Constellations at the Zenith

We don’t often look “straight up”. Unless you are laying down, it’s not a comfortable viewing position. However there is lots to see and the Big Dipper (Ursa Major) is right overhead this time of year.

Setting up a camera with a 10 second exposure can capture quite a good deal of the sky, and you don’t have to worry too much about star trails. What stands out is the large variations in the colors of stars, from cooler deep reds, to hot bright blues.

Constellation Near Zenith 08May2018

Constellations right above in the May evening.

17mm f/4
Canon 80D (ISO 3200)
12 x 10sec (2 minutes)

M44 Beehive Cluster

Image

In one of my previous post I mentioned how Messier 44, the Beehive Cluster, would be an easy find the evening of April 22-23, so even if I took this photo on the 21st, the same evening that I took a photo of the Moon, all I needed was to moved a few degrees north after observing the Mooon to image this large open cluster.

Messier 44 - Beehive Cluster. Benoit Guertin - with Skywatcher 80ED and Canon 80D

Messier 44 – Beehive Cluster. Benoit Guertin – taken with Skywatcher 80ED and Canon 80D

Photos of open clusters with small refractors always lack the diffraction spikes that really make the stars stand out.  So a little photo editing did the trick to spice up the image.

Skywatcher 80ED
Canon 80D ISO 3200
Stacked 22 x 10sec

Never Wait for a Full Moon

The Moon should be the first thing you look at the day you get to peer through a telescope. It should also be the first thing you photograph.  However don’t wait for a Full Moon.  Sure a large round moon over the horizon can be breathtaking, but most of the subtle details of the lunar surface disappear under a Full Moon.  The lack of shadows blends away the peaks and valleys, crevasses and ridges. It is really this dance of light and shadows that makes the craters stand out.

Click on the image below for full resolution.

April 21, 2018 Moon. Benoit Guertin

Lights and shadows….

The photo above is a single shot with Skywatcher 80ED telescope and Canon 80D (ISO 200, 1/125s)
Wavelet analysis with Registax.

Photo – Sun April 21st 2018

After a weeks of clouds, rain and even snow, I finally get a sunny weekend without a cloud in the sky.  With the warmer temperatures, time to take the telescope out. Unfortunately no significant sunspot happening on April 21. Just a small region (AR2706) on the western part of the sun.

Canon 80D (ISO 100, 1/400s)
Skywatcher 80ED (80mm F/7.5)

Sun with sunspot AR2706 (21-apr-2018). Benoit Guertin

Sun with sunspot AR2706 (21-apr-2018). Benoit Guertin

This Weekend – Beehive Cluster

The Beehive cluster, also known as Messier 44 (M44) is one of the nearest open clusters, and therefore one of the largest in the night sky. While open clusters are often too dim to be seen with the naked eye, all you need is a pair of binoculars or a camera with long exposure to see it.

What makes this weekend special is that in the night of the 22nd to 23rd of April, the Moon will pass within 1-1/2 degrees of this cluster.  So finding it will be child’s play.

Messier 44 - Open Cluster

Messier 44 – Open Cluster Benoit Guertin

On April 22nd, simply look for the Moon once the sky is dark, and just above it you will find the Beehive cluster with its 1000+ stars. OK, even with a telescope you won’t be able to see all the stars, but take some time to notice how this group of stars stands out with regards to background stars further away in our galaxy. And while you are at it, consider that nearly 410 years ago, Galileo made the first observation of these stars.

M44_Moon_22Apr2018

From there you can also hop over a to the west and observe the color difference in bright stars Castor and Pollux in the Constellation Gemini. And if you instead decide to go east, the smaller Messier 67 open cluster is also accessible with binoculars.

Blue Moon Lunar Eclipse

The second Full Moon in a month is generally called a Blue Moon. And yes the old saying “once in a Blue Moon” is in reference to this rare event.  Well… if you consider every 2 to 3 years rare. However this one will be extra special because it won’t be blue at all!  It’ll be blood-red because we’ll have a lunar eclipse on our hands!

