Affiliation
American Association of Variable Star Observers (AAVSO)
Sun, 06/25/2017 - 11:04

I use a light box with low voltage "grain of wheat" tungsten light bulbs to create my flats.  For years I've been annoyed by the fact that the B flats take much more exposure than the rest of the filters due to the poor output of the tungsten bulbs in the B band.  I've occasionally looked for alternative light sources for my light box that have a more balanced output but could not find anything that I liked. 

Does anyone know of any newer types of small light bulbs that would be suitable for a small light box?  I know that there are newer LED lights that appear white but heard that their output is not smooth across the visible spectrum.

Here's what I'm looking for:

1) Balanced output from the B band through Ic, with a smooth continuum in between.

2) Low voltage (3 to 12V DC).

3) Little or no waste heat (I don't want to start any fires inside my light box!).

4) Low cost.

Appreciate any advice on this subject from other CCD photometrists.

Thanks,

Bob

 

 

Affiliation
American Association of Variable Star Observers (AAVSO)
light box

Hi Bob,

There are several ways to address this problem that I can think of quickly.

- use a second bank of the bulbs that can be switched on when you are doing flats.  You might even run this bank at higher voltage to increase the color temperature.  At worst, you double the light and halve the exposure time.

- use quartz-halogen bulbs.  They run hotter/bluer and so your B exposures would be shorter.

- add a second bank of white or blue LEDs, and use those when you make your B flats.

- do your blue flats as twilight flats rather than lightbox.

- replace the light box with an EL screen, like a flip-flat.  More expensive, but gives good BVR flats.  Of course, then your long-exposure flat becomes the Ic flat!

Arne

Affiliation
American Association of Variable Star Observers (AAVSO)
Arne,

Arne,

Thanks for the tips.  Since I already have some mini tungsten bulbs, I'll probably just run then at a higher voltage to get the light level I need and maybe a slightly hotter color temp.

Bob

Affiliation
American Association of Variable Star Observers (AAVSO)
Flat Panel Voltages for Grain of Wheat Bulbs

In a paper published in May  2011 discussing the various flat panel schemes for the AAVSO  Photometric All Sky Survey, there is a graph prepared by the late Donn Starkey that shows relative flux levels for "underpowered" and "overpowered" incandescent lamps. The "overpowered" spectrum is flat compared to the one that is "underpowered."

Does this "under" and "over" correspond to particular voltage levels used to power the grain of wheat bulbs? The bulbs I have are nominally 12v although I have raised the voltage to 15v without any noticeable detriment.

I've constructed an array of bulbs placed in the holes of a peg board like from Home Depot, Lowes etc. I then put several ceiling panels of different types between the bulbs and the scope to produce what appears to be a uniformly illuminated surface.

Thanks,
Bob

Affiliation
American Association of Variable Star Observers (AAVSO)
Hello! I crated a lightbox…

Hello! I crated a lightbox using the plans in Berry's book for AIP4 win. It is a two stage box using milk glass as a diffuser with foam board as the side walls since that keeps the weight down and were easy to cut to size. A bit og glue and tape keeps everything very sturdy.

    In the top (or second stage), I made a string bright white LEDs for blue and V. For infrared, I use a string of small incandescent Christmas tree lights. At first, I used a controller to vary the light output, but I eventually just settled on adjusting the exposure time for each filter. At first, I ran the LEDs on a battery, then I added a 12-volt supply  and I run both the LED string and incandescent string from the same extension cord from my 110 volt outlet with a single simple on-off switch on the lightbox.

    Also, I use morning twilight flats. When seeing is good, I'll typically combine the lightbox and twilight flats. I don't know if that gives superior flats, but it is easy to do since I stop imaging at twilight, and with my LX200, I only need flats a couple of times a month since the system is enclosed and does not get dust on the mirrors, even on those occasions when I've been able to run my system every night of a month, all night long.

    In my work flow, I'll do lightbox flat during evening twilight. For me, once I get good focus, there is usually not enough time to get enough evening twilight flats. As a result, l get my twilight flats in morning twilight since focus is assured from my evening's run. Best regards.

Mike