Focus is one of the most frustrating problems when shooting in the dark. The number of astrophotos ruined by poor focus is legion. Worse, the problem is often not found until afterwards, when hours of effort have been wasted. Very slight errors in focus can spoil an otherwise great photo.
Taking photos in the dark creates a situation where the normal solutions do not work… Autofocus simply will not operate without enough light. Taking the lens off and attaching the camera to a telescope results in a completely manual focus. Simply focusing through the viewfinder is not accurate enough.
A Bahtinov focusing mask in use on the front of the AT6RC telescopeThe simplest method of focusing in the dark is to use the live view feature of the camera. By placing a bright star or distant streetlight in the view and magnifying you can achieve a reasonably good focus much of the time. This is not perfect, even the magnified image is a little mushy near ideal focus, making perfect focus difficult to judge. If you find yourself in an after-dark situation, without any special equipment along, this is the method to use.
Do remember to switch off the autofocus feature of your lens. Once you get focus set correctly you do not want to lose it the moment you touch the shutter button. Not that I have ever done that… Never? Well, perhaps maybe.
Today an amateur astronomy icon passed away. John Dobson popularized the very simple design of telescope that came to bear his name, the Dobsonian. As a Vedantan monk John possessed few material means, pursuing a passion for telescope building in the monastery garden shed he designed a telescope that could be built from whatever scrap parts he could scavenge. He could often be found around San Fransisco showing the wonders of the night sky to anyone who would look through one of his telescopes. His infectious enthusiasm for astronomy led him to help co-found the San Francisco Sidewalk Astronomers.
Chris Fuld using his monster 40″ dobsonian at Oregon Star Party 2013
The Dobsonian is a telescope that is characterized by an extraordinary simple and robust design. Made of plywood and other hardware store parts, there was nothing in the design that could not be built by hand.
The optical layout is a standard Newtonian design with the eyepiece at the front of the telescope. This allows the heavy primary mirror to be located quite close to the ground. The entire telescope rotates on a simple lazy-suzan azimuth bearing made of plywood, formica and teflon blocks. A simple set of trunnions allows the telescope to be raised and lowered in elevation.
Steve Dillinger’s 20″ Dob awaiting full dark at Sentinel, AZ with Venus and the Moon shining behind
The Dob brought large aperture astronomy into reach of thousands of backyard observers. Anyone with a modicum of skill could build a Dob in a garage with simple hand tools. Commercial designs soon appeared at very affordable prices.
Amateur telescope makers have built upon John’s ideas, creating elegant designs that far surpass those simple telescopes made from scrap. Aircraft grade plywood, machined aluminum frames, carbon fiber and computerized controls are common in modern Dobsonians. The design can be scaled up, Dobsonians are sometimes enormous, with telescopes of 30 or 40 inches aperture seen at many star parties. At OSP last year I setup next to a 40″ built by Chris Fuld, a monster telescope built by hand.
John spent much of his later life touring wherever dark skies, telescopes and people could be found. This often included national parks and regional star parties. I met John a few times across the years, at Grand Canyon Star Party and at an evening observing session at Starizona, an astronomy shop in Tucson. His signature graces the secondary cage of my 18″ f/4.5 Dobsonian, Deep Violet, beside the signature of David Levy.
John Dobson’s signature on the secondary cage of Deep Violet
John was also a proponent of a decidedly non-standard cosmology, believing that the Big-Bang model had fatal flaws. His alternate ideas make… Uh? Interesting reading. He describes a recycling steady state cosmos heavily influenced by the teachings of eastern religions and mystical thought.
John Dobson died today, 15 January 2014 at the age of 98 in Burbank, California. John leaves behind a son, many friends, and a community indebted by his contributions to amateur astronomy. My friend Dean Ketelsen knew John far better than I did, I suggest you read his notes on his passing.
I spent a few moments and put all of the photos of dobsonian telescopes that have appeared here on Darker View into a gallery. The photos are just a little sliver of what John Dobson meant to amateur astronomy…
Gluing the mirror box with a cage ring atop to check the fit
Preparing to set up the telescope
Cliff at the eyepiece of his 24″
Deep Violet fully assembled and nearly ready for dark
The 18″ setup at 9000ft on the side of Mauna Kea
Detail of an elevation bearing and encoder
Looking down the observing line at Oregon Star Party 2013
Olivier beside his new telescope, a 12″ Orion Dobsonian
Almost all of the big dobsonian telescopes to be found on Hawai’i show up at the VIS for the night.
