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What is wrong with my images

astrophotography refractor CMOS beginner
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#1 Evan Huffaker

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Posted 30 November 2020 - 11:57 AM

This photograph of Andromeda highlights a major issue I have been having with deep space photography.  I am taking this photograph as an example because it is taken at the zenith, with the thinking that there would not be all that much atmospheric refraction.  Yes, I am aware of the piece of grass or hair that got in the photo.

Setup:

  • Camera: Nikon D5100
  • Exposure time: ~5 minutes
  • Telescope: Sykwatcher EvoStar 100ED (refractor)
  • Mount: Skywatcher EQ6-R Pro
  • Eyepiece: Badaar Morpheus 17.5mm Eyepiece

I do not have any other equipment yet, so no active star tracking with a separate camera.  I also am only using two adapters to connect up to the camera.  No extenders between the camera and the eyepiece are being used.

 

When I did my star alignment with the camera, I did notice the stars tended to deviate slightly, after a few seconds, from the center of the live view.  If this was the cause though, I would have expected my Andromeda image to have unidirectional smearing, not radial light diffraction.  I noticed a similar problem when I was trying to fine tune my focus with a star.  I had slight, but noticeable radial light diffraction around the star, which I had centered on my screen.

 

I should also note that, the diffraction looks less on my Orion Nebula photograph and I only exposed it for about a minute.  But it was also near the horizon when I took it, and I have not yet done anything with color filters.  Also, the diffraction is not present when I visually observe, skipping the camera.

 

Any idea what might be causing my images to look like this, and any possible solutions? 

 

 

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#2 Eddie_42

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Posted 30 November 2020 - 12:09 PM

What is your setup between the camera, eyepiece and optical tube?   From what I am interpreting, you are using the eye-piece, and then having the camera take a picture looking into is (called afocal imagery).   before proceeding too far down the discussion, it is important to know how you have all the gear setup.



#3 Evan Huffaker

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Posted 30 November 2020 - 12:58 PM

First I have the camera.  Next in line is the Nikon to T2 Adapter, followed by a T2 to M43 adapter, which is connected to the eyepiece.  The adapters are thin, and so the eyepiece is close to the camera.

Between the eyepiece and the telescope, I have a 2" diameter extender and the focuser which came with my telescope.  The only optics I have are the telescope, eyepiece and camera.  I decided to go without the right angle for photography because it added too much weight to the focuser, and if I do not secure it super tightly, it has the potential of rotating if I nudge it.



#4 Madratter

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Posted 30 November 2020 - 01:08 PM

The problem is a combination of using an eyepiece and the lack of a field flattener.


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#5 RedLionNJ

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Posted 30 November 2020 - 01:10 PM

I don't think going afocal is the optimum way to capture a large target like M31.

 

Try it without the intervening eyepiece - this will create a much brighter image, too - and perhaps remove the rainbow effect.



#6 Evan Huffaker

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Posted 30 November 2020 - 01:26 PM

I don't think going afocal is the optimum way to capture a large target like M31.

 

Try it without the intervening eyepiece - this will create a much brighter image, too - and perhaps remove the rainbow effect.

I am more interested in trying to correct for this problem before I try to photograph smaller objects.

 

 

The problem is a combination of using an eyepiece and the lack of a field flattener.

So that is what I am missing.  Thank you so much, I was really confused!



#7 klaussius

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Posted 30 November 2020 - 01:53 PM

With that eqiupment, you'd be better off getting a T-ring and doing prime focus. That is, connecting the camera directly to the OTA, skipping the eyepiece entirely.

 

You have considerable field curvature. You'll need a flattener to correct this. It's best if you get a flattener specifically designed for that telescope, but generic ones will work as long as they're designed for the same F/ratio.

 

You also have considerable chromatic aberration. I suspect it's the eyepiece, you shouldn't have CA that bad in that OTA, and I imagine a lot of that will be corrected by going prime-focus, but you will probably still have a little bit of CA in the end as that's an ED doublet.

 

In essence, your assembly would look like: telescope - flattener - T-ring - camera.

 

You may need extra adapter between things if they don't mate properly, but that's the general idea.

 

Also, most flatteners require a specific distance between the flattener itself and the camera sensor to work correctly, and you may need to add spacers to get that distance right. Some work out of the box with standard adapters, but not all.


Edited by klaussius, 30 November 2020 - 01:58 PM.


#8 Evan Huffaker

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Posted 30 November 2020 - 02:38 PM

I would be surprised if it was chromatic aberration because it is not visible with visual observations.  Next clear night though, I will try out skipping the eyepieces altogether and seeing if I get any issues.  As I said before though, I am more interested in taking photographs through an eyepiece so I can photograph smaller objects too.

 

If instead I wanted to use the eyepieces, would I do: telescope - flattener - eyepiece - adapters - camera?



