Let there be affordable Light Box's!

[netwolf]

Hi All,

I came accross this website when looking for Qhy8 shots and found this chap is using a nice and cheap Light Box solutions that seems to work well for him.

http://astroanarchy.blogspot.com/search/label/equipmentshttp://astroanarchy.blogspot.com/search/label/equipments
Be Warned his Images may make you buy expensive toys. No seriously this guy has taken some fantastic images with the Qhy8 and Canon and Tokina Lens's.

Lightbox from.
http://www.posterpoweruk.co.uk/page5.htm

Anyone know a source for this in Australia?



Regards
Fahim

[[1ponders]]

Interesting. I'd like to see the spectrum, could be a bit light on in the red, but looks like a great idea.

[RB]

I agree with Paul, looks very interesting indeed.
Nice find, Fahim.

Will check out if available in Oz.

[Tandum]

http://www.elaust.com.au/ looks like them.

[Ian Robinson]

We all have LCD flat panel screens .... some monsters 24" and bigger.

Why not clean up the computer screen , no dust of muck on it , and the open up something like MS Paint and create a full screen sized object that is as pure white as you can get and use that as the "light box" to get the custom white balance (in a darkened room) ? (would the screen luminance be sufficient ?)

Or alternatively get hold of nice cheap whiteboard and take that outside on a sunny day and use that to get the custom white balance ?

[Tandum]

Because you really need to take the flats at the time you are taking the images. I have already been caught out using flats from a previous shoot which have not resolved issues from the current shoot. That won't happen again, I'm not wasting another night imaging. I always run a set of flats before I packup, it takes no time at all with a light box.

A Flat removes imperfections in the imaging train. It is not for CWB.

[RB]

Ian you don't use a light box for Custom White Balance.
Flats are used to remove imperfections in the optical train that you're imaging through, like dust spots and vignetting.

[AlexN]

Also, here... http://www.elec2go.com.au/category7_1.htm

A3 electroluminescent sheets are $150AUD If your scope requires more area than A3, then move to A2... however, the prices get silly above A3..

For those who image with smaller scopes, the A4 sheet at $75 would be perfect. frame it somehow to keep it flat, sit it on the aperture of the scope and run of 20~30 flats... I would assume you could modify them to run of 12v power too... :)

I'll be ordering an A3 sheet in the next few days.. :)

[[1ponders]]

How would the brightness be controlled? Fully illuminated looks wayyyyy tooo bight.

[AlexN]

take a shorter exposure? cover it with a white sheet?

[Ian Robinson]

Ah .... I see , you use them to a means to compensate for vignetting .

If you are using the same scope all the time , you need a set per imaging session .... not started using my DSLR for astroimaging yet .... got a bit to learn I see.

[Ian Robinson]

If you don't ask (dumb questions) you never learn ... I think I see now.

Those electroluminescent sheets look pretty spiffy , would be nice if someone did a spectral response on one .... to find out how white they actually are.

[AlexN]

For anyone interested, the guy Fahim referred to and linked to got an APOD a few days ago with that lens + QHY8 and using that thing for flats.. So it must work... I just need that el sheet, and I have everything he used to get an APOD... so I'll have no excuses (Damn..)

[[1ponders]]

Exposures shouldn't be too short Alex. 2-10 sec is what is usually recommended, though lots of people use shorter. I suppose the old T shirt trick could be used with it.

[h0ughy]

lol ordered the A3 and the customiseable one for 33 dollars - all work off 12v -- should have them monday

[AlexN]

sweet houghy, make sure to let us all know how it goes..

[h0ughy]

i am not the only one, i know of another who ordered and will receive exactly the same as me------> it shall remain nameless unless he fesses up here:whistle:

[Tandum]

Same here, at $33 that's a real plus for us, so long as they are balanced white.

[Octane]

I have a sneaky suspicion I know who it is as I was conversing with them back and forth at work today. :whistle:

This, just after I went out and bought a whole heap of goodies to make my own light box! :)

Regards,
Humayun

[RB]

LOL, poor Mandy from elect2go told me yesterday she didn't know what hit her.
I had already ordered mine and she asked what I was going to use it for so I told her.

She said afterwards when I called her back to confirm the order that she was inundated by orders, most were for astro use. :lol:
I told her their details were posted on the forum so Mandy said she'll be stocking up on this product. Hope it works. :whistle:

Mine should arrive soon. If it doesn't work, I can use it for something else I have in mind so all is good.

