When you want to see the stars, find someplace dark
A small design detail in a dobsonian telescope is a method to restrain the mirror box in the center of the rocker box, to keep it from sliding side-to-side in the elevation bearings.
I have had trouble with these pads in Deep Violet. They do not stay put! Sometimes when inserting the heavy mirror box into the rocker I would catch a pad and simply shear it away from the wood. I ended up using a larger pad and using small wood screws to secure it instead of the adhesive.
Every astronomer has a first telescope, mine is a 6″ f/5.1 Newtonian I first built as a teenager. As life progressed I was forced to dismantle the telescope and it dwelt for a time as a pile of parts in a box. Eventually I had an opportunity to rebuild the telescope, but as both my technical capability and my financial means had increased I was able to do a substantially better job.
The optical design of the telescope is standard Newtonian with a f/5.1 primary mirror of 6.0 inches giving a focal length of 777mm. The mirror was hand ground when I was a teenager and thanks to expert help during figuring is an excellent mirror.
The RFT design is deliberate and has proven to be a good choice, particularly with modern eyepieces that perform so well in short focal length scopes. With a 35mm Tele-Vue Panoptic eyepiece the scope provides a 22x image with just over a three degree field.
When doing any sort of public astronomy, showing folks the beautiful sights available to a telescope, I often hear the question “Can I take a photo of that?” The person asking the question is usually holding the ubiquitous compact digital camera. They are often surprised when my answer is “Yes”.
It is indeed possible to manage hand held shots of bright astronomical objects by simply holding the camera up to the eyepiece. There are a few tricks to making it work, but nothing that can not be demonstrated in a minute or two. The resulting photographs can be quite pleasing, definitely worth showing to friends and family along with the rest of the Hawai’i vacation shots.
The method of positioning a camera with a lens in front of an eyepiece is called afocal photography, or sometimes digiscoping. Afocal has been around for a while, but was not considered a practical photographic method by most. The advent of common digital cameras without removable lenses has changed this.
Continue reading “Afocal Photography”I have been getting a few questions about the video. To answer a few of them I have compiled a guide to the scenes. Some quick explanations to what you are seeing, information on the camera used as well as the exposure information.
The video is a combination of two techniques. Many scenes were filmed as standard video then accelerated during editing to allow the motion to become clear. Examples of this are scenes of telescopes slewing and the interferometer delay lines moving.
Slower subjects, such as clouds or the stars moving across the sky, were photographed as time lapse. Here a large number of still images were taken. These are then processed and converted to video using Photoshop CS5 before loading into the video editing software, Adobe Premiere Elements. To construct the time lapse sequences sometimes required thousands of separate images, quickly filling memory cards and exhausting batteries. After dark it is long exposure time lapse that is used, with individual exposures often 15 seconds to one minute long.
Keeping the telescopes on-sky every night is the task of the Keck Observatory Operations and Infrastructure Department. A great crew of guys that I am honored to work alongside. This video is dedicated to the guys of the Keck daycrew who make it all possible.
My friend Dean Ketelsen has posted another great bit of video from the Steward Mirror Lab where he works. This time it is time lapse of polishing one of the eight meter GMT mirrors.
Somehow the video does not do justice to the sheer size of an eight meter mirror, it looks smaller than it really is. Be sure to read his description for all of the technical details.
The Astro-Tech 6″ RC is a wonderful little telescope… A true Ritchey–Chrétien design, at a size well matched to DSLR astrophotography. It was get one now or never, these were the last of the production run, and now they are gone.
Best of all, Astronomics was letting them go at a fire sale price… Less than $300 each!! It may have taken months on a waiting list, but I finally received one. Then it took another two months of waiting for some necessary accessories to make it work! No problem with the wait, I was patient… mostly.
With the final parts, the extension tubes, in hand. I determined to spend part of my four day Thanksgiving weekend getting the new ‘scope into service.
A few images later and things were not looking all that bad, maybe even functional. Better yet, my venerable ST-4 autoguider seemed to be tracking well. Maybe take a real picture? What to shoot? Something easy… M42 was rising over the garage… Why not? Just a test for the new setup, a fair chance something will go horribly wrong.
But still, not bad for a first real attempt.
The stars look nice across the frame, showing that the collimation is decent, always an issue with the RC design. I suspect the optical quality of the telescope is quite acceptable. The brighter star images are a bit “fat” but that is due more to the mediocre seeing over Waikoloa. After all of these years shooting with a refractor, I had forgotten how pleasing diffraction spikes can be. Better yet, with the scope positioned for north up on the tube, the spikes are neatly at 45° to the cardinal directions. I am looking forward to some more imaging sessions with a new telescope.
