Friday, October 24, 2014
“Finding expression, to say what you want to say, comes with a wonderful sense of freedom. Then, having written, the responsibility for one’s words clearly defines that no man is free, completely! Bound by the words of our mouth and that of our pen - keyboard - we answer to all, and God.”
Thursday, May 1, 2014
This thread has its origins in Craig Stark’s article about the non-linearity of DLSR RAW data, as well as discussions on various forums about preprocessing of DSLR RAW frames.
The non-linearity of DSLR RAW data (compared to 16bit CCD linear data), is a product of camera firmware corrections, which are designed to create a RAW image that is easily processed, out of the box! Non-linearity is a technical term for data that has been manipulated in some way from the RAW linear state captured by the CMOS sensor. This is the case with DSLR RAW images (data) to produce a nice picture.
Because DSLR RAW data is manipulated in camera, it has already been processed to a point which requires unique handling during preprocessing; that is, during the calibration process. To avoid further data loss, typically experienced by applying 16 bit processing methods to DSLR RAW data; the following is recommended
1. Do NOT bias subtract the dark frames or the light frames, as you would if scaling the dark frames.
2. DO bias subtract the flat frames only.
3. DO subtract the master dark frame from the light frames - bias in the dark method.
4. Do NOT apply pixel rejection algorithms to bias, dark or flat frames - avoid data loss.
5. DO take flats and bias at the lowest ISO.
6. DO apply pixel rejection algorithms appropriately to calibrated light frames.
The issues discussed here were more evident when preprocessing cooled, temperature regulated, DSLR images, and does not rule out the same application to uncooled DSLR RAW data. However, if you have one of those supercooled DSLRs, perhaps dark frames are a thing of the past. A carefully constructed master bias and flat may be all that is required.
“The point of this discussion is, that problems arise with DSLR RAW data when dark frames are bias subtracted and then the master dark frame is subtracted from the light frames, as well as subtracting a master bias - as one would do if scaling darks. This is a process normally applied to linear data and is not suited to DSLR RAW data calibration. Linear processes applied to DSLR data may produce less than optimal and sometimes devastating results due to data truncation.”
Because DSLR RAW data is processed in the camera, it is “clean” to begin with, and applying pixel rejection algorithms to calibration frames is unnecessary. However light frames are exposed to unfriendly light sources and pixel rejection is necessary.
A note on bias frames - CMOS sensors do not have a bias and instead produce a pattern of fixed noise, which, to all tense and purpose, may be considered bias and applied in the same way.
The practice of not calibrating dark frames is common place, and has been for years; “the-bias-is-in-the-dark.” However, the advantages and reasons, applied to DSLR RAW data, may not be clearly understood - “it’s always been done that way.” Some software defaults to dark scaling and is not suitable for DSLR RAW data.
“A typical DSLR RAW calibration data set, comprises uncalibrated darks and bias subtracted flats. Flats and bias frames can be taken at the cameras lowest ISO setting.” Denoising the master flat frame is recommended by some imagers.
AstroArt preprocessing is a one time operation, and uncomplicated. In PixInsight, master bias, master dark and master flat frames are prepared separately to avoid bias subtraction of dark and light frames. Either way, pixel rejection is applied to light frames only.
Thursday, October 31, 2013
This is a sample entry, posted to show you some of the features of FlatPress.
The more tag allows you to create a “jump” between an excerpt and the complete article.
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Saturday, December 29, 2012
For something completely different. “The Black Generic” (say that with a thick Scottish accent). Presented here for posterity. See further down, for a small snail pattern that was very effective in small dams, too.
“The Generic” (which never had a name, until yesterday) was an attempt to accentuate the form of nymphs - and it worked. The hook shank bent upwards, giving the body a curved posture, the long flared tail, tight skinny abdomen and bulbous bushy thorax were intentional. I noticed that variations were less successful. A thinly tied or short fiber thorax being the main flaws. It needs to look, curvy, leggy, cheeky and thoroughly provocative - hey! look at me, I’m here, come and get me!
My rough sketch doesn’t do it justice and I no longer have my fly tying gear.
Fished, in shallow runs, deep, sinking and on the rise, this fly caught plenty of fish in streams North of Melbourne, some 25 years ago, when I had the time to fish - Deep Creek and Jacksons Creek in particular, when they were healthy, before the drought devastated them. I understand that recent rains have regenerated the area and that fish are more plentiful.
The appearance is a long bulging thorax with a very bushy hackle to 2/3 of the body length. The abdomen is thinly wound single layer of silk with tightly wound silver wire segments and a long flared tail.
Hook size: 14-16 longshank - slightly bent upward ~2/3 from the eye.
Body : Black silk.
Tail: Soft black hen 6-9 curved fibres
Segment: Silver wire - anterior only.
Carapace: Black crow wing.
Hackle: 3 dark brown - black Ostrich herl with long fibres.
Head: Tie off crow wing with 3 half hitches and clip to cover eyelet - a flared finish.
Starting near the hook bend (not the 2/3 bend), tie in the tail, flaring with turns of silk. Tie in the wire and wind the body tightly (a single layer) to the bend (2/3 bend). Wind in narrow segments with the wire and tie off and clip at the 2/3 bend.
At and continuing from the 2/3 bend, tie in a good bunch of crow wing and several Ostrich herl. Wind silk to eye and half hitch. Wind Ostrich herl to within ~1/2mm of the eye and secure with half hitches. Bring crow wing over and tie off with half hitches immediately behind the eye and trim to cover the eye.
It may be necessary to tease out the hackle under the crow wing with a dubbing needle.
You may wax the thread, but I don’t. I prefer the fly waterlogged and sinking fast in moving or deep water.
Tying the crow wing shiny side up is an alternative presentation.
Small Snail pattern:
A second and very effective little fly, was a very small snail pattern. Hook size 16. Olive thread. A few winds of olive chenille with an olive carapace and a few turns of yellow chenille. About half and half.
Fished above weed beds in particular. Very easy fly to make.