Tuesday, May 28, 2019

May 28, 2019: Progress on the Astrophotography Front: Part 2

Progress with Star trails, Nebula and Galaxies and the Milky Way























 

May 28, 2019: Progress on the Astrophotography Front: Part 1

Astrophotography is a huge challenge.   In retirement, overcoming all the little challenges will keep me actively engaged for probably several years.   I've learned a lot in the past 6 or 7 months -  how to align an equatorial mount withe the celestial north pole, how to  align with enough stars (3) to have adequate tracking.  how to focus on things you can't see (i.e. how to use liveview in low light situations) - and the biggest challege of all:  post processing images.   

What I've learned is that capturing the images is about 20% of the task.  The other 80% is aligning and post processing.   Three tools that stand out are Astrometry  (a tool for figuring out where your image is located in the heavens above and annotating significant objects in your image,  Sequator (a Deep Sky Stacker equivalent)  for aligning and stacking images and Roger Clark's rnc-color-stretch for post process an aligned and stacked image.  For making star trail images, I used StarStax. The challenge with post processing is taking an image with a small range of intensities (from the image histogram) and stretching it to bring out the details without destroying the colors.  Clark's essays about all things about astrophotography are insightful and provacative. 

My camera skills are also dramatically improves.   There are so many things to understand when photographing the sky: how long an exposure (to minimize trails and maximize the signal/noise ratio), what ISO (turns out that the same image taken for different ISO values usually does not reveal a graded change in noise - and some cameras are ISO invariant (Sony A7).  Then there is the challenging of focusing in low light and maintaining focus over several hours of imaging - and the environmental challenge is eliminating condensation on the lens / filter.  

Here are recent images highlighting what I've learned:


















 

Tuesday, February 26, 2019

February 24, 2019 - Cloud and star trails


This morning about 2am, I work up - and looked outside,  it was clear so I thought I'd get a couple of hours photos of the northern sky (30 sec eposures).  I set up the camera and timer and went back to sleep.  About 5am I work again and the fog was horrible.  I retrieved my camera and though it would be a total failure. Turns out it was quite interesting:  watching the clouds go by as if in a Star Trek warp drive.
Star Trails around Polaris (actually around the celestial north pole)

Clouds flying by as if in a time warp

Building the star trails using StarStaX  2 hours of 30 sec exposures.




Friday, February 22, 2019

February 21, 2019 Questions of the Celestial North Pole and Polaris

I've been trying to capture rotation of stars around the celestial north pole.  I expected the stars to orbit around something close to Polaris  - as the earth rotates.  But I found a surprise.

Early morning sky - overlay of 250 images revealing the apparent star trajectories around the celestial north pole

Early morning sky (single frame)



Night Sky (requires imagination) with Polaris and Kochab labelled

While other stars are rotating aound some axis, the Polaris trajectory is flat.  I don't understand this
 

February 22, 2019: Falcon 9 Launch (Beresheet, SpaceIL lunar lander)

Launch of Falcon 9 with SpaceIL lunar lander

Launch time was 20:45 and the launch we on time.



SpaceIL will use a gravitational slingshot to position their lunar lander on the surface of the moon.  Here is an amazing animation of the mission


and animation of the gravity slingshot propulsion



And my images of the launch trajectory

Falcon 9 trajectory for SpaceIL Lunar Lander

My backup camera image of Falcon 9 launch trajectory for SpaceIL Lunar Lander


 

Wednesday, February 20, 2019

Feb 19, 2019: Super Snow Moon

  What a Challenge between the appearance of the Super Snow Moon and the clouds that were appearing each night.  For a few hours, there were no clouds and I managed to get some good results.

Rising Super Snow Moon (about 10pm)

Rising super snow moon, Feb 19th evening


Almost Super Snow Moon (apparently because its February and snow is everywhere - except Florida)  February 19 about 3am

    
A sequence of the almost full moon setting Feb 19 about 3am.  The darker images are when clouds were competing for equal time   
 

Monday, February 11, 2019


February 6, 2019:  Understanding vignetting and post processing astrophotos

I've been plagued for months with the light region in the image (below).  I was using Deep Sky Stacker to produce the images and skipped the flat and bias images.  Turned out to be a huge mistake.   The flat images are obtained by shooting an evenly illuminated subject (sky, through a paper towel or tee shirt) at the same ISO used for the light (astrophotos) images.  The bias image are obtained by shooting a dark field with the lens cap on - typically 1/8000 sec, to measure the dark current associated with no exposure (hence the high shutter speed). Dark images are also taken at the same settings as the light images with the lens cap on.  This gives a measure of the dark current associated with the exposure and ISO used for the light images. 

This article discusses how to adjust for vignetting: Photoshop techniques to correct for vignetting




DSS image of NGC2024 and IC434 - I did not understand the white region in the middle
An image of the morning sky as seen through a paper towel covering the front of the lens

Stretching the histogram of the above flat image reveals dark corners - vignetting
Deep Sky Stacker corrects for vignetting (using the flat images) and here is the result - a stack of 150 images of NGC 2024 (flame nebula) and IC434 (horsehead nebula) - no highlighted area in the middle  


Another Example:   M31
 
M31 with significant vignetting
M31 with flat compensation followed by GIMP autobalance (histogram stretch)