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In the general direction of Capella
Nov 26, 2021 07:19:43   #
Curve_in Loc: Virginia
 
At some point, I'd like to get a nice image of a Nebulae. I'm going to need lots of help with this project so I'm including as much information as I can. Here's my start, for this image, according to the Bortle scale, I'm in a zone 7 shooting towards a zone 6. Equipment wise, I used a Nikon D750 with a Sigma APO 120-400mm lens shot at 400mm and f/5.6, a tripod and a gimble style head. I shot 789 light frames, 44 dark frame and no usable bias frame at iso 4000 with a 2 second exposure. I processed the files with Siril which took about an hour and a half and I forgot to use the dark frames. The resulting .fit file was adjusted in GIMP for exposure and cropping and exported as a .jpg.


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Nov 26, 2021 08:35:50   #
Marc G Loc: East Grinstead, West Sussex, England
 
Curve_in wrote:
At some point, I'd like to get a nice image of a Nebulae. I'm going to need lots of help with this project so I'm including as much information as I can. Here's my start, for this image, according to the Bortle scale, I'm in a zone 7 shooting towards a zone 6. Equipment wise, I used a Nikon D750 with a Sigma APO 120-400mm lens shot at 400mm and f/5.6, a tripod and a gimble style head. I shot 789 light frames, 44 dark frame and no usable bias frame at iso 4000 with a 2 second exposure. I processed the files with Siril which took about an hour and a half and I forgot to use the dark frames. The resulting .fit file was adjusted in GIMP for exposure and cropping and exported as a .jpg.
At some point, I'd like to get a nice image of a N... (show quote)


Astro-photography is going to be difficult without some kind of tracking mount but not impossible as you will require lengthy exposures to capture faint nebula & galaxies.
Looking at your image I do see a few issues that can easily be corrected to produce a better image.
Firstly I wouldn't use a zoom lens at full focal length as many lenses produce softer images when extended.
The focus does look a tad off too, I do appreciate focusing lens can be tricky at the best of times.
There is some CA / field curvature of which is common with lens too.
This curvature has also produced some elongated stars towards the edges of the frame.
The use of dark frames is really beneficial for astro-photography & will most certainly assist you when it comes to processing stacked images.
When it comes to processing I would recommend leaving your black point setting between 10-20 upon the histogram thus producing a more natural looking sky. This maybe an issue when shooting in heavy light polluted skies.
Finally, keep at it, practice your focus & most of all enjoy
kind regards Marc

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Nov 26, 2021 10:04:15   #
Curve_in Loc: Virginia
 
Marc G wrote:
Astro-photography is going to be difficult without some kind of tracking mount but not impossible as you will require lengthy exposures to capture faint nebula & galaxies.
Looking at your image I do see a few issues that can easily be corrected to produce a better image.
Firstly I wouldn't use a zoom lens at full focal length as many lenses produce softer images when extended.....
......
Finally, keep at it, practice your focus & most of all enjoy
kind regards Marc


Thanks Marc,

I appreciate the straight forward comments. I will be getting a rotating mount, but I wanted to work on the editing process first. I'll try again tonight with a prime lens if the clouds don't roll in....

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Nov 27, 2021 14:48:10   #
Ballard Loc: Grass Valley, California
 
Curve_in wrote:
At some point, I'd like to get a nice image of a Nebulae. I'm going to need lots of help with this project so I'm including as much information as I can. Here's my start, for this image, according to the Bortle scale, I'm in a zone 7 shooting towards a zone 6. Equipment wise, I used a Nikon D750 with a Sigma APO 120-400mm lens shot at 400mm and f/5.6, a tripod and a gimble style head. I shot 789 light frames, 44 dark frame and no usable bias frame at iso 4000 with a 2 second exposure. I processed the files with Siril which took about an hour and a half and I forgot to use the dark frames. The resulting .fit file was adjusted in GIMP for exposure and cropping and exported as a .jpg.
At some point, I'd like to get a nice image of a N... (show quote)


Hi Curve_in
I agree with Marc on his comments particularly with regard to the need for tracking, 2 second exposures won't get much at F/5.6 even with stacking lots of images (with a perfect senor it would be possible but with real sensors there just isn't enough signal relative to the noise with very dim objects). At 2 seconds the stars do show some elongation at 400mm (normally I use the rule of 400 were you divide 400 by the focal length of the lens in millimeters to get the number of seconds to expose before star trailing becomes an issue without tracking (of course the closer to the pole the longer you can expose, and the closer to the celestial equator you go the shorter the exposure will need to be without trailing).
When stretching the image, you normally don't want to make the sky completely black, this will be important when trying to pull out the faint sections of nebula. The stars in your image seem to be very harsh and with little variation in pixel intensity. Note unless you purposely made this black and white there should also be some slight variation in star color from blues to yellows to oranges to reds.

When I use a DSLR for astrophotography I also use the laptop to focus the image. For my canon the laptop can be connected to the camera and set to a live view to display and magnify the image (10X) on the laptop screen which helps to focus more accurately (I usually crank the ISO way up while focusing to see the stars better, particularly if there are no bright stars in the field). I assume there is some similar functionality with your Nikon.

I order of importance the frame types to use are lights (the image itself), darks to cancel out noise (taken at the same temperature, ISO and duration as the light frames, Flats to remove vignette and dust circles (see comments on flats below), and Bias to remove electronic read noise, taken with the lens cap on with as short exposure as possible at the same ISO. Stacking software can use all these frame types to improve the image (A couple examples of stacking software include DeepSkyStacker (freeware), Pixinsight (Not free but much more powerful with a stepper learning curve).

To take flat frames take a clean white pillow case (or T shirt) and strap it over the front of the lens so that it is nice and taught (no wrinkles or seams in front of the lens, using the appropriate size rubber band to hold the cloth over the lens shroud works well). The focal point should be set to infinity. Then take 20 or 30 exposures aimed at blue sky pointed away from the sun (or pointed at evenly illuminated clouds) with the histogram peaking near the center (make sure that the upper end of the histogram has tailed down close to 0).

Below is an example of what a flat frame looks like using the sky as the light source. You need a separate set of flat frames for each lens and each F stop you plan to use (For zoom lens, for each focal length you plan to use). I you use any filters take sets of flats with each filter you plan to use. The flat below was using a mono camera with a luminance filter.

500mm F4.0 Canon lens flat frame. Note the vignette across the image.
500mm F4.0 Canon lens flat frame. Note the vignett...
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