===== Night Sky Images - 25/10/2021 =====

Clear for most of the night. After successfully using the tracker a week or two earlier I managed to get hold of a remote shutter release cable to allow the tracker to automatically trigger the camera - my previous testing was several manual shutter release image (I don't have an external intervalometer at this point).

I took two different images of the night sky - roughly pointing in the same direct, just at a different elevation; one at tree-top level, the other almost vertically up.

Gear:
  * Canon EOS 200D
  * Tokina AT-X Pro SD 11-16mm f/2.8 (IF) DX
  * Tripod
  * Star Adventurer 2i tracking mount (with shutter release cable)

Weather:
  * Clear (zero cloud for most of the night)

Location:
  * Garden

Software
  * [[https://www.rawtherapee.com/|Rawtherapee]] - RAW file importing, optical correction & image processing
  * ASTAP - Image stacking / general astrophotography toolset
  * GIMP - RAW/TIF to JPEG, cropping

==== Images ====

First image was taken from a sequence of 5x 30 second exposures at ISO 1600. This was my usual 'at the tree tops' image composition for the garden.

<imgcaption 1|EOS 200D, Tokina f/2.8, 11mm, ISO1600, 5 x 30 seconds, processed from RAW>{{:blog:photography:night:2021_10_25:final_a2.jpg?550|}}</imgcaption><imgcaption 1|EOS 200D, Tokina f/2.8, 11mm, ISO1600, 1 x 30 seconds, uncorrected JPEG>{{:blog:photography:night:img_3751.jpg?550|}}</imgcaption>

The second image is comprised of 6x 120 second exposures at ISO 3200. Something that I could have technically done without the shutter release cable, but not something I'd want to stand around counting down and pressing the button for; the app for the tracker did it all brilliantly.

<imgcaption 1|EOS 200D, Tokina f/2.8, 11mm, ISO3200, 6 x 120 seconds, processed from RAW>{{:blog:photography:night:2021_10_25:final_2.jpg?550|}}</imgcaption><imgcaption 1|EOS 200D, Tokina f/2.8, 11mm, ISO3200, 1 x 120 seconds, uncorrected JPEG>{{:blog:photography:night:img_3759.jpg?550|}}</imgcaption>

There was a fairly lurid coloured moon this night, so I also switched out to my only other lens available my 105mm Sigma macro:

<imgcaption 1|EOS 200D, Sigma f/2.8, 105mm, ISO200, 1 x 1/60 seconds at f11, processed from RAW>{{:blog:photography:night:2021_10_25:img_3788.jpg?550|}}</imgcaption><imgcaption 1|EOS 200D, Sigma f/2.8, 105mm, ISO200, 1 x 1/60 seconds at f11, centre crop>{{:blog:photography:night:2021_10_25:img_3788_centre_crop.jpg?550|}}</imgcaption>
==== Image Processing Notes ====

Since this was my first time stacking and processing multiple images, it took a while until I found a solution that **(a)** worked properly on Linux, **(b)** I was comfortable with and **(c)**, was happy with the output from.

First I tried [[https://siril.org/|Siril]], but I just couldn't get to grips with the interface - I found it entirely unintuitive **and** the tutorial to stack your images just would not run - throwing an error about a line in the pre-processing script which made no sense. 

Then I tried [[http://deepskystacker.free.fr/english/index.html|DeepSkyStacker]] which, although Windows only, seemed to load and run via //Wine//, but ended up crashing repeatedly when processing images. 

Eventually I looked at [[https://www.hnsky.org/astap.htm|ASTAP]] which did everything I wanted and was clearly laid out.

I tried loading images into the applications as the came from the camera, but all of my results were over-exposed, suffering varying levels of optical aberrations and just generally didn't look particularly great - even though stacking brought out far more detail than I had seen before.

Eventually I settled on the following routine (which works well for me, in my fairly light-polluted garden):

**1. Load RAW files into Rawtherapee and adjust as below**

{{:blog:photography:night:2021_10_25:rawtherapee-initial-exposure-settings.png?300|}} {{:blog:photography:night:2021_10_25:rawtherapee-initial-chromatic.png?300|}}

//The chromatic aberration settings are likely particular to my lens which is of course in this case is the ultra-wide 11mm Tokina. At 11mm and f/2.8 it has quite bad chroma in the corners of the image. It would likely not need such aggressive settings stopped down, or at the other end of it's focal length (16mm).//

**2. Save as TIFF, 16bit. Load into GIMP and apply gradient as below in the direction of any light pollution**

{{:blog:photography:night:gimp-gradient-settings.png?200|}} {{:blog:photography:night:gimp-applying-gradient.png?600|}}
   
//Depending on the level of light pollution, increase/decrease the opacity of the gradient, as well as the length it is applied to.//

**3. Load all of the processed TIFF files into ASTAP as your light files. There will be some warnings about missing metadata, but this doesn't appear to impact the final image.**

**4. Let ASTAP do the stacking, along with any dark, flat and bias images.**

{{:blog:photography:night:astap-stacking-settings.png?600|}}

//For me, the best results were with the AstroSimple option and Sigma Clip stacking options as above.//

**5. In the resulting stacked image, if the star colours look //off//, try another stacking/colour de-mosaic algorithm, or try the 'Autocorrect image colours' option from the tools menu.**

**6. Export as TIFF from ASTAP and load back into your image editor of choice. I used Rawtherapee to do final image/colour enhancements; playing with the colour and black/white highlight balance.**

{{:blog:photography:night:rawtherapee-final.png?600|}}