DSLR PI Process Flow for Dithering?

[jwaters125]

I think I am getting mixed messages for PI Dithering Process Flow on CN.  Its been recommended that I don't include BIAS and Dark Calibration Frames when I Dither. 

What's the correct PI process flow for Dithering when I use my DSLR or even a OSC and can this be done with BPP?

[John_Gill]

Hi,

Dithering is the processing of moving the scope a couple of pixels (in random directions) in-between capturing each image. 
To implement Drizzle you need to check the box when doing a StarAlignment, and when doing a ImageIntegration add the Drizzle file (and the L.Normalization files).  After the ImageIntegration then close all images and do a DrizzleIntegration with the Drizzle and LocalNormalization files.

space is not black
John

[jwaters125]

I have been doing Dithering for many years.  My questions is in relation to processing the calibration files - BIAS, Darks and maybe Dark Flats.  Some say to exclude Darks, BIAS, Darks and BIAS, only use Darks Flats ...etc.

What's the correct method and process flow?

[pfile]

i don't know why you'd exclude calibration frames when drizzling. i can't find the thread at CN... what arguments are people making saying that you shouldnt calibrate images you are going to drizzle?

rob

[jwaters125]

I have always included Darks, BIAS and Flats when I process my DSLR images - Dithering or not.  I always include Drizzling.  Some Astro Club members say to do Flat-Darks and not Darks, others say to exclude BIAS or Darks and list continues for Dithering - too many experts....   This question came up because I was getting Walking Noise in one of my wide-field (200mm) imaging sessions.  It turns out that I may not have been moving the mount RA and DEC enough.  The PHD 2 and Backyard EOS parameters were too low.

I want to make sure I understand the 'recommended PI work flow' for processing subs that use Dithering.

Some background info
https://www.cloudynights.com/topic/663665-what-kind-of-noise-is-this/

[pfile]

ok, i think that guy is saying that canon RAW files are actually cooked, meaning the camera firmware is attempting dark signal suppression, and that it can't be turned off. i think that is broadly true but i'm not sure it's settled science that calibrating a DSLR frame makes the pattern noise worse.

generally speaking calibration of frames from uncooled cameras is somewhat difficult as you can't really be sure that the temperatures match properly. usually optimizing the darks will mostly take care of this, but then of course any hot pixels in your frame end up being under-corrected by the scaled dark. then i suppose if they are now not bright enough to be rejected as outliers, the hot pixels make the "walking noise" even more obvious. i guess if this is the situation you are in, you could try running CosmeticCorrection to clean up the 'warm' pixels before proceeding to bayer drizzle.

rob

[jwaters125]

Thanks Rob.

[1DegreeN]

I gave up using darks when I was imaging, and dithering, with my Canon 700D. I found they sometimes made the integrated image worse and CosmeticCorrection was the tool I used to remove hot pixels. I think the issue with my DSLR darks was that they were not temperature matched to the lights. Now I image with a cooled CMOS camera and use temperature and exposure length matched darks and they work like a dream. I still include CC in the workflow but really only for old time's sake because it picks up only a handful of hot pixels now. I still dither but have been gradually reducing the dither settings since my DSLR days and have not yet encountered walking noise.

[pfile]

yeah, i agree, the lack of temperature regulation in DSLRs is a real killer... i also switched to a cooled CCD camera and life got a lot easier.

rob

[jwaters125]

I may end up switching to OSC also.

[STEVE333]

You have received good advice from knowledgeable individuals.
Just my 2 cents worth based on about 4 years experience using a DSLR.

I have a Canon T3 (modified) and always use dithering. My Precal includes Bias, Darks and Flats. I don't have any "walking noise" issues.

I've built a Darks Library as follows: (make sure to use the same ISO and Exposure Time you use for Lights)
1) Put the camera (cap on) into a ziplock bag and put the bag into an insulated Lunch Bag along with some artificial ice.
2) Wait until the camera temperature rate of change slows down, then start taking Darks. Continue until the camera has cooled all the way down and then has warmed up again to the highest temperature you will need.
3) Sort the Dark Frames into folders by temperature. For example, put all the 46F Darks into a folder named 46F, all the 48F Darks into a folder named 48F and so on. My Darks library has Darks from 46F to 73F.
4) Having all the Darks sorted now group the darks into 5C ranges to make Master Darks. For example, 
a) Combine all the Darks from the 46F, 48F and 50F folders to create the MasterDark46_50.
b) Combine all the Darks from the 48F, 50F and 52F folders to create the MasterDark 48_52, and so on.
c) This completes your Darks Library.

When calibrating Lights, group them into 5F groupings that have a matching MasterDark. I've had very good success with this approach.

Hope this makes sense.

Steve

[jwaters125]

Steve - Thanks for the info.  How often do you have to redo your Dark Library?

[STEVE333]

Steve - Thanks for the info.  How often do you have to redo your Dark Library?

I have two sets of Mast Darks, one set for 180 sec exposures and a second set for 540 sec exposures (both at ISO 800). I've been using the 180 sec exposure Darks since Nov 2017 and they still seem to work OK. The 540 sec exposure Darks are considerably newer.

I have heard some people suggest that Dark frames should be redone every 6 months. So far I haven't needed to, but, that's just my experience. Wish I had a better answer for you.

Steve