Astrophotography – Page 19

SBIG ST-i Autoguider

A box in shipping and receiving with my name on it. A much awaited box. The new SBIG ST-i autoguider!

For those who are uninitiated into the mysteries of astrophotography, an autoguider is the secret to taking hours of exposures without having the manually correct the telescope position constantly. For almost two centuries, from the first time a camera was attached to a telescope, through the invention of auto-guiders, guiding was a supremely tedious task. The photographer would spend hours on end, peering through an eyepiece watching a single star, if the position of the star started to drift, he would press a button to correct the position of the telescope during the exposure. This was necessary to achieve any sort of long exposure, as the telescope, no matter how precisely made, would drift a little during the night, leaving streaks in place of pinpoint stars.

I have done this, it is no fun at all.

Then came the autoguider. A small digital camera that could take a picture of the star, then check the image for drift in the star’s position and, if necessary, send a command to the telescope to correct the position automatically. The first commercially available autoguider appeared in 1989, the Santa Barbara Instrument Group model ST-4. This little device revolutionized astrophotography, allowing far longer exposures with much less effort and much better precision.

SBIG ST-i Autoguider — The SBIG ST-i with the optional accessory kit including mount and lens

It is an ST-4 I have been using to guide my photographs for over a decade. Any of the long exposure astrophotos seen here on DarkerView have used this venerable little device. I have had some issues with it as of late. Trouble acquiring stars, struggling with the cryptic displays to lock onto a star, the unit does require a great deal of experience and intuition to return good results. Modern autoguiders are so much easier to use with far better performance. I have been looking at the advertisements with envious eyes for some time now.

No more, I have broken down and purchased the new ST-i from SBIG.

The newest, latest and greatest, just released model.

Unpacking the box I am happy with what I see. Everything looks good.

It does seem like a small device for the $595, smaller than most of my eyepeices. Fit and finish looks good. A nice small package that will be easy to mount to any of my telescopes.

I setup the software and drivers on the laptop with no issues, simply following the provided instructions. The SBIG software for the camera , CCDOps ran first time, connecting to the camera and taking a frame. You can hear the soft click of the mechanical shutter in the camera. The bias frames look quite nice, a smooth field of salt and pepper noise with no gradients or other artifacts.

Attaching the lens from the accessory kit I take a few images of wood-grain on the kitchen cabinets across from the table I am working at. Another task will be to properly evaluate the imaging performance of the camera. It does have a decent CCD in it, the specifications indicate a proper 16-bit A/D system. A real photon transfer test will reveal if that system lives up to the specifications. A subject for another post!

Wrong Size — An improper fit of a button head screw into the mounting plate on the SBIG ST-i accesory kit

I did find one problem. Someone in engineering or purchasing screwed up when setting up the accessory kit. The screws meant to secure the rings to the mounting plate do not fit into the provided recessed holes. The head is notable larger than the machined recess. This leaves the head of the screw above the surface preventing proper mounting of the whole assembly.

A Better Fit — A proper fit after modification on the SBIG-ST-i mounting kit

Not to be deterred in setting up the camera I quickly set about a solution… I clamped the screw into the chuck of my cordless drill and removed the outer edge of the head with a file. A minute or two per screw reduced the head diameter enough to fit into the recesses. The remainder of the assembly went well. The mounting appears to be very solid, with little chance for flexure and the resulting image issues that can cause.

The package offers a nice solution, on paper at least. With the included 100mm lens the camera should provide a 165 x 123 arcminute field of view, or a 2.7 x 2.0 degree field of view. The literature promises sub-arcsecond guiding accuracy with this setup, with the ability to use 7th magnitude stars with one second exposures. These are claims that will have to be checked as well. I intend to do a proper job of verifying these numbers, I expect to get many years of service out of the guider. The new camera has quite an act to follow, replacing my classic ST-4!

I also hope to be able to guide on the Sun for the Transit of Venus. I will be trying the included software as well as Dave Solar System Recorder in the coming week. I wonder if either package will be able to guide on a large, non-stellar object like the Sun, perhaps with a shorter focal length lens.

This leaves only one real question… How well does it work? Unfortunately that will have to wait for another day. In true astronomical tradition, the receipt of new astro toys invariably occurs when the weather precludes their use. In other words… I have clouds!

