Help Joe With Flats

[JoLo]

I built a light box for my AT106 and it seems to be working well. I have 4 bright white LEDs in the box, typically my flats produce 1/3 to 1/2 histogram peaks at about 1/6 of a second at ISO 400. At 800, around 1/20.

I built a smaller frame and velcroed it to the telescope end of the box, hoping i could use the box with my AT65 as well (I use it as both guide scope on the 106 and an imaging scope). The fit is not tight, and the end of the tube does not rest against the diffuser.

Anyway, I took 2 hours worth of lights on M31 last weekend with the 65, as well as 15 darks and 15 flats. When I got everything calibrated and stacked, I noticed vignetting, gradients, and a shift in color balance to red. I was able to produce a decent image, but processing was difficult, had to deal with all above, and the hump had trouble moving off the left side of the histogram. I was disappointed.

Then I thought about the vignetting (which the flats should take care of) and thought i would restack and calibrate, without the flats, to see what happened. Voila! Vignetting was still there, but the colors were in balance and the gradient was much less troublesome. The image was easier to stretch, and I managed to keep the hump off the left side. Nicer image, less noise, more detail.

So, is the lesson here to build a new light box just for the 65? Has anyone had similar problems? Could it be something other than the fit of the box on the tube? Am I fretting too much?

Any advice on flats is most appreciated. I am going to clean them up this weekend and will post both early next week.

Thanking you in advance........Joe

[ollypenrice]

Hmmm...

I'd be inclined not to make things complicated so far as sheilding the the lightbox is concerned. I would try to get the scope pointing just at the illuminated face of the lightbox, the suface of the box perpendicular to the OTA. They don't have to be all that close but they should be orthogonal. And then I would just shoot the flats in the dark. I often have dodgy flats if I don't shoot them in the dark. It is hard to control gradients otherwise.

Your other problem may be the spectrum from the LEDs. Can you see a histogram for each colour? Perhaps the averaged RGB spectrum looks OK but what about the individual channels. If they are skewed this could explain the damage done to your colour balance. I had all sorts of hassle finding bulbs which gave a decent broad spectrum. For example, the low energy bulbs I tried gave absolutely nothing in the blue channel. I just got a dark image with one bright corner.

Olly

[JimStan]

HMMMM - White LEDs are really NOT white ! They act like little florescent lamps in that there is a "white" light emitting material ( appears yellow when not energized ) that glows when the ultra-violet diode is excited .

Just like in a gas tube florescent lamp, this phosphorescent material does not have a broadband spectrum, and may be missing light in very important wavelengths, even though the light appears "white" to our eyes.

I would think that normal automotive tail lights would have a more continious spectrum, and they could even be lit with a higher than "normal" voltage for a short while to raise their color temperature.

I have not tried this, but I suspect that it might work. I would LOVE to hear from anyone who does give this a test. Is the idea valid, or not?

Jim S.

[JoLo]

Olly / Jim, thanks for the replies, I appreciate it.

For the light box, I do take the flats in the dark, usually after I finish imaging. The flats look like they are supposed to (I think), they seem uniformly illuminated, with some vignetting in the corner. I don't see the dust donuts I have seen on other flats posted here, maybe I just keep my stuff clean....LOL.

For the spectrum, I do see skewing on the blue histogram, red and green seem well aligned. When I use the light box on the 106 (the one it was designed for) I see less skewing, and none of the color balance and gradient issues I saw with the 65. I read several DIY procedures before I put mine together, and went with the bright LEDs as that seemed to be the concensus. Jim, I did some some that use "flashlight" bulbs as well, but thought the LEDs would last longer.

I am going out tonight to see if I can get the supernova in M101 and will use the box on my 106. I may post some of the flats, if you guys would take a look. Like I said, seems to work with the 106, so I shall see.

Thanks again for your help.

Joe