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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?
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.
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.
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?
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,
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.
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.