WATS ALL THE NOISE ! ! ?

Hi John,

Thanks for your reply. I cant imagine it is atmosphere moisture.. I had never seen a more moisture free night... lens didnt even fog up in 5 hours and all equipment bone dry...
My set up is a Canon 400d prime focus on celestron 9.25 SCT... iso 800 ...100 x 1 min exposures light ... 50 x 1min darks ... 50 bias ... and no flats... stacked in imagesplus and adjusted.. I think it is the lack of flats that is problem....(im gonna find out next clear night anyway!! :lol:

Lots of reflected light from the brightest parts of the image

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I think that the noise is perfectly in centre of frame but galaxy isnt so hopefully moisture not culprit.

I use a cable release... do u think is there a need to use mirror lock up as well?? Ill let you know how i get on.

Love your photos on flikr by the way...

kevin

Try the mirror lock-up anyway - it basically replaces the hat trick. Raise the mirror wait a second or two then fire the shutter.

There is a distinct pattern to the "noise". I see horizontal bands running through it and it is quite distinct at the bottom of the frame, which leads me to think that it is not just vignetting. Has this problem only occurred recently? or has it always been like this with this camera? Your Bias frames should have dealt with any readout noise, so maybe the camera is faulty? A 400D should still be under warranty. If you can borrow a different SLR and try it on the same set up then it could help show where the problem actually is.

Try the flats anyway, it may help rule it out as the cause, but I can't see it removing the problem, as the primary function is to remove spots of dust etc. in the optical train.

Hi all,
any results yet?

because this problem still puzzels me, I came to a new thought:
Vignetting only occurs as light, reaching the chip through the lens system,is not evenly distributed.
Noise on the contrary is not a result of external light but just generated in the chip.
to my idea this noise can not be vignetted it must be evenly distibuted, so i think that this is not the problem.
if noise has a clear pattern then it must be a fault of the chip or surounding components due to heat generated
somewhere near the chip or underneath it and it must show up in the dark images just as clear.
My D70 has a problem like that in the upper left corner, a very nasty purple area, but shows in the darks as well
so i can highly get it minimised
maybe you can see this if you blow up light levels of a dark image.

The collor reminds me of pictures i took of orion just above the horizon the same sort of redish-brown.
So i also think it is a false light problem or more, of moisture high up in the sky as mentioned above.


And I used the 'HAT' trick as well with a black t-shirt.
i tried the mirror-up trick of my D70 but it blocks the whole camera
(it's a shame...)but it is only intended for maintenance (cleaning of the ccd etc.)

an orange light in distance was reflecting off lens cap

You know that may just be your problem.Do let us know if you get it sorted.

Try the flats anyway, it may help rule it out as the cause, but I can't see it removing the problem, as the primary function is to remove spots of dust etc. in the optical train.

John - the primary function of flats is to determine the responsiveness of the pixels on the imager chip. Therefore flats take account of many things - vignetting, dust and other optical "defects" as well as the (varying) sensitivity of individual pixels on the imager.

I have vignetting in my optical system and flats remove the problem very well. I also have dust donuts - and they are removed too.

See the definitions section of this page:

www.saratogaskies.com/articles/cookbook/index.html

HTH
Dave

Dave - I stand corrected - but I still don't think that the sole problem here is vignetting, whats more I think you are spot on in all your assertions below:

The circular part of the image, center, is vignetting. This can be reduced by taking flats (if you don't want to go down the route of hardware changes).
I have the same issue with my setup and flats largely remove the effect. Post processing will help but not as much as taking flats (and flat darks as well).
The image displays what could either be coma or tracking errors or both. Coma seems evident in the uppr right corner. Tracking seems evident in the central parts of the image.
There also seems to be some vertical and horizontal banding going on in the central portion of the image - that may be down to the imager or processing or both.
There's a lot of noise in the image - longer images and dark/bias subtraction will help reduce that.
I can't quite figure out what is going on at the base of the image - that may be due to a light leak or not...
Other than that, you've certainly captured some detail in the galaxy which is no mean feat. Its a great start - work on the calibration frames (bias, dark, flats and dark-flats) and you'll progress.

Just that after perfect pre processing techniques he will still be left with the banded noise - odd that darks and bias didn't clean them up. I think he should borrow another DSLR and try it on the same setup to rule out a fault in his 400D.