#9 klaussius

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Posted 30 November 2020 - 02:53 PM

As I said before though, I am more interested in taking photographs through an eyepiece so I can photograph smaller objects too.

 

I don't think the eyepiece helps with that.

 

You might get a barlow, but given you're at 1.1"/px image scale with your equipment already, I wouldn't expect better images unless you use lucky imaging, for which a DSLR is suboptimal.

 

DSO imaging resolution is usually limited by seeing and guiding, which won't let you achieve very high resolution except for exceptional sky quality conditions that happen only rarely. Using a barlow rarely ever helps.

 

In short, prime focus and no barlow is the best way to achieve optimal results for DSO imaging. You can get a barlow for planetary if your DSLR can do high speed raw video to try lucky imaging and small bright DSO like some planetary nebulae.



#10 charlieb123

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Posted 30 November 2020 - 02:54 PM

Keep it simple.

Scope -> Camera

or with FF.

Scope -> FF -> Camera.

Capture images using a program like APT. View images as you take them and make ISO and exposure adjustments from there.

 

I took many pictures without a FF and ended up with some great images.

A FF is not required to see whether or not you're capturing good images. A FF only improves the lens distortion from the scope.

JMO, YMMV.


Edited by charlieb123, 30 November 2020 - 02:56 PM.


#11 Pauls72

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Posted 30 November 2020 - 03:16 PM

Get a T2 to 2 inch or a T2 to 1.25 inch nose piece and eliminate the eyepiece.

Then you will go straight from the scope to the camera, no eyepiece and no diagonal.

 

Something like one of these:

https://agenaastro.c...14-2458105.html

https://agenaastro.c...16-2408150.html

 

 

This should make it quite a bit better.

After that you may want to consider a Field Flattener to make it even better.



#12 Evan Huffaker

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Posted 30 November 2020 - 03:27 PM

I don't think the eyepiece helps with that.

I am not sure I understand completely, but I trust you.  I might have to review my optic-physics again.

 

So if I wanted to photograph at a smaller field of view than at prime focus, I either need to get a different telescope or a different camera?



#13 klaussius

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Posted 30 November 2020 - 04:23 PM

I am not sure I understand completely, but I trust you.  I might have to review my optic-physics again.

 

So if I wanted to photograph at a smaller field of view than at prime focus, I either need to get a different telescope or a different camera?

Or a barlow, or just crop the image.

 

All of these are viable options.

 

When imaging, zooming in doesn't get you any extra resolution beyond what you can resolve given the diffraction, seeing and guiding-imposed limits. At 1 arcsec per pixel, you're close to what those limits would normally be for most people's skies, so zooming further in optically won't get you anything you can't get merely by cropping.

 

There are lots of resources about this. You want to google "image scale" and "oversampling". If your image scale is much lower than your seeing, you're "oversampled", which means all those extra pixels don't help, you just get a bigger yet blurry image. If your image scale is much higher than your seeing, you're "undersampled", which means you could potentially get better resolution by increasing the FL (with a barlow for instance).

 

Your image scale depends on both your camera and your telescope's FL.

 

All this happens because, with long exposures needed for DSO imaging, atmospheric turbulence (seeing) blurs the image regardless of your telescope's magnifying power. Seeing is usually measured in arc-sec FWHM, which is the blurring effect it has on the image. The only way to counter seeing is to do lucky imaging, which is tough on DSO but commonly done for planetary imaging.


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#14 Evan Huffaker

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Posted 30 November 2020 - 04:51 PM

Or a barlow, or just crop the image.
 
All of these are viable options.
 
When imaging, zooming in doesn't get you any extra resolution beyond what you can resolve given the diffraction, seeing and guiding-imposed limits. At 1 arcsec per pixel, you're close to what those limits would normally be for most people's skies, so zooming further in optically won't get you anything you can't get merely by cropping.

I think you explained it really well, thank you.  I knew with my telescope I had something like a 1 arc-second resolution, but I did not think that the pixel size was close to hitting that limit, but I guess so.  If I did magnify an image with optics, I would not see any more details.  I am also surprised that turbulence can cause blurring of images with long exposures, I was under the assumption that it only made a difference with short exposures.  But that makes sense: by averaging the turbulence "noise," you are not removing it, resulting in the average scattering effects caused by it.



#15 idclimber

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Posted 30 November 2020 - 04:53 PM

I would be surprised if it was chromatic aberration because it is not visible with visual observations.  Next clear night though, I will try out skipping the eyepieces altogether and seeing if I get any issues.  As I said before though, I am more interested in taking photographs through an eyepiece so I can photograph smaller objects too.

 

If instead I wanted to use the eyepieces, would I do: telescope - flattener - eyepiece - adapters - camera?

This is classic chromatic aberration. The camera is just a lot more sensitive to the shift in color than your eye is. 

 

If you have the matching flattener, use it with a t-ring to mount the camera with the correct spacing for that reducer. 




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