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.
Monday, September 24, 2012
There has been a lot of interest in this thread in recent times so I’ve provided a short summary, mainly for the benefit of newcomers to DSLR astrophotography - I hope it’s helpful. Dithering, is in my view, one of the most useful and effective techniques applied to image acquisition for increasing SNR, applied to DSLR cameras, saving hours of processing time and frustratingly poor results.
There has been a bit of discussion on various forums about substituting dithering for calibration. Sounds attractive, but;
***dithering is not intended to be a substitute for calibration***
Dithering and image reduction serve different purposes with the same aim - increased SNR. Temperature must be considered when using DSLRs, requiring dark subtraction. Dithering however, will hide the majority of temperature related calibration errors/inaccuracies as well as several other types of artifacts… read on.
You can also read about dithering in Berry and Burnell’s, “Handbook of Astronomical Image Processing,” where they recommend displacement of images by at least 12 pixels. There are several informative academic papers on-line, as well.
Backyard EOS has dithering capability, however, I have never used the program. My setup is Arduino based, controlling the RA or DEC axis between images, simply by slewing to present the camera to the target, displaced by 10 - 15 pixels or more between images - it’s that basic.
The comparison image, is intended to accentuate the underlying issues with the image on the right. No attempt has been made to minimize the effect with post processing. The image was stacked and stretched - please note that the red streaks were not evident in individual subs and only appeared after integration. In-fact, I naively spent hours trying to salvage that image - a complete waste of time. The image on the left was taken with the same camera, dithered.
Rather than spending time eradicating/covering up unsightly problems, time was spent lifting out detail, which in the image on the right was partly obliterated by poor acquisition - no dithering.
Here is the pattern I follow… it keeps the image within the sensor boundary. I use a look up table in the Arduino program to schedule the correct hand controller button activation.
Struggling to produce a decent image, it became apparent that the physical and electronic characteristics of DSLR CMOS sensors demand more attention and an understanding of the limitations of the camera’s sensor was essential.
I went to the trouble of replacing the factory IR filter with a special purpose astronomical filter to increase the transmission of Ha wavelengths and built a sensor cooling system to reduce dark current/thermal noise - this all worked quite well. But, there is more to sensor technology and manufacture than meets the eye.
For instance, the anti-aliasing filter, which forms part of the dust reduction system in Canon DSLR cameras is designed to reduce Moire, an artifact produced by the Bayer (RGB) colour matrix. However, the reduction of moire tends to soften the image. Even with a focus mask at ‘perfect focus’ - consequently, the AA filter should be removed.
But that’s not the end of the story. Among the millions of pixels that make up the sensor light gathering matrix, a small percentage are dead (don’t work). Some are on all the time (hot) and overall, pixels differ in their ability to convert photons to electrons. For daylight photography this isn’t a problem, as a rule.
There’s more. The pixel matrix and associated electronics produce fixed pattern noise. Heating of the sensor during extended operation also increases noise. Random noise is a function of arriving photons and is different for every frame. The optical properties and cleanliness of the sensor, filters and lens also produce artifacts.
Combined, this all conspires to ruin the image to which you have dedicated copious amounts of time at the wrong end of the day, when perhaps, you should be sleeping.
So what’s the solution? In principle, reducing the effects of the deleterious electronic and physical influences inherent in the optical system is quite simple, in practice however, it involves another layer of complexity; that is, DITHERING!.
Dithering is an authentic solution because it addresses noise suppression, optical and sensor artifacts - dithering does not replace proper image calibration techniques. It will however, greatly improve results and avoid problems that no amount of calibration or sensible image processing can resolve.
So, what is dithering?
Dithering is the practice of shifting the sensor (camera) between images, so that each new image is slightly offset from the previous image. The image is sampled by different pixels; that is, the image moves, ideally, by 10 - 15 pixels, from the position of the preceding exposure. With careful management the target image remains well within the sensor boundary.
