Then and Now: Astrophotography on the Simple Side?

I spent some time this morning with PixInsight on a stack of M-42 images. This is the result. PixInsight is an impressive, though oddly challenging, piece of software. The interface still eludes me at times. The results are splendid, however.

This image was taken through a Nikon D-810a at f/4, 200mm, tracked on an iOptron mount in gusty winds. This piece is the result of three major processes:

  • All images were aligned using stellar centroids.
  • The images were then stacked… this is an image integration of 100 seconds worth of exposures.
  • PixInsight was then used to do a Dynamic Background Extraction to essentially perform a flat field thus removing the lens’ vignetting. I still can’t get over this process: no flat fields required… though I bet real flats would result in a better overall image.

The camera does its own internal bias and dark subtraction. The image was then brought into PhotoShop for adjustment to levels and cropping.

M42 color stack using Nikon D-810a

Now… compare that colorful image with the monochrome one: that was taken way back in 1986 on Tri-X Pan film pushed to about 1000 ASA by boiling it in nitrogen. The image is a 20 minute exposure through a Celestron C-8 at f/10, manually guided with an illuminated reticle eyepiece. I developed this in my bathroom using duct tape and towels to block all external light from entering.

M42 film

What a difference! New technology brings better sensitivity and a whole new world of imaging…. but we knew this. I’ve been playing with CCDs since the early 1990s. No surprises. The real surprise? Cost! All this tech adds up in cost. I am not really sure that it saved me a whole lot of time to make the new image with the new tech… perhaps if both images were color? Then, yes, the new tech has saved me time. Simple? M’eh. It’s about the same level of technical detail. It ends up being about one’s knowledge base: software or film developing? You choose. Certainly some of my best images were taken with film. Which do you prefer? It’s totally up to you. Like vinyl records, film is making a comeback, but hasn’t made its way to the realm of astrophotography again. I am pretty sure that CCDs and CMOS sensors are here to stay for astro-art imaging.

  • PixInsight sounds interesting: check out their site here.
  • iOptron? Check out their site here.

Comatic Aberration and Chromatic Aberration

These two phrases are pretty much guaranteed to raise the blood pressure of optical aficionados: Comatic Aberration and Chromatic Aberration. There. Did your blood pressure go up? Then it is likely you have dealt with one or both of these issues before… and it is likely that you do not need to read further! For those looking around the internet for an example of these aberrations, seek no more!

Let us start with an image. This shot is of the December sky taken through a wide angle 20mm AFS Nikkor 1:1.8G ED lens on a Nikon D-810. The images were raw NEF files without any processing (except resize), either on board the camera or using software. Click on any image to see it in larger format.

Taurus and comet Wirtanen
Taurus and Comet Wirtanen

The image is a pretty typical night shot: 10 seconds focused at infinity and using 5000 ISO at f/2 (a little stopped down). The constellation Taurus is dominating the right side of the image. There is an airplane top-center moving to the lower left. If you follow the airplane’s future trail it leads to a faint greenish fuzzy object, Comet 46P/Wirtanen. This image is reduced in size…. but upon close, full-scale viewing, this image displays two of the common issues that astronomers and photographers aim to rid themselves of. Funny thing is that this lens gets fabulous reviews on sites like Amazon, and when I complained about these issues I was actually chastised! “Are you kidding? This is such a great lens!” Well, no. It’s not, and for the price, it really should perform a lot better. Add to this story the fact that the Nikkor 16mm fisheye actually is BETTER than this 20mm lens, and there you have an argument to not buy this 20mm lens. So, read on….

May I present to you comatic aberration:

comatic aberration in star images
Classic coma on the images of stars. These should be small round dots, not winged things….

This aberration is off to the sides of the image, off-the central axis. The further from the center, the worse this aberration gets. Some systems sprout seagull like wings from stars. This lens sprouts more than that. Ugly. The cause of this problem is in the optical design and is usually found in parabolic mirror systems like Newtonian reflectors. Alas, it also happens here in lens designs.

May I now present to you chromatic aberration:

Chromatic aberration in star image
An example of chromatic aberration: note the violet halo surrounding the bright star. The violet wavelengths focus at a different distance than all the rest. The fuzzy green object to the left is Comet Wirtanen.

Chromatic aberration has been the bane of the optical world for a long time, starting with those who first pointed telescopes up at the stars (i.e. those like Galileo, etc). A single lens acts very much like a prism in how it bends (refracts) light. The angle of refraction has to do with the light’s wavelength, so not all colors of light will come to focus at the same spot. This is usually handled with complex, multiple-lens systems like Petzval lens groupings using unique glass recipes than minimize chromatic aberration. Well, this lens? It suffers. When pointing at a bright white star, this lens gives an image very much like that of a simple two-lens refracting telescope, what is called an achromatic refractor. Well, they are notorious for having a violet to blue ring of light surrounding bright objects… and halos of blue around the moon and Jupiter. Not fun. Nope. This is why we have monstrously expensive systems like apochromats and Petzvals. We are talking expensive!

Cleaning Time!

Every observatory needs basic maintenance, and those here at PEA are no different. I usually cringe at the thought, but cleaning is a part of the requirement… not that I dislike cleaning. I actually really find it meditative, and a clean observatory dome makes me smile. The cringe-feeling comes from the prospect of kicking up a ton of dust, pollen, cob webs, and such… all of which will have to come to rest some place: Hopefully not on any optics! EEEK! Scheduling the cleaning is a whole other game to play, as well. School ends in early June. A few weeks later, the summer school program begins, and then runs for 5 more weeks. Grass is growing and getting cut throughout June and summer, so, why clean if it’s going to get even more dusty and grassy and pollen-dusty…? So… I wait until the end of summer, when there is a cool, dry, sunny day, like today!

Step – one – cover the optics. Then cover the telescope tubes and mounts with trash bags. Open the dome and aperture.

Two – Vacuum the whole place from top to bottom. We have open studs, so there are a lot of nooks and crannies to work through.

Three – Damp wipe of surfaces, and then a scrub of the floor.

Four – wipe down the ladder and other step-stool devices used by observers throughout the year.

Five – wait for everything to be dry. A light breeze and sunny, dry weather help here. Today was a perfect day.

The result? A clean observatory with a bunch of displaced spiders and no more wasp nests. Webs are gone. Pollen and dust are gone. Happiness!

IMG_1947

A clean machine!