Moon and Venus on May 17th

Came home from my piano lesson (yes you can still learn a new instrument past 40) and the sight of a 2-day old Moon and Venus in the dusk sky was stunning. Unfortunately by the time I got home to grab the camera, the sky had darken quite a bit, so I lost my opportunity for some color in the photo.

Venus 6 degrees from the Moon (May 17, 2018) - Benoit Guertin

Venus 6 degrees from the Moon (May 17, 2018) – Benoit Guertin

While I did take more close-up photos, I find adding the rooftop in the foreground helps establish scale.

Notice the Earthshine, it was easily picked up to naked eye.

Canon 80D
85mm F/8
ISO3200 (1/15sec)

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


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

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 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.


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.


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



Astrophotography in the City – Part 2

Continuing with my series on how to do astrophotography in the city…

In Part 1 I described how to set up the camera and take pictures for astrophotography. So if you’ve followed up to here you should have the following 40 images stored on your camera in RAW format.

– 20 LIGHT frames
– 10 DARK frames
– 10 OFFSET frames

The next step is relatively simple, entirely performed on a computer, you simply have to set it up with the right parameters, the right files and off it goes. The purpose is to register (align) the LIGHT frames and stack them to improve the Signal/Noise Ratio (SNR) such that we can adjust the dynamic range and “tune-out” the unwanted bright sky while keeping the stars.

Register and Stack

There are lots of software out there that can perform the task of registering (aligning) and stacking images.  They all look for pin-point stars in an image and use those as references to align your LIGHT frames such that when they are added, the pin-point stars all stack up correctly.

I’ve used three different software, all of which are free:
IRIS – Very powerful, but not exactly user-friendly. If your camera is 2015 and newer, it may not decode correctly the RAW files. However if you know how to use IRIS, the results can be quite amazing. I will still use IRIS, but that will be in Part 3.
Registax – Works best with planetary and lunar images, especially video is used instead of individual images. However cannot open RAW files.
DeepSkyStacker – (aka DSS) Simple to use, but the resulting image has to be post-processed in an image editor. This is what I use for the Canon 80D and what is described below.

With the Canon 80D, I have to use DeepSkyStacker as IRIS does not correctly decode the  Canon 80D RAW files. With my previous camera (Canon EOS Rebel XTi) I would have gone straight to IRIS for all the processing.

The first step is to open each of the LIGHT, DARK and OFFSET frames with DSS using the upper left menu.


Click on Open picture files and select your LIGHT frames. Then select dark files for your DARK frames and offset/bias files for your OFFSET frames. Once that is done, be sure to select Check all on the left-hand side such that all your files are selected and will be used for processing.

You should see in the lower portion of DSS all your images, tagged respectively Bias/Offset, Dark or Light. More importantly, they should all be checked-marked.


The next step is selecting the Register checked pictures from menu on the left which will bring up this pop-up.


Normally the default settings are good. Essentially DSS will remove the DARK and OFFSET frames from your LIGHT frames, look for stars in each and computer the translation/rotation required to align the stars frame to frame. There needs to be 10 or more stars in each LIGHT frame to be able to align and stack.  If that is not the case, it’s possible to play with the threshold in the Advanced settings in order to detect sufficient number of stars in your LIGHT frames.

After that has completed running, DSS will have evaluated all your images, selected the best one as your reference and unchecked any image that could not be aligned. Next is the stacking.  The following was established through trial and error with my Canon 80D.  You may experiment with different settings to see what each parameter does.

Upon selecting Stack checked pictures, and then selecting Stacking Parameters, the following is presented.

Standard Mode will align and stack the images without cropping.  By default this is selected, and cropping can be done at a later time in photo editing.

For wide-angle DSLR images, don’t bother with the Drizzle options. It’s only good when you want to focus on a small galaxy or nebula within your image. If you use this, you better to select an area of interest to keep the file-size and processing time small.

As a DSLR or consumer camera takes one-shot color images, no use to select Align RGB Channels. This would make sense with a monochrome camera, where individual color filters need to be used

The next tab, Light, is where you can have a good say on the final resulting image. Each setting controls how individual pixels are added between each LIGHT frame.


Average is the fastest, and most basic. However random events that show up in 1 or 2 frames like a satellite, meteor or a plane will still be visible in the final image.  This is a good setting for a quick preview of the final result.

Maximum is perfect when you want to do things like star trails, or see if among your many LIGHT frames you caught something a moving object such as a comet, asteroid, satellite or meteor. It essentially keeps the brightest pixel from each LIGHT frame.

I tend to use Median Kappa-Sigma clipping. For every pixel, it does a distribution of the intensity, and if in a frame that pixel falls out of the standard distribution, the pixel gets replaced by the median value.  It essentially avoids extreme values to mess things up, so  a plane passing in 1 or 2 images, or a satellite streaking by will be eliminated in the processing.  It also makes for more pin-point stars.  In the end, it removes random events from your picture.

From experience, a very important parameter to select is Per Channel Background Calibration.  Light pollution in the city tends to have a pink hue, and this can cause the final image to be skewed into the wrong color with the result being either too red, too green or simply grey.  By selecting Per Channel Background Calibration, each RAW image is decomposed in its RGB components and calibrated to have a BLACK background sky (because the night sky should be black, and not pink from high-pressure sodium lights).

The remaining parameters in the other tabs should be kept as per default, and you are now ready to let DSS do all the data crunching.

Once completed it will load the resulting image, and by default saved it as a .TIF file. This is a 32-bit image, it will be large (over 234MB with the Canon 80D RAW files), and not many programs will open it. Luckily the Win10 default photo viewer can preview it. But what is important is that the registering and stacking process has kept as much of the useful data (light photos entering the camera) while removing the random and sensor electronic noise. As we are not done processing the image, no point is throwing out data just yet by using compression or lower dynamic range.


DSS offers capability to adjust the Levels, Luminance and Saturation, but it is best to keeps as is and do this fine adjustment in another program like Photoshop or GIMP.

The next steps will be to continue the processing in other programs:
– IRIS to remove the sky gradient
– GIMP (or Photoshop) to adjust levels, curves and saturation

Continue to Part 3