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SD14 in Astrophotography

Steaphany

Well-Known Member
One of the subjects of photography that I enjoy is Astronomy and astrophotography. Prior to purchasing my SD14, I was well aware of the Nikon and Canon dSLRs being the predominant choice, but the capabilities of the Foveon imager, regardless of the reviewers complaints about high ISO noise levels, would still provide the functionality I was looking for. My decision was also based on wanting a general purpose camera flexible enough to handle all my photography needs.

I know that imager noise in astrophotography can easily be handled by the multitude of image stacking software packages which are commonly used to clean up and combine multiple frames. A common image medium used by amateur astronomers is adapting a low resolution webcam to a telescope and recording and stacking from AVI video files. I was sure a SD14 would out perform any webcam.

This is the results of my first astrophotography test. I used my Sigma 28 mm f1.8 EX DG Aspherical Macro which provides a 40.57° x 27.69° field of view. My SD14 was on my Velbon DF50 tripod which offers no means to track the sky. The used a manual focus and manual exposure was set to 16 Seconds with an f1.8. I set the ISO sensitivity to 100, to keep the noise down and to see just how well the SD14 would perform. I chose 16 Second exposure since it was the trade off point between maximum light collection while keeping the stars looking like point sources and not streaks across the sky. The White Balance was set to Overcast since my earlier attempts to photo a Moon rise turned out very inconsistent results with auto. To trigger the shutter, I used my RF remote control that I purchased through a vendor on ebay (I know I need to update the thread on Gear-Talk/Accessories).

I chose to aim the SD14 to Cassiopeia since it was on the meridian and nicely high in the sky, well away from the effects of the atmosphere near the horizon. Cassiopeia also has the Milky Way passing through it along with several star clusters and faint nebulae. It was also a small enough constellation to completely fit in the field of view.

I never bothered to let my eyes adapt completely to the dark, so I found it difficult to focus the 28mm lens on the bright stars of Cassiopeia since they were too faint. I knew they were in the center of the field of view, but not clear enough to properly focus. I knew that the 28mm lens, at f1.8, had a hyperfocal distance of 24.23 meters, so I temporarily aimed the SD14 towards the lights of a cellphone tower roughly 3 miles away, which would have the stars, effectively at infinity, in perfect focus.

This is what the SD14 recorded:

View attachment 871

This is a decent star field, low noise, with good focus. The bright stars of Cassiopeia are easily visible as is some of the color variations between the stars.

I then continued to shoot a total of 24 light frames while leaving the tripod and SD14 exactly in the same position. I also placed the lens cap over the lens and shot 2 dark frames. This took me roughly 15 minutes from start to finish. Keep in mind that the tripod has no means to track the sky as time passed by. Each frame had the stars in a slightly different position when compared to the other frames. The image stacking software combines the light frames, subtracts the effects of the imaging system recorded in the dark frames, and aligns each field to produce the final image.

I used IrfanView, http://www.irfanview.com/index.htm, to bulk convert the 26 X3F files into PNG format and I used DeepSkyStacker, http://deepskystacker.free.fr/english/index.html, to align, stack, and adjust the final image. An advantage of DeepSkyStacker is that it is able to identify a star from the surrounding sky making alignment automatic.:

View attachment 869

I was really surprised to see just how many stars where captured. DeepSkyStacker discarded several low quality frames and built this up from just 17 of the original 24 light frames. Remember this was at ISO 100 and the total exposure time was only 4 Minutes and 45 Seconds.

I did trying using RegiStax, http://www.astronomie.be/registax/, but found it taking a lot longer and the alignment algorithm brought the stars tagged by multi-point references into proper alignment but the surrounding faint stars were dragged into arcs facing the nearest reference point, caused by segmentation of the image. By contrast, DeepSkyStacker uses an alignment algorithm which positions and rotates each frame as a whole. Both have their place as I found Registax will accurately align and combine terrestrial and non-star field photos.

To help those here who are not astronomically inclined, I created a composite of the DeepSkyStacker image with a screen capture from my Sky map program. The alignment is not 100% but close enough for each of Cassiopeia's features to be identified:

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DeepSkyStacker discarded several low quality frames and built this up from just 17 of the original 24 light frames.
View attachment 931


Hi, Steaphaney!

