Refractor with Flip-Mirror to Smaller "Finder"

Back in 2005 I created a six-inch rich-field refractor that was "bent" so the eyepiece position was always convenient. It was the subject of a Sky and Telescope article in December 2006 and you can read that article here.

At the end of the article, I mention that

One modification that does strike me as potentially worthwhile is a Questar-like built-in finder utilizing a second, smaller finderscope objective. If you placed the finder objective (with its own bending mirror either in front or behind it) opposite the focuser, you could then engage the finder by switching the main bending mirror out of the way ...

I ended up creating such a telescope. The result does not match the optical layout of a Questar, because I decided that moving the main folding mirror was a bad idea, because it is large and heavy and alignment is important. Rather, I created a separate folding mirror placed between the main bending mirror and the focusser -- that is the "finder" is on the focuser side of the main OTA, not on the far side of it.

This telescope is the result: an eight-inch f/6 telescope which can be converted into a four-inch f/6 without moving your eye away from the eyepiece. I find it a joy to be able to switch to a twice-as-wide field when star-hopping or just to observe a larger context.

And this telescope was discussed in the Gary Seronik column in the September 2010 issue of Sky and Telescope.

You can read about my Rich Field Telescope design notes here.

The Main Telescope and Basic Design

The telescope started out as a eight-inch f/6 "TMB refractor" OTA that I purchased from APM Telescopes. The "bend" keeps the eyepiece in a comfortable position: see the December 2006 Sky and Telescope article.

The smaller telescope is the objective from an Orion four-inch f/6 refractor -- I could not find a good source for a f/6 objective in a cell, so I bought the short-focus refractor and sold the tube and focusser.

Both objectives are f/6, so the exit-pupil stays the same when you switch between them. A 35mm Panoptic eyepiece gives me 35x with the main objective and 17x with the four-inch. The true four degree field of the latter is big enough to use as a finder field, in conjunction with a small 1x red-dot finderscope

The four-inch objective is supported on three "stand-offs": crude but effective, and it easily enables adjusting the position.

The Outboard Portion and the Connection

The red square tubing contains the flip mirror, and holds the focusser at the far end.

It is attached to the main OTA via an aluminum shape (also red) that is the only piece that I had specially made. A machine shop used CNC milling to create a six-inch square with a cylindrical shape on the bottom to match the main tube, and a through-hole for the light-path. It is bolted to the main tube from the inside.

In use, the six-inch square tube is attached to the OTA using three simple luggage-style clamps and two locator pins in opposite corners.

Adjustments

You need several adjustments to make all this work. You want to be able to tinker with:

  • The alignment of the main eight-inch objective so the optical axis of the eight-inch is at the center of the focusser
  • Similarly, the alignment of the four-inch objective
  • The orientation of the four-inch so the two subsystems see the same field
  • The distance of the four-inch from the focus, so you do not have to refocus when switching
  • These four alignments are performed by:

  • Tipping the bending mirror side-to-side and rotating it. (One could tip and tilt the bending mirror. The way it is supported, it is more convenient to tip and rotate, which also is effective.)
  • Tipping and tilting the four-inch objective via the stand-offs.
  • Adjusting the position of the flip mirror when it is in the "down" position - more on this below.
  • Adjusting the distance from the four-inch cell to the red square tube by adding washers to the standoffs.
  • The only tricky alignment in the above is aligning the flip-mirror in the down position, to make the "finder" field of view align with the main-telescope field. You can see in the photo a small off-white machine screw at the bottom of the red six-inch tube. That is a nylon screw and the flip-mirror rests on it in the "down" position. That handles one degree of freedom. The other one is harder. The flip-mirror rotates via a piece of piano-hinge at the top (objective-side) of the six-inch tube. I alter the orientation of the hinge by loosening the two bolts that hold it