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Another Example of Better Results with Lower Gain

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#1 jrbuchanan

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Posted 20 July 2019 - 05:09 PM

As I stated in an earlier post with examples from M8, I think that I am getting much better results with imaging at slightly lower gains.  From my understanding this is giving me a greater dynamic range, especially when it comes to Nebula.  Here is a livestack that I go of M20 last night with my Celestron NextStar 6se with .5 focal reducer and ZWO 224 camera.  The image is a livestack of 480 seconds (5s exposures) at 300 gain.  

 

I will post an earlier attempt at higher gain in the replies for reference. 

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#2 jrbuchanan

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Posted 20 July 2019 - 05:11 PM

Here is my previous best attempt at M20.  This image was taken with same equipment but is stack of 430 seconds (5s exposures) at 350 gain.  

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#3 PirateMike

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Posted 20 July 2019 - 05:30 PM

Is there a reason that you take 5 second exposures as opposed to 1200 second exposures or 900 second exposures or even the lowly 60 second exposures? Why not just a single exposure at 480 seconds?

 

If so, can you please explain to me what reason that may be.

 

 

Miguel   8-)

 

.


Edited by PirateMike, 20 July 2019 - 05:49 PM.


#4 jrbuchanan

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Posted 20 July 2019 - 09:09 PM

Miquel,

Yes the answer is simply tracking.  I am using a goto alt az mount rather than a GEM. 



#5 Adun

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Posted 20 July 2019 - 10:25 PM

Mike probably didn't notice which forum this thread was part of. It has happened to me too.



#6 Roger Corbett

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Posted 21 July 2019 - 09:05 AM

JR, as i noted elsewhere, fantastic image!  The comparison is telling and staggering.  Great improvement!



#7 chilldaddy

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Posted 21 July 2019 - 01:10 PM

JR,

You're making fantastic progress so far!  It's fun to see each of your images getting better and better.

 

The older image of M20 actually looks better in some ways and I'm wondering about the conditions between the two nights. 

 

The newer image at 300 looks brighter and redder but that wouldn't be explained by a lower gain.  It appears to have too much red in the image so I wonder if you stretched the levels differently and bumped red and maybe saturation?

 

I would expect the lower gain image to be a little smoother but it seems slightly grainier and has halos around some stars. You might consider doing a test on the same night and see how much change you get between gain settings trying to match the color between the two as much as possible.  It's very helpful to post a screen capture showing the controls as well as the image.

 

Ultimately, you are taking great strides, and I agree that it's amazing to be able to see things with the moon out.  It's obviously not as good as a moonless sky but in the past, I wouldn't have even tried.

Keep up the good work!

 

Greg


Edited by chilldaddy, 21 July 2019 - 05:57 PM.


#8 Rickster

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Posted 21 July 2019 - 02:08 PM

Much better.  Quite good.  waytogo.gif  Good enough that we can start picking on some fine points.  smile.gif  I have noticed that all of your images (that I can remember) contain stars with shadows (red shadows in this case).  And the shadows are consistently on one side of the stars.  This is typically due to a misalignment in collimation or camera tilt.   Without knowing your collimation method, all I can say is that when I have this problem it is because I didn't tweak the collimation with the camera in place.  All you (probably) need to do is find a bright star with the camera in place, and defocus until it turns into a big doughnut.  Set your exposure to something fast like 0.5 seconds.  Then tweak your collimation until the doughnut hole is well centered (not off to one side or the other).


Edited by Rickster, 21 July 2019 - 02:09 PM.


#9 jrbuchanan

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Posted 22 July 2019 - 11:35 AM

Thanks Rick. That is something that I've never done but will certainly do some research and check it out.
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#10 Ptarmigan

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Posted 22 July 2019 - 10:23 PM

Much better.  Quite good.  waytogo.gif  Good enough that we can start picking on some fine points.  smile.gif  I have noticed that all of your images (that I can remember) contain stars with shadows (red shadows in this case).  And the shadows are consistently on one side of the stars.  This is typically due to a misalignment in collimation or camera tilt.   Without knowing your collimation method, all I can say is that when I have this problem it is because I didn't tweak the collimation with the camera in place.  All you (probably) need to do is find a bright star with the camera in place, and defocus until it turns into a big doughnut.  Set your exposure to something fast like 0.5 seconds.  Then tweak your collimation until the doughnut hole is well centered (not off to one side or the other).

I have had issues with camera tilt. Is there a way to reduce or prevent it?



#11 Rickster

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Posted 23 July 2019 - 08:09 PM

I have had issues with camera tilt. Is there a way to reduce or prevent it?

I also have had trouble with tilt.  I can't say I have all the answers, but I can tell you what I have learned. 

 

First off, I think most of my camera tilt is created by using a laser to get the initial collimation.  The laser and the camera don't sit in the focuser exactly the same, and that difference results in misalignment of the camera with regard to the collimation.  So I have to retouch the collimation once the camera is in place.

 

My Newts have adjustment set screws in the focuser base that can be used to adjust the focuser tilt (and therefore, camera tilt).  I have attempted to use them with a program that gave me a real time readout of camera tilt.  The problem was that constantly changing atmospheric conditions had me chasing my tail.  So I got it as close as I could and quit.  Since then, I have decent luck cancelling it out by adjusting the secondary mirror (collimation).  Doing it that way means that the camera tilt and the collimation will both be off, but the errors cancel out (for the most part).  I also find that shimming the camera extension with tape helps by taking up slop between the focuser tube and the camera extension tube.  It has occurred to me to put a few spots of fingernail polish on one of the tubes to tighten things up, but haven't actually tried it.

 

I don't recall having as much of a problem with my SCTs as my Newts.  But then, I am more particular these days (my SCTs have been gathering dust for quite some time). I don't have any ideas for SCTs other than the obvious.  That is, to make all of the extensions fit snug so that there isn't any appreciable sag or slop.  And do what you can about mirror flop (like crank the focuser back and forth full travel a few times to redistribute the lube between the primary mirror and the support tube). 

 

Basically, I just do a final collimation with the camera in place and call it good.


Edited by Rickster, 23 July 2019 - 08:13 PM.

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#12 Stargazer3236

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Posted 05 August 2019 - 12:55 PM

Try using exposures of 15 seconds and a gain of 300. That will get you a nice stack in about 10 minutes worth of live stacking. Tweak the black and mid level sliders (use Horizontal Positioning Axis), the white sliders will darken the image. Try using Unsharp Masking under enhancement, that is if you have SharpCap 3.2 PRO. Try Sigma clipping at the default settings or use 2.0. Color saturation is also nice to use to bring out some more of the color.



#13 jrbuchanan

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Posted 05 August 2019 - 02:53 PM

Thanks




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