A Holder for Your iPad So You Can Use It To Make Flats

This will help turn your iPad (10th generation) into a flat frame illuminator for astrophotography. As currently designed, it works with a small aperture telescope whose dew shield outside diameter is a bit less than 93 mm (snug for 93 mm).

Print two of these, do a little cutting and gluing, screw them together and there you go.

A 10th gen iPad is 9.8" by 7.1" and those are the dimensions this device supports.

I'm providing my FreeCAD design file. Feel free to adjust the parameters for a different size dewshield or iPad.

There are at least two iPad apps you can use to create the illumination:

• HSB Light (a plain aqua icon)
• Light Panel Pro

Warning: In the procedure below, it's best to not make the exposures too short because I've heard that the iPad backlight brightness is controlled by PWM and if exposures are too short, you'll get banding in the flats. Better to use longer. I don't go below 0.75, but YMMV.

Mono Camera with LRGB filters

• Set up the iPad with one of the apps. Since my mono camera has lower gain in the red region of the spectrum, I add a bit of red to an otherwise equal RGB balance. You can play with this.
• Set the filter to LUM, exposure to 1 second or a little less, like 0.75 sec and take a shot. Adjust the intensity of the iPad app until you get something around 75% of your camera full scale. For an ASI2600MM, I go for about 38,000 ADU.
• Go to RED and do the same, but now you'll have to adjust to a longer exposure. Keep the iPad intensity the same and keep track of your exposure times.
• Repeat again for GREEN and BLUE

With your exposure times in hand, you can create a sequence with your image acquisition program (Ekos, NINA, SGP or whatever) for collecting all the LRGB flats with a single brightness setting of the iPad. So then set your sequence to collect however many you like to collect (I go for ~25), then sit back and wait.

Note: for cameras with a mechanical shutter, you will need to make sure the exposure time is long enough for the shutter to completely open. No doubt you've had to deal with this already when trying short exposures with your camera.

Mono With Narrowband

You can use a similar procedure for narrowband, but don't include LUM because you will definitely need to make the iPad brighter. Again, once you have the relative exposures right, you can create a sequence and shoot them all at once with no intervention.

This is obviously not 100% automatic because you might need to readjust the iPad brightness again next time you take flats, but at least since you have the relative exposure right, you just need to get the brightness correct for the first filter in the sequence, then let 'er rip.

One-Shot Color

For one-shot color, it's simpler because you don't take flats for each color. But it is tricker due to the different gains per color. Red will probably be the lowest, so add red to your illumination. Try to get green the brightest and then adjust the red content.