4/15/2024: I added two files for a solar filter. I attached some Thousand Oaks solar film to Part A using a two-sided adhesive sheet, then attached Part B using a few drops of super glue. This filter threads into either the bezel or the dew shield.

This is a small spotting scope based on the 76mm Maksutov lens/mirror combo available from The Surplus Shed:

https://www.surplusshed.com/pages/item/PL1298.html

NOTE: This is a hugely popular item. Whenever Fred gets a new shipment, they sell out the same day. I recommend that you go to the site and click "Notify Me" to get a heads up when they're back in stock.

This is a remake of a previously posted design:

https://www.thingiverse.com/thing:6431085

That design was based on a length of 4" schedule 40 PVC tubing. This design is based on a length of 3-1/2" sched. 40. In addition, I've made a couple of changes:

• I tightened up the tolerance between the objective slide and the slick strips. This has significantly reduced image shift when focusing, but there is still a bit.
• I added an optional dew shield. The dew shield screws onto the bezel and is threaded to accept the cover.
• Modified the objective plate to reduce weight and speed cooling of the objective mirror.

The project consists of the following 3D-printed parts:

Meniscus cell: Holds the front meniscus lens and secondary.
Meniscus retainer: Screws into the meniscus cell to hold the lens in place.
Objective plate: Mounting plate for the primary mirror. The hole in the mirror fits over a threaded tube.
Mirror washer: A beveled washer that goes over the threaded tube on the objective plate between the objective mirror and the baffle tube.
Baffle tube: Threads onto the objective plate.
Objective slide: Mount for the objective plate using three screws and springs, allowing for collimation of the primary mirror. The slide moves inside the tube to achieve focus.
Rear cover: Mounts to the rear of the optical tube. Receives the diagonal holder. Has three holes to access the collimation screws and one hole for the focusing screw.
Diagonal holder: Screws into the rear cover. Has two screws for retaining the diagonal.
Collimation cover (3 req'd): Plugs the collimation holes in the rear cover. NOTE: These are printed with TPU.
Focusing knob: Holds an M5 nut to engage the focusing screw.
Bezel: Mounts to the front of the optical tube. Threaded to receive the cover.
Cover: Threads into the bezel to protect the meniscus lens.
Strap top (2 req'd): Top half of the strap securing the scope to the dovetail bar.
Strap bottom (2 req'd): Bottom half of the strap.
Mounting bar: Vixen-style dovetail bar. Attaches to the strap bottoms.
Dew shield (OPTIONAL): Screws into the bezel.

Two other printed parts are needed to assemble and adjust the scope:

Drill guide: Used to locate the mounting holes for the rear cover.
Collimation tool: Sized for an M3 nut. Used to collimate the scope.

The following additional parts are required:

3.5" Schedule 40 PVC pipe, 6-7/8" long
M5 X 40mm hex-head bolt and nut: Used to focus the scope by moving the primary mirror.
0.6 X 7 X 30mm compression spring: Moves the objective slide when focusing.
M3 X 15mm screws, nuts & washers (3 req'd): Holds the primary mirror on springs, allowing for collimation.
0.5 X 6 X 10mm compression springs (3 req'd): Part of the collimation mechanism.
45 deg. diagonal: Available from Ali Express or Amazon - https://www.amazon.com/SVBONY-Refractor-Catadioptric-Telescope-Astronomical/dp/B071CKG619/
M4 X 10mm screw and nut (2 req'd): Retains the diagonal in the diagonal holder.
3/4" X 4-1/2" Slick Strips (3 req'd): The objective slide rides on these strips spaced 120º apart on the inside of the tube. https://www.amazon.com/Slick-Strips-Width-32-Thick/dp/B0035Y46GM
M4 X 16mm flat-head machine screws, washers & nuts (2 req'd): Attaches the bottom straps to the dovetail bar.
M3 X 12mm screws, nuts, & washers (4 req'd): Attaches top straps to the bottom straps.
No. 4 X 1/2" flat-head wood screws (6 req'd): Attaches the rear cover to the tube and the bezel and meniscus cell to the tube

I bought this compression spring assortment from Amazon: https://www.amazon.com/Compression-Assortment-Stainless-Different-Individual/dp/B09VZH6NS1/

Assembly procedure:

OTA:

1. Apply the slick strips to the inside of the tube spaced 120 degree apart. Use the objective slide to position them. Make sure the objective slide rides on the strips with minimum resistance.
2. Align the drill guide with the slick strips. Drill three 3/32" holes through the holes in the guide. Countersink these holes to accept a No. 4 flat-head wood screw.
3. Slide the bezel over the other (front) end. Align the holes with the drilled holes at the back end. Drill three 3/32" holes through the mounting holes on the bezel.
4. Mask off the outside of the tube and the three slick strips. Spray paint the interior with flat black paint. Make sure you use a paint rated for plastic.
5. (Optional) Mask off the ends of the tube and paint the exterior with color of your choice.

Meniscus assembly:

1. Slip the meniscus into the meniscus cell with the concave side facing front. Secure with the meniscus cell retainer.
2. Slide the meniscus cell assembly into the front of the tube. Align the holes in the cell with the holes in the tube. This is a bit tricky; try using a paper clip to find one of the holes and verify that it's aligned.
3. Slide the bezel over the front of the tube, taking care not to disturb the meniscus cell. secure with three No. 4 x 1/2" flat-head wood screws.

Rear assembly:

1. Insert three M3 X 15mm screws into the recesses of the objective plate. Temporarily retain them with a bit of blue painter's tape.
2. With the objective plate upside-down, install three 0.5 X 6 X 10mm compression springs over the M3 screws.
3. Insert the M5 hex bolt into the objective slide, seating the hex head in the recess.
4. Holding the objective slide by the M5 bolt, lower the slide over the objective plate, making sure that the three M3 screws come through the holes.
5. Pressing the objective slide down to compress the springs, secure with three M3 washers and nuts. Only thread the nuts until one or two threads show. Remove the blue painter's tape.
6. Insert an M5 nut into the focusing knob.
7. Slip the 0.6 X 7 X 30mm compression spring over the M5 bolt on the objective slide.
8. Lower the rear cover onto the primary mirror assembly with the M5 bolt through the large hole and the three collimation screws aligned with the three collimation holes.
9. Screw the focusing knob onto the M5 bolt.
10. Install the mirror over the central tube of the objective plate.
11. Place the mirror washer over the central tube of the objective plate, beveled edge down.
12. Thread the baffle tube onto the objective plate. Tighten finger tight, no tighter.

Final assembly:

1. Insert the rear assembly into the tube, aligning the grooves in the objective slide with the slick strips. Secure the rear cover with three No. 4 X 1/2" flat-head wood screws.
2. Secure the bottom straps to the dovetail bar with two M4 X 16mm flat-head screws and nuts.
3. Secure the top straps to the bottom straps with four M3 X 12mm screws, nuts and washers. Leave these loose.
4. Insert the telescope into the straps and tighten the M3 screws.
5. With the telescope pointing at a bright, featureless surface, look through the rear opening and adjust the three collimation screws until the image of the baffle tube, mirror, and secondary spot are concentric.
6. Insert three collimation covers into the collimation holes.
7. Insert two M4 nuts into the diagonal holder. Thread two M4 X 10mm screws into the nuts.
8. Thread the diagonal holder into the rear cover.
9. Insert the diagonal into the holder and secure with the M4 screws.

You may screw the cover into the bezel (or dew shield) when not in use.