A writeup of the operation of the new autoguider will be another post, when the skies provide me a chance to use it.

A Dark Night

Another new Moon weekend, another observing outing to Mauna Kea. With some shiny new astro equipment to play with I decided to load for photography instead of visual. Disassembling the photographic mount I realized it had not been taken apart in over two years, setup in the garage and used in the driveway. I did not expect to be alone, a few other local observers had indicated plans to get out this night.

Worry on the drive up, the cloud deck seemed quite high. I was concerned that the Mauna Kea VIS, at 9,200ft, may be in the clouds. With wipers and headlights on, through the heavy fog I drove, not encouraging when you expect to use a telescope. The worry persisted until the last switchback, ascending through the last wisps of cloud just a few hundred feet below the VIS to behold a cloudless blue sky overhead.

As expected the Mauna Kea Visitor Information Station was a zoo. A dozen summit tour buses added to a heavy Saturday night crowd. The parking lot was full with even more cars parked along the road. Not a real problem, the tour buses use reserved spaces right in front of the building. They pull out about an hour before sunset, headed for the summit with their customers. We swoop in on the vacated parking spaces and set up our gear.

Astrophotography — A trio of telescopes set up for astrophtography at the Mauna Kea VIS

The crowd is an interesting experience. Well over a hundred folks waiting for dark and the evening program. There are the usual issues of headlights and camera flashes. It may not be all that dark for a while, but we gain a nice paved place to setup, and there is electrical power available. The crowd will slowly wane as the night progresses, mostly gone by 9pm, driven off by the cold. At 10pm the VIS closes and the remaining lights are turned off.

In addition to the VIS telescopes, there were quite a few local amateurs taking advantage of a moonless Saturday night. Cliff and Tony brought Cliff’s 24″ Dob, a two person job to move that ‘scope. Wayne brought photography gear. Mike was likewise set up for photography, planets instead of deep sky, using a Flea 3 and an ancient 10″ Celestron. Olivier brought his 19″ Pricilla, providing plenty of glass for visual astronomy while the cameras exposed for hours.

It was Malalo o Ka Po Lani, cultural night at the VIS, with a special lecture. This meant a large crowd, most of whom stayed to enjoy the perfect skies the mountain provided this night. They wandered around the telescopes asking lots of questions. While the photographic ‘scopes did not offer views through the eyepiece, there was still a lot of interest in the process. I chatted with many folks as I worked, fiddling with the complex equipment necessary to take photos of the sky.

It was quite the gathering of Losmandy G-11’s! Wayne brought two, Mike brought one to carry the old 10″ Celestron, I had mine setup for photography with the AT6RC. Add the three that the Mauna Kea VIS uses! A testament to these well-built mounts, some of which are two decades old.

The astrophoto gear was working nicely. There were a few issues to deal with at the beginning of the night, the usual new gear stuff. A few things I need to fix before next time… need to remount the guider so I can co-align it with the imaging ‘scope. I need to mount a real finder, and make it easier to hunt down the targets. But overall I was pleased, the new setup worked as I hoped it would.

I will write more about the SBIG ST-i later. The short version… Much easier to use than the ST-4, accurate, painless acquisition of guide stars, it just locked on and stayed there. The only real drawback is the inability to dispense with a computer. I recently re-built one of my older laptops for use as the astrophoto machine. But since the computer is going to be there, I may as well use it, time to integrate the DSLR with the computer and shoot tethered. I have the software, just that I had tried to keep it simple in the field.

Another pleasant surprise was the ease of focusing with the AT6RC with a Bahtinov mask. I have been wondering about the stock focuser on the new ‘scope, how well does it handle the heavy DSLR camera. One lesson is that it locks the focus quite well, I noticed no drift each time I checked focus. Indeed, at one point I pulled out the 60D, swapped the focal reducer and adapter onto the 20Da and checked focus again, it was still perfect.

While my camera took exposure after exposure, I took in the views that big glass can produce. Bouncing back and forth between Cliff 24″ and Olivier’s 19″. No surprise for a spring session, galaxies were in rich supply. We viewed a lot of the showpiece objects, from Ursa Major to Virgo and Corvus.

This night was the type of night we hope for when planning an observing outing… Not too cold, no strong wind, and no clouds. Just a dark sky to delight the imagination all night long.