Dithering can be random using a hand controller and estimating the offset by timing the button push in DEC or RA. Better still, an automated system that dithers in a box shaped spiral, or maze pattern. The goal is to avoid a succession of images occupying the same or neighbouring pixels (which produces poor results) and to prevent the target object moving out of the FOV.
Dithering, particularly with DSLR cameras improves signal to noise ratio for very little effort. It can be win win for the astrophtographer.
Executed properly, dithering deals effectively with random noise, hides hot and cold pixels, improves flat fielding and sub-pixel sampling; that is, capturing the image over a range of pixels means that we are not sampling the same and possibly less efficient pixels repeatedly for the same object location.
Calibration is not always as effective as we would like with DSLR images. And even if the images weren’t calibrated, a dithered stack would produce pretty good results.
Monday, July 2, 2012
For something completely different.
One day in May last year, I was walking the dogs along a Bass Strait beach, just near Torquay (Victoria, Australia). There is a little fan shaped cove with rocky promontories at either end. The sun was warm, the sea lapping gently on the shore, at low tide. Perhaps it was the light, but as I traversed this section of the beach I noticed a small creature, perhaps 5 - 6mm (leg tips,) hurrying along the waterline, occasionally covered by a light wash. Once the water had receded, this leggy little organism, having resisted the water flow, continued on its way.
A wisp of life, it seemed to be feeding near the low tide water line over smooth packed sand. It hurried here and there, stopping suddenly, then moving on with equal energy. Taking a camera with me that day was fortuitous. I managed to squeeze off one shot in focus while chasing this little fellow around the beach, losing sight of him, standing up to find him again, a bright little speck on the sand.
I made several inquiries to various institutions without much joy. Without a ’sample’ there was little to identify. Eventually, however, and almost a year later, it was suggested that the arthropod is most likely a species of Sea Spider or Pycnogonida, which inhabit the oceans from the shore to the deep, worldwide.
I’m intrigued by what appears to be a proboscis attached by a socket to the front of a dorsal appendage sweeping backward over the body, like a trowel handle. The other legs appear to be arranged asymmetrically. The abdomen gives the appearance of a terrestrial Daddy Long Legs. The eyes, blue and arranged either side of the base of the dorsal appendage. Fascinating little guy!
Wednesday, May 16, 2012
The Bow Tie focus mask, described here, is derived from Carey and Lord focus masks, which are types of diffraction gratings, similar to the well known Bahtinov mask.
The bow tie mask was purpose designed to suit a small aperture, short focal length lens. The four obstructions are intended to produce splayed double spikes, similar to the Carey mask, while eliminating the grating typical of focus mask designs. The wide obstructions and absence of grating increases the brightness of the diffraction spikes - discernible with a small lens.
The junction of the obstructions also provides an area of certainty. A central spike perpendicular to the double splay is generated at focus. This spike is not present otherwise. Another phenomenon of this design is the presence of red and/or blue fill within the splay of each pair of spikes.
The bow tie mask is easy to make. A flat section of rigid plastic is easily cut to shape with a hobby knife and steel rule. The clear plastic can be coated with black indelible marker. Sharp straight edges are essential.
Using the bow tie mask is straightforward. Equal spacing of each pair of spikes and the presence of the perpendicular spike indicate focus.
Saturday, December 10, 2011
The links below provide a more complete description of the base cooling system. Additions will be forthcoming, describing other functions.
Appendix 1 Canon 1000D Thermoelectric Cooling Conversion - PCB etching. This is the basic board.
Why cooling - a very basic explanation
For anyone not familiar with the reasons for cooling a digital camera sensor. The purpose is to reduce dark noise (thermal current) generated during long exposures - the result of sensor heating.
Reducing the temperature at which an image is acquired improves its quality because signal to noise ratio (SNR) is improved.
The cooling system described here, dependent on Thermoelectric module (TEC) and heatsink rating, is capable of reducing sensor temperature between 18 and 30C.
Sample images taken with a cooled Canon 1000D 200mm prime lens f5.6 and f6.3.