JATObservatory

The Adirondack Flat Fielder Light Box

The Adirondack Flat Fielder light box is really more of a panel than a box. The panel is only ½ of an inch thick and weighs in at either 2lbs or 3.5 lbs, depending on if you choose the 8” or 13” diameter model.

The “Flat Fielder” panel was designed to be used in place of  the sky or the inside of an illuminated dome, as flat field light source. Flat Field images are used as part of the calibration process when taking digital astronomy images. Light entering the telescope does not evenly  illuminate the camera’s chip. This can cause darkening on the corners of the pictures (vignetting). Also unless you keep your optics perfectly clean dust will manage to find its way on to the cover glass of your camera’s chip, the filters, or other parts of your scope’s optical chain. The closer to the camera’s CCD or CMOS chip the dust motes are the more dramatic the affect it will have. The dust motes on the chip show up as doughnut holes, ones further away on filters or diagonal with show as dark circles on the image. (Flat field images are supposed to be taken whenever anything in the optical chain changes, that includes focus.) 

In addition to the 2 sizes options mentioned above you also have the option of an analog power supply, or a digital one. Both the analog and digital units function as a hand controller. The controller is pretty simple (fig 1), it uses 2 buttons (up & down) to control the luminescent light source inside the Flat Fielder light panel and has a LED* that displays the current setting. The buttons allow the user to select 10 different calibrated output levels. The LED display indicates the output setting by displaying 0 through 9 or “F”, with “0” being “OFF” and “F” being full output.  The digital version has the same features with the added option of  control via an RS-232 port of a PC. In addition to the 10 calibrated settings the computer controlled version allows the user to select any one of 255 un-calibrated settings. But this can only be done via the computer interface.

Fig 1. Simple light box controller. The labeling was added to identify what the controller was for. The connectors on the top side of the controller are (from left to right) RS-232, 12 VDC power-input, Power out to light box.

The computer controlled version is supplied with a simple program that will emulate the LED display of the hand controller on the PC. The user can then click on the up and down arrow keys to step through the 10 calibrated settings, or use a slider to select one of the 255 un-calibrated settings. If any of the un-calibrated settings is selected a “U” appears on both the hand-controller’s LED display and the PC screen. The software package also includes a Visual Basic script. This allows the user to select the step size and output setting by typing the numbers directly into a box that prompts the user for an input value when run. The VB script is simple and easy to modify. This was the method I used to semi-automate control of the Flat Fielder for remote operation. Using the single supplied VB script I remove all the required input prompts the user would normally get and replaced them, with a fixed value in the script. The a script for each of the 10 setting was created. I then use a program like MPO Connections to execute the script in order to control the light Box.  An  example of  a modified script "10_on.vbs" is below. The simple script will set the light box output to the Full (calibrated) setting.  To turn the box OFF, x.Step would be set to "0" (zero).

' Set Light Box output to FULL
set x = CreateObject("FlatCtrl.FlatControl")
v = x.FirmwareVersion
v = x.ProgramVersion
x.Step = 10
x.Level = 0
l = x.Level

If you are a programmer you can use the info supplied my the manufacturer to write your own code. The Flat Fielder talks to the com port at 9600 8,N,1 and uses hex values to send data to the box. For me the ability for remote automation was a key selling point, but because my Flat Fielder is been mounted on the rotational section of my dome (fig 2) the power cable needs to be connected and disconnected, so some operator interaction is still required.


Fig 2. - Flat Fielder mounted on the dome wall. Note: The picture angle makes it appear the diameter of the light box is smaller than the scope's aperture. This is not the case, the light box diameter is 13" and the scope is 10"

Current Operation:
After the imaging session is completed the scope is parked and the dome is homed. This sequence places the center of the telescope mount in-line with “Flat Fielder. Depending on which telescope was used for imaging, the adjustable height Pier Tech-2 is either raised or lowered in order to place the telescope’s aperture directly in front of the Flat Fielder. In the case of the figure above, if the piggy-backed scope was the imaging scope, the pier would be lowered about 9". That would place the smaller 4" refractor's aperture in front of the light box.  The light box is then plugged in.

To ensure only the light from the light box enters the telescope, any lights that may have been on are turned off and the shutters are closed. Even the computer display is turn off so the interior is darken. Once the observatory is empty a script is then executed remotely to perform the acquisition of the flats, and darks for the flats. The script currently performs the following:

Connects the camera (SB Scripter only)
Sets the camera's temperature to -5C
Selects the filters in the filter wheel
Selects the output setting of the light box
Deletes any previous files
Takes a series of 20 Bined 1x1 flats for each filter
Takes a series of 20 Bined 2x2 flats for each filter
Takes a series of 20 Bined 1x1 darks that matches the exposure time of each 1x1 flat
Takes a series of 20 Bined 2x2 darks that matches the exposure time of each 2x2 flats
Turns the light box off
Shuts down camera
End script
 

The Preparation:
Before a script could be written to set the light box output,  the correct values had to be determined first. The output needs to be one set to a level that will not saturate the camera's pixels by more than 40% of their well-depth. Each camera/telescope/filter combination is different, and each one needs to be measured. So far I have only created a calibration table for the Stellarvue SV102A and the Williams Optics ZS80 telescopes, both with the SBIG ST-2000XM camera attached. The current calibration table can be found  here.

The nice thing about this setup is I can now take flats. I had not been taking flats before because I had no way of taking the number I wanted for all the filters within the 15 minute window during dusk or dawn. Now I can take them whenever I want. If the image session ends after I have gone to bed I can just plug in the light box and take them the next day. I can then create my master flats for the any filters I may have used during the imaging session. In the future I want to create a script that will automate the creation of the masters. I also will create a number of other scripts that only take flats for either a single filter or just a few. Currently for a full series of flats and darks for all the filters it takes about 2 hours, this is why I wanted a computer controlled light box.

Eventually I will relocate the panel on one of the lower dome walls. This would allow me to leave it plugged in, and the flat field process would now be completely automated. All in all the Adirondack light box is a nice tool.

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Updated 01/20/2007 - Please report broken links webmaster@jatobservatory.org