Dithering and the Iris Nebula, a Blue Gem of Beauty

HOW TO GET BETTER ASTROPHOTOGRAPY IMAGES

Recently, I focused my telescope on the Iris Nebula for over four hours.    It is an amazing brilliant blue gem in the night sky.     

Getting great images in the night sky is a lot of fun, but there are a number of techniques you can do in order to get better images.   Dithering is one of them.   What is dithering and why it is important?   

ALL THAT NOISE NOISE NOISE!

When you are recording, whether it is audio or visual, most of us want the true representation of what we are hearing or seeing.    In technical terms, we want the signal to be faithfully recorded and reproduced.    Anything that deviates from that signal is noise.   Unwanted fluctuations.   Additional information that should not be there.

When I am recording audio outside, I am doing my best to eliminate all sources of noise from my videos.   A passing vehicle, a random dog barking, crickets humming, or wind noise in the background.   Other types of audio noise are hisses, hums, crackles and pops.

In visual imaging, noise can come from a lot of sources.   Temperature differences in your imaging equipment and camera sensor, electronic circuit noise, shot noise, ect..

Although we cannot eliminate noise completely, dithering is a great method to help reduce the noise in your images in post processing by dealing with hot sensor pixels, bad columns, and unwanted artifacts.     Here is a closeup image of one of my light images I took while shooting the Iris Nebula recently.   The close up image features at least three stars.    You will notice that the background is not the color black, but a random pattern of noise.

 This images was taken with my ZWO ASI183MM Pro CMOS Color Camera.

I zoomed even further into this above image on the pixel level and brightened up the image.   As the Grinch who stole Christmas would say, "noise, noise, noise!!"



The noise in the background is quite apparent.   The pixels on my camera sensor registered something other than the pitch black of space. 

WALKING NOISE

Stacking software can get rid of some noise in your images.   But what can happen is that you can get "walking noise".    Due to imperfect polar alignment of your equipment and other reasons, a dark fix pattern of noise (DFPN) can show up, appearing in each of your subsequent light frames in a constant direction.   Once you stack your images, they show up as very faint drifts or steaking as seen in the example below.


HOW TO DITHER

So how do we dither?   If you are shooting your image without the ability to dither, you will have to make very small manual adjustments through your mount.   Only the slightest and smallest movements are needed every few frames.   

Most guiding imaging software has the ability to dither with a separate autoguiding camera.   The new ZWO ASI2600MC Duo Pro camera features a both the primary imaging sensor and a guide sensor.   This also works as well, eliminating the need for a separate guide camera and guidescope. 

On my equipment, I am using the ZWO ASIAIR Plus controller with my guidescope camera, and the ZWO 220mm mini astrophotography camera.  Let's explore how to activate the dithering settings in the software.

Once you have everything turned on and the app activated, select the guidescope setting.   Scroll down to the bottom to dither settings and press the arrow.     Slide the bar to turn on dithering.     Adjust the settings for pixels, interval, and "RA only" to your personal preference

My preference is to dither by 2 pixels and setting the interval to every 3 frames.   If you set the interval than 3 frames, this can extend your imaging session a bit over the night as your telescope will have to settle after each dithering movement prior to imaging again.   On the RA setting, I leave that off.

Next, go back to the guidescope settings, and select the Guide Stability Settings.   I have my equipment set to 2" for Stability, 10 seconds for settle time, and 60 seconds for timeout.    Your settings may be different depending on your equipment and location.   These settings work for me as long as my polar alignment and guiding accuracy are good, which I'll dive into in a future video.

IRIS NEBULA

With another dark night, I focused my telescope on the Iris Nebula (NGC7023) in the constellation Cepheus.   It is a bright blue reflection nebula being illuminated by a luminous young star.   It resides around 1300 LY away and is six LY across.   The nebula is surrounded by cold clouds of interstellar dust, blocking the stars behind it.

I took 120 images at 2 minute exposures of the Iris Nebula over 4 hours.    Pulled out 5 images due to airplanes crossing over my images.   I used the dithering feature, stacked it in my ASI DeepSkyStacker software, and processed it in Photoshop to get rid of the light pollution and bring out the amazing beauty of this night sky gem.


I hope you enjoy this amazing nebula. 

Enjoy learning more about Astronomy and Astrophotography?   Come explore with me with my newsletters and YouTube channel.

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