LunarEclipse_27sep2015

September 27th 2015 Lunar Eclipse

The lunar eclipse will be visible from most of North America, but people out West will be better placed to see it.  In the East, the we’ll only get a partial eclipse as the moon sets in the early morning on Wednesday the January 31st around 6:48am EST.

If you do plan to photograph a lunar eclipse, a tripod is strongly advised, and if you are using a telescope, an equatorial mount is required. The above photo is a single frame at 2.5 second exposure and ISO400 with a Skywatcher 80ED. Yes those are a few stars popping into view during the eclipse.

 

 

 

Astrophotography in the City – Part 3

In Part 2, I explained the steps involved in improving the signal to noise ratio (SNR) by stacking multiple images and removing camera sensor noise (DARK and OFFSET frames). In this third article I will deal with sky gradient removal and white balance.

IRIS is a powerful astrophotography tool, and learning how to use the numerous commands can lead to fantastic photos. You can find good documentation and procedures on the IRIS website, so I won’t go in too much detail here.

While IRIS can process images in 32-bit, it cannot open the 32-bit FIT files generated with DSS. With my image still opened in DSS from the previous step (or by opening the Autosave.fit created by DSS), I select to save the image as a 16-bit FIT such that it can be opened in IRIS.

Below is the result in IRIS, and two things become apparent: 1) the sky has a gradient due to the light pollution from city lights; 2) the sky has a pink hue. These two elements will be corrected in this article.

iris sky gradient

Note, when I opened the image in IRIS, it was inverted, I had to flip it horizontally (menu bar – Geometry/Flip/Horizontal).

The sky gradient removal tool works best when two elements are addressed: 1) nice clean image edge, 2) the background sky is black

Trim the Edge

The image needs to have a nice edge around the border (i.e. be smooth all the way to the edge). Hence any dark bands, fuzzy or slopping edges needs to be trimmed. Zooming in on the left part of the image, I will trim at the yellow line, keeping the right-hand part.
photo edge trimming

Typing win at the command prompt within IRIS will give you a cursor to select the two corners to crop your image.

A Black Background

The background needs to be black and have an RGB value near 0. To do that, select a small area in a dark portion of your image, with no stars, and use the black command. This will offset the RGB values to be 0 based on the average within the square you selected. Essentially what you are telling the program is that the darkest portion of your image should be black.

White Balance

The sky gradient removal tool can also correct the background sky color, but before doing so, we need to adjust the white balance such that white stars appear white. To do this correctly you will need a star map (Cartes du ciel, C2A, Stellarium) and locate a star in your image that is as close to our own star color: G2V. This is not exactly for beginners, if you don’t know how, skip and do the white balance later in a photo editor. Once the star located, simply selected it with a small box and use the white command in IRIS.

We perceive a white piece of paper in sunlight to be white, hence light coming from a star of the same spectrum as our Sun should also look white in photos. It’s essentially a white balance exercise, but selecting a star in your image to calibrate instead of most programs which uses the average of the whole image.

Sky Gradient Removal

With that done, you can now select from the menu Processing / Remove gradient (polynomial fit) to get the following pop-up

remove gradient

If you have just stars in the image, a Low background detection and Low Fit precision will work.  However if you have intricate details from the Milky Way with dust lanes and all, then a High setting will better preserve the subtle changes. Try various combination to see what works best for your image. You can also do one pass with Low, and then follow it with a 2nd pass at High.

The result of all this is presented below: the sky gradient is gone, and the sky background is now a nicer black instead of a pink hue. And if you did the white balance, then the stars are also of the right color.

iris-completed

I should mention that the two most important dialog boxes in IRIS are the Command prompt and Threshold. When viewing and performing the various operations, the threshold values (essentially the min/max for brightness and darkness) often needs to be adjusted to get a good image and see the required detail.

iris-command-threshold

The next step will be importing the file in a photo editor for final adjustments. Color saturation, levels and intensity can be adjusted in IRIS, but I find a photo editor to offer better control. And because I will continue my editing in a photo editor do not set the Threshold values too narrow. I prefer a grey sky and then do a non-linear adjustment in a photo editor to get a darker sky.

More to come in another article