if still not happy! After that going to find another object to shoot, i'm still only 4 months into this and it's a target rich environment! :)



#14 Deesk06

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Posted 09 April 2020 - 06:12 PM

Speaking from a perspective somewhere below 'amateur' and from someone who also took a first try at the Pinwheel this past Sunday, the background speckling you're seeing may be a result of trying to process the stacked image beyond what the data you collected actually supports.  This has been one of my problems for a while ... I want to get as much detail as I can from the image, but the software can't distinguish between DSO data and background, and I tend to over-process the image.  If you try to bring the DSO details out too much, you can't help but also bring out the background.

 

When I took my subs on Sunday, the moon was at about 95% full and was not all that far from the Pinwheel, so I had a lot of background signal.  I may be wrong, but I think this is going to wash out some of the detail in your target, and you're just not going to be able to get it back without creating background artifacts like speckling.

 

By the way, I really like your photo of the Pinwheel.  I'm just hoping mine turns out half as good.

I just partitioned my drive and added windows for free. It's pretty simple to do if you have enough room. APple even integrates a program into your mac already to make it even easier.  I then downloaded DSS



#15 BQ Octantis

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Posted 09 April 2020 - 06:40 PM

Wish I could... 64 hours!!!!! Going to try again at 8 hours then 12 hours, if still not happy! After that going to find another object to shoot, i'm still only 4 months into this and it's a target rich environment! smile.gif

 

You'll definitely get better results with more time.

 

To answer your question about darks, ideally you should stack the same number of darks as lights to match the sensor noise statistics, but I've found the average to settle out at around an hour at a given temperature. As to separate sessions, you can combine the data from multiple sessions at a given location (vice stacking separately), as long as the environmental noise statistics weren't that different:

 

  • The sky conditions were roughly the same.
  • The light pollution was roughly the same.
  • The ambient temperature was roughly the same.

 

You also want the target's position and orientation on the sensor to be the same, or you'll get pixel registration (overlap) errors that will smudge the stack.

 

BQ




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