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.
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
Rewriting this section, I am conscious that the original text was based on experience and generalised in a number of areas.
Dithering, the practice of repointing the camera image sensor for each subsequent exposure, provides enormous benefits in image quality. The uneven performance and faulty operation of photosites, as well as the production of random and fixed pattern noise on top of imperfect field flatness are issues nicely managed by dithering.
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. I cannot say that dithering is necessarily a replacement for image calibration though some modern low thermal noise sensors may be an exception.
Dithering and image reduction serve different purposes with the same aim in mind - increased SNR.
Dithering is very useful for hiding artefacts produced by temperature mismatch between light and dark frames. It is also an effective method of hiding hotpixels and other sensor related misfunction artefacts, as well as optical train and field flatness imperfections.
You can 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.
Various programs provide dither capability. My setup is Arduino based, mechanically controlling the RA or DEC axis between images, slewing to present the camera to the target displaced by 10 pixels or so- quite basic.
Pressing hand controller slew buttons between exposures, whether in a predetermined pattern or random scattering is just as effective. These methods are not precise, which is not an absolute requirement for dithering DSLRs, providing the target image is kept within the sensor boundary and images are not displaced such that image alignment during preprocessing is not effective.
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.
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 describe version 5 of the base cooling system, using a full spectrum modified Canon 1000D/XS/Kiss F (or 450D, which is of similar construction), fitted with an Astronomik UV/IR Clip-in filter. The notes are divided into 3 main parts and sub-sections, mainly to keep file size reasonable.
UPDATE New Teensy code. This version also runs a small 1 inch OLED - which is not essential. It also has pin allocation for the automatic switching of sensor heating and telescope dew heating, plus a wider setpoint range, +15 to -10C. Sensor heating switches on at 7C cold-finger temperature, while telescope dew heating switches on at 6C air temperature.
UPDATE 2: The new software routine does away with PWM and employs an on-off routine based on the Adafruit Arduino Machine State code, which is just a modified version of the blink without delay sketch. I have found this approach much quieter and more accurate than using PWM.
The on-off routine is based on a ‘profile’ of the cooling system in which time-on and time-off values have been recorded at 1C intervals and used in a look up table.
The idea is based on just meeting the energy requirements of the cooling system so that energy oversupply is inhibited and the system cant cool further. This makes it much easier to control the set point temperature by avoiding the inevitable over swing and subsequent under swing, of setpoint, we see with other routines.
Because the energy required for thermoelectric cooling is relative to an air temperature datum on any given day, the amount of energy required to meet a given differential does not vary. I guess that goes without saying, but for clarity, irrespective of the starting point on the day, 20C cooling is still 20C cooling. Therefore, the lookup table remains valid for all occasions, even though the system may not be able to reach very low temperatures on a warm night.
Control is as simple as increasing the time-off. There is no need to manipulate time-on with this routine. Cool down is slower but only by a few minutes. The look-up table is only valid while sensor heat (condensation protection) is operating.
Presently, I am working on a modification to tell the system to increase the off-time if the sensor heating is not on. I haven’t encountered a time when it wasn’t required - perhaps in warmer climates. Though, in humid climates the sketch should be modified to turn the sensor heating on at a higher temperature than the default 7C.
This is the 1000D version. Heat load is about 40% less than the 450D, consequently the profile is different.
Previous 450D Teensy code with delay() function. Use this if not getting results with the new version.
Otherwise stick with this version Teensy Code - .ino file This should work for 1000D and 450D but not as accurate.
Note: Please read this.
Note: A better resistance heater for the camera sensor. I have removed the sensor heater notes. The one shown in the following link is far superior, as is the use of clear glass in place of the low pass filter. The advantage is less energy and less reduction in the maximum cooling differential (<1C from my measurements).
If the low pass filter is retained, keep in mind that it is also an anti-aliasing filter and may soften images - maybe that isn't such a bad thing? This works a treat - tested at -10C.
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.
Having completed this prototype, short of 3D printing the electronics compartment and light exclusion shroud, which is black gaffer tape at the moment, the electronics package is performing surprisingly well.
Cool down from 16C to -5C took approximately 2 minutes. Once at temperature (-5C), the on-temperature/setpoint LED came on and remained on, except for a few brief moments, which I suspect were sensor read errors/spikes, throughout the first run of dark frames. The changes to the circuitry have been more successful than anticipated.
An advantage of regulated cooling is the acquisition of dark libraries. Typically, the system presets, 5C, 0C and -5C, at 800 and 1600iso and various exposure times. However, most of my imaging seldom exceeds 210 seconds, unguided. This is extended to -10C with the improved sensor defogger or the use of an argon bag - links above.
Note: The notes that follow are dated in some places. The battery compartment idea incorporates camera power and is less intrusive. It is experimental at this time and up for redesign of the PCB.
It was possible to include all the cooling, heating and power supply requirements on one double sided board. The hardware is otherwise unchanged, except for a number of surface mount components. The board is better manufactured professionally.
If anything, the code could be improved and I will work on that in due course.
Appendix 1 Canon 1000D Thermoelectric Cooling Conversion - PCB etching. This is the basic board - updated.
Appendix 2 Canon 1000D Thermoelectric Cooling Conversion - Arduino/Teensy Code txt file. New version - corrected error to pwmV code.
Appendix 3 Canon 1000D Thermoelectric Cooling Conversion - Arduino/Teensy Code - .ino file New version - corrected error to pwmV code.