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Shutter Effects, exposure times, and Flats?

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Shutter Effects, exposure times, and Flats?

Hello! I'm trying to improve my flats. I use a Meade 8 in LX200 classic and an SBIG ST-402ME with BVIC filters. I made a light box as described in AIP4Win with four bright white LEDs.

    Integration times to range for V=7.3 sec, B=20sec, and I=100sec.

    I experimeted with using a string of 6 small incandescent bulbs without the white LEDs. The bulbs have an orange hue. The integration times are V = 6sec, B=60sec and I=0.6sec. I used a dimmer switch and the I was obtained on the lowest setting and the V and B on the brightest setting.

    I'll try to mix and match the bulbs with the LEDs to get a more balanced integration times, and see about replacing several of the orangish bulbs with white incandescent bulbs.

    I've read about trying to get exposures between 2 sec and 10 sec, presumably to balance noise and shutter effect. Is there a shutter effect when taking flats with the ST-402 using short exposures that would create artificial light gradients in the flat? Thank you and best regards.


HQA's picture
shutter times

Hi Mike,

You are seeing the difficulty in getting a good light source!  The white LEDs have very little output at Ic, which is why the exposure is so long.  The incadescent bulbs have very little output at B, which is why their exposure is so long.  My usual solution is to add more incadescent bulbs in another string of lights, and then use the computer to turn on one strand for V/Rc/Ic, and two strands for B.  That gets the exposure in the blue down to a reasonable time.

Regarding shutter effects: the ST-402 has a "photometric" shutter, which is actually the rotating filter wheel if I remember correctly.  This means it has a short exposure limit somewhere around 0.2sec, but anything longer than that, the exposure across the field is uniform.  I would consider exposure times in the 1-2second range to be fine with such a setup.  The ST-402 has only a single stage of cooling and therefore has higher dark current than other cameras, so I'd try to increase the light output so that you can get reasonable exposures in all filters within 10-20sec rather than 60-100sec.


WBY's picture
The attached article on dome

The attached article on dome flats has interesting information on using LEDs as light sources.  I had trouble using while LEDs and ended up going to halogens. I found some small halogen "pancake" lhalogen fixtures with 20 watt bulbs at home depot. I Took off the UV filter glass thinking that would help the blue end of the spectrum, based on the attached spectrum of a supposedly typical naked halogen desklamp bulb.  I am not 100% sure if that was the right thing to do. It seems to work well, however.

I installed the pancake fixtures on foam blocks cut on compound anglesin the corners of the light box near the opening end pointing at the closed end of the box. I can get exposures in blue with the lamps turned up almost all the way in 10 seconds or less for my old ST7E. 

I use a smart home programmable dimmer that I can set multple "scenes" one for each filter. Of course as the lamps heat up the light output changes so I generally let the lamps warm up for 5 minutes or so before starting flats. I generally start with the Blues at high lamp power and work my way down the list. 

Another thing I did to check flatness is to take ten flats, rotate the box by 90 degrees take 10 more and repeat for a total of 4 sets. Average each of the 4 sets and compare normalized pixel values between the averaged images over a 12x12 matrix of pixel positions at the same locations in every one. You should get the same (certainly less than 0.1% different among the extremes of the values at the same location among the four averaged images. You have to do this with normalized values because you may have overall variations in light output between images depending on constancy of your dimmer, changes in lamp temperature, line voltage and other probably other factors I haven't thought aboout.  Most household type electronic dimmers work on the principal of time averaging on and off to get areduced illumination level, and that averaging operation can be affected by temperature variation. A variac control is much more accurate and repeatable, BUT unless it is equipped with a stepper motor, which multiplies the cost of the variac by about 5X not including the electronic controls, you have to manually set the dial for each filter. There is no problem doing that unless you are trying to remote control your installation.  Even If you are operating manually,  the Smart Home controller is inexpensive compared to a manual variac but you do get significantly more variation in overall illumination among exposures. 

I haven't done it yet, but I have thought about putting a slit in front of the camera and putting my Staranalyzer 100 grating in the filterwheel to see if I can get spectra of the light box at the various dimmer settings. It would be intersting to see What the spectrum is and how it varies with dimmer setting, if I get anything useful at all. I will have to do some sort of eyepiece projection to get  the slit in focus, but it might work. 

Brad Walter

WGR's picture

Does anyone know a substitute source for the Souriscreen mentioned in this Paper.  Tried to find them a year ago, and seems like they went out of this business.  Any proven substitutes?