:thumbsup:

[bluescope]

Hi Dave

Do you mean you bought an A3 sheet and the custom sizeable sheet both for $33 ?

:thumbsup:

[h0ughy]

no the A3 sheet is 150, the customisable one is 33 - dont get your hopes up LOL:help:

[AlexN]

I ordered the customisable one for now.. I'll see how well it works on the APO's if its all good, I'll pay up for the A3 one for the C11

[bluescope]

That's what I thought Dave ....

Looks like a great techno gizmo that could be perfect due to it's light construction and low power needs etc. Would be easy to fashion a custom built frame for a scope just like a solar filter.

:thumbsup:

[monoxide]

nice!
be sure to post some flats guys :)

[RB]

Will do, as soon as we get a chance.
I'm very interested to see how bright these are.
The cuttable version would be ideal for use on camera lenses when doing widefields (flats).

[monoxide]

im interested in the exposure times too, although i do have a gadget here for my QHY8 when running off a battery to dial the voltage down, mabee it could be used to adjust the brightness of the screen?

[AlexN]

apparently they can be run from a 9v battery for around 3hrs, this lowers their brightness.. so yes, a voltage regulator would be great

[netwolf]

Wow this thread really took off and good going finding a local source Alex.
I too intially found this chap through the QHY Site and seems he has made it to APOD and another on SkyantTelescopes editors Choice . So i thought he must to know what is doing, and wanted to look at his equipment list and thats when i found what he was usign for flats. Once i get my act together and get back out there i will certainly be looking at getting one of these.

Have a look at his APOD work
http://apod.nasa.gov/apod/ap081107.html
This was done with a Tokina 300mm f2.8 lens.

[AlexN]

Fahim, I must admit, I didnt find the local source... :) It was Hagar (Doug) who pointed me to it 1 day before you posted this thread... I was stoked to think that for $33 I could start taking flats, and better my images...

I'll be sure to post up a few flats as soon as it arrives.

J-P's spent a hell of a lot of time on his setup, and its paid of brilliantly.. The 300mm F/2.8 lens is perfect for what he needs, and at F/2.8, shooting 20 minute subs... The man goes DEEP! :) The first time I looked at the pics on his site and saw the light board, I didnt read the text right, and assumed he back lit the white sheet with a difuse light source...

[Hagar]

Hi Guys, Just found this thread. I have ordered an A3 with inverter and 12V socket. Cost $140 + GST + Post from a source in Melbourne www.elau.com.au (http://www.elau.com.au) I have spent quite some time looking into to this item and had many email discussions with users in Europe. It appears the true colour of the light source is almost irrelevant. The real power in the sheet is the fact that the light source is of equal intensity accross the entire surface. The European users all seem to be of the opinion that these sheets are bu far the greatest light source for flats any of them have ever used.
I may be wrong but I am under the impression that flats are converted into monochrome images when being setup to make a master flat frame for calibration. Most software also carries out a few other processes on the base images to soften the image prior to combining. If the intensity of the sheet is uniform the final image created will be much better than say a sky flat which will always exhibit a slight gradient accross the field due to small changes in the earth/sun position.
I was actually waiting to firstly make the purchase and test the sheets before starting a thread. Looks like Fahim is following the same tak as me.

All I can say is if the results I get are as good as the European results I will be over the moon and will have a ready source of flat frames which can be rolled uo and stored in a post tube or fitted to a flat panel with double sided tape and will run on 12 volts at a fraction of the cost of custom made light boxes and probably give a more even illuminated flat frame.

I will post some images as soon as it arrives.

There are quite a few suppliers in Australia so you should be able to find one in your own area. Just google "Electro Luminous sheets"

[citivolus]

My biggest concern with these is that they are driven by (relatively) high voltage AC. Combine that with dew & moist ground and you could get a bit of a buzz when you bump your scope, or it could potentially feed back through your mount electronics. Make sure you isolate your terminals well!

[h0ughy]

12 VOLTS WITH A CIG LIGHTER SOCKET?:shrug:

[AlexN]

they can be powered through 12v Ric... As a matter of fact, the small customizable one only comes with 12v power option.. The bigger ones come with the choice of either..