Postcard from the Universe – Omega Centauri

I love it when everything actually works. As I have often stated… Astrophotography is an art of details. After dealing with each detail, one by one, sometimes it works…

Photographing the Transit of Venus

With the Transit of Venus looming on the calendar, a discussion of solar photography is in order. Taking good photos of the Sun is not that difficult, but can be aided with a little information. There are some unique challenges in solar photography.

The one obvious problem is dealing with the sheer intensity of the Sun. An intensity that can easily damage a camera if placed behind unfiltered optics. A proper solar filter is the easiest way to reduce the light to a safe level.

A solar filter will also produce the most pleasing images of the Sun. Indirect techniques like projection can be used. But for good solar photos, a proper filter in front of your optics is the single best method.

Solar filters for optics are constructed with a thin film of metal such as aluminum or stainless steel vacuum deposited on a substrate. This substrate is usually glass or a thin mylar film. The resulting filter allows only a small fraction of the light through, about 0.01% or 1/10,000 of the unfiltered value. Importantly, the filter blocks the ultraviolet and near infrared part of the spectrum as well. The result is a safe filter than can be used on a telescope or telephoto camera lens.

Sufficient magnification is needed if details of the Sun’s surface are to be well recorded. A few hundred millimeters focal length, found in common telephoto lenses will produce a reasonable solar image. The image will still be fairly small. To fill the sensor requires more. For an APS-C sized sensor (Canon T2i, 60D, 7D, Nikon D5000, D3200 or similar) a telescope with 1,000mm focal length will create an image filling a good portion of the image.

Image sizes for APC-C Sensors
Focal Length		Image Size (arcmin)
100mm		760×510
400mm		190×128
800mm		95×64
1000mm		76×51
1500mm		51×34
2000mm		38×25

The table to the left shows the resulting images sizes, in arc-minutes, given various focal length lenses, on an APC-C sized sensor. Recall that the Sun is about 30 arcminutes across as seen in our sky. With 100mm the resulting image is 510 arcminutes from top to bottom in the frame. This is 17 times the width of the solar image, a pretty small image indeed. With 400mm this improves to about 4, thus the Sun will reach about 1/4 the height of the image. At 1000mm this is about ideal, the Sun will reach more than halfway across the frame.

1500mm will just fit the solar image. While this may seem ideal, there is an issue. A small amount of drift will put part of the Sun out of the image, cutting off part of the disk. Sizing the image to fit in the frame with a good margin will allow some drift, while still giving a good image scale.

If you have a full frame camera (Canon 5DMkII, Nikon D800, etc.) a larger image can be used to fill the larger sensor, thus a longer focal length can be used. A telescope with 2000mm focal length will produce an image 17mm across, neatly fitting in the area of a full frame sensor.

Few compact cameras can boast a lens that will zoom far enough to produce an image of the Sun filling the frame. For these cameras another technique can be used, afocal photography. This can also produce good images, but will require experimentation to find the right combination of telescope, eyepiece and camera to produce a correctly sized image.

Solar Framing — The full frame solar image with about 1000mm of focal length and a Canon 60D

If you want to calculate the image scale for your optical combination, lens and camera, I suggest downloading the CCD Calculator from New Astronomy Press. You can enter the optical parameters and see exactly what the resulting image will look like with a sample image of the Sun, Moon or other selected objects.

Another issue is resolution. Our atmosphere usually limits the practical resolution to about one or two arcseconds, blurring any finer detail through atmospheric distortion. This can be much worse in the daytime with solar heating of the ground and air around the telescope. Thus the limit for resolution will be reached with about 1000mm focal length and a modern 10-15 megapixel camera. Any further magnification beyond about 1000mm will simply result in magnifying the blur. There are techniques for overcoming this (image selection and stacking), but if you know how to do that, you already know what you are doing.

Just a bit of summing up… You need a proper solar filter or other method of safely reducing the solar intensity. A long telephoto (400mm or more) will produce a reasonable solar image. A small telescope with about 1000mm of focal length is ideal for photographing the entire disk of the Sun with a DSLR camera.

