Category Lessons Learned

Full Width Half Maximum: A Focusing Lesson While Imaging M81 and M82

The Bode’s Galaxy (M81) and the Cigar Galaxy (M82) are located in the Ursa Major constellation, which is right outside my backyard.  Taking advantage of clear night on 2/14, I decided to see if I could capture these two galaxies.  I got everything set up, ready, and started imaging.  Two and a half hours later, I started to process the images and boy was I disappointed.  M81 and M82 were severely out of focus.  I told myself no big deal and I could just wait for another clear night and image them again.  Here is the output from the night of 2/14:

M81/M82 out of focus

M81/M82 out of focus

Three days later , on 2/17, I tried it again.  Thinking I had a much better focus, I imaged the two galaxies for another 2.5 hours.  As I was processing the images from that session, the same thing happened!  I was out of focus again.  How could this happen two times in a row?  Taking a step back and do some research on focusing.  Turns out, there was still a great deal to learn.  The clouds rolled in after that night so this gave me some time to do some research into how to achieve a better focus.

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YouTube Direkt

Turning to youtube, I came across this fantastic tutorial by John Blackwell on understanding a Star’s Full Width Half Maximum (FWHM).  Huh?  What is FWHM?  Turns out this four letter F word (not the one you are thinking about) is very important for determining whether your image in focus or out of focus...

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A Lesson in Field of View and Apparent Size of Deep Sky Objects

It was another crystal clear night on February 10th so I decided to do some deep sky imaging.  I really did not know what to shoot that night so I turned to the hand controller on my Celestron CG-5 GOTO mount and I started slewing to various objects in hopes I could find an unobstructed patch of sky.  After spending 30 minutes trying to find something to image, I slewed my telescope to  NGC 3395 and 3396, a pair of interacting galaxies in the constellation Leo minor.  After focusing the image, I programmed the number of exposures in in the Luminance, Red, Green, and Blue (LRGB) filters and I 2 hours and 26 minutes later, I began processing 34 sub exposures.  In my mind, I was thinking that the pair of galaxies I was shooting would fill quite a bit of the frame I was shooting.  To my surprise, after successfully processing the image, I noticed that the pair of galaxies was quite small.  After getting over the initial shock, I started to look closer at the image.  Not only did I capture NGC 3395 and 3396 but also NGC 3340, 3424, and 3413 showed up in the image as well.

NGC 3395 3396 3340 3424 3414

NGC 3395 3396 3340 3424 3413

After discovering how small the galaxies were given my apparent field of view (FOV), I decided to calculate my FOV for the telescope and the CCD camera.  After plugging in my telescope focal length and and my CCD sensor size, I calculated my full field of view of my images are 74.8 x 99.7 arcmin (an arcmin is a unit of measurement in astronomy)...

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The Importance of Focusing When Imaging Deep Sky Objects

Focusing a telescope is not a trivial task to be overlooked when imaging deep sky objects (DSO).  There are many factors that go into focusing, especially when you do not have an eyepiece and you have to rely on a computer screen to determine if what you are trying to image is in focus or not.  Most photos of DSO’s take a couple hours to image and it is pretty disappointing to process out of focus images after you have spent a lot of time capturing them.  Relying on your own eyes would seem to be a good solution but your sense of sight can fool you into thinking that an object is in focus when it is completely out of focus.  This is where looking at Histograms and Full-Width Half Maximum (FWHM) comes in.  In the Astrophotography program, MaxIm DL 5, there is a focusing tool that gives you a readout of of the object’s brightness and plots the brightness on a histogram, like the one in the image below.

Ideally, if image is in focus, the histogram should have a sharp peak and the FWHM should be less than 2.  Not knowing this handy piece of information when I first imaged the Orion Nebula on January 29, 2013, I was left with exposures that were out of focus.  Thanks to clear skies tonight, I was able to re-image the Orion Nebula to see if I could get a better focus, which you can see the comparison photos below.

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Autoguided versus Unguided Long Exposure Astrophotography

I wanted to demonstrate, visually, the difference between an unguided versus a guided exposure.  When taking photographs of deep sky objects, stars can move as fast as 500 kilometers per second or as slowly as a few kilometers per second.  This makes taking nice round images of stars a bit difficult.  To counter the movement of the stars, an Autoguider is needed, which is an additional camera sensor (either on the camera that is taking the photo or an independent camera strictly used for guiding).  In short, an Autoguider is a sensor that constantly makes short exposures to take a picture of a star and put it in memory.  A software program calculates the stars position, monitors its movement, and calculates the necessary correction to bring the star back to its original location.  The software then issues commands to the telescopes drive system to make the correction.  Since the sky is constantly moving, the telescope mount is able to move with the sky and object you are photographing looks like it is frozen in time.

Below, you will find an Autoguided, 4 minute exposure of the Andromeda Galaxy.

Below, you will find an Unguided, 90 second exposure of the Andromeda Galaxy

If you notice the difference in the stars from guided and the unguided exposures, you will see that the stars look smudged in the unguided exposure and the stars look like round balls of light in the guided exposure...

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The Art of Telescope Balancing, a Lesson in Physics, for the Celestron CG-5 GOTO Mount

Astrophotograpy is not a hobby you can jump right into!  Although eager to capture my first deep sky object, I jumped in head first.  When dealing with telescope mounts, telescopes, and autoguiding, I spent many a frustrating night over the past few weeks trying to figure out why my telescope was off in polar alignment when I was trying to take long exposure photos.

Celestron CG-5 Mount Unbalanced

The art of telescope balancing involves physics plain and simple.  The mount that I have can hold up to a 35lb payload sitting on top of it.  It only took me a couple months before I actually weighed all of the individual pieces I had siting atop the mount.  Turned out I have 21.5 lbs on my mount, which is no big deal right?  Wrong!

Given that the mount only came with an 11lb counterweight, my telescope was way out off balance, which caused problems with alignment and autoguiding (which I will describe in another post).  A telescope must be balanced in Right Ascension (RA) and Declination (DEC).  As you can see in the photo, the 11lb counterweight was NO match for the 21.5 lbs payload, consisting of two telescopes, dovetails, saddle plates, red dot finder, and a clamshell.  Who knew that all that stuff added up to so much weight?  So what is the solution?  Physics will tell you that you need to add more weight to the other side of the mount.  Luckily, they make extra counterweights...

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