That is a great post and of immense value to me.

I am possibly missing something though (pardon my ignorance in advance), but if all frames are exposed equally, why were the results of several unsuccessful?

Keep up the great work!

Sincere regards, Jim R
 
Hi Steaphany,

I am deeply impressed, too ... although absolutely ignorant in this issue.

I once did experiments with Image Stacker.

http://www.tawbaware.com/imgstack.htm

It is a very nice tool for artificial long time exposures. Now, that the SD14's BULB-mode became extended from 30 seconds up to 120 seconds .... this tool is hardly necessary any more. ..... practically, the former 30s were absolutely enough for my purposes.

However,

thanks a lot for this very interesting post. :z02_respekt:

See you with nice stars

Klaus
 
Both DeepSkyStacker and RegiStax employ a means to evaluate the quality of each frame. As the frames are processed, they are assigned a Score value which is based upon characteristics of image content. For example, if camera vibration causes the stars to be in a non-round shape, the quality is lower and the score is not as high as a frame with perfectly round stars. By default with both packages, any image with a quality below 80% is discarded from inclusion in the stacking process.

From the DeepSkyStacker's FAQ:

What is the score, and what is its meaning?
The score is a measure of the picture quality. To put it simply, the higher the score, the more round and not too big stars were found.

Is the score a measure of the absolute quality of a picture?
No. The score is a relative measure that is only used to sort the pictures of roughly the same area in order to keep only the best pictures for stacking.
From the RegiStax pdf manual:

At the same time as alignment is going on, RegiStax is also estimating the quality of each frame, and sorting the frame set into order of quality. There are five different methods by which this can be done, and you can choose the method used before clicking the Align button. Of the five however, the best results are usually achieved with the "Gradient" method. It is also the simplest to use as no additional settings are required. You can choose the method used by selecting it from the drop-down list in the Quality Estimator section of the Alignment options tab.
 
Thank you...

Steaphany,

Thank you....I have been slowly aqiring several pieces to be able to do night time shots....I will have to play a little now...

Tony C.
 
Very interesting!

Thanks very much for the thorough description of your work. It was very interesting to me, because I have been thinking about trying this myself.

I have a couple questions, if you don't mind:

1. Why the dark frames? I was under the impression that the SD14 automatically subtracts a dark frame from long exposures, is that incorrect, or insufficient?

2. Have you yet looked into shooting with a telescope? What method of connection are you considering?

Robert
 
Robert,

To your first question, I have to say that I have not experimented to see the distinction that the additional dark frames would make. At the time when I did the Cassiopeia photo, I saw how my SD14 would take the exposure, close the shutter, and sit for a period before writing the data to the CF card. It was only later when I learned what it was doing during the closed shutter wait.

In some ways, I find it problematic that I can not turn off the SD14's internal dark frames. I enjoy photographing lightning and the internal dark frame means that every exposure carries a corresponding period where the storm activity is missed. I also want to shoot time lapse star videos and static camera star trail images, to achieve this with out having the stars blinking or the star trails becoming a series of dotted lines, the internal dark frames mean I need to use short exposures. I also question how raw the X3F files are when the SD14 has already performed it's own dark frame compensations. I'm aware of most photographers not being experienced with the common tools to process astronomical images, but for those that are, I would prefer that the SD14 had an option where the photographer can choose to shoot included dark frames, to shoot and store dark frames separately, and control how they are used with the light frames. Leaving this flexibility to the photographer may also increase the capability for the SD14's high ISO's which are inherently noisy.

As the evenings get warmer, I plan on answering some of these questions.

To your second question, Yes and I've had information in three threads on the subject using a SD14 with a Telescope:

SD14 Bulb & telescope adapter

First photos through an Orion 90mm Ø 1250mm Focal Length Maksutov Cassegrain

The Moon

I hope this helps you out in your efforts.
 
Amazing and fascinating! Although I probably won't find myself delving into astrophotography, I am an itinerant "armchair astronomer", and the physics and astronomy books are what I usually buy or check-out from the libraries.
 
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