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I have a Celestron Edge HD 8 inch. I am beginning to get my head around taking images. I can understand why there is a need for Dark and/or Bias frames but flat frames are still a mystery. Does my Edge HD need them when taking images?

Still sorting the (new to me), Atik 16200 imaging train as I try to shift from my trusty SBIG 8300, Mac to PC for mount control/capture and from a separate guide scope to the Atik OAG.... The camera needs to go back to Atik (awaiting the email from Vince) as there is dust inside the chamber, so this is a good time to get everything checked - ready for the autumn season. After some imaging/testing time at the rear of Leo in the last week, I noticed on my flat frames a strange half moon light - by the dust mote (that was over-correcting the lights). Eventually I worked out it was the screws surrounding the sensor cover window (or the 3x rounded cap screws that attach the EFW3), bouncing the light onto the back of the filter (Baader L in this example) and I suppose onto the cover window and onto the sensor. To test the theory, I opened the imaging train up and added a Sharpie pen to them. Couldn't get into the cross-heads with the pen, but with re-testing, the reflection had gone! Perhaps it would never be an issue with the actual light frames, but you never know with a bright star in the frame of a future target...? So today, after shifting slightly outwards the OAG stalk, I addressed the stainless steel screws 'properly', by (again), taking everything apart and lightly painting a cover of matt black acrylic paint over them and into the x-heads (a bit of overspill), nothing too heavy-handed as I didn't want to glue the things in with paint! The finished effect is duller than the pic here and the reflection has gone after another round of testing. Always something to catch us out, hey!? Why Atik can't use black screws is another matter..... Perhaps this may help others out at some stage.... Damian

Hi all, I've run into a bit of a strange instance when calibrating my OSC subs using my master flat generated from a white tablet screen, especially with my L-eXtreme filter. One corner seems to overcorrect using these flats, however, when I use skyflats I don't see this overcorrection and the end result looks pretty good. Whilst the answer is clearly "take skyflats", I'm not always fortunate enough to take flats first thing in the morning (weather wise, it is the UK!) so it would be great to understand what's going on with my tablet and how to avoid this issue. Details are below, but my questions are: How can I prevent over-correction with tablet flats? Am I over-exposing them? If I change my light source, will I see this same issue if using an EL panel (e.g. like this one )or a bespoke flat field generator (like those provided by Geoptik, Pegasus Astro etc.) Is there anything in the PI WBPP 2.0 script which can help identify overcorrection and adjust the amount of correction applied? Acquisition For the tablet method, I capture my flats using the auto-exposure tool in my ASI Air Pro (it calculates the optimum exposure time based on the light source presented). The light source is my tablet display at 100% brightness, no blue-light/night time filter, and displaying this blank, white page provided by Covington on the screen. The display is large enough to cover the entirey of my Redcat 51. I attach 2 layers of t-shirt to the front of the telescope and the camera used is an ASI533. It's a small sensor, so I wouldn't expect to see a significant amount of vignetting compared to a full frame camera. For the skyflats, I used the same approach but used 4 layers of t-shirt isntead of 2 to ensure I don't end up with too short of an exposure time. The exposure time has been adjusted to ensure the average value of the individual frame is ~30,000 ADU for a 14-bit image. For the tablet flats, this results in a 7 second exposure using 2 layers of t-shirt. For sky flats (overcast day), this results in 180ms flats using 4 layers of t-shirt to ensure the exposure time wasn't too low. The optical train had not been disturbed in this time, and both approximate exposures were determined by the ASI Air Pro auto-expose feature (I rounded them up when shooting). There were 30 x flats for each case. The tablet flats were taken at midnight, the skyflats were taken in the morning. Other than this, there was no difference in imaging conditions. Processing All stacking is performed in PixInsight using the WBPP 2.0 script. All settings under flat are left at default settings. The script was executed twice: the first was using tablet flats, the second was using skyflats. No other changes were made in the stacking process. Flat Images and MasterLights Below are four screenshot images. The top row shows debayered master flats (purely for illlustration purposes), the bottom row shows the final stack. The left side shows the tablet flats, the right side shows the skyflats. The statistics are from the master flat, pre-debayered, and show there there is a small sifference in mean values. However, the images show much brighter corners for the tablet flats, especially in the lower left where I see over-correction. You can see this clearly in the master light images. Looking at the master light which was calibrated using tablet flats, you can see a red cast over the lower left corner which I suspect is overcorrection from the flat. However, you do not see this on the master light which was calibrated using skyflats, the background looks fairly uniform.

