Thursday, December 28, 2006

Going deep...

Over the Thanksgiving holiday I got very lucky with the weather - four days off work and four clear nights forecast. This kind of thing is typically unheard of for someone with lots of new astro equipment, so I was praying that the forecasters would not let me down. The night before Thanksgiving I setup my equipment before it was totally dark and got prepared for some imaging. I haven't mentioned it before, but there's a lot of stuff that needs to be connected to make this whole system work:

USB cable from laptop to Canon Rebel XT USB port
USB to serial convertor to Canon Rebel XT shutter port
DC adapter from power strip to Canon Rebel XT
USB laptop port to Shoestring Astronomy GPUSB unit
GPUSB interface cable to EQ6 autoguider port
Firewire cable from laptop to firewire hub
Firewire cable from firewire hub to DMK camera
Power cable to firewire hub
Power cable to mount
Power cables to dew heaters on each scope

So you can see that there's lots of stuff to trip on, and I have on more than one occasion. All these cables are necessary to automate the process as much as possible. The ImagesPlus software uses the USB and serial cables connected to my Canon Rebel XT camera to control camera settings, download images, and control the camera's shutter. ImagesPlus makes the process so easy. The software has a routine that measures focus, so you can quickly dial in the focus before getting started. Once you have achieved the best focus, you simply enter the number of exposures, the delay between exposures, the ISO setting, and any prefix to the file name. Then you click "Release" and the software begins taking exposures.
But there's a little more to be done for longer exposures. My mount can typically allow exposures up to 90 seconds without any guiding. That's without any precise polar alignment or drift alignment. To go longer we need something to keep the mount pointed precisely. That is where autoguiding comes into play. There are three basic elements to the autoguiding system:
1. A camera such as a dedicated CCD guide cam or webcam
2. Software to read the camera images and issue correction signals
3. A hardware interface to the mount's guide port
In my case I use the DMK21AF04 firewire camera as my guiding camera. I purchased this camera because it excels as a planetary imager with it's high frame rate and low noise. I tried to pick equipment that can have more than one use, and the DMK fits that role perfectly. The DMK is mounted in the C8, which has the f/6.3 reducer installed. Without the reducer my field of view would be very small, and the f/10 ratio would require much brighter guide stars. Going down to f/6.3 gives me a bit larger field of view and requires less exposure time for dimmer guide stars. On my laptop I am running MetaGuide software which works with lots of different webcams. Metaguide captures the frames coming in from the camera and stacks several to overcome small movements due to poor seeing. The software then determines which way to correct the mount, and issues guide corrections to the GPUSB interface from Shoestring Astronomy. This little device converts the guide commands to signals that the mount can recognize. What all of this boils down to is that I can run my exposures much longer. What makes it so convenient is that I can get everything setup in about 30 minutes, then start a long image sequence and walk away. While I am inside sipping cocoa or having a snack, the rig is snapping away pictures and keeping itself guided along the way. I just go out from time to time to check the progress and make sure nobody has ran off with any of my equipment. Sometimes I even catch a quick nap, which is hard not to do when you are imaging through the night.
Over the holiday weekend I managed to spend two nights out and capture three objects very nicely. The first was M45, which was in the previous post. The second was the Rosette Nebula, which consisted of 80 exposures of 3 minutes each at ISO 800 (top picture). The third was the California Nebula, which consisted of 50 exposures of 2 minutes each at ISO 1600 (bottom picture). Beyond just capturing a bunch of images, I learned a lot in the process. Every night I go out I learn more, and that's a big part of the reward of this hobby.

1 comment:

Charles said...

Hi Tim,
I have a friend who needs a powercord for the clockdrive on a 1971 Celestron C8 fork mount, we have searched the web in every way we could think of, to no avail. Any ideas on what kind of cord/connector this might be, standards wise or other equipment cords, like coffee pots, TVs, etc. that might fit? We really want to get the clockdrive going so that photography is easier, because right now its very limited exposure time wise.
Any help would be appreciated.