Postcard from the Universe – Sol

Set up the gear again to get ready for Venus Transit. Attempting to autoguide on the Sun. No luck with the guiding, the software just does not like a non-pinpoint target. I did take a few photos in the process of messing about…

The Sun 19May2012 — The Sun on 19 May 2012, 90mm APO and Canon 60Da

Preparing for the Venus Transit

Time to start preparing the gear for Venus Transit! This means dismantling the astrophotography rig in the garage and reconfiguring for solar work. Taking the the AT6RC telescope off and remounting the 90mm APO. The APO has just the right focal length to produce a nicely sized solar image on an APC-C sized sensor, such as the sensor in the Canon 60D.

First up? just setup the ‘scope in the driveway and take a few photos of the Sun. Just checking the photographic setup, the necessary parts and pieces. Nothing misplaced? Where did I store the solar filter? A nice focus on the camera? Perhaps take some nice photos of the large sunspots that current grace the surface of the Sun while I am set up. I hope we have some nice spots during the transit, they make focusing so much easier!

Next step is to get autoguiding operational, this will be a seven hour event and I really do not want to manually guide for the entire duration. Particularly with a telescope that was setup in the daytime and is not properly polar aligned.

A couple other steps remain in the preparation. Automate the camera to take photos at a regular interval. Insure I can provide a good video feed to the computer sending out the webcast. I do have a few more weeks to accomplish this. I am certain those weeks with speed by surprisingly quickly. Time to get ready!

A Driveway Telescope

Astrophoto gear takes a while to setup. A lot of little bits and pieces are required that make up the mount, the telescopes, and the cameras. This is all connected with cords and cables, a computer, an autoguider, extension cords for power and more. Coming up with a way to minimize the setup is a big advantage. The best solution is to build a personal observatory where the telescope can be setup and remain setup. Since I am not likely to have that solution available any time soon I use the next best thing. I assemble all of the gear and simply roll it out of the garage to use and roll it back when I am done. This reduces the setup time to about 10-15min, in place of almost an hour if done from scratch. The computer and support gear sits on a table small enough to pick up and carry out intact. connect an extension cord for power, connect three cables from the table to the telescope, turn it on and go.

The mount sits on a custom wheeled dolly designed just for moving the telescope. To align and level the scope there are three jackscrews one at each leg to level the mount at setup. Leveling and aligning the scope is one of the longest steps in the process, about 5-7minutes to level and align properly to the Earth’s axis.

Continue reading “A Driveway Telescope”

Postcard from the Universe – Rising Milky Way

MIlky Way over A'a — The rising Milky Way silhouetting and old a'a lava flow on Mauna Kea, Canon 60D @ ISO12800, 17mm f/4, eight 10 second frames aligned and averaged

Canon Introduces the 60Da

Most of my astrophotos are taken with my venerable Canon 20Da, a special version of the EOS 20D that was produced for astrophotography. Normal DSLR cameras work quite well for astrophotography, with one major drawback… The filter placed in front of the sensor blocks much of the Hα light emitted by many nebulae.

NGC1976 or M42, the Great Orion Nebula, taken with the Canon 20Da and a AT6RC telescope.

This light, emitted at 656mn, a wavelength deep in the red, give emission nebulae their characteristic shades of rich red. Hα is the strongest component of light produced by emission nebulae. Without this light, the nebulae will often appear bluish in photographs, as the next strongest component OIII dominates.

Specifically for astrophotography, Canon produced a special version of the 20D with a re-designed filter that allowed Hα light to reach the sensor, the Canon 20Da. The camera also featured on-screen focusing, a feature now found on most DLSR cameras, but unusual back in 2005.

The 20Da was discontinued in 2006. Astrophotographers wanting a DSLR camera with a filter that admits Hα light must buy a standard camera and remove the filter, or have it modified by specialist that offers a conversion service.

Continue reading “Canon Introduces the 60Da”

Postcard from the Universe – C/2009 P1 Garradd

Currently passing through Usra Major, comet C/2009 P1 Garradd, is bright enough to appreciate in small telescopes. It is well placed for observation, well up in the sky through much of the night. At about magnitude 6.8 it is also bright enough to make an attractive photographic target.

I am not totally happy with the photo, I could have really used more time on target. Another issue, the exposures I used were a bit too long for this fast moving target, the coma is elongated in the frames. I will probably re-shoot the comet if I get a chance soon.