Oh, Those Stars at Night

Milky Way Galaxy as Seen From Chambers Lake in Northern Colorado

I’ve always had a fascination with night photography.

Most of the photographs I’ve taken of the night skies over the years have been fairly simple. A wide angle, fast aperture lens mounted to a camera, supported by a sturdy tripod. Great for landscape photos, star trails, Milky Way galaxy. But this is a fairly limiting approach.

I broke out my telescopes this winter and was looking at ways to mount a DSLR to them. I quickly discovered that the optical quality and construction of my telescopes, while quite suitable for viewing, did not really equate to high quality photographs.

The basic formula for photographing celestial objects is called the rule of 500, sometimes referred to as the rule of 600.  The rule is a mathematics rule that tells you what the maximum exposure time one can use to photograph stars or the moon or other fixed night sky subjects without a visual blur to the stars due to the earth’s rotation. I’ve found the rule of 600 works fairly good with wide angle lenses, and when using a super-telephoto lens I revert to the rule of 500. Simply stated, divide 600 or 500 by the focal length of the lens, and you get the maximum exposure time you should use to prevent the blur.  This simple rule works fairly well on a full frame sensor and crop sensor. With the crop sensor camera, you must calculate the effective focal length of the lens by multiplying the crop sensor crop factor by the focal length.

For example. If you wish to photograph the moon with a 500mm lens on a full frame sensor I would use the rule of 500. 500/500 = 1 second exposure time max.  The same photograph taken at a 250 mm focal length on a full frame sensor would be 500/250 = 2 second exposure time max.

For a crop sensor camera, lets say a Nikon D7200 (it’s what I have), you multiply the focal length of the lens by the crop factor. In my case with the D7200, the math is 500mm x 1.5 (crop factor), so the effective focal length is now 750 mm, Using the rule of 500, you divide 500/750 = .66 seconds maximum exposure time. The crop factor increases the effective focal length of the lens and consequently reduces the amount of time you can expose without apparent movement of the night sky objects. Simple stuff.

For wide angle photography, one gets a little more exposure time by virtue of the wider focal angle. For example, if I wish to use a 20 mm prime lens on my D7200, I multiply 20 x 1.5 and get an effective focal length of 30mm. Using the rule of 600 (my wide angle rule) I calculate the maximum exposure as 600/30 = 20 seconds. The wider focal angle reduces the relative movement of the stars over time, so you can expose for a longer period of time and/or reduce the ISO, thus making for less noise in the image. There are trade-offs and tricks you lean once you understand the basics of night photography but these are the foundations we start with.

The moon is a sunlit object, so a 1 or 2 second exposure time is more than adequate to get a good exposure at just about any ISO setting.  For stars, there is less light so longer exposure times are preferred to capturing more light at the sensor and getting more stars in the photo. This is all beginners stuff and something easily learned.

Milky Way photos are fairly easy if you are in a dark sky area but again, you’re stuck with the calculated time limits if you want sharp images.

But, stars and the moon aren’t the only objects out there in space. There are galaxies, nebula, and other things. Most of which can’t be seen with the naked eye, so a telescope is really required to view them.

The very best telescopes have what are called equatorial drives attached to them. The equatorial drive is essentially a motor driven device that will move the telescope in synchronization with the objects rotating around the earth.  If you want to photograph something like the Orion Nebula, or a distant galaxy, which are very dim objects, one must find a way to increase the exposure times on photographs. The equatorial drive provides a stationary view of a moving object, thus the exposure times can be increased dramatically.  Light is additive, so longer exposure times means more light gathered and better noise free photographs.  Using special software these longer exposure images can be filtered and stacked together using computer software to get really nice results. So I’ve heard.

I’ve never attempted this before, but I do have a few friends who have thrown their efforts at this and I’m convinced that I too can do it if I have the proper equipment.

I’ve heard tales of photographers dropping tens of thousands of dollars into astro-photography equipment. A good quality telescope with equatorial drive can run several thousand dollars all by itself. Start throwing in the different gizmos and doodads and like any expensive hobby, it can become a money pit.

My telescopes don’t have equatorial drives and their optics aren’t very good for quality so what to do?

Well, I have high quality optics in my camera lenses and the best approach for me at the moment would be to take advantage of the gear I do have.  I can cover any focal length between 20mm and 750mm, so all I really need is an equatorial drive to wade into the money pit.

My wife was kind enough to give me an equatorial drive as a Christmas gift this year. A gizmo made by a company called iOptron. The gizmo is called the SkyGuide Pro camera mount. Essentially, it’s a single axis equatorial drive that attaches to my photography tripod and camera and once properly aligned to my photographic latitude and true north will provide me with a way to actually track and photograph a space object that moves with the earth’s rotation.

iOptron SkyGuide Pro With Nikon D7200 & 70-200mm f/4 Lens

The idea is that it will allow me to increase exposure times and that gives me more light to see things with the camera for longer periods of time.

Couple the equatorial drive with a good selection of high quality lenses on a high quality camera, the sky is the limit so to speak. No, it’s not the Hubble Telescope, but from the images I’ve seen on the internet using the SkyGuide Pro and similar camera equipment to mine, I should be able to get something nice out of it.

Night sky photography is tough in cities and areas with high light pollution.  Those city lights and smog and other things in the atmosphere degrade the visibility from the suburbs. Lucky me though, I have a cabin in the mountains of Northern Colorado and the night sky at my cabin is DARK. A perfect place to see stars and such and a perfect excuse for me to spend more time at my cabin.

This coming year is the year I learn astro-photography. Stay tuned in, I’ll report back with my results and experiences as I improve my knowledge and skills.

I suppose this is what old photographers do when they get bored.