I've attached an example image that shows the problem. The first is unaltered, the second I forced the levels to emphasis the issue.
Ignore the black on the right - that's the OAG prism holder poking in.
What I'm interested in is the concentric rings, that are particularly concentrated in the centre. Are these gradients something I can fix by using flat frames (I haven't got a setup for this yet), or do I have some other flaw in my system?
If it's vignetting a flat will take care of this. Otherwise it could be dew somewhere in the imaging train. This is harder to get rid of. As it is now your best bet is to use a tool such as Gradient Exterminator or other to artificially get rid of the gradient.
Thanks for the replies everyone. I did some checking to see if I could rule anything out.
Reflections: I'm fairly certain it's not this: I've seen the results of reflections before and like your image, they tend to be skewed to one direction, not centered like mine.
Cleaning smudge: Just pulled it apart to check. Nothing obvious there. Recleaned it anyway. :p
Vignetting: I am fairly confident that it's not this. The back of the focuser is 2.5" in width, the connected field flattener is also 2.5". The next spacer is also 2.5" before dropping to a 2" spacer to connect to the OAG (2") to finally the 6D. With the OAG prism just poking into the image on the right, I think this means the 6D is getting fully covered. Or if it isn't, it would be an extremely small amount in each corner that's missed.
Dew: I now realise that this is the most likely culprit - but I'll have to do specific tests to be completely sure. I went back over the images I took from the beginning of the night until near the end, plus reviewed some other nights as well. It's looking like there's some differences - but the hassle is that I can't tell for sure since I didn't examine for dew on the field flattener or secondary mirror at those times. I remember looking at the primary at one point, but it was ok.
I'll cross check further by replacing the FF with a FR in the same session - it has slight vignetting, but didn't exhibit this behaviour from memory.
I'm in the process of sorting out a dew heater system (I had the primary dew up one night), so if it's the issue I'll get it fixed eventually.
I've also started getting similar rings since switching 2" nosepieces from a Baader to a Moonlite nosepiece. In my case, I think it's internal reflections off the Moonlite nosepiece as it's quite shiny (the Baader is much duller). It completely disappears with flat frames, so I'm not terribly worried about it.
Flat frames are easy to take using the "white T-shirt method". A couple of hours after dawn / before dusk, point the scope at a patch of clear sky in the opposite direction to the sun, place a white T-shirt or towel over the scope, and adjust your exposure duration until you get a middle-histogram exposure. Take a reasonable number of exposures (say 30+). It'd be a good idea to take "dark flats" too - same as normal darks, but with equal duration to the flat frames.
Does the 6D have a shutter on the viewfinder? I got a similar image last year when light actually got in through the viewfinder of the camera.
No shutter - you just put the lame cover on it that's part of the neck-strap. I've forgotten it before, but I just checked - it's still secure.
Quote:
Originally Posted by naskies
I've also started getting similar rings since switching 2" nosepieces from a Baader to a Moonlite nosepiece. In my case, I think it's internal reflections off the Moonlite nosepiece as it's quite shiny (the Baader is much duller). It completely disappears with flat frames, so I'm not terribly worried about it.
I just checked my focuser - it's not shiny inside, it's quite matt really and didn't seem to overly reflect the light.
Quote:
Originally Posted by naskies
Flat frames are easy to take using the "white T-shirt method". A couple of hours after dawn / before dusk, point the scope at a patch of clear sky in the opposite direction to the sun, place a white T-shirt or towel over the scope, and adjust your exposure duration until you get a middle-histogram exposure. Take a reasonable number of exposures (say 30+). It'd be a good idea to take "dark flats" too - same as normal darks, but with equal duration to the flat frames.
Who wears white t-shirts? However, I'll see what I can dig up in the short term. I was planning on creating a light box, but I need to work out what this problem is now, so I'll try that way.
Who wears white t-shirts? However, I'll see what I can dig up in the short term. I was planning on creating a light box, but I need to work out what this problem is now, so I'll try that way.
Hrmm actually since you're using a DSLR you probably don't need the T-shirt/towel. It's used mainly to create a diffuse light source (if you're using an artificial light source), or to block out enough light so that a CCD camera can take a >= 1 sec exposure - many CCDs have problems with ultra-short exposures, but DSLRs don't.
Just wait say a few mins before/after dawn, point the scope at a featureless patch of sky at say 45 deg alt directly opposite the sun. Turn sidereal tracking off (or use the hand controller and slew east slowly), and take lots of flat frames. Combining a decent stack of flat frames will cancel out the stars (but avoid bright constellation stars).
Light boxes can work great, but they're a bit trickier to do right. Given the choice, I prefer to take sky flats - but if I'm packing up at say 1.30 am because clouds have rolled in, the light box is the only way
Hrmm actually since you're using a DSLR you probably don't need the T-shirt/towel. It's used mainly to create a diffuse light source (if you're using an artificial light source), or to block out enough light so that a CCD camera can take a >= 1 sec exposure - many CCDs have problems with ultra-short exposures, but DSLRs don't.
Just wait say a few mins before/after dawn, point the scope at a featureless patch of sky at say 45 deg alt directly opposite the sun. Turn sidereal tracking off (or use the hand controller and slew east slowly), and take lots of flat frames. Combining a decent stack of flat frames will cancel out the stars (but avoid bright constellation stars).
Light boxes can work great, but they're a bit trickier to do right. Given the choice, I prefer to take sky flats - but if I'm packing up at say 1.30 am because clouds have rolled in, the light box is the only way
I've got an old sheet I'm trying with tonight, but the light is fading fast. It's another reason to use a lightbox, plus if I want to change the configuration during the night, I can take some flat frames at that point.
I can see though, given the extra time and hassle involved with the flats, that I'm most likely going to pick one configuration for the night and go with it. It's going to be bad enough to have to take flats for each ISO I use. Another bonus to the lightbox - I should be able to consistently predict the exposure duration for each ISO, and therefore easily build the dark flats library.
Quote:
Originally Posted by naskies
By the way, this article on flat fields may be of interest:
However, I'll see what I can dig up in the short term. I was planning on creating a light box, but I need to work out what this problem is now, so I'll try that way.
