Astrophotography Using Very Short Exposures with Lightweight Mounts
Article 1. Alt-Azimuth Mount Astrophotography Applications
by Joseph L. Ashley
Jeff invited me to prepare a series of articles based upon my book “Astrophotography on the Go, Using Short Exposures with Light Mounts” which is part of The Patrick Moore Practical Astronomy Series published by Springer Books. This is the first article of the series. If you have an interest in using an azimuth mount for astrophotography, putting together a portable observatory that you can easily hand carry across town or to a dark spot, no longer are physically able to setup and use a traditional equatorial mount, just don’t like heavy kit, or are only curious; Welcome to Astrophotography on the Go.
My Observing Spot, Black Arrow on the roof marks the spot
First, let me say what my book and this series of articles are not. I am not contesting the role an equatorial mount has in the field of astrophotography. I think the capabilities of an equatorial mount versus an azimuth mount are well documented and need not be debated. For anyone wanting to debate the issue, I give up. You are correct.
However, some circumstances and applications do exist where a lightweight and very portable astrophotography kit can play a major role or is superior to a traditional heavy equatorial mount. Here a lightweight azimuth mount may often be the difference between being able to photograph or not. This series of articles will discuss in detail astrophotography with very light, highly portable azimuth, goto mounts using a DSLR and very short exposures.
You can’t do astrophotography with an azimuth mount. That’s an astronomy myth that I’ve heard plenty times over the past several years as I clicked away photographing nebulae, galaxies and a few clusters with my entry level goto mount. Like most myths, this one does have a kernel of truth; in fact at one time this myth was very true. Its genesis dates back to when film was king and the king demanded perfection. No azimuth mount ragamuffins with their field rotation deficiency were permitted entrance into the kingdom of film.
Times change, the digital world was born. With the digital revolution came computerized goto telescopes. Now, just like its equatorial sibling, an azimuth mount can track objects in the night sky for hours on end. However, its handicap, field rotation remains. The new rulers of astrophotography, CCD and CMOS cameras, have sensitivities far deeper than film and gave a dispensation to azimuth mounts known as very short exposure astrophotography. The handicap of field rotation still exists and limits what an azimuth mount can do, especially in relation to an equatorial mount. However, field rotation does not prevent using an azimuth mount and successfully imaging a very large number of objects in deep space.
Even though years have passed since the amateur astronomy world embraced digital technology, two aspects of this digital revolution in astronomy are not fully appreciated by many in the astrophotography community:
- The sensitivity and internal noise characteristics of modern digital single lens reflex cameras as well as some of the new mirror less cameras are sufficient to capture images of many deep space objects using very short exposures of around 15 to 20 seconds and to do so with a useful signal to noise ratio.
- Very short exposures have a major impact upon the quality of the mounts and tripods required for astrophotography. No longer is a mount and supporting tripod or pier required that is vibration and movement free for long periods of time. All is needed is a mount that can consistently provide short periods of stability about 50% to 60% of the time while it tracks an object. This requirement is within the capabilities of many relatively inexpensive, lightweight GOTO azimuth mounts used today.
The phrase “lightweight and portable mount” will mean different things to different people. So we all use the same sheet of music here is the definition used in my book and used in this series of articles:
- Weight not to exceed 7.5kg (16.5 pounds) total (mount, tripod, center tray, and, if applicable, the counterweight)
- Easily separated into two or more pieces
- Has a standard Vixen dovetail saddle
- Has a collapsible tripod with extendable legs.
This definition is purely arbitrary in many respects but it does describe the parameters needed for a portable observatory that can be hand carried on public transportation.
Just what is a lightweight portable astrophotography kit? One good example is a SkyWatcher SynScan AZ GOTO mount, Orion ST80A 80mm refractor, Canon 1000D single lens reflex camera. The complete kit, except for the tripod and folding camp stool, is easily put inside a salesman’s sample case or in a 40 liter backpack. The total weight of the mount, tripod, OTA, finder scope, and camera is 13.8 lbs (6.3kg)
ST8OA, SkyWatcher AZGOTO, Canon 1000D
Portable Observatory Ready for Packing
Portable Observatory Packed
What circumstances are favorable for lightweight azimuth as well as lightweight equatorial mounts? While these lightweight mounts can produce excellent images for a wide range of deep space objects, they cannot compete with a traditional German equatorial mount. If this is the case, then why use a lightweight mount? That is a good question and one that should be resolved before anyone decides to abandon the traditional approach using a heavy German equatorial mount for astrophotography.
In today’s world over 50 percent of all people on earth live in a city with as much as 80 percent of the populations of some countries living in an urban setting. A large percentage of these people live in mid-city condominiums, apartments, and other high density housing were storage space is scarce and the rules, security lighting, or lack of viewing areas require travel to a nearby park. In this setting, a small, portable kit including a viewing chair that can be carried in one trip from the apartment to the subway and then to a local park can make the difference between pursuing the hobby of astrophotography or not. Unfortunately, few city dwellers realize that they too can get out at night and either view or photograph the night sky.
Urban astronomers often pack-up their kit and drive to a dark spot away from their light polluted sky. Often a good spot is not used because security and other reasons require that the entire kit including power supplies, observing chairs etc. must be transferred in one trip from the automobile to a spot some distance away; something impossible to do with a traditional German equatorial mount. With a portable observatory using a lightweight azimuth mount, these locations can become viable observing sites.