WBY's picture
Substitute for Souriscreen

See Attached article about mixing your own Barium Sulfate paint. Also at the CCD school last summer, ARNE advised that the foil backed foam insulation with the foil removed to expose the rough white foam worked well. I suppose that you could paint the reflective circle with some of this white paint. You may remember he commented that paint applied to create the black border interacted with the white foam and ate into it somewhat. Therefore, I would try whatever paint you plan to use as a base on a sample first. If it is water based latex it may not interact. 

As an alternate to using black paint as a non reflective border, I suggest cutting out some black funny or funky foam sheet to form a circle of the required size, you can get it at Walmart or Hobby Lobby and gluing it onto the foam with the glue that is made for arts and crafts with foam. alternately you could make a valence with a circular hole the correct size spaced in front of the reflective surface. Face that with the same black non-reflective foam and put your lights behind that pointing at the reflective surface rather than mounting them on the telescope. that way you don't have "Christmas LEDs strung around the front of the OTA. 

This latter method is what I plan to use in my recently upgraded "telescope shed to avoid the need for mounting a light box over the top of the telescope. I can go to park and let the flat sequence go on its own while I sleep. I really hate taking flats in the wee hours of the morning when I am "stupid tired. " I mess up the flats and have to leave the camera on in the Texas humidity until the following night which isn't very good for my glass J-C filters. The B and V in particular tend to absorb moisture and get crusty from the edges in even though the edges are sealed. My new 50 mm filters will be the Astrodon interference type to avoid this problem as well as increase B throughput and get a camera independent cutoff on the long end of the Ic filter. 


Brad Walter, WBY

GFB's picture
Re: shutter effects...

Hi Mike,

As Arne noted, there isn't really a shutter in the ST-402, it uses solid areas between filters on the filter wheel to act as a shutter.  That means the detector is covered and uncovered uniformly as the blank area swipes across the detector, rather than an iris type shutter that covers the central portion of a frame last.


Another test you might consider to check the uniformity of your light box flats is to take a few twilight T-shirt flats one evening for comparison.  Between 15-30 minutes after sundown, put a single layer of a T-shirt over the scope and point it about 15 degrees east of the zenith.  Take a few frames to test for saturation until you can get a nice unsaturated frame about 2 seconds long.  Just do it in V for your comparison.  Compare this T-shirt flat to the V flat that you took with the light box.  Are you happy with the similarities or differences?


I've found that twilight flats give a nice uniform illumination and in this case it's absolutely a free test to try!


Bill Goff

Thank you

Thank you all for the comments. I appreciate them. Best regards.



HQA's picture

As Gary mentions, the Canadian company that used to produce this (expensive) material appears to have gone out of business.  When I was looking into screen material, I considered projection screen material, as this tries to have laminar reflection.  Probably the glass bead type would be best.  You can often find projection screens at flea markets, as people try to sell off their old slide projectors.

Other than that, the options mentioned by Brad work.


STHO's picture
Flat Screens and "Styrofoam"

As Arne mentioned previously, we test out our flat field system for APASS using a couple of different methods. Obviously our final choice was based upon the overall best results but for the money and time needed to create a flat screen I (we) found that the common "Styrofoam" insulation sheets from the local hardware store worked pretty darn good and had results that rivaled the more expensive painted targets that we currently use. The trick was to peal off the silver-looking reflective sheet from the smooth side of the "Styrofoam" sheet and then to mask off a circle that would fully contain the light supplied. It was true that the flat black paint I used etched or ate the "Styrofoam" a bit but it didn't affect the reflective target areas.

We use "grains of wheat" tungsten bulbs powered from a fully regulated 12 Volts DC power supply controlled in various banks or strings of light made either into parallel strings for some banks or series strings for others. I use the Phidgets 8/8/8 USB IO device to toggle relays that then power the lights in the desired configurations while taking the flats in the 5 colored filters (B,V, g', r', i'). One of the key things we found in this process was the distance that the flat screen (target) is from the OTA opening is pretty critical in getting a nicely illuminated target. We did affix the "grains of wheat" bulbs to a short flat black dew shield exterior and mount then just behind the OTA aperture so we didn't get any direct light into the OTA but all came from the reflected illuminated targets.


For just a couple of bucks you can build the flat targets and for the cost of a regulated power supply (not a big one just one capable of handling the 2 or so Amps to drive the bulbs) and of course the "grain of wheat" tungsten bulbs (available from Edmond Scientific) you can put together a really nice flat fielding system.

We published a paper and I gave a presentation at the Society for Astronomical Sciences (SAS) two years ago on this exact system. The paper is part of the SAS proceedings.

What I think is of major importance is getting a nicely illuminated flat target with repeatable results and one that is automation-capable for remote operations like APASS.


Thomas C. Smith

Dark Ridge Observatory

Weed, New Mexico

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