A snippet of overheard conversation. I was in Keck 2 control talking with the operator when a few words drifted over the video conference link from Keck 1. Some sort of trouble. I headed for the other end of the building.
Sniffen, our night attendant was looking for the cause of a drive fault. For some reason the computer refused to move the telescope, displaying a drive fault error. It would not move manually either, drive power would not come on. As he explained this to me I was a bit worried. This could be really simple, or really bad. Possibly something that would cost us the night.
Sniffen and I looked at each other… How had the computer driven into a limit? This was not supposed to be possible. While we occasionally put the scope in a limit during maintenance, I had never heard of this occurring during the night. Was it really in the limit? The computer was indicating we were well clear of the limit. Something was lying to us.
Walking out into the dome and looking up confirmed we actually were on the limit switch. The telescope had driven as far around in azimuth as it could go, stopping just before the hard stop, a massive steel assembly that will definitely stop the telescope.
Oh !#%$! We have to push.
Yes, it is possible to push 300 tons of steel when it is floating on a film of oil. I have never had a chance to do it myself, but years ago someone had shown me how to do it. With Sniffen pressing a pair of pencils into the electrical contactors to release the brakes, I shoved the telescope out of the limit.
It moves surprisingly easy. I had braced my back against the laser enclosure and put my feet against the steel rails used to move the instruments. One good shove and it was moving. I really did not need to brace so well, just a good hard lean against the railings would have done it.
Once you get 300 tons of steel moving, it keeps moving. After I stopped pushing it continued to glide for another few degrees. Low friction creating a clear example of Newton’s first law of motion.
A surreal experience… 2am, in a darkened dome with stars overhead, pushing one of the world’s largest telescopes by hand. Life is interesting sometimes! Twenty six minutes of precious dark lost, but with a quick reinitialization of the telescope we were back on-sky.
Planning a night of observing is a challenge. There is the choice of equipment, setting up observing lists of objects to target. And then there is deciding where to go.
Finding a dark spot can be a challenge in Hawai’i. Almost every bit of land is gated and tied up in bureaucratic rules. We often use the area around the Mauna Kea VIS to observe. Located at 9,000ft on the south side of Mauna Kea the area has much to recommend it for amateur astronomy. This land is under the administrative control of OMKM, who actively support astronomy, both professional and amateur. But the area does have a number of lights, and there is regular vehicle traffic, even in the middle of the night. Thus I have been actively looking for other places.
State land is an interesting issue in Hawai’i. No camping is allowed outside of designated sites, period. But, according to the DLNR administrative rules it is not camping unless you are… “in possession of a backpack, tents, blankets, tarpaulins, or other obvious camping paraphernalia, any time after one hour after sundown until sunrise in a forest reserve” (Section §13-104-2 Hawai’i Division of Forestry and Wildlife). I made certain I had no “camping paraphernalia” with me. I am merely picnicking… in the middle of the night.
One of the tasks I have helped with on the K1 AO Laser is aligning the Launch Telescope Assembly (LTA). This is not so much an electrical engineer’s task, but a mechanical one. So how did I end up with the task? Simple, I was present the first time it was done. Since the mechanical engineer who was first responsible has now left the observatory, I get the job by default. Just the way things work around here.
The launch telescope is a small telescope, about 0.5 meter in aperture, that projects the laser into the sky. Mounted behind the secondary mirror of the Keck 1 telescope, it must be precisely aimed to exactly the same spot in the sky the main telescope is aimed.
In practice this adjustment is about one to two hours crouched in the secondary assembly of the Keck 1 telescope. Each time the bolts are tightened the assembly moves about 10-15 thousandths of an inch. Thus I have to guess how much to offset the measurement so it ends up correct when the bolts are tight. It takes anywhere from three to five repetitions to get right sometimes.
This is where I curse the mechanical engineer who dreamed up the mounting for the launch telescope. I look at the dial indicators, shift my stance against the cold steel to stop the cramping, loosen the bolts and try again.
The last adjustment was a mere 0.004″ (four one-thousandths of an inch) to move the pointing about 30 arc-seconds on the sky. I am now only about 10 arc-seconds from the optical axis. Keep in mind that a single arc-second is 1/60th of an arc-minute, which is in turn 1/60th of a degree. Ten arc-seconds is pretty good, but we want closer. Here I go one more time…