Hi all. I've created my own darks library for my ASI1600mm Pro. Various gains and various exposure times all at -10 Celsius. I'm just wondering what people do with them? Once you've taken darks and processed an image you get your master darks. Do you guys just keep the master dark, in a master darks library, or do you process your darks again each time you stack a new image? I'm just thinking to save storage, could I discard the dark sub frames and just keep the masters? I don't want to throw them away if that is wrong though. Also, with this camera I take dark flats rather than bias frames. Because the scope is covered, do I need to do dark flats each time I take flats? I have an even illuminated flat box which I always use at the same exposure lengths. I guess the only thing that could vary is the gain value. I understand the need for flats every time you change focus, for each filter etc., but if I'm using the same exposure time can I just use a previous set of dark flats as long as the camera settings are the same? If this is possible, can I then create a dark flats library for any difference in gain values? Not sure if I gain values affect dark flats though. I've read that back, and that's a lot of questions! All help greatly received.

Hello, when applying the flats taken in my last session (to find out what is causing the strange diffraction spikes) with Siril, the final stacked result still shows the vignetting and the dust spots. I also did the whole preprocessing with Nebulosity, same result. I took the flats as follows: same iso as my subs camera and focus not touched I use a homemade flatbox combined with the a white t-shirt with Ekos took test shots till the histogram was half-way to the left checked all my flats, they all show vignetting and the same dust spots as in my subs I tried using them with and without using a bias frame, same result, the final result looks as if no flats were used. Anybody any idea what is going on? An other question I have, will the vignetting and dust spots also show in the master flat (flats stacked)? Thanks for your help, AstroRookie

Hey everyone, Over the past few days i've been gathering data on M31 due to the battery not lasting long and andomeda dissapearing behind a tree, therefore the mulitple imaging sessions. So far i've been out and me being me, only took dark, bias and light frames for the first 2 sessions but for the last one i also included flats... (The lights all have very slightly different settings cause i have been experimenting slightly... that being 45sec @ ISO 400, 45sec @ ISO 200 and 50sec @ ISO 100) Now DSS assinged all the correct bias and dark frames to the corresponding light frames but because i only have flats for my last imaging session it applied those to all the different light frames and not just to the last set...(and due to me moving the telescope / taking the camera off, the dust spots have obviously been moving around and that therefor dont work at all for the other light frames...) So my question: Is there any way in DSS to apply flats to only one set of light frames and if not are there any other apps with which i can do this with? Many Thanks!

Hello, i am having so many difficulties as far as i remember to make some nice flats to match my images.For some reason it it impossible for me to find the secret spot in time and adu for my Flats. This is my equipment : Telescope: Redcat 51 Camera: Atik 383l+ mono (i know the shutter problem that's why i always aim for 3sec and above for taking my flats) Filter Wheel: Atik EFW2 (36mm) Filters: Astrodon Ha & Oiii 36mm 3nm FlatBox: Geoptic flat field generator 164mm (dimmable) Image aquire software: APT Tool v3.71 Software: Astro Pixel Processor This is what i get...i am uploading some images converted in jpeg for the site... Photo A is a MASTER FLAT Ha (Bin 1x1) created with 30 Darks, 50 FLats (3.7sec adu 22000), 50 Dark Flats, 100 Bias Photo B is a MASTER FLAT Oiii (Bin 2x2) created with 30 Darks, 50 FLats (5sec adu 46000), 50 Dark FLats, 100 Bias Photo C is an example result of stacking my fits of Helix Nebula at Bin 2x2 , with the above calibration master frames (this is the Oiii in this case) Are my calibration frames Over correcting ? if yes what can i do ? Is the Optical path problematic ? as i can see at photo A ? what can i do , what to check ? I have checked also the filter wheel position related to filter change , but it comes centered...i couldn't see a fault on it. i Believe i have a problem also finding the correct ADU value....i made any experiments by changing the adu value and i have tried at 22000, 32000, 47000 by using the CCD Flats Aid.......but nothing all come out the same. I hope someone can help cause i am loosing my mind and my fits

