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Building a Blinking Filter Slide

by Ed Stewart <>

Link to: Filter Slide Modification

Filters, such as Lumicon's Oxygen-III (O-III), H-Beta, and various manufacturer's light pollution and colored filters, are effective in improving the visual appearance of selected objects; however, it is bothersome to remove the eyepiece, screw in a filter (especially fun with a $200 filter on a cold night while wearing gloves), and re-insert the eyepiece to see if any improvement occurs. If there is none, then doing the exercise all over again. Or a nebula filter can be blinked (shifted in and out of the line-of-sight) in front of the eyepiece by hand to test its effectiveness, but there is a risk of dropping it. I decided to design and construct a means to rapidly and safely change filters for my Newtonian reflector by sliding the filters on a thin piece of modeler's plywood between the eyepiece and the secondary mirror.

Photo of filter slide

***Photo showing the filter side-note the unpainted 1/4" strips
down each long edge and the grasping hole on the right end.

My Favorite Accessory

With the Lumicon O-III and Deep Sky filters in my slide, I can check the effectiveness of applying a filter with a simple two-second push/pull motion of the slide. And, by rapidly blinking the filter, I can detect a two- arc second, 13th-magnitude planetary from the surrounding stars. At public star parties, even first-time observers are amazed at the difference in some of the popular nebula as I blink the O-III for them. At club star parties, experienced observers will come over to see the effects with and without filtration on some personal favorite object, usually with the comment that they really need to take the time to make their own filter slide. Although a few have commented that it "makes it too easy."

Photo of filter slide in the telescope tube    Close-up of Guides

***The tracks have been painted white for illustration purposes to demonstrate their orientation to the focuser and upper tube assembly. Closed tube designs can grasp the slide from the front of the tube, while open tube designs, such as this one, can choose to grasp it from below the upper tube assembly.

Drawing It Right

The first order of business is to draw on paper a full-size arc of your tube's inside radius to represent about 6" of the arc below your focuser. Add to the drawing any part of the bottom portion of your focuser that may extend into the tube during inward focusing. The slide is going to be 3" wide X 12" long X 3/32" (3mm) plywood although some larger scopes may need to allow for more width. Draw the 3" width centered below the focuser and about 3/8" down from the tube wall (see diagram). Allow more space if your focuser intrudes into the tube.

The size of the pair of tracks that the slide moves in is 3/4" X 1/2" X 12" with the side touching the tube cut at an angle measured off your drawing so that the tracks will be in alignment with the slide when the tracks are screwed to the inside of the tube.

Diagram of filter slide in the tube

***Looking down the tube, filter slide moves up & down the tube's length.


Cutting Some Wood

After purchasing the 3/32" plywood at a hobby shop, cut it to 3" X 12". Allowing for 1.25" and/or 2" diameter filters and 1/2" between each down the length plus enough space at one end of the slide so that you can still reach it at its furthest inward position down the tube. My scope is an open tube so that the slide can be reached from either side of the secondary cage.

Using an adjustable beam circle cutter in a drill press, use trial-and-error attempts in scrap to first find the perfect hole size into which the filters can be forcefully threaded-this plywood is so tough that you can actually cut threads into it-then drill the filter holes into the slide. Also drill a finger-sized hole into the end of the slide to be gripped as an aid to grasping and moving the slide. Sand the edges smooth and round-off the corners for easier insertion into the tracks. After masking off the 1/4" slide area down each side of the plywood, spray it with flat black paint. Then use car wax on the unpainted slide area for a smoother movement.

Cut the two track pieces to the dimensions determined from your drawing out of ordinary pine or other wood on hand. Paint the tracks flat black, and then cut a 1/8" groove (the width of the saw blade on my table saw) down the length of the tracks 1/4" deep. The difference between the groove width and the plywood's thickness is 1/32". This gap is filled using adhesive-backed Velcro (fuzzy side only) on one side of each groove. The tension and friction of the Velcro allows for easy but not loose movement. Test this motion; and if too tight, cut the groove slightly wider. Now screw them to the inside of the tube. The slide should stay in place even when the scope is pointing at the zenith.

Final Thoughts

To make the focusing position consistent when no filter is needed, I installed a clear UV filter into one of the filter holes. More than likely, your slide will not be perfectly parallel to your eyepieces, but this is actually a benefit! When threaded onto an eyepiece, these filters will often reflect back any stray light, including starlight and moonlight, but with a very slight tilt, the reflection is not directed into the observer's eye.

As to the obvious question of how will you know when a filter is centered under the eyepiece, you will be able to see the circular opening moving across the field of view so it's easy to position with a little practice.

Parts list:

(1) 3/32" or 3mm plywood, 3" X 12"
(2) 3/4" X 1/2" X 12" pine strips
(2) 1/4" X 12" strips of Velcro, fuzzy side only
(1) can, flat black paint

Publication Credit:

This article appeared in Amateur Astronomy #16, Fall, 1996, issue. I recommend this magazine for those interested in the people side of the hobby and for its excellent ATM articles. Check them out: <>


Subject: Filter Slide Modification

From: Kurt Maurer <>

I modified my Ed Stewart Filter Slide with the addition of a simple indexing, or "click-stop" feature for increased ease in centering each filter under the focuser. Here's how i did it:

First, cut V-shaped notches in one side of the filter slide on the centerline of each filter 'station'.

Then drill a hole in one of the slide rails, centered on the slide's slot and exactly in line with the focuser's centerline, and perpendicular to the focuser's and the telescope's optical axis. This hole accepts a ball bearing (bought at the local hardware store). Under the ball bearing, place a small compression spring (also bought at the local hardware store). Then cut a small piece of thin aluminum sheet (from a soda-pop can), and using tiny wood screws (again, bought at the local hardware store), fasten this to the bottom half of the rail, clearing the slide's slot, and partially covering the hole to "trap" the ball bearing in the hole.

So here's what happens: the ball bearing is constantly being pushed against the edge of the filter slide by the spring, but kept from exiting the hole by the brass retainer when the slide is removed. It rolls along the slide as the slide is advanced in the rails, until it is pushed into one of the notches. A slight tug or push is all that's reqired to disengage the ball bearing from the notch.

This requires careful measurement of the ball-hole location in the slide rail as well as the mounting holes, to ensure the ball's alignment with the focuser. I drilled slightly oversized mounting holes in the rails for final adjustment of this, as well as the spacing between the rails for best slide fit.

Finally, don't forget to round off the ends of the slide to help it go past the ball bearing when installing it into the slots.

Commercial version of this design from AstroCrumb: <>
  even has a dew heater available for it.

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