Optical Tube Assembly OTA

Peanut Butter Jar Lid Focusers The problem with sct focus controls is that they are too small in diameter. If they were larger they would deliver finer focusing, which would be handy during imaging or high power planetary observing. Fine focus controls like the Feathertouch focuser that replace the stock SCT knob are readily available and work great. Unfortunately, they cost close to $150. Is there a low-cost solution? Actually, the lids from many food jars are just the right size to "enlarge" the focus knob and make focus action noticeably better. The tops found on some popular brands of peanut butter are perfect; they are just the right size and even have textured ridges around their circumferences to make it easier to grip the lid—or focus a telescope. To make a peanut butter focuser, cut or punch a hole in the lid slightly larger in diameter than the focus control. Take care to center this hole as accurately as possible. Wrap a few layers of masking tape around the focuser if necessary to ensure a non-slip fit, slide lid over knob, and enjoy precision focusing for the cost of a jar of peanut butter (Plate 79).

Hartman Mask Does it seem overwhelmingly difficult to focus the CAT accurately enough for CCD or webcam imaging? The focus aids on CCD camera control programs help, but they are not perfect. On less than stable nights the numbers they display tend to jump around as the seeing changes. Isn't there some easy way of obtaining exact focus? You bet: a simple tool called a "Hartman focusing mask" (Plate 80), an aperture cover with two small holes cut in it.

Plate 79. (Peanut Butter Focuser) A focus knob made from a peanut butter jar lid makes SCT focusing finer and easier. Credit: Author.

Plate 80. (Hartman Mask) The Hartman mask turns every star in the sky into a double star. Adjust the CAT's focus control until two star images merge, and the telescope is in perfect focus. Credit: Author.

What does a Hartman mask do? Point the scope at a single bright star with the mask in place and look thorough an eyepiece. Due to the twin apertures, two images of the same star will be visible in the eyepiece unless the scope is in perfect focus. They'll be separated by a small distance, and the star will look like a binary. Turn the focus control experimentally, and the star images will either move closer together or farther apart. If the stars move farther apart turn the control the other way. When the knob is turned in the proper direction, the two star images move closer and closer together and finally join. When the images are perfectly merged at high power, focus is close to perfect.

A Hartman mask is simplicity itself to make. Obtain a piece of heavy cardboard or poster paper (black is nice). Cut a circle just a little larger than the end diameter of the telescope's tube. Then, cut two small holes in this circle. Use a compass to draw the holes and cut them out with a sharp blade. An Exacto knife works perfectly. These holes should be directly opposite each other and should be positioned near the edge of the circle so they will be away from the secondary mount. The size of the holes is not critical, but about 1/5 the diameter of the primary mirror works well. That would make each hole 1.5-inches in diameter for an 8-inch telescope. To finish the mask, glue or tape a cardboard strip around the circumference to act as a "lip" to hold the mask in place at the end of the tube. If that seems like too much work, just tape the mask over the end of the scope (the Harman mask works best when it's mounted over the end of the tube, not the dew shield) with tape.

Diffraction Spike Focusing A Hartman mask sounds good, but making one sounds like too much trouble? There's an even simpler method for achieving exact focus. Tape two lengths of cord across the front of the dew shield in crosshair fashion. Think "Newtonian telescope secondary mirror support" (a "spider"). What this will do is to add a cross shaped diffraction pattern to bright stars as seen in a Newtonian reflector. The stars will show four "spikes." Focus until these diffraction spikes are sharpest and skinniest, and the scope will then be in good focus. This method of assisted focusing works especially well with CCDcameras. If lengths of cord don't seem fancy or neat enough, wide rubber bands or pieces of fabric work even better and can be secured to the dew shield with Velcro if desired.

Motorized Focusing on the Cheap "Motofocus" is super if you're an imager or a high power planetary observer. No need to touch the focus control and induce shakes while focusing. Unfortunately, motofocus units made for SCTs and other CATs are fairly expensive—usually about $150. There's an out: motors sold for the rack and pinion focusers of small refractors and Newtonians cost only a fraction of what CAT motor focusers go for. These devices, almost always with included push-button control boxes, can often be found on swap tables at star parties often for less than $10. Of course some cuttin', fittin', and cussin' will be required to adapt one to an SCT's focus knob. It's possible one of these motors can be fitted to the focus control with its supplied coupler and bracket in a set up similar to that of commercial SCT motofocus units. Unfortunately, that's unlikely to work without a lot of bracket and coupler modification and fabrication. Don't give up, though.

The most elegant and simplest solution might be a belt drive. Not only does a belt installation avoid coupling and bracket issues, it allows the focus control to be turned manually with ease when desired. The focus motor itself is mounted to the rear cell of the scope via the telescope's accessory mounting holes. It may be necessary to drill one hole in the focus motor's mounting bracket, but that will probably be it. The focus knob and motor are coupled together with a belt of the proper size. That's the only marginally difficult part of this project: finding a workable belt. Look around, especially at auto parts houses—an engine timing belt is a possibility—and something will turn up eventually.

Rear Cell Plug and Dehumidifier If the plastic cap that seals the telescope rear port opening goes missing, a plastic 35mm film canister inserted into a visual back makes a good substitute. Leave the cap on to prevent the canister from slipping into the OTA interior accidentally. Kick it up a notch and add functionality by turning the rear port seal into a humidity reducer. Drill a few holes in the canister bottom with a 1/6-inch drill, drop a few small packets of silica gel (often found packed with consumer electronics and available from a variety of sources) into the canister, and insert that into the visual back. Not only will dust be kept out, the tube interior will be kept dry.

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