Hi All, I am posting what I suspect is a newbie mistake question but hoping that someone can assist with the issue of flats. Although I have been fumbling around the sky, taking snaps at leisure, recently I became serious. I have read up about the different calibration files (flats, darks, bias) and they seemed to make sense; different ways to capture the image defects and extract those from the image of the sky. After a few weeks (months) of further fumbling I went back to the very first target to receive my attention, M42 Orion Nebula. In short, I took 20x 30s exposures in LRGB and ran these along with 20x LRGB each of darks, bias and flats. To obtain the flats I used a diffuse sheet of perspex (lightbox material) and an LED video lamp that has 180 white LEDs, turned to its lowest setting. Attached below is the stacked Luminance flat and the light image. In the lights I am getting very strong marks from dust and I had thought that the flats would subtract this but looking at the flats the marks are completely different shapes and do nothing to remove them from the lights. The attached has been further stretched to show the issue. Now, I am obviously doing something wrong but I have no idea what, any pointers from the vast pool of knowledge will be much appreciated. Thanks, Anthony

I've finally got around to making my flats box. I decided to go for a cylinder rather than the normal square as I thought it would maximize the amount of reflected light and limit any 'dead' areas. I could also use the Celestrons dust cap retaining pins to lock the flats box onto the 'scope. I purchased some of the craft board that has a thin foam sheet sandwiched between two sheets of thick paper/thin card. In order to bend the card into a cylinder, I creased the board every 20 mm by pressing the edge of a steel ruler into the board. It took two of the sheets to make a cylinder big enough to fit my C9.25, with only a couple of cm trimmed off. I then made a reinforcing ring/defuser holder from two strips of the foam board; this time creasing them at 15mm intervals. I stuck these level to the bottom edge so the joins were 90° to the main cylinder joins. These strips were cut wide enough to ensure that the diffuser cleared the secondary housing. The cylinder was designed to lock into the C9.25s dust cap retaining pins so next I cut two keyways into the bottom outer side. They looked a little weak so I reinforced them with some Christmas chocolate reindeer plastic packaging! Although the foamboard is quite shiny, I wasn't happy with all the grooves, so I lined the inside with white A4 paper. The Perspex sheet was cut to shape and hot glued into place onto the ledge. Next, starting at the top, I notched the edge of the cylinder to run the LED string lights cable through and then started to spiral the LEDs around and down the cylinder. The top cap/reflector was made from two discs of foamboard. One to go inside the cylinder and one to sit proud of the edge. They were glued together before being hot glued onto the top of the cylinder. The LED light string that I bought has an integrated on/off button as well as both up and down brightness buttons with a 3M sticky pad on the back, so I stuck this to the top cap. As I had previously made myself a 'scope mounted power distribution box with aircraft sockets for power, I removed the 3 pin UK plug/ac-dc converter and soldered on an aircraft plug to match my 12 volt DC supply socket. The lightbox illuminated.

Hi All, I have recently converted from an old Canon DSLR to a ZWO ASI1600MC-Pro. The individual subs look great with the new camera, but I am having seemingly insoluble problems processing, so I need the assembled wisdom here! The first image shows a single sub on the left (auto-stretched), and the result of stacking some 25 subs on the right, combined with flats. The flats have corrected quite nicely for the dust doughnuts, but given me inverted vignetting. (I should say that I have tried processing with darks and bias as well, but the basic problem of the reversed vignetting remains. Processing was done in Deep Sky Stacker, although I get the same results using PI and the BPP script). I initially thought this was down to using the wrong ADU values for the flats, so I've tried various from 5,000 up to 30,000 in 5,000 increments, but nothing improves - the higher the ADU the worse the effect seems to get. I've also tried different techniques for taking the flats - grey skies (easy at the moment) with t-shirt layers in front, or a tablet screen on white. Flats are seemingly ok - here is an individual sub on the left and master flat on the right. OK, they are rather brownish, but apart from that they look ok to my (DSLR-attuned) eye. Different ADU values seem to fail to correct for the dust doughnuts, as well as making the vignetting worse. I have one further thought / problem to add. The over-corrected centre of that processed image (top, right-hand side) shows some odd details. Here's a zoomed version. There are fairly broad swirly lines showing up in there. Actually the lines are across the whole image, but most visible in the centre where the vignetting shows it up. My only thoughts are that this could be condensation or similar? Might this account for the flats not correcting properly? As I said I'm using the ASI1600MC-Pro, and have it set to cool to -20C for all lights, darks and flats. I've not encountered a problem like this with flats failing to work so miserably, so I'm after any and all knowledge at this point. To a fair extent I can take out the vignetting with ABE/DBE, but that then leaves me the dust doughnuts. I really don't want to have to 'edit' my pictures to that extent, and feel that the flats should take care of the vignetting. Thanks for looking. Phil

