Photographing Deep Sky Orion


armando_m

I have been going to a little island in the pacific coast of Mexico for the last few years, recently I was there for my 6th time, one of the most fascinating aspects - for me at least - is the very dark sky one is able to enjoy at night, and I specifically choose dates close to the new moon dates, to avoid the moon hindering the star gazing experience.

 

While shooting the milky was my initial fascination, at this time of the year it is only visible close to sunrise, very low on the horizon, so the opportunity to get a decent shot is limited.

 

My eyesight is not particularly good for star gazing, and my night vision is also not particularly good, so using he camera to discover things I can not see without aid has fascinated me since I started with digital photography.

 

During this trip I tried a tracker - a device that moves the camera at the same speed as the earth rotates - allowing the use of long lenses and long exposures keeping the stars in a steady position relative to the camera.

 

The tracker has to be properly aligned with regards to the rotation of the earth, this translates into three adjustments:


1. The tripod where the tracker will be mounted  has to be perfectly horizontal.

 

2. The tracker has an elevation adjustment which has to match the latitude of the location where you are taking the photos. 

 

3. The tracker has to point towards polaris, the north star. 

Once the tracker is aligned the camera is mounted on a tripod head that is installed on the tracker rotating head, the camera then has to be pointed toward the object you are interested in and a number of long exposures can be taken.

 

Before continuing describing all the caveats of the process - and perhaps bore you to death with all petty details - here is the final result of my attempt to capture Orion. 

 

processed.jpg

 

 

1. Tripod and tracker alignment 

 

Before going on the trip I did try aligning the tracker in my backyard, a rather impossible task as I was not able to see the north star given all the light pollution from the city, but using my mobile phone and a sky map application I got it pointed towards the right direction, adjusted elevation to the 21° 44' latitude, the tracker has a rather imprecise scale, but I "fined tuned" it with the mobile app, this initial attempt gave me exposures with a 150mm lens of only 5 seconds before the stars started to trail... hmm rather mediocre. 

 

On my next attempt I used a bubble level to make sure the tripod base was horizontal, adding this step and my rather rudimentary alignment with the mobile phones pointed towards the north star gave me good exposures for 15 seconds, much better !  Then a business trip, bad weather, and a bad cold put the practicing on pause for a couple of months, I just said, please do not forget to bring the level to the island. And what did I forgot  on my trip? You guessed it, the level. One more thing I did was to tie some 2kgs weight to the bottom of the tripod. 

 

At the island, with beautiful dark skies, no clouds and little wind, the north star is clearly visible, even with my poor eyesight, but surprise! When doing the alignment one has to look through a scope that is mounted on the tracker and then it is not only the north star that is visible but a number of faint stars, 5 or 6 in the field of view. Oh, and the view is reversed, so I had to concentrate and make the inverse movements to what I was seeing. Still I could not tell which one was polaris. 

 

Lucky for me I wasn't not alone. A number of enthusiasts of the night skies came along and some of them have green laser pointers. I asked for help and one of them pointed their laser towards polaris as I adjusted the tracker. OK! Tighten the screws and do not breath too hard to avoid disturbing the adjustment which obviously was disturbed, but at least then I knew how to get it back to the proper adjustment without to much fuss.

 

Before all this I borrowed (yet more help from the team) a mobile phone with a bubble level app and got the tripod horizontal, with a heavy rock tied to the center post

 

2. Lens focus to infinity

 

I had with me the 70-300 zoom which wide open at 300mm is only f5.6 and not very sharp, another alternative was my 85mm lens using f2.8. One of the guys lent me a 80-200 f2.8 zoom and I gladly used it, mounted it on the tracker using the tripod mount from the lens, which gave it a nice weight balance. Focus was done manually in live view using Venus as it was the brightest dot in the sky.

 

3.Mounting and aligning the camera

 

If pointing to a bright object such as Orion, it is not a complicated matter, only requires some patience as the tracker and camera alignment will have to be done multiple times until everything is just right. Some test shots at 5 seconds, then 10, then 30 seconds showed the alignment was good and no star trailing was evident.

