Jon. The reason for not addressing calibration of Pat's frames in this instance, is that the striations in the image can occur with or without calibration. Hopefully, there will be improvement with calibration.
I will go so far as to say that DSLR frames should be dithered aggressively. But dithering is not a replacement for calibration - bias darks and flats. Dithering will sample as many different pixels as frames and separate any sensor artifacts by a respectable distance, so they never meet up when stacking.
In general - Increased sub-pixel sampling - pixels vary in performance. Flat field improvement. Masking of sensor artifacts - hot and cold pixels and cosmic ray artifacts. Noise reduction. And, a biggie for Canon users, banding is eliminated, as far as I can tell, completely, in the stacked image. Overall improved SNR and less post processing overheads
In short, if you can find a way to dither with your gear, do it. It's for beginners as well as the pros.
BTW. I have corrected the sequence in the previous image. Been a long time since I've had to think about it, as dithering has been sequenced with image capture, automatically for some time now.
Jon. The reason for not addressing calibration of Pat's frames in this instance, is that the striations in the image can occur with or without calibration. Hopefully, there will be improvement with calibration.
Understood. I guess there are two questions we are replying to here:
1) how can we avoid striations in the future? For which the answer is dithering, and matching temperatures.
2) What can Pat do to improve the outcome of his data (which is *excellent data, great focus, guiding and detail IMHO).
My answer to this second question is - take some darks. Maybe even try and get some flats with the camera as close as possible to how it was positioned in the original.
Finally, I'm going to guess that the really high sensor temperatures happened at the beginning of his imaging run. Pat, it might be worth going back and checking whether the striations can be seen in all your subs, or just those at the start.
Understood. I guess there are two questions we are replying to here:
1) how can we avoid striations in the future? For which the answer is dithering, and matching temperatures.
2) What can Pat do to improve the outcome of his data (which is *excellent data, great focus, guiding and detail IMHO).
My answer to this second question is - take some darks. Maybe even try and get some flats with the camera as close as possible to how it was positioned in the original.
Finally, I'm going to guess that the really high sensor temperatures happened at the beginning of his imaging run. Pat, it might be worth going back and checking whether the striations can be seen in all your subs, or just those at the start.
ah i think i understand much better now, really not sure on the process of dithering but some study on that is easy to do, but mainly as you have all said the matching of lights darks and flats are all important
i will have a look at the individual captures to check whether or not the striations are present
i am really pleased that it is possible to improve the images i already have too
and the camera in the fridge!? great idea if you want your beloved to think you have finally flipped ha ha, i cant wait to try that one!!
1) how can we avoid striations in the future? For which the answer is dithering, and matching temperatures
The great thing about dithering is that it masks small temperature mismatch speckles and is a way of accommodating float in a cooling system overnight. 5C is a reasonable limit, I think.
I took the attached picture a year or so ago, and i think i probably took the camera inside, did some flats, then did darks, giving the sensor a while to warm up. That was with a 40D and Pentax Takumar lense
Awesome picture otherwise - I clicked on it looking at the thumbnail, not the topic.
I've not used APT. Backyard EOS has a dithering function if you're using PHD - tells PHD to dither between exposures. I've not used it, but am going to after this discussion.
James, I've almost always had these striations before in darks, not lights. Shooting a long run of darks inside in the warm is not always a good idea!
The other time I get them is I I have been using live view to frame and focus. Live view pushes the temp though the roof! Now I always go to the object, fame and focus, THEN calibrate PhD, before imaging. Gives the sensor time to cool off a bit.
That said,I've just bought a drinks cooler from Jaycar which I am going to cannibalise into a camera cooling box.
It's a pain, but the most reliable way to ensure relatively consistent temperature of your frames is not to let the sensor cool too much between shots and follow up right away with darks and flats. Bias frames - take lots.
If you are competing with cloud cover, take darks at these times. Keep imaging and that way you have fewer to take at the end of the night.
Remember that box coolers are not as effective as cold finger mods when it comes to minimizing sensor temperature increase. So the principle of getting the sensor to temperature and keeping it there applies, as it does with a camera that is not cooled. But with less dark noise!
had a good look at a few single images and one of our members was probably right
besides the eos clip in LP filter seeming to make the sky background extremely blue(?) it took a while to realise from the first frame there seemed to be an awful amount of extra white dots which i assume are hot pixels
the last image in the set has maybe half the amount
i dont know how this can be saved at all
thanks for all the responses and i have certainly learned a lot from my post and i am glad i asked
pat
Pat. I think this thread has run its course, but I used an Astronomik CCD-CLS clip in filter for a while and it too produced a strong blue gradient - easily removed with the right software. It was good for contrast and enhancing red in nebulas.
A modified DSLR at its optimal iso/gain and exposure time for the conditions and the lens at its best resolution, plus good calibration and dithering are the fundamentals. Once these factors are under control filters become useful.
Frankly, I doubt that I will resort to filters for DSLR work again. The exposure times are too high except for a guided set up and unless you get a good exposure with a filter, the frame will be dominated by noise - which may be the issue you referred to.
Careful selection of iso and exposure time around your aperture and sky condition is essential for good light frames using a DSLR.
E.G. My Canon 1000D - light pollution at f5.7/6.3 iso400 210 seconds. It's different for different sensors. I'm still experimenting with my Astrodon Inside mod, cooled 5D MKII, but it looks like iso800 and 180 seconds is good at f5.7 under light pollution.
There are lots of things to consider, but with the basics under your belt, it is much less dissapointing and less frustrating when things don't go well - easier to narrow down the problem.
