Almost at my last nerve
Sorry to read that this doesn't work for you @astrogee. For me, on the contrary, it always works for my 80 mm refractor. If I don't use a t-shirt then I need to use exposure times below 1 sec and then my master flat ends up badly. However, with a t-shirt folded twice I can use exposure times of 3 seconds and then the master flat comes out perfectly fine. For the Ha filter I need to take 12 second exposures otherwise they are too faint.
I'd prefer a flat panel that I can dim, like the one I use for my 203 mm RC telescope, but I don't have one so I need to use a t-shirt.
Clear skies, Wouter
OK, after battling for what seems like an eternity I may possibly be getting somewhere.
Firstly, my imaging train is all 2" right through to either my CMOS, (fairly small sensor) or my DSLR, (crop sensor).
Taking measurements & even just looking through from my OAG to my attached DSLR after removing the setup from my scope, the prism doesn't look like it overlaps my DSLR sensor.
I did notice that my Flats had this shadow from the OAG prism, I know it is the prism because if I remove it the shadow is gone. However on closer inspection this shadow is not exactly central as I would expect it to be, in fact my Flats do look slightly asymmetrical.
In an attempt to rule certain things out I downloaded a trial of PixInsight & noticed that it had a Flat Contour Wizard, so I ran this wizard on one of my MasterFlats, this is the outcome:
as you can see, the Flat is definitely not symmetrical at all, so something in my imaging train is out of whack.
Now bear in mind that when I take my Flats I have the scope pointing straight up and all the imaging train is hanging straight down with no lateral strain on it.
My thinking now is that this asymmetry is due to poor collimation.
But I first thought that if my scope is out of collimation, it also is when capturing Lights & the Flats pattern should still match up. Is it the fact that I use a light panel placed directly on top of my scope that exacerbates the issue?
Is it possible that my Focal reducer is playing a detrimental part in this too as maybe the light cone through the FR is too narrow or ever so slightly off for the sensor of my DSLR.
I really can't say that I have noticed these issues at all when using my CMOS camera where the sensor is around a quarter of the size but I will have to run test with that at the next available oppotunity.