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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.
I first made a 6" f/10 telescope back in 2002. It was, as everyone called it here "the minimum size you should make". So I went ahead with it. The f/10 was because some gentleman had ground the glass to f/10 and abandoned it in the local astronomy club and the president of which handed to me.
After having used the scope until 2009 (I went to UK for my Masters and bought my GOTO setup at the Telescope showroom I was working. I am primarily an astrophotographer, but one can never forget ones roots can we?
So, long story short, I am back to grinding a 6' f/8 this time, The FL now stands at 49" (f/8.1). I had finished fine grinding with 1000 grit SiC and moved to 1200 grit SiC but only to realize (after 30mins of grinding) that the seller had incorrectly labelled the powder as 1200 grit. I ended up with large pits all over the mirror and had to resort back to the "actual" 1000 grit SiC. This was yesterday. Now, after about 2 hours of 1000 grit, there are about a dozen pits of varying depth. I think another 30 minutes should get rid of them.
I tried making a polishing tool from White Portland Cement using the mirror as the mold and aluminum foil as separator. Disaster struck as the cement ate away the foil and got stuck to the mirror in the form of a thin layer (thankfully) and I was able to remove all of it by mild scrubbing and later grinding with 1000 grit SiC. The misadventures that I deliberately get into. 😕
Hopefully I can start polishing soon.
Wish me luck folks. I will post al developments here.
I am planning to build a Mirror-O-Matic machine, but since the designer Dennis Rech is not responding to emails anymore, I would be very grateful to anyone who could kindly share the plans of the machines you've built. Anything would be helpful. Images, plans, cur lists, advice etc.
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....
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?