[MrB]

Yes, but the 12v is bumped up with an inverter unit to hv ac.
Having said that, I can't see any problem with it, as long as everything is well sealed/insulated.

[RB]

Also, it doesn't take that long to take flats, so I don't see dew being too much of a problem especially if the unit is placed in a box enclosure of some sort.
Normal precautions should suffice.

:thumbsup:

[AlexN]

yeah, my small one will be laminated, the placed into a plastic frame of some sort, I'll silicone up the area where the cables come out of it, and bobs your uncle!

[RB]

Hopefully they'll arrive in the next day or two, can't wait to see if they'll perform well for flats.

:thumbsup:

[AlexN]

indeed! (will be my first time taking flats! fingers crossed it works well

[JohnH]

Any reason why a flat needs exposures to be 2-10s long? I must admit to using my laptop screen set to display a pure white 1024*768 image to take flats. So long as focus and imaging train are not adjusted it seems to work ok from nigh to night...my exposures are typically .05 secs though!

[RB]

OK my package just arrived from Elec2go.
They look terrific and I think will serve this purpose well !!

The first pic is of the A3 sheet, it looks pink when it's 'off'.
The second pic is what it looks like when it's 'on' in the dark.

I then took a CWB (Custom White Balance) of the sheet and applied the CWB to this quick test flat frame using the 5D and the 24-105mm L set at 24mm f4 and ISO100 for a 1/25th sec exp.

The last shot was of the cuttable version when it's on.
I'll be using this for another purpose.
(BTW this shot was with the CWB on).

Can't wait to test it with the Tak and build up my light box now.

[Hagar]

Hi Guys, Mine arrived today. Quickly shot a serries of flats and made a master. I didn't bother to clean the CCD of the QHY8 which needed a clean.
The info below is from one of the FITS headers. The final Master is a combination of 20 such files combined in IP3.


SIMPLE = T / file does conform to FITS standard
BITPIX = 16 / number of bits per data pixel
NAXIS = 2 / number of data axes
NAXIS1 = 3040 / length of data axis 1
NAXIS2 = 2016 / length of data axis 2
EXTEND = T / FITS dataset may contain extensions
COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H
BZERO = 32768 / offset data range to that of unsigned short
BSCALE = 1 / default scaling factor
CREATOR = 'Nebulosity v2.0.3' / program and version that created this file
INSTRUME = 'QHY8 ' / instrument name
DATE = '2008-11-17 01:09:16' / UTC date that FITS file was created
XPIXSZ = 7.8 / X pixel size microns
YPIXSZ = 7.8 / Y pixel size microns
EXPOSURE = 0.6 / Exposure time [s]
GAIN = 32 / Camera gain
OFFSET = 104 / Camera offset
BIN_MODE = 1 / Camera binning mode
ARRAY_TY = 1 / CCD array type
XBINNING = 1 / Camera binning mode
YBINNING = 1 / Camera binning mode
EXPTIME = 0.6 / Exposure time [s]
INPUTFMT = 'FITS ' / Format of file from which image was read

At this stage I havent been able to try it on an actual image but hopefully tonight.

[AlexN]

Looks good Doug... Doesnt look like your camera needs a clean... Theres only really 2 spots on that (both of which look to be on the camera, although its always hard to tell) but they are small...

[h0ughy]

LOL mine arrived will play with it later

[h0ughy]

So the second one must be for that underwear fetish you have:whistle: nice an customisable - - is there an anniversary coming up soon RB:lol:

[AlexN]

Doug - a quick question, do you shoot your flats at the same gain and offset as your images?

[Hagar]

Alex... Everything the same except the exposure duration. Trial and error seems to be the way to go with duration. Try to keep the histogram to the left of center and the readout for the center of image at about 20,000. I don't know how critical it is but I am going to explore the best or at least the theoretical best settings as posted by TerryB a while back.