David Nemo and his handmade 20″ string telescope
A 12.5″ dobsonian telescope after the refit
Chris Fuld using his monster 40″ dobsonian at Oregon Star Party 2013
The author’s setup awaiting a dark December sky with Deep Violet
Telescopes at Oregon Star Party 2013
The rocker box between coats of polyurethane
Ken Jones observes with his 18″ dobsonian
Sunset at Arizona City awaiting dark
Under Arizona stars at Las Cienegas
Gluing the laminate to the rocker box to create the azimuth bearing
Deep Violet packed in the rear of my Ford Explorer
Gluing the gussets into the mirror box
Assembling the truss tubes and secondary cage
Chris Tribe with his 20″ f/4.5 built from ultralight materialsChris Tribe with his 20 inch f/4.5 built from ultralight materials
The ground board with center pivot and teflon pads in place
First light atop Kitt Peak
Ken and Ann Jones oberving with their 18″ at Oregon Star Party 2013
Secondary Cage with the focuser, digital setting circles and Telrad
Tony shows a group of enthusiastic student views with his 12.5″ dob.
Looking into the mirror box without the primary and mount in place
Bob Clements demonstrates his 10″ dobsonian with an equatorial platform
The Sky Commander digital setting circles mounted to the secondary cage
Deep Violet at the Mauna Kea Visitor Information Station
Steve Dillinger’s 20″ Dob awaiting full dark at Sentinel, AZ with Venus and the Moon shining behind
Setup among the saguaros of the Tortillita Mountians
Looking into the mirror box with the mirror mount in place
Routing the top of the rocker box to create the elevation bearing
Mark Folkerts preparing his equipment for the night
The 18 point mirror cell, built to the plans from Barry & Kriege
The author waiting for dark at the 2005 All Arizona Messier Marathon with my usual visual setup including Deep Violet
Deep Violet set up neat the Dragoon Mountains in Southern Arizona
Looking at the mounted mirror cell and primary mirror
I have only had three telescopes given to me this year. Telescopes in various states of disrepair. I usually fix them up, clean them up, and find a new home for them.
It was Julia who gave me this little bit of fun… A Celestron 62mm f/4.8 Cometron. A small refractor intended for low power viewing. Prefect for viewing comets or other wide field objects.
A Celestron Cometron 62mm f/4.8 telescope.These little refracting telescopes were sold in the 1980’s to capitalize on the comet Halley mania. Sold bearing the Celestron name, they were actually built by Vixen. They continued in production for many years as they proved relatively popular. The Cometron name has been used for a number of small telescopes over the years, but, as far as I know, this is the original.
The ‘scope I was given was in pretty good shape. Nothing broken or badly damaged. The optics dirty but free of coating damage or uncleanable grime. All that was require was disassembly and a good cleaning to remove dirt, spider webs, and a few cockroach egg cases.
Supposedly twins, each of our two telescopes has its own peculiar personality. Anyone who works on the crew can tell the telescopes apart at a glance. I do not need the caption to know this is Keck 1. Each telescope has a unique configuration, a unique set of instruments, plus many little differences that take time to appreciate and come to love…
The Keck 1 Telescope awaiting lights out and release for the night
We have had a couple electrical engineers leave Keck this summer. As a result the few of us left are stretched a little thin. To help cover this, one of the engineers in our group has transferred to the TSD department to continue the telescope drive upgrade project. Unfortunately this moves some of his responsibilities to me.
The segment jack lifting a Keck primary mirror segment during segment exchangeTelescope drives, dome and shutter drives, primary mirror active support system, and more. I have simplified the description of my new responsibilities to one line… If it moves I get to fix it.
This also means I am spending a lot more time at the summit. Over the last weeks I have almost forgotten what my desk at HQ looks like, two weeks and only two days spent in Waimea. This is not going to change any time soon.
I have a few projects left to finish up. This month will see the shipment and installation of the TRICK camera and support gear. We started this process last week with the pedestal and focus stage. The dewar and detector will be here next week. A few more days of hands-on work remain to install everything.
There is also the installation of another TBAD system on Keck 1. I will have to get one of the techs to do that for me. All of the engineering is done, what remains is the fun part, installing the gear. I expect that will be the pattern, I get to do the paperwork while the techs have the fun.