Hi all While I've been largely locked under clouds for a couple of months, I thought I'd write some software that I've been meaning to get around to for a while. I use a standard LCD computer monitor for taking flats. I plug it in to my imaging laptop. I started off using a blank HTML page in a full-screen browser window to provide the uniform grey colour on the display but I needed to vary the brightness of the display for different filters. So, I wrote some software that does it. The software is called Second Monitor Lightbox and is available free of charge from my site (paw-print.com). If you use a CCD with filter wheel, it'll link to the filter wheel via the ASCOM driver so that, once you've set the perfect brightness for each filter, it will change brightness automatically as the filter changes. It also has a 'preset' mode for non-ASCOM set ups so that you can just re-call a pre-determined brightness to match a certain configuration (filter, binning, etc). There's a tutorial based around using SGP Pro on the site, so if you want to try it have a look through that. When you install it, Windows will warn you that it's from an Unknown Publisher. This is just because I don't pay Microsoft an annual fee for a developer's certificate. Just tell Windows to install it anyway - you may need to click 'More information' to get the option to go ahead. Let me know what you think. I hope some of you will find it useful. I know it saves me a load of hassle at the end of a long session trying to get the right brightness level to give decent flats for each filter. You can get it from here: http://paw-print.com Steve

So after Carole helped me to realise that I was taking my flats wrong, I've reprocessed this image and finally the funny rings have vanished. Original thread TL;DR - I was underexposing my flats, which resulted in failure of calibration to remove vignetting, as well as it introducing weird red concentric oval rings. Last night was supposed to be clear, so I set up in the garden. Cue the Irish weather. I only managed to get PA and focus (you know, the most fun bits) and then the clouds rolled in. Anyway, so instead of imaging, I took a bunch of calibration frames. The setup was slightly different to the lights - 1) temp 4 C instead of -1.5 C (no big deal) 2) camera at different angle to telescope (can't correct dust motes) 3) new camera angle adjuster in path (different vignetting and internal reflections) so I didn't hold out much hope that it would correct the problem. But it did! I used the new bias and darks to correct the new flats, and then the old bias and old darks and new flats to correct the lights. I'm so pleased I've finally figured out what was going on as it's ruined many images for me over the past year+. Thanks again Carole, Barry

Hi again! I posted a query about flats a while back, but have now run into a more specific issue so starting a new thread for this. I had a decent clear night couple of days back and a chance to give the ASI1600 a bit more informed testing. Still finding some oddities with flats generally though, so hoping for some further thoughts please. In the past when doing flats with the DSLR, I've pointed the scope at an evenly illuminated area of the ceiling and run off a number of exposures sort of mid-gray middling histo no problems - always worked very nicely. With the ASI1600MM a couple of oddities though. First, opinions seem to vary on what gain to use - some say a low gain, some say the same gain as the lights. Same gain as the lights seems sensible, so thats what I went with. But with quite high gain (300 in this case) the flats only need very short exposures. This seems to cause horizontal banding, possibly due to flickering in the source light, and if I cover the scope (T shirt method) to reduce the light and extend the exposure to say 60 seconds, same as the lights, I get a lot of noise - I guess this is shot noise. So I settled for just doing lots of flats and hoping the noise would even out - I settled for 100 - though subsequently I see some folk use more, maybe for this very reason. Superficially the master flat looks OK, but at high mag it still looks noisy. See below and magnified section 300%. So what is the solution? Even more flats? Or maybe reduce the gain - all I need is the flat to replicate the vignetting. The mid gray/mid-histo used for flats isnt really the same light level as used in the lights anyway, so does the gain matter? With the DSLR I always set the ISO to 100 for flats and that worked just fine. Grateful as ever for any thoughts and advice!

Does anyone seen the new Ikea LED lighting lineup ? They have launched a new LED product, FLOALT, which is some sort of light box and it comes in three different sizes, from which one is really interesting: FLOALT 30cm. Does anyone already acquired it, would this work as a flat box ?