This is the post. http://www.iceinspace.com.au/forum/showthread.php?t=37175
Doug.
As for the flats, the flat needs to be in the linear part of your sensor otherwise when you divide by them they will give an incorrect result. This was discussed on one of the photometry groups as the linearity is important for photometry.
Your sensor on the QHY8 is an antiblooming sensor and is probably not particularly linear above about 1/2 the saturation level.
You could try and measure the linearity by taking a series of images of the same star field with a bright star that will saturate the sensor and some dimmer ones that don't. Expose for increasing times in ~10 sec increments. ie 10,20,30,40,50 sec etc until you have saturated the bright star. You need to start with an exposure that doesn't saturate the stars.
You then measure the flux of a few stars in the images and graph the results against time.
This assumes that the stars you are imaging are not variable in the short term and that no clouds etc have stuffed up some of the images.
The results you get will look like the attached charts.
After doing this you can pick what intensity is linear for your CCD and keep the flats within that range.

Cheers
Attached Thumbnailshttp://www.iceinspace.com.au/forum/attachment.php?attachmentid=49314&stc=1&thumb=1&d=1224847547 (http://www.iceinspace.com.au/forum/attachment_browse.php?a=49314)37.5 KB7 views
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=49315&stc=1&thumb=1&d=1224847547 (http://www.iceinspace.com.au/forum/attachment_browse.php?a=49315)32.1 KB5 views
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=49316&stc=1&thumb=1&d=1224847547 (http://www.iceinspace.com.au/forum/attachment_browse.php?a=49316)37.7 KB5 views

[pvelez]

Does this mean that you still need to build a box to hold it? Or am I missing something?

Pete

[RB]

I was thinking of building something simple so I can have the EL sheet in a tidy frame, but I suppose I can just lay the sheet directly on the dew shield of the scope as a bare minimum.

I'll have to do some experimenting and see.

[pvelez]

Thanks Andrew - that was what I was expecting.

I have just built a lightbox (very dodgy construction but it does the job) - I am in danger of being seduced by the prospect of a simple illuminated screen.

Looking forward to updates on how it goes

Pete

[AlexN]

I'll just build a small frame similar to what people make for full aperture film solar filters... Seems simple enough, and will do the trick

[RB]

Will keep you updated Pete.



I'm thinking similar, something simple will suffice.

LOL Houghy :P

[Hagar]

I have just stuck mine on a suitably sized sheet of plywood with double sided mounting tape (Cloth type). I will eventually get some clear perspex to go over the top and edge it with plastic C section. Should do the job. I may even get rash and make a fabric protective bag to keep it in.

[Hagar]

I have finally done a few tests on the light panel. The test images consist of 2 X 10 minute images of NGC253 Calibrated with flat frames only. The flats were 5 images combined in IP3 with a median combine. IP3 was used for the full calibration and stacking process.
It is obvious that these still require a bit of work to sort out the best settings with the QHY8. I think I will have to experiment a lot more to get things correct but the panel does show promise.

The images were stretched in PS CS3 way more than normal to show the extent of the correction.

The shots in order are calibrated using the following calibration files.

0.001 sec, 0.005 sec, .01 sec, .05 sec

Your comments welcomed

[AlexN]

somewhere between .005 and .01 I think...

[Hagar]

Thanks for the feedback Alex. I have a bit more work to do and I an investigating the best settings for taking Flats with the EL Panel. I will post the results when I work it out.

[JohnH]

Have a look at:

AstroPhoto Insight Feb / Mar 2008 which you can find here:

http://www.astrophotoinsight.com

Quote follows:

"One way to test the quality of the flat field frame light source is to create a master flat field frame and create a second master flat field frame with the telescope or light
source rotated 90 degrees. The first master flat field frame is used as the test image
which is calibrated by the second rotated master flat field.

A good flat field calibrated test image is a uniform noisy image with no dark or light areas. A good test image histogram is a normal probability distribution. The normal probability distribution is caused by at least three types of noise that are in the master flat field frame and the test image. The noise sources are photon noise, dark current noise and readout noise. A nonuniform light source causes dark or light areas in the test image."

[Hagar]

Thanks for the comments John. I don't think the light source quality is the issue. I think the intensity of the flat frames is the item in question. The length or duration of the exposure is the hard thing to get right. Once this has been scoped for my camera I think the panel will produce excelent frames which correct abberations in the light frames and don't tend to add a few from an over exposed or under exposed flat frame.
I'll keep at it when the weather clears and I can find the linear exposure for this camera.

Thanks John I will look it up and see what they have to say. Interestingly the internet is full of information on flat frames but some directly opposes other information. Looks like trial and error is going to be the best method. At least for me.


