Astrophotography

Rosette Nebula in Ha

Always a bit tricky imaging under a bright moon, but even more so when you’re in clumsy mode. Last night I managed to disconnect the power from my mount whilst aligning. Twice! Then I dropped an eyepiece by slewing the scope without having it fixed properly (fortunately it landed on the rubber eye cup). Finally I spent ages trying to work out why I couldn’t focus my guide-scope until the penny finally dropped that I was twiddling with the locking ring and not the focuser. So I’m taking this picture as a victory of the scope over its owner!

15x 10 min subs – Canon 600d – 130 pd-s- 7nm Ha filter.

 

Image processing lore

There is an article in January’s “Sky at Night” magazine under “Skills” and “Image Processing” entitled “Improve your deep-sky images with luminosity layers”. While not entirely BS, it seems to me that it is a complicated way of doing something relatively simple. There is nothing new about the point that most of the resolution information in a colour image is in the luminance part of the image. After all, this is how colour TV signals have been transmitted for ever. With colour TV, the image is decomposed into a luminance, or black and white signal and a chrominance signal that contains the colour information. Luminance is then transmitted on a high-bandwidth channel, and the chrominance on a low-bandwidth sub-carrier. They are combined again inside your TV receiver.

The author of the article used Photoshop, but you can do essentially what he did (and to me, much more simply) with GIMP. How to do it is in an article from 3 years ago on our own web-site at http://www.thornett.net/Rosliston/Astrophotography/DSO.pdf.

AND, of course, as we keep reminding everyone, GIMP is FREE!

Venus and the Schroter effect

Dichotomy, or half-phase for Venus was predicted to be 6th January but due to the Schroter effect may appear late. This is a phenomenon in which the observed phase of Venus appears less than the predicted phase. As a result, dichotomy (half phase) occurs early at evening elongations, and late at morning elongations. The time difference between calculated and observed dichotomy is about a week. The effect was discovered by the German astronomer Johann Hieronymus Schröter (1745–1816). The cause may be that the region near the terminator is darker than the rest of the illuminated disk.

Here is a rather poor image of Venus from the window-sill this morning.

 

Here is the general view from this morning (9/1/2019). I didn’t manage this yesterday. Early risers can look forward to the Venus-Jupiter conjunction on 22nd January.