Note sure what to make of this. I was watching a video on improving flats for my WO Star 71 and using CCD Inspector to help with this. I downloaded the free trial and performed a Flat analysis on one of my Flat frames. I was fairly alarmed by the results because when I looked at them in SGP they looked fine. CCD Inspector appears to show at least two issues. One is the centre of optical illumination is quite a way out from the centre of the CCD chip (KAF8300) Also not sure if the high percentages around the image are an issue? Though they seem to be fairly consistent though. When I perform a high stretch on a Light frame the image in SGP does seem to follow the off centre illumination issue. I have looked at other settings on CCD Inspector but have to say I am not too sure on whether the results I get back are good or not. How or do I need to adjust the optical illumination train to be more central? Is this a massive issue? Basically I have the CCD camera fixed with extension tubes to allow me good focus with not much drawer tube extension (11mm). It is a good fit with no slack. I have included a picture of one of the flats. Thanks

I need to get/adapt an ELD for use with my Explore Scientific 80ED and Mak 127 to make Flats. What is the current thinking on the best choice of panels to base one on (uk) and any advise on power supply. Many thanks David

As this is my first go at doing flats, I am going to go into a bit of detail, as almost any step I have taken in this could be the cause of the weird result I got. I purchased a 10cm circular white ELP (sticky back) which I attached to a circular piece of card. I covered this with a piece of plane white paper (to diffuse the light) and attached a ring of card so that it would sit over my small refractor or camera lens. It is powered through the mains cable. After completion of my lights, I moved the axes of the mount so that the camera lens was pointing vertically, placed the mounted ELP over the lens hood (having made no alteration to the settings) and switched it on. I switched the camera from manual to Av (it is a canon 1000D) and went into EOSutils, adjusted the aperture to f/4 (the aperture I had been using to take the lights) and took 10 subs. The screen advised me that it was taking 1.3s subs. I then rotated the ELP through 90 deg and took another 10 subs. Upon loading the subs into DSS to produce a masterflat - the description file produced by DSS is attached. MasterFlat_ISO800.Description.txt My first surprise was that it was telling me that it was only stacking 17 out of the 20 flats loaded. Now I do have it set to only stack the best 80% lights - is it automatically doing the same for flats? It also told me that the exposure time was 1/32s, compared to the 1.3s I was advised when taking them. These two surpises, however, paled into insignificance compared with what it produced. Firstly, a jpg of one single sub (20%). Secondly, a jpg of the masterflat produced by DSS (20%). To me it looks incredibly dark compared to the single sub. And finally a closeup of the masterflat (500%) - this is only a small bit of it, but it looks the same all over. Now, I am no expert on flats, but I have seen a fair number on SGL over the years, and I have never seen one that looks anything like this. And, on the basis that it is far more likely that I have done something wrong than that the entire astro-community has been doing something wrong all these years, I am guessing that this one should not look anything like this either. So my question is: what have I done wrong? And (even more important): how do I not do it wrong next time? Thanks.

I have been generally pleased with my Moravian camera and filter wheel. However, I noticed I was getting some very odd results when trying to calculate filter offsets for my autofocus routine. I found that the wheel was not rotating to the filter requested - indeed it seemed random. Of course it was not random .... I have a 10 position filter wheel. However, it seemed that the Moravian Ascom driver was convinced I had a 12 position wheel. To get the Moravian wheel correctly configured, you have to run a configuration program. It tells you to do this in the manual. I just hadn't read the manual. As you can imagine, this led to erratic results. If I shot filters in sequence, things would go OK. However, if I tried to go back to a filter I had shot before then it would not work. Now, I shoot the flats from my 7 filters all in one go, BUT I test out the required exposure times first which means a cycle through all 7. So when I come to shoot the actual flats, I am shooting through the wrong filter or (sometimes) through no filter at all. I was never all that happy with the M31 I did back in early December 2016. There seemed an odd gradient on the left side of the image: https://stargazerslounge.com/topic/282827-m31-revisit/ So, having got the wheel correctly configured, I redid the flats and recalibrated the data. Lo and behold, the bright patch on the left disappeared. Data: Baader Blue 36 mm: 16x300" Baader Green 36 mm: 16x300" Baader Luminance 36 mm: 61x300" Baader Red 36 mm: 16x300" Chroma Ha 3nm: 6x1200"

What sort of exposure length do people use for their flats? In particular with a flat field panel. I'm concerned my Gerd Neumann panel is too bright. He now sells grey filters to cut the light level. I use Skyflats4 to ctually take the flats. My exposures are around 0.03 seconds for LRGB. Is this too short? I'm using a SXVH694. Thanks Anne