[/LEFT]

[tornado33]

Hughy bought his electroluminiscent sheets round, heres 2 pics I took of the sheet, it gives an even bluish white glow. The histogram on the camera shows that there is plenty of red light present as well, so it would be good for flatfielding in any colour band. As a guide to its brightness the full frame view was with the camera lens slmost touching the sheet, F5.6, 1/60sec ISO400, with unmodded 400D, on Auto.
Scott

[RB]

I'll be putting the finishing touches on my lightbox later today.
I'm very happy with the result.
Pics to come later.

:)

[AlexN]

Sigh.. my small sheet arrived today, And due to my own error, it is not quite as wide as my main imaging scope (the Megrez 102) Although the dimensions on the elec2go site say its big enough, due to the tabs at the edges where the wires connect, it actually falls about 5mm shy of completely covering the objective... Looks like I'll have to design a box, use this to illuminate a diffuser (white translucent perspex) I Can only hope this will work... (Feel free to make your assumptions, inferences etc)

Else, Does anyone want one of these smaller cuttable sheets, never used? never out of its plastic slip? I may have to go straight to the bigger sheet for use with my C11 and the M102... Cheers.

Alex.

[Hagar]

Sorted out my flat fielding problems at last. It is not the EL sheet at fault.
A bit of research and I found out flat frames don't work very effectively with a QHY8 without using Bias files as well. I tried another exposure run using some slightly less aggressive settings for both the lights and flats and went on to shoot and calibrate with Flats and Bias shots and it worked, and it worked realy well. The final result was way better than I had expected.
I have been pleasantly surprised at the way this inexpensive and simple sheet works to improve my images.

[RB]

Yes that will work, it will illuminate the diffuser screen completely as long as you set the distance right.
A few inches will do.

I used a diffuser screen in my box and got a beautiful evenly lit screen.

[AlexN]

Doug : Good to hear its all sorted, and working beyond expectations!

Andrew : I had planned for ~3" (+/-.5") between diffuser and sheet... I'll get to building asap.

The box will also serve to protect the sheet from dust & dew..

[RB]

Ok, here's some pics of the lightbox I made using the A3 EL Panel mounted in a box made with balsa wood and foam-core.
LOL a bit crude but works brilliantly (pardon the pun).

I simply used hot glue and pins to assemble the box.
Also, I inserted 6 'diffuser screens' made from Drafting Film and A3 translucent cover binders.
These help diffuse the light and give an even glow right to the edge.

I then attached a tripod adapter to the box so I can mount the lightbox on a tripod and swing the scope into the box to take my flats.

The only thing left to do now is to attach a white shroud on the end of the box to enable any of my scopes and lenses to be put in place to take the flats.
I think this is an easier/simpler way than having individual diameter foam cutouts for each one.

I've also attached a master flat of 7 raw flats, taken with the 5D and the 24-70mm at 70mm f/2.8, ISO 100.

As I said, it's crude in it's construction, I didn't spend much time designing it so it's not exactly symmetrical, but it works perfectly.

1. On the tripod.
2. How it looks when lit (I used a custom White Balance).
3. Tripod adaptor under the box.
4. Master Flat (7 raw frames)

:)

[h0ughy]

looks good mr RB

[AlexN]

good indeed!! well done

[Ian Robinson]

Think I'll hold off until more people have used them and mad rush dries up.

[h0ughy]

suit yourself - demand may drive up prices - -remember these are probably imported and the existing stock was bought at older prices. BTW they are fantastic and you have absolutely no idea how good they really are until you have tried one. i have been looking for years for a good light box, i bought one when the US dollar was at 96 cents and it was a bargain - i had that custom made for the 127ED and could use it with cameras and standard lenses, but the stupid thing was I should have got the adapters at the time for the C8 and the meade 10" and the vixen R200SS but i didn't. to get that is a small fortune, but this is a gift of light, for next to nothing in comparative costs. a source of even constant light that you can choose to use at a moments notice- priceless. you add together the cost of all your gear, and the observatory you want to build and all the imaging gear you have then go and take astrophotos only to find everything goes pear shaped because you skimped out on getting a decent light box - get real! on a comparative basis this is chook feed (sorry RB), and it is the basic requirement for decent image processing. if your happy doing a few rough shots then processing them to reveal hideous vignetting or terrible crop circle of your final image then you hold off.... besides the government wants us to go out and spend, spend, spend and spend some more to keep the economy flowing----- so go on, make a move and spend a dollar and post some images, processed with a light box in tow:thumbsup: I have been keeping the economy going for years:whistle:

[AlexN]

Here Here!! :) Two enthusiastic thumbs up for keeping the economy alive! :)

[Octane]

Andrew,

That is wonderful! Beautiful flat frame all the way across. :)

I'm glad to see there's no dust bunnies on the 70-200. :)

Regards,
Humayun

[Ian Robinson]

Bugger the economy , it's never done me any favours .... like all new stuff ... they'll drop in price once the novalty wears off.

