We installed a “flat panel” for the second pier (Pier 2). “Flat field correction” is another facet of image calibration in astrophotography and attempts to remove artifacts and other aberrations that occur as light travels though your specific optical train. These could be anything from dust to stray light or shadows occurring due to various equipment components and set-ups. The goal is to achieve a uniform field across the target image. The “flat field” is itself an image of a uniformly lit area which when exposure is adjusted properly, yields a “flat image” showing the artifacts present in your set up. When you subtract this image from the actual astronomical image, the result will be (hopefully) a clean image of your galaxy, nebula etc. Several options exist for a uniformly lit target to produce adequate flat frames. For pier 1 up here at the observatory I use what are called ‘sky flats’. At dusk or dawn if it is clear there is a region of sky where brightness is uniform. The 16″ scope routinely takes these sky flats before and after the imaging session. They work very well- when it’s clear. The other potential issue is that you have to keep changing exposure times as the sky continues to darken or brighten during dusk or dawn in order to maintain the same peak intensity of your flat, typically 40-70% of the saturation point of your sensor. While this exposure adjustment happens automatically with the control software for Pier 1, at some point it either becomes too dark or too light in the sky to continue so you will always be limited with the numbers of flats you can acquire using the sky flat method.
Flat panel which is placed in this case in front of the FSQ106 refractor for obtaining flat frames
Enter the “flat panel”! This is a really neat device which is a uniformly lit artificial light panel where you can adjust the brightness until you have reached the desired level for your equipment. Then you can fire away and take as many flats as you like whenever you want! I decided to try this out for pier 2 and I have to say it works very nicely. Now it may be difficult to automate this feature because the flats in this case are obtained when the telescope is in the parked position and the imaging session is completed. However it is easily carried out through a PC software interface and thus can be obtained remotely!
“Spika flat fielder” is available through a company called All Pro Software and is very easy to set up and use. I purchased their standard panel which is about 15″ and can be used for scopes up to 12″ in diameter. Larger ones can be purchased for bigger scopes if needed. I modified an artists easel to accommodate the panel and this enables me to move the panel around if needed. A standard AC power converter enables a usb connection to your PC and also powers the panel. The software control is quite simple. An example of a flat frame is shown below.
The flat panel is mounted onto an easel. The entire assembly is fairly light weight and can be easily positioned in front of your telescope
This is all there is in the control panel! Just click up or down on the panel brightness.
An example of a flat frame. The round spheres are dust particles in the optical train. Notice also the darker “shadows” in the corners. This frame is subtracted from the target image to arrive at a uniform result without the artifacts present in your system
Happy Flat Fielding!
Thanks for reading!
My hats off to those who are truly remote imaging from a distant site! I have spent probably the last 2 weeks troubleshooting the focuser, the guiding and was having trouble with the usb connection from the mount to the laptop timing out. Lots of things can go wrong! I suppose a lot of these remote areas people use from 2,000 miles away have tech support on site available to do this, but I am happy I am just 50 feet away! Anyway last night I ran the system an entire night unattended for the first time in the new permanent location. We took about 5 hours of luminance, some dark frames, bias frames and dawn flats. Our target this time is NGC7331. This is a bright spiral galaxy in the constellation Pegasus. Many believe the structure is similar to our own Milky Way.
One of the things we have to do when processing images is remove all of the stuff in the image that isn’t actually part of the object but is artifact either from the camera electronics or from extraneous light. Light gets into the optical tube and can create odd patterns of illumination due to the way the mirrors are postioned inside of the tube. There is also dust that gets on the mirrors, the camera chip and filters that also can be problematic. The way to eliminate this is by using what are called flat frames. These are created by taking an image of a flatly illuminated, uniform source of light. The result will be an image of all the imperfections in the optical train. Here is an example from last night’s data:
This is a 10 minute raw image of NGC 7331. It was actually a better than average night and we captured a lot of excellent data! However you can see the donut shaped light patch surrounding the galaxy. This is artificial and the result of excess ambient light and the way the light bounces around inside of the optical tube with the 2 mirrors there.
Here is what is called a “flat frame” . This is a picture of a uniformly lit area, in this case the dawn sky just before sunrise. These are called “sky flats”, a subject for a later blog entry. For now we are just illustrating the ability to take an image of the artifacts in your system. You see the donut patch of light and you can also see a small dark circle on the right. This is from dust in the system. Now one glitch with the sky flat is that if you take them too early at dawn or too late at dusk, you can get stars in there which you don’t want! I have pointed these out! You will then see what happens when you subtract this flat image from the original
And now you can see how nicely this works! By subtracting the flat field image we have eliminated the donut patch and created a pretty uniform image! Unfortunately we have the 3 black spots from the subtraction of the stars. Typically I will go through the flats and throw out the ones with star images in them so this doesn’t happen!