As one completes 70 orbits or so around the sun, time takes its toll for many people. Even with the younger crowd, an accident or illness can greatly reduce physical strength. Lifting heavy mounts and telescopes is often no longer possible. Here, an astrophotography kit based upon a lightweight mount can make the difference between enjoying photographing the night sky or watching television.
Many beginners own a telescope on a lightweight GOTO mount and want to explore astrophotography without spending much money. They can try astrophotography to see if it is something they want to do without buying an expensive mount that they may not use later. All they need to get started is a digital single lens reflex or mirror-less camera, adapters, interval timer, and possibly a focal reducer. For them, using their existing lightweight mount is an inexpensive entry into astrophotography. This is especially true if they already own a digital single lens reflex camera.
Unguided, very-short exposure astrophotography using an alt-azimuth GOTO mount is ideal for the “casual astrophotographer;” a person who primarily wants to visually observe the night sky and occasionally photograph what they see in their telescopes. For them the ease of use of an alt-azimuth GOTO telescope for visual work whether it be a heavy 10 inch Meade LX200 or a portable Celestron 6 SE is more important than the additional photographic capability provided by a wedge or a German equatorial mount that is seldom used.
Lightweight mounts are ideal for people who want an astrophotography travel kit to take on business trips or on vacations. A lightweight mount, small telescope, digital single lens reflex camera, and accessories can easily fit into a small case to minimize space in the family car or to take aboard commercial airliners as carry-on luggage.
Many astronomers simply prefer small aperture telescopes and lightweight mounts even with the challenges and limitations associated with using this equipment. I must confess that I belong to this group.
Last but not least is economics; a short tube refractor on a lightweight, portable, alt-azimuth GOTO mount is the lowest cost entry into the world of astrophotography. This is important for people on a severe budget. However, as we will discuss in a later article, the magnitude of the savings is not as significant many people may think.
M31 ST8OA, 4SE mount on wedge, Canon 1000D, 120×60 seconds
So what is astrophotography on the go about?
“Astrophotography on the Go” is portability; astrophotography with a portable observatory having everything you need including a stool for imaging away from home. Here we are talking about a kit you can easily pack up and carry in one trip from where ever you store your equipment in your home to your observing site whether you must carry it down several flights of stairs, take it aboard city buses, back-pack to a remote spot by walking or on a bicycle, carry it in your car, etc.
“Astrophotography on the Go” is about using unguided, very-short, exposure photography to exploit an ignored paradigm shift created by the marriage of affordable digital single lens reflex cameras and computer controlled mounts. No longer are heavy, sturdy, expensive mounts and tripods required to photograph deep space.
Astrophotography on the Go” is a story telling how to photograph the night sky without spending thousands of dollars, pounds, euros, etc. All the processes, techniques, and equipment needed to use inexpensive, lightweight azimuth and equatorial GOTO mounts and very short exposure photography to image deep space objects are explained. Currently available light-weight mounts and tripods suitable for photography are examined from an economic versus capability perspective. A similar analysis is presented for entry level telescopes and mounts sold as bundled packages by the telescope manufacturers as well as some customized bundles that I especially like.
“Astronomy on the Go” is about helping people decide what camera, telescope, and mount is the best fit for them. Currently available light-weight mounts and tripods are identified and examined from an economic versus capability perspective. A similar analysis is presented for entry level telescopes and mounts sold as bundled packages by the telescope manufacturers as well as for some customized bundles that are interesting.
“Astronomy on the Go” is about the logistics requirements for transporting lightweight astrophotography systems whether it is in the family car, a short hike to a local dark spot, or flying abroad on vacation or business. The wide variance in major, global, commercial airline, carry-on baggage allowances are discussed along with suggested options for carrying telescopes, mounts, and cameras with you as you travel. Examples of air transportable portable observatories are presented and evaluated from a capability versus weight and volume perspective.
8 inch SCT on a SkyWatcher AZ Mount
In closing, why a lightweight Alt-Azimuth mount and not a lightweight German equatorial mount like the iOptron SmartEQ? The simple answer is that Synta and iOptron GOTO azimuth mounts can take a heck of a lot of abuse and keep on ticking. Take a look at this photograph. That is a Meade 8 inch LX200 OTA sitting on a SkyWatcher SynScan AZ GOTO mount. The OTA weighs 12 pounds (~5.5 kg). The SkyWatcher mount has no difficulty with this load for gotos and tracking. And yes, it’s sitting on the same much maligned tripod used by the SLT mount that is supposed to vibrate, shake, and lean and there is no 5 pound bag of sugar on the accessory tray.
Try overloading a typical German equatorial mount’s stated payload by 50%. I have never tried the 8 inch SCT on the SkyWatcher AZ mount for photography but have used the SkyWatcher mount with my C6S OTA and a DSLR many times.The C6S OTA and the SkyWatcher SynScan AZ GOTO mount are soul mates, a perfect match.
NGC 0253 C6S @f/6.3, SW AZGOTO Mount, Canon 1000D, 123×30 seconds
NGC 0253 C6S at f/6.3, SkyWatcher AZ GOTO, Canon 1000D, 123×30
The SkyWatcher SynScan AZ GOTO mount is not available in the USA. Electro/mechanically is it very similar to the SLT mount but does have a different hand controller with very spartan programming; a two bright star align that no one can get to work, and a typical two star align that works very well. It gives very little information about the 42,900 objects in its data base.The SkyWatcher AZ mount tripod is identical to the SLT tripod. In Europe the SkyWatcher AZ GOTO mount is 25% cheaper than the SLT mount and 50% cheaper than the SmartEQ PRO.