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Colored T-shirts for flats

astrophotography ccd
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#1 Steve Solon

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Posted 10 December 2019 - 02:17 AM

Hi all,

I recently installed a 24"x 24" LED flat panel in my observatory for flat frames. It is mounted ~ 16" from the front of the dew shield of my SCT. Unfortunately, even at the panel's dimmest setting and with a 2-layer white T-shirt, L (1x1) and RGB (2x2) exposures for the correct ADU are sitting between .6 and .8 seconds. I would like longer exposures, and am considering using a dark-colored T-shirt instead of the white one to increase dimming of the panel. My question is: If I'm using a mono CCD camera (QSI 683) should the color of the T-shirt make any difference? My thinking is no, but would appreciate your thinks.

 

 - - Steve



#2 happylimpet

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Posted 10 December 2019 - 05:24 AM

Probably wont make much difference for RGB, but it might for L - ideally the colour of the light used for your flat field should approximate to the colour (spectrum) of the light you're correcting for with your flat field. This is just in case there are some low order chromatic effects going on, which there may be.

 

For me, this means the colour of light pollution. So a T shirt which is light-pollution coloured would be best. Maybe a revolting orange-brown.


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#3 RaulTheRat

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Posted 10 December 2019 - 12:13 PM

Happylimpet makes a good point that I wouldn't have thought of that flat field light should probably be broad band so that chromatic effects are in the flats, but my first instinct would be to think as you initially did that it shouldn't matter at all with a mono camera.

I still can't quite imagine what chromatic effects would be present that could affect the flats significantly when shooting luminance flats, but just to be belt and braces I'd probably rather cover the flats panel with some paper or another layer of fabric to attenuate it more whilst keeping the light white(ish).

#4 freestar8n

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Posted 10 December 2019 - 03:51 PM

If there is any dust it probably isn’t purely gray and would block light slightly differently based on wavelength. So it’s possible there could be issues. But if you try it and the slats seem to work then it’s probably ok.

Frank

#5 richorn

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Posted 10 December 2019 - 04:54 PM

Wouldn't it make more sense to get some Arch D paper to dim the panel?  That would be a more un-attended solution.



#6 Steve Solon

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Posted 10 December 2019 - 05:32 PM

Thanks all for the suggestions. I processed the flats last eve in PixInsight - they look ok, but still think they could be better. I had never heard of Arch D paper, but it certainly seems worth looking in to. Covering the panel with either paper or some sort of material to dim the light seems to be a majority consensus, so will try that first. This panel from Home Depot certainly works well (especially at $49 US), although it is a bit too heavy to lay on top of a vertically pointing scope. I've shot flats with 'bounce light' white panels before when I had a small AstroHaven dome. The results were ok, but produced annoying gradients that were a pain to mitigate. I'll look at all these wonderful suggestions, including the light pollution-coloured T (Thanks, UK!)


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#7 richorn

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Posted 10 December 2019 - 05:41 PM

Arch D paper is 24x36 inch paper.  Often used by architects...  



#8 dayglow

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Posted 10 December 2019 - 08:52 PM

try looking for Staedtler Vellum Paper in "D" size



#9 MapleEve

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Posted 11 December 2019 - 04:08 AM

It matters much more with OSC since VNG is simulated debayer.
Will destroy the color balance, but when you are using mono it doesn't matter much.

#10 jhayes_tucson

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Posted 11 December 2019 - 10:00 AM

First off, with a t-shirt you can move your scope further from the panel.  Second:  Have you actually tried a white t-shirt?  One layer will greatly reduce the brightness and two layers will certainly work--if you can't get further back.  It's just a matter of how you set it up.

 

John




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