Techniques
Telescope Guiding etc
My observatory is conveniently located in my back garden and my control room is attached to my garage only a few metres away. I first open the control room and power up the PC. Using the PC I can remotely turn on the power to the telescope, dew heaters, cameras and lights. While the cameras are cooling down, I open the roof of the observatory .
I have my scope permanently mounted, so that a simple one star alignment is all that is required. The SXV-H9 and HX516 cameras require only a few minutes to cool to operating temperature, and this can be done while the rest of the equipment is being prepared. As soon as it's dark enough, I turn on the telescope and synchronize on a known star.. The finder, guidescope and APO refractor are already aligned with the main telescope so that all four optical systems are centered on the same object.
The CCD cameras are left attached to the scopes so that focusing is a simple affair. Although the LX200 is supposed to be an f10 instrument, it appears to be working at f12 normally. I usually use an f6.3 or f3.3 focal reducer for deep sky imaging and add a 2x Barlow to the scope to give me a measured f25 for planetary imaging.
For deep sky work, most of my integrations are 600 seconds. I would take a series of maybe 8-10 of these which would then be stacked to produce the required total exposure. Guiding is done by using the separate 80mm guidescope.
For wide angle imaging, I attach the CCD camera to a 300mm or 500mm camera lens and this is piggybacked on the main scope.
For focusing, I use FocusMax software which operates via a RoboFocus system to produce very accurate focusing in a short time. This RoboFocus is temperature compensated to allow for slight changes in focus as the temperature of the air changes during an imaging run.
I usually use AstroArt for imaging although I also have MaxIm DL/CCD.
I use a home made lightscreen to produce flat field images. It is a simple matt white screen, evenly illuminated and attached to the wall of the observatory and seems to give me acceptable results. I take multiple flats, and then average or median combine them, before storing the Master Flat as a file for use in the image processing
Home Made Dew Heater
INTRODUCTION
After putting up with ending several observing sessions early because of the inevitable dew problems, I decided to reduce my frustration in this area. Having researched several different home-brew and commercial solutions, I decided to build my own.
The Kendrick and Orion systems looked attractive, but were expensive and not available locally. Most of the home-brew systems involved soldering a number of resistors together and this was too "fiddly" for my liking. After searching for sources of resistive wire etc I hit upon the idea of using the heating element of an old electric toaster. This would give me the best of both worlds - a neat, effective end-product, yet inexpensive to build and easy to use.
DEW PROTECTION
The objective of this project was not to not to "heat up" the corrector plate on my telescope but to replace the heat lost due to radiant heating of the glass to the night air. A large glass corrector will quickly radiate its heat away causing it to drop below the current air temperature. When the heat loss is great enough that the temperature of the glass reaches the dew point, dew will form on it, and will have to be removed by some means. One way is to use a hair dryer but this can cause hot spots on the corrector plate and distort the image. It also means having to keep an eye on the corrector plate to determine when the dewing was taking place. A dew cap offers limited protection by slowing the heat loss of the glass but will not prevent it entirely. A dew heaters job is to actively replace the heat that is being radiated away, keeping the glass at a constant temperature above the dew point.
In producing a dew heater, there is a delicate balance to be struck so as not to introduce too much heat in the system which will cause optical distortion. The scope should not feel warm to the touch but should simply not be as cold as it would have been without the heater.
The most effective solution would be a dew heater and dew cap combination. The dew cap keeps the corrector plate away from the night air and reduces the amount of heat needed from the active dew heater system. My dew cap is made from a sheet of polystyrene, about 8 mm thick, painted black and formed into a tube which fits over the end of the scope.
CONSTRUCTION
I dismantled the old toaster and unwound the heating element from one side. I then measured the resistance of the element and found it to be 24 ohms. I came across a length of self adhesive draught excluder strip in my garage and measured it carefully so that it would fit around the metal rim of the corrector plate on my scope. With the strip lying sticky side up on the table, I carefully laid the heating element on to the strip so that the element stuck to the strip. After this the combined element and strip was stuck to the metal rim of the corrector plate as can be seen in the photo. the ends of the heating element were then connected to a length of twin core wire, the other end of which I connected to a 12 volt power supply.
Using the simple power equation:- W=V^2/R (W equals V squared over R) where W is the power in Watts, V is the voltage and R is the resistance, we get W=6 watts. The current drain was half an amp which was easily supplied by my little power supply.
I found the dew heater to work very effectively and, in fact, have recently produced a similar design for my refractor guidescope. In this case the element is simply wound around the outside of the refractor close to where the objective lens is positioned.
This project was completed in about 4 hours.
Focusing
Focusing an image on a CCD camera can be a tricky task for the inexperienced. Certainly when I started out on this journey, I used to spend much of the available observing time acquiring and focusing the image.
One of the techniques I eventually found was to use a Hartmann mask. This is a simple device which can be bought under the trade name of Kwik Focus or made from basic materials. They can be made with 2, 3 or 4 holes and this description will apply to a 4 hole version.
When focusing is required, the mask is placed over the objective in the same way as the lens cap or objective cover. The telescope is then be pointed at a bright star or group of stars and a series of short exposures is taken with the CCD camera.
If the telescopes is not in focus, each star in the field will appear four times - once for each hole in the mask. Adjust the focus control and as the telescope is brought into focus, the four images will move closer together and will finally merge into one image. As the stars get closer together, it would be best to use dimmer and dimmer stars to judge the degree of merger. When the faint stars merge, the system should be very well focused.
Once this has been completed, the mask can be removed. Further checks can be made by checking the brightness readings of a particular star. When the value is at its greatest, the star is in focus.
Typical sequence using a Hartmann Mask
The mask itself is easy to make. I made mine from plywood. The disk should be made just slightly larger than the circumference of the scope body ( e.g. the diameter of the dust cover) so that the disk does not slide down into the scope. The outside edges of the holes are located very close to edge of the corrector plate or lens. The hole size is not critical but I made mine about 2" diameter to suit my 10" scope. As a guide, they should be approx 1/5th the diameter of the corrector plate or lens. Too big and the star images will not separate enough while too small and the images will be very dim.
To hold the mask in place I used two strips of Velcro tape. One end of the tape is secured to the mask while the other mates with its companion on the body of the scope.
I have also made one these masks for my 300mm camera lens which I use for wide angle images with my CCD camera.
The number of holes is not critical, and the photo shows one of my masks with 3 holes. It has been suggested that using triangular holes can give a more accurate in-focus position, as diffraction spikes are produced and, when they overlap, the system is in focus.
The photo below shows two of the masks I made, one in position on a 300mm camera lens.
