Solar System Week — Two Planets and a Comet!

Learning to Photograph Planets

I’ve been working very hard over the past few years to understand everything you need to know in order to take good astrophotos of the deep sky; things like big cloudy nebulae, supernova remnants and distant galaxies. This has been very challenging but eventually I’m getting the hang of it although there is still plenty of room for improvement. The two best planets to see in telescopes are Jupiter and Saturn and they are both nicely place for viewing around midnight.

Opposing Forces

Early July this year had the “opposition” dates for both Jupiter and Saturn. When a planet is said to “be at opposition” that means that the Earth is on the same side of the solar system as the planet (Jupiter and Saturn in this case) and is as close to those planets as we’re going to get that year–you could draw a straight line from the planet to the Sun and the Earth would be right on that line. So the Sun and the planet in question are opposite each other in our sky, thus “at Opposition”.

You might think that since you use a telescope with a camera at the back hooked up to a computer that photographing the planets would not really all that different from deep sky stuff but as it turns out it’s a completely different way of going about things.

When shooting the deep sky your basic goal is to take as many long-exposure images as you can; something like one hundred 5-minute shots if you can make that happen. Your telescope mount has to be tracking the sky perfectly both in speed and alignment with no jerks or hiccups. You need the long exposures because the deep sky objects are so faint that their light is just barely brighter than the self-noise of the camera so you need to gather as much as possible so you have something decent to work with later in the computer after combining (“stacking”) the best of your one hundred shots into one master shot that has the combined exposure time of a couple hours or more. You also want a wide field camera/scope setup to take in as much sky as possible.

Well when shooting planets it’s a completely different approach. The planets themselves are quite bright so your exposure time is now in the thousandth’s of a second instead of minutes. Since your exposure time is so short mount tracking doesn’t have to be anywhere near as accurate but you do have one big obstacle to overcome: the smearing effect of the Earth’s own atmosphere which makes the image you’re trying to photograph to appear that it’s “boiling” like it’s hovering over a Texas road in the summer! This is readily noticeable in the eyepiece when visually viewing planets low in the sky.

The planet only takes up a tiny bit of the camera frame so you want to use a long focal length telescope for more magnification and a camera with as tightly packed pixels on the sensor as possible for maximum detail. Also, using a black and white camera will help your resolution as well.

Time For Your Closeup

So my setup for this was the 2000mm Ritchey-Cretien Astrograph scope I used to shoot a bunch of the galaxy shots I took back in April and May.

Now the camera I would normally shoot with is wide-field and in color (both undesirable qualities for planets) so I decided that I would use one of my guide cameras as the main camera for this shoot. This would also give me a chance to try out the new filter-wheel that I bought in preparation for getting a wide-field monochrome camera later this year.

The way you get a color photo from a black and white camera is to take the same picture three times, once through a red filter, then a green, and finally a blue one. Then you combine them together as a color image in Photoshop.

Getting Lucky

So how do you compensate for the Earth’s turbulent atmosphere? You need some “lucky imaging”. There’s no possible way to know when. for a split instant, the seeing will be undistorted so what you do is take a thousand images a minute as a movie file. Movies are nothing more than a chain of still images so this is the most convenient way.

So I had to find, download, and learn how to use a program called “FireCapture” which is written to do exactly this. It’s pretty easy to understand and after watching a couple YouTube video tutorials I was up and running in one night.

So I zoomed in on the planet Jupiter as it was getting to it’s highest point in the sky and took a one minute movie which ended up looking pretty horrible overall:

You just had to trust that in 1000 still photos there were going to be at least a few that were going to be in focus and undistorted. That was through the red filter. Then switch to green and shoot for another minute and then blue for a third minute. You really can’t shoot much longer than a minute on each filter because in the 4 minutes it takes to get set up and shoot those three movies, Jupiter itself is rotating so fast your photos from the RGB colors won’t line up anymore!

Stakking

Once you’ve got your movies you have to select the best frames, process and stack them into a single “best of the best” image for each of the three colors. So I had to get another brand new piece of software called “Autostakkert2” which would do all of those tasks. I told it to take the best 12% of the frames and use only those, align them with some sharpening into single Red, Green and Blue images for each of the movie RGB sets, of which i had about six of Jupiter and twelve of Saturn. Once you’ve got your settings it’s pretty easy to burn through the raw movie files.

Coloring Time

There are a bunch of ways to combine these images into a color images so I thought I’d try Photoshop since I’m pretty familiar with that already. So I created a new color document and opened each of the Red/Green/Blue master images from one set and pasted the red filter image into the red color channel of the color image. Same for Blue to Blue and Green to Green. Then I held my breath as I clicked the button to show me the full color image. Would they align automatically? Would the color be more or less natural? Well here’s what I got:

Jupiter at Opposition, 2020. The Moon Io (top) has just cleared the planet but you can still see its shadow on the cloud tops..

Saturn is a little smaller than Jupiter but it’s almost twice as far away. Even so, there’s a decent amount of detail.

Needless to say I was pretty chuffed with myself since this was the third planetary imaging session of the week and the first two really didn’t yield good results although I didn’t really ever find out why. Focus was definitely an issue so I spent a lot of time with that and I also double checked my collimation (mirror alignments in the scope), planetary imaging is very touchy about bout that.

I did some basic color adjustment and contrast enhancement (Saturn had a pinkish tinge to it I had so neutralize) but it was just a couple of quick adjustments and it didn’t seem like it needed any more. Quite refreshing since the deep sky images usually take hours to process and then sometimes you have to throw it all away and start over with different ideas.

But that’s not the end

We have had the first decent comet in 25 years in the vicinity of the Sun and I went out with just a regular camera on a tripod to get a shot of it, low in the west just after sunset. I was able to shoot for about an hour before it got too low. Here are some of the better shots:

Even though this would be considered a “bright” comet it was a lot less obvious to the naked eye than the pictures would indicate. I found it fairly hard to find each time I looked for it and it was just barely discernible over the remaining skyglow in the west. There have been so many false alarms about “the next big comet” recently that it was good to actually be able to go see one and get a pic.

Carpe Noctem!

Bill Gwynne
aka Bill the Sky Guy