Results of sea trials for software, hardware, optics, et al. . .

 

December 1-2.

Optical notes: There's a lot of glass exposed to the sky in the 200mm and not much dew shield. F2 is still really hard to focus, at least in white light; F2.8 is much easier. Best white light focus is with the "5" in " 50" just barely inside the leftmost DOF mark. Best H-a focus is near the point at which the same line bisects "50". See pix. [Refined: based on another night's experience, the line should be almost tangent to the left side of the "0".] Note the red dot Nikon provided for IR photography; H-a is quite a long way out on that end of deep red and so is the empirically determined point of best focus. Since white light photos are combos of all colors to which the CCD is sensitive... it's no wonder we get bloated stars with the clear filter despite its IR blocking coatings. The presence of that red dot says that not all the focus adjustment required when going from clear to IR filters is owing to the different thicknesses of the plano-plano "lenses" that filters are.

Electrical experience: Turn everything on, then plug it into the computer. Use the same USB slot every time and you don't have to reload drivers. 8x sidereal is too fast when autoguiding this way; use 2x sidereal on the control box and specify 5s or longer autoguider calibration runs in Maxim.

A voice from a few weeks in the future: keep the guiding at 1x sidereal or slower so that the guider never reverses the RA motor. This avoids problems owing to backlash in the worm. Keep the telescope slightly weighted to one side of the meridian so that the same side of the worm's threads are in constant contact with the worm gear. A good many other imagers recommend keeping weight to the east. Might as well. At these slower rates, use a much longer guider calibration run in Maxim -- 10 or 15 seconds. You may need to run one axis 50% longer than the other to get solid calibration. Why? I don't know. As you were.

Mechanical points: Sight the finder to coincide with the ST80, then adjust the ST80 to align with the main instrument. Mount the 200mm so that the micrometer is outboard rather than between the guidescope and the telephoto -- easier to reach without bumping stuff in the dark or snagging wires on sleeves. When you get half a chance, whittle down and smooth the aluminum beside the micrometer drum. It makes getting a good grip on the micrometer much tougher than it needs to be. It'll be a real problem with gloves. [Done]

 

White-light focus point for the
200mm F2.0 EDIF on the ST2000XM.

 

Approximate hydrogen-alpha focus point.

 

 

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Text & Photos by David Cortner
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