Can anybody suggest what might be wrong with my flats calibration process? I created a master flat from 40 images taken by placing an ipad over the end of the scope displaying a photo of a featureless wall (maybe not the best method?) A stretched image of the master flat shows a number of very faint dark circles which I think are grease spots on the objective lens of the scope. I can see these clearly when the lens is starting to dew up after an imaging session. Anyway after using these flats in DSS or Atik Dawn, these artifacts start to come through on the processed image, see M81 image below. Any suggestions or thoughts would be much appreciated. Steve

I've had continual problems with reflections in my flats. I have an Atik 383L, an Atik EFW2, and I use 36mm unmounted Baader filters. It seems that when using the camera and wheel 'as intended' I get reflections. I see these especially in my flats, but I think they are there in the images too. They do not calibrate out - I am left with lines in my images and I either have to crop them heavily or use blunderbuss processing techniques, which inevitably leads to some degradation in image quality (and my images are already quite degraded enough). I have posted about this problem before: https://stargazerslounge.com/topic/260450-horizontal-lines-in-flats/ and https://stargazerslounge.com/topic/269119-does-atik-efw2-work-with-atik-383l/ There is a solution, but it is not ideal. I thought that I would provide an update as to my progress and post my solution. If anyone has any other ideas I would consider trying them out. I am struggling to believe that I am the only person having this problem. First off, here is the issue - it appears to me to be a reflection of either the optical window edges or the sensor itself (I've pushed the contrast a bit to make the issue obvious in a reduced size JPEG): The issue is most obvious in the NB filters, but it is there in the RGB too - perhaps worse in the blue filter. I have tried the set up on 4 separate scopes - 3 x refractors and 1 x SCT - there is no difference I have tried two different 383L cameras - they both cause this problem I have tried 2 different sets of Baader LRGB filters (thanks to FLO for supplying the second set) and a set of Baader NBs - no difference I have tried reversing the orientation of the filters in the carousel (suggested by the chap from Baader) - no difference I have tried a different filter - a Chroma Green kindly supplied by Bern at Modern Astronomy - no difference (indeed maybe the reflections were a little more obvious - I've posted the Chroma version) I have tried 3 different methods of taking flats: an LED panel; a Gerd Neumann EL panel; and sky flats with a pillowcase I had one camera back to Atik who tried blackening the optical window edges. The chap from Baader also suggested putting a spacer between the camera and the wheel. This started to work. As I added spacers, the lines moved to the outside of the frame. Eventually they went away altogether. I was able to lose the lines at 7mm of spacing on one 383L. I needed 7.5mm of spacing with the other. The spacing does not seem to make the vignetting any significantly worse, so this is a 'fix'. On the other hand if you are considering using an OAG, then being forced to add that 7.5mm could be significant. Am I really the only one having this problem?

Hi All, I have just made myself a light box for doing Flats which on my first play with it last night, seemed to work okay, but I am not sure what brightness / exposure level I should be aiming for? Attached are a few screen shots taken at different exposures (0.2 seconds up to 1.0 second), all taken at the same brightness level. Now, rather than changing the exposure, I can make the lightbox darker or brighter, but I will see similar images to those below. So my questions is, which one of these exposure times is the 'best'? I have included the histogram in each screen shot, as I understand that this is the important thing, or is it the number in the White / Black box which is important? Hope someone can offer some advice here 0.2 Second Exposure 0.4Second Exposure 0.6 Second Exposure 0.8 Second Exposure 1.0 Second Exposure

Hi all, Last week I made some photos using my canon 500d on fixed tripod, with also the darks and bias. The results were not bad, but no milky way really visible, and I've been trying a little bit to fiddle with DSS parameters, or ACDSee, but no luck. I thought maybe the lack of flats was partially the cause of this, so I made some, yesterday. But probably I haven't understood flats at all, because instead of cancelling the vignetting, it added bright vignetting...... So my question is, if my lights are 30 20sec subs, 1600 ISO, f/4.5 at 18 mm, what should my flats be like? I did 30 subs at 1600 ISO, f/4.5 18 mm, but left the exposure automatic, which meant 1/320sec... (I used a tablet screen using a flashlight app). The flats show vignetting. If I had kept the 20sec exposure, they would obviously be completely white, overexposed... I attach two photos, one with, and one without the flats... NB: the one with bright vignetting is the one WITH the flats... :-/ thanks for any help!! Gerhard.