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Bambusman
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Astrophotography

09 Apr 2017 02:27

A-L-E-X wrote:
Source of the post They are great- what kind of camera and lens and focal length did you use? I do wide field imaging and I find that I can do 13 sec exposures at 14mm (28mm equivalent) and still have round stars.  I use DSS for stacking and then a program called Sequator to remove light pollution.Did you know you can even detect exoplanets with simple camera equipment?I don't do dark frames and all the other stuff as I merely keep dark frame subtraction on in my camera.  It takes the dark frames automatically.Bias frames are for dust on the sensor (I think) and Olympus cameras are legendary for dust removal (I wouldn't use any other brand and I only use mirrorless because I want as lightweight as possible.)

I Have a simple canon 7d  with a 55-250mm lens (i used 250mm).
i also used DSS for stacking, i deactivated this automatic dark frame subtraction because for me it feels like im losing detail with it. Bias frames are used to remove the readout signal from you camera sensor and dark frames also have these signals. For me flat frames is a must have because of vignetting, without it i have a huge donut ring around my images which needs more time for removing in the processing.
I use PixInsight or StarTools for processing and afterwards gimp.
 
A-L-E-X
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09 Apr 2017 03:00

Bambusman wrote:
A-L-E-X wrote:
Source of the post They are great- what kind of camera and lens and focal length did you use? I do wide field imaging and I find that I can do 13 sec exposures at 14mm (28mm equivalent) and still have round stars.  I use DSS for stacking and then a program called Sequator to remove light pollution.Did you know you can even detect exoplanets with simple camera equipment?I don't do dark frames and all the other stuff as I merely keep dark frame subtraction on in my camera.  It takes the dark frames automatically.Bias frames are for dust on the sensor (I think) and Olympus cameras are legendary for dust removal (I wouldn't use any other brand and I only use mirrorless because I want as lightweight as possible.)

I Have a simple canon 7d  with a 55-250mm lens (i used 250mm).
i also used DSS for stacking, i deactivated this automatic dark frame subtraction because for me it feels like im losing detail with it. Bias frames are used to remove the readout signal from you camera sensor and dark frames also have these signals. For me flat frames is a must have because of vignetting, without it i have a huge donut ring around my images which needs more time for removing in the processing.
I use PixInsight or StarTools for processing and afterwards gimp.

A great camera.  I see most people either use Canon or Sony as well as Nikon (Sony makes mirrorless full frame cameras).  It's impressive that you get round stars at 250mm and 2 sec exposures!
Are you getting vignetting from hooking up the camera to a telescope or do you get it with the lens too? I get the donut ring effect when I hook my camera to my SCT and the secondary mirror of the telescope causes shadowing.
Have you checked out Sensorgen?  That site posts sensor data on the sensors of all different cameras.
 
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Bambusman
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09 Apr 2017 10:00

A-L-E-X wrote:
Source of the post Are you getting vignetting from hooking up the camera to a telescope or do you get it with the lens too?

its coming from the lens i guess because its a zoom lens. I dont use my telescope for astrophotography because i dont have a tracking mount but i use it for Luna and planetary photography.

A-L-E-X wrote:
Source of the post  It's impressive that you get round stars at 250mm and 2 sec exposures!

actually they are not round they have little tail, at 1 sec i get round stars like the double cluster.
 
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09 Apr 2017 19:27

Same here.  Equatorial mounts are pretty cumbersome to use with counterweights and all, and I just have a computerized goto mount.  You can still do photography with it, especially if you're just going to piggyback the camera on the mount.  You can get exposures up to one minute with it without much trailing.
 
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pzampella
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10 Apr 2017 08:34

Watsisname wrote:
I wonder if we should have a thread specifically for the techniques of astrophotography?  Where people can talk about their equipment, strategies for image calibration, software and tutorials. :)

I absolutely agree with this!

A-L-E-X wrote:
Source of the post Did you know you can even detect exoplanets with simple camera equipment?

How?!?!
Last edited by pzampella on 10 Apr 2017 08:39, edited 1 time in total.
 
A-L-E-X
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10 Apr 2017 17:40

 
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pzampella
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16 Apr 2017 12:29

I have a couple of questions and maybe somebody can help me. I live in Madrid and there is a lot of light pollution, however, I have the Astronomik CLS filter. I understand that that's not ideal, but it's all I got. I've been using it with my camera on my SkyWatcher BlackDimond 150/750 telescope with an equatorial and motorized mount, and I'm trying to get a nice picture of M51. So:

1.- I would like to know how should I pick the relation between ISO and exposition time. First, I tried with a 6400 ISO and 15s and it was too noisy when I processed it with PixInsight. Then, I tried a 800 ISO with 30s, but the result after processing it the galaxy is too faint. Instead of keep trying night after night, I would like to know a good combination according to your experience.
2.- Is it better to use the CLS filter, allowing me to increase the exposition time, or should avoid using it and even if I'm forced to decrease the exposition time?
3.- Any other raccomendation?

Here there are my first two tries:

6400 ISO and 15s:
► Show Spoiler


800 ISO and 30s:
► Show Spoiler
Attachments
1.jpg
2.jpg
 
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Bambusman
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16 Apr 2017 12:47

pzampella wrote:
Source of the post I would like to know how should I pick the relation between ISO and exposition time. First, I tried with a 6400 ISO and 15s and it was too noisy when I processed it with PixInsight. Then, I tried a 800 ISO with 30s, but the result after processing it the galaxy is too faint. Instead of keep trying night after night, I would like to know a good combination according to your experience.