Got higher priorities - ie getting my roll-off observing shed done and then I'll think about getting a 70-200 f/2.8 L IS and doing something about restarting my astroimaging ... bit of luck the AUD will recover a bit by then - I'm in no hurry.

[Octane]

Good luck in buying your lens at the price you want to.

You better buy it now, whilst it's hovering at $2,460, before it goes up. Prices are set to rise in January and won't come down until at /least/ the end of next year.

How do I know? I've been keeping in touch with the manager of a retailer (in the process of ordering a Canon EOS-5D Mark II) who is also an avid photographer.

You can procrastinate and make all the excuses in the world for not buying this magnificent piece of kit for a very reasonable price (all up, building my own light box has come to very nearly $100 with all the tools and materials purchased), however, it will make /all/ the difference in the world for when it comes to processing your images. I have shot and processed all my images with flat frames since day dot, and, whilst I'm not in the league of Mike, Eddie, Peter, Greg, or Martin, I think my images can speak for themselves.

Regards,
Humayun

[Ian Robinson]

The likes of Harvey Norman "Discounts" , and their ilk and the camera shops want you to buy now , in panic to beat the "price rises" ....

I'm in no rush , and am prepared to wait as long as it takes .... and the money is not going anywhere and is nice and safe where it is. I reckon there may be some good bargains after Xmas and the AUD may just sneek back up a bit in the NY after Obama takes the reigns, and the stores find themselves stuck with a stack of stock they couldn't shift at "inflated prices" in the run up to Xmas. Yes - I'm a buyer who is on strike , and bugger the economy.

Just ordered 100" long roll of self adhesive flocking material - once that turns up I'll ready to rebuild the 10" newt OTA , then I'll do my permanent concrete pier , and my deck or slab in readiness to build the SORO.

In the meantime no major expendatures are on the agenda.

[Octane]

You're wrong.

I'm not dealing with Harvey Norman, even though, I have bought from them in the past, and they have price-matched Internet prices for me.

So, if you were not going to buy a big ticket item from a retailer such as Harvey Norman, who are you going to buy from?

Retailers have purchasing contracts with big vendors which usually are of a six month duration. The price of consumer electronics is set to rise as the current batch of goods that is being sold is being sold on the basis of having been purchased by the retailers prior to the economic crisis.

This same retailer has price matched the Canon EF 70-200mm f/2.8L IS USM, and even dropped a further $200 off the cost, as compared to a popular grey importer.

So, yes, I'm sure he has his own vested interests in this sale and isn't looking out for me, as you've suggested. :screwy:

Regards,
Humayun

[multiweb]

Here's my lightbox. EL sheet wasn't cheap but all the rest is off the shelf stuff. Nice and compact, almost "pocket size" :)

http://www.multiweb.com.au/astro/lightbox.jpg

[h0ughy]

interesting use of the photo frame. any other images of the actual light box ?:D

[multiweb]

:lol: nope. That's it. Light enough to stick on the scope and still keep on trackin'. My better half still hasn't noticed the wedding picture is missing a frame :whistle: Just kidding...

[peter_4059]

I picked up a cuttable sheet but unfortunately there appears to be a manufacturing defect within the laminate which shows up as a small dark area so probably no good for flats on its own. Might experiment with a diffuser. :(

[RB]

hmmm that's unfortunate.
My cuttable version is fine.

If you use a diffuser it will elevate the prob, I'm sure.

[AlexN]

Peter: Have you tried giving it a clean? with something like spray & wipe or the like? Might just be some greasy/dirty substance?

[peter_4059]

It's inside the laminate - like a dark patch - possibly some dust.

[multiweb]

If it comes to worst you could rotate your light box when taking flats. The dark patch will go when combining the master flat.