My LIGHT Table Solution

In 2021 I bought a LIGHT Table on Amazon which I believe works very well. It also caused me to develop a method for taking FLATs that is quick, accurate and easy.


A key aspect of Astrophotography is taking quality FLATs. Vignetting caused by lens or telescopes is not simple. Even though your telescope, focuser, filters and camera all look like they are perfectly aligned…they typically are not. The center of your LIGHT frames is not always the location where the vignetting is at it’s minimum. This “sweet spot” is typically offset from the center. This varies with every different combination of equipment. The standard caution is to assume that your FLATs need to be re-taken every time you rotate your camera or any other piece of equipment in the optical path. Dust motes collect on glass surfaces and imaging chips. Compensating for dust motes is a key goal of taking FLATs. The dust that might be on your filters is another reason why you must retake your FLAT if you switch filters.

The other issue with taking FLATs is the criteria for the light source you use. The main criteria is that the light source needs to be evenly illuminated…both in brightness and color. This can be difficult because the color aspect of this task refers to the color that your camera records and NOT what you see. This is especially true when using Narrowband filters. It’s very difficult to determine whether the Ha light is evenly output by the light source you use.

CaLIGHTs has a Light Table feature which allows you to use your computer screen as a light source with adjustable color. Over the past few years I realized this can work for only LED type monitors. The compact fluorescent type monitors look like they are evenly lit but there is a significant drop-off of illumination that occurs as you move away from the center of the screen. When I was using the CaLIGHTs Light Table I would carefully take in my telescope at night while being careful not to remove or move anything in the optical path. The next morning I would place the telescope in front of my computer monitor and take my FLATs. I really liked being able to boost the red and reduce the green so that all three colors (R, G, B) were collected by my camera equally. With a white screen the red pixels receive 1/2 of the light as do the green pixels. The blue pixel receive 75% of the light that the green pixels receive. This causes the FLAT compensation for the red pixels to be noisier than for the blue or green pixels.

My Light Table Solution

In February 2021 I discovered a Light Pad that is designed for tracing artwork or designs. I looked at it’s features and decided to buy it. Here is a link to the item on Amazon

This is the Amazon webpage for the Light Pad. It’s description has changed over the past year as more ways to use it have become more popular. The main features are that it is very evenly lit and there are no dots visible on the surface. The brightness is completely variable and full brightness is very bright. It is USB powered and it takes much less than 500mA at full brightness which means it can be easily powered by any USB port. I plug it into a spare USB port located on a powered USB hub mounted at my telescope. The A3s/B4 size is perfect for my 8″ Celestron EdgeHD scope. It is very light which makes it easy to lay it on the aperture of my scope without worrying that the RA or DEC axis will slip.

How do I mount it on my telescopes

I have two scopes…an 8″ EdgeHD SCT and a Skywatcher BK80ED refractor. So I needed to solve two problems. I am a woodworker and I own a small bandsaw. Many times I have used the bandsaw to cut sheets of 1/8″ plexiglass to make cutting templates for my wife’s quilting projects. To create a pattern for cutting the plexiglass I used a rectangular piece of cardboard that was wider that the aperture of the telescope and was the same size as the height of the Light Pad.

I placed the cardboard on a table and then positioned the telescope on top of the cardboard with the aperture of the telescope touching the cardboard. For the small refractor I just used the Dew shield. Next I used a tape measure to ensure that the telescope is exactly centered on the cardboard. Then I used a pencil to trace the perimeter of the telescope aperture onto the cardboard.

I realized that the plexiglass templates don’t need to completely enclose the aperture of the telescope as you can see from the following photos.

On the left are my plexiglass templates for my BK80ED. The four paper clips do a great job securing them to the Light Pad. The middle and right images show my templates for my 8″ EdgeHD SCT. You can clearly see that the A3s/B4 size is just big enough for my SCT.

Here I show how I position the mount so that I can place the Light Pad on my SCT. I remove the dew shield and place the Light Pad right on the aperture. The templates hold the Light Pad in place. The RA balance weight bar is horizontal to the ground.