I always use an ISO of 1600 i think its the best for any astrophoto, exposure time i would keep between 15-30 sec

pzampella wrote:
Source of the post Is it better to use the CLS filter, allowing me to increase the exposition time, or should avoid using it and even if I'm forced to decrease the exposition time?3.- Any other raccomendation?

i dont know how the night sky in your location looks like, but i would keep the filter on. On your images i see alot of vignetting and gradient that needs to be removed there is alot of detail that is still hidden there, it needs alot of processing. as for anyone i can always recommend stacking,for your images i would take atleast 30-50 lightframes and 30-50 darkframes with iso 1600 or 800 and 15 to 25 sec exposure. also if you are not really good at processing i suggest you use StarTools for that, its not free but it gives you alot of processing tools and a really good noise reduction. you should definitely experiment with easier objects first like the Orion nebula or some star clusters.
If you like you can give me the raw file over private message and i can look what can be done in processing.
 
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midtskogen
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16 Apr 2017 13:03

A simple and effective way of removing light pollution is to blur the image and subtract that from the original.  Here's what I get for your image (the 800 ISO one):
x.jpg

It can probably be improved more by adjusting light curves.
EDIT: In a Unix shell using ImageMagick:  convert 2.jpg -blur 128x128 - | convert - 2.jpg  -compose minus -composite -contrast-stretch 0 out.jpg
You can try different blur sizes.
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DoctorOfSpace
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16 Apr 2017 16:05

midtskogen, thats pretty awesome advice
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pzampella
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16 Apr 2017 16:31

First, thank you very much for your advices!

Bambusman wrote:
Source of the post as for anyone i can always recommend stacking,for your images i would take atleast 30-50 lightframes and 30-50 darkframes with iso 1600 or 800 and 15 to 25 sec exposure.

Sorry that I didn't say this in the original post, but those images are both after pre-processing (60 and 30 lights, 30 darks, bias and flats), but before post-processing. The problem is that even processing it I can't get something that I consider acceptable.

Bambusman wrote:
Source of the post you should definitely experiment with easier objects first like the Orion nebula or some star clusters.

Believe that I want to try with Orion, but I will have to wait like 6 more months for that =/

midtskogen wrote:
Source of the post A simple and effective way of removing light pollution is to blur the image and subtract that from the original.

I've done that before, but it also takes away a lot of detail and some of the dust of galaxies or nebulae.
Today I took 40x90s lights, I'll let you know how it turns out.
 
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Watsisname
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16 Apr 2017 18:57

Midtskogen's trick is pretty simple and powerful with a good choice of blur size.  Another technique you could try is to characterize the light pollution and remove it by subtraction or difference modes in an image editor.  Here's a quick summary of how I do it using GIMP: 

1) Open the image

2) Choose the "color picker tool", with the settings "sample average" over a large area (a good portion of the image area), and "set to foreground color".  Then click on the image over an area that is relatively empty of galaxies/nebulae/stars (mostly background noise).  The idea is that you're detecting the average color of just the light pollution in the image, which in a deep sky image should be pretty uniform.

3) Create a new layer filled with this foreground color.  Set the layer mode to "difference", and then slide the opacity until it best cancels out the light pollution.  I find this is usually around 50%.

This is what I get using this method + a bit of curves adjustment on your ISO800 image.  Scaled up 200% to see effects a bit better:

► Show Spoiler


This is a really nice capture by the way.  After the little bit of processing you can even see the star formation regions!


As for the question of ISO vs. exposure time, always go with the lowest ISO / longest exposure time combination that you can manage while obtaining a neutrally exposed image.  Higher ISO can achieve a neutrally exposed image more quickly, but it will be more noisy.

The theory is that you want to maximize the signal to noise ratio (S/N), which represents how much of the light being recorded is actually from the object you want to image, vs. noise from random fluctuations. S/N increases as the square root of the exposure time.  Raising the ISO itself does not do anything for S/N -- it's just acting as an amplifier of both signal and noise by binning pixels together.  But with higher ISO and shorter exposure time, you'll get a noisier image.

Other ways to improve S/N are to stack several images together, which acts to average out the noise.
 
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pzampella
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17 Apr 2017 03:14

WatsisnameI like that idea. So as soon as I get more lights to stack I'll try it, because right now I'm enjoying my 45 days trial of PixInsight, but after that I will have to go back to Photoshop.
Using DSS I stacked the lights from last night and the ones from the night before that (all of them with 800 ISO). Last night were 90s exposure images and the others with 30s exposure. The improvement is incredible to me (you can even see two blurry points right of M51 that are in fact two distant galaxies):


► Show Spoiler

Unprocess. 

► Show Spoiler

Processed (I still need to learn a lot here).

I'll keep you up as soon as I get something better.
Attachments
M51.jpg
M51.jpg
Last edited by pzampella on 17 Apr 2017 04:11, edited 1 time in total.
 
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Watsisname
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17 Apr 2017 03:18

Yeah, that sure does look like an improvement. :)  Looking forward to more!
 
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Bambusman
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17 Apr 2017 04:38

pzampella, there are many tutorials for processing on the internet, this is what i get processing your non processed jpg file:
M51 (1).jpg

after stacking in dss i wouldnt touch the image in that Programm, save it as a tiff or fit file and start processing somewhere else. everyone has his individual way of processing you have to find out how you like your images.
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