 

4. Exposure

 

The 80-200mm zoom was set to an aperture of f/4, this gave some added sharpness and reduced coma. I set the camera to 30 seconds ISO 800, then proceeded to take 20 similar exposures. Zooming in on the camera LCD I was able to clearly see the Orion nebula. I (and all the spectators) was delighted with the results.  

 

So during the shot I had help to level the tripod, point the tracker and borrowed a lens. If I were alone then this wouldn't have been possible. 

 

One more thing, humidity was a problem as there was condensation on the lens which had to be wiped every few shots. This probably caused the glare in the bright stars. 

 

5. Processing

 

The images are fed to a program called DeepSkyStacker. It takes a few minutes to complete the alignment and stacking. The levels adjustments are rather unique and obtaining usable results takes some patience.

 

Here is how the stacked image looks - somewhat cropped - but is a 32 bit TIF, so there is a lot of information in the file. 

orion unprocessed.JPG

 

For comparison sake here is 100% crop of a single NEF file, with some curves adjustment.

orion single file.JPG

So, there you have it! If you are still reading, thanks so much, I hope you enjoyed it. 

 

My shopping list: a 300mm PF lens, a couple of right angle viewfinders, a laser pointer. Finding dark locations, preferably not involving a 4 hr boat trip, are also in my "to-do" list. Good weather is also key to success.  

 

Edited by armando_m


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I have dark skies here whenever there's no moon (nearest city is small, and 40km away behind a mountain range), but I prefer to take the skies as part of another picture anyhow, usually involving a fisheye or ultrawide. This makes the use of a tracker mostly unnecessary.

 

The other fascination I have is for the ISS when it tracks overhead at that perfect time when the skies are almost full night from the ground but the ISS is still in full sunlight. It would be interesting to see what a tracker does to the line that the ISS tracks across the sky (almost horizon-to-horizon at the perfect time after sunset) - while the stars may appear still, I thing the ISS track would probably look really odd, given that it is straight and the rotation compensation is circular. The ISS can be visible for over six minutes when it passes on a directly overhead track. On Monday morning it will do that here, but unfortunately it will be around dawn so a six minute exposure will not be possible as the sky will be too bright.

 

It may be of interest that at least one IBIS-equipped cameras has a rudimentary tracking accessory that works within the scope of the distance the sensor can move (Pentax "Astro Tracer" being the one I know of, which gives up to five minutes of exposure time).

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Outstanding work, Armando! Thanks for letting me put this amongst our articles. :) 

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Thank you.  I missed it too. And just today I was asking technical information from a questar (3.5" telescope) group member about some technical aspects of star shots.  I think this just answered them for me. But one more question of you:  Does i only take 5 seconds to get small star trails? I have been dealing with coma from various lenses, but now use a D500 at ISO of 4000, which gives me exposures of less than 5 seconds even with f4 opening.

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On 3/25/2017 at 18:48, waltonksm said:

Thank you.  I missed it too. And just today I was asking technical information from a questar (3.5" telescope) group member about some technical aspects of star shots.  I think this just answered them for me. But one more question of you:  Does i only take 5 seconds to get small star trails? I have been dealing with coma from various lenses, but now use a D500 at ISO of 4000, which gives me exposures of less than 5 seconds even with f4 opening.

When the camera is on a fix tripod there is a rule to determine the maximum exposure before trailing becomes apparent, that is dividing 600/ focal length, this applies when talking about camera lenses. 
I do not know how to translate the telescope 3.5" to this rule, but using a telescope 5 seconds with a fixed mount sounds like a long time , that would be equivalent to using a 300mm lens

The coma problem goes away when closing the aperture, but then you loose light, both problems go away if you use a tracker 

 

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Thank you for posting this interesting article.

 

Reading up to and including the shopping list ;)

 

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Really impressive Armando and thank you for providing so much of the information about how this was achieved. Mongo has tried this kind of photography with mixed success (without a tracker). It seems clear that the tracker is really necessary to achieve the much better and worthwhile results. 

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