How do I take my FLATs

I set up my telescope/mount every night I use the system. My procedure is to build my system outside before sunset so that I can power it up at sunset.

1)The first activity is to polar align the mount. I have written software that uses the video from my guidescope so that I can be inside at my computer. I will move the DEC axis back and forth until I pick up Polaris on the guidescope video. Once I do this I take a breather until the video or polaris is stable with the star significantly brighter that the darkening sky. This normally takes 5 to 10 minutes. Once I have a good video of Polaris I rotate the RA axis which causes the video of Polaris to trace out a circle. My program keeps track of the position of Polaris and determines what the polar alignment error is and displays this error as a graphic overlay on the live video. I then go to the mount and make adjustments using the AZM and ALT knobs so that the video of polaris moves to align with the graphic overlay. The polar alignment error is also displayed on my tablet which I have beside me at the mount. All I have to do is make the manual adjustments until the polar alignment error display on my tablet becomes zero. I am typically finished polar aligning 20 minutes after sunset.

2)The next activity is to calibrate the mount which I do inside at my computer. The guidescope video is used once again. I use Nexremote to perform a 2+4 alignment. When I need to align the mount to each star I use the guidescope video as my eyepiece. Once again, I have written software to make this step easier and quicker. Normally the 2+4 alignment takes a further 20 minutes.

3)The next activity is to perform a focusing of the imaging camera. I have a live video of the imaging camera that displays the last star used in the 2+4 alignment. I have written a program to focus either telescope. The focus is adjusted and the FWHM values are determined so that the focuser can be positioned to the optimum focus position. This typically takes 5 minutes.

4)Now I take my FLATs. I command the scope to move so that the scope is pointing straight upwards. Once positioned I remove the dew shield for my SCT. I leave the dew shield in place on my BK80ED. Next I install the Light Pad, plug it into a USB port and turn it on. If I am using a Narrowband filter I will use full brightness on the Light Pad. If I’m using a light pollution filter I will dim the Light Pad. I have written a program to position the mount and to sequence the QHY294C imaging camera. My program will set the GAIN of the camera to zero which is the lowest gain for the camera. For a DSLR, this would correspond to the lowest ISO possible. Then my program will start taking images and adjust the exposure until the mean value of the center 25% of the image has red, green or blue pixel values that are at a preset value. For my QHY294C, I use a preset of 50,000. I adjust the brightness so that the exposure is more than 1 second but less than 4 seconds. Once the desired exposure is obtained my program will take FLATs until I stop it.

5)Now I will remove the Light Pad. If I am using the SCT I will reposition the mount to it’s home position (pointed at Polaris). I do this because it’s the best position for re-installing the dew shield on the SCT.

6)Now I will reposition the mount so I can get PHD guiding up and running.

7)Finally I will position the mount to the DSO and plate solve for fine positioning so that I can start imaging.

Details about my FLAT frames

The biggest change for me is that now I take my FLATs before I start imaging. This gives me the opportunity to inspect my FLATs for bad dust motes or anything unusual before I take hours of LIGHT frames. The other important detail is that I take my FLATs at the lowest possible GAIN. After studying my FLAT frames I realized that the signal-to-noise ratio (SNR) is at it’s maximum when the image is composed of the largest possible number of photons.

Here I used CaLIGHTs to display what the pixel values look like in my FLATs. The High GAIN FLAT has collected fewer photons which results in the pixel values looking noisy. The Low GAIN FLAT had to collect a lot more photons which improves it’s SNR and causes the pixel values to look smoother. I think the net result is that I can achieve superior masterFLATs with fewer FLATs. I typically take 15 low GAIN FLATs every night. I have a masterDARKFLAT that was taken with a one second exposure and a GAIN=0. This masterDARKFLAT combined with a masterFLAT generated from the FLATs I just took gives me FLAT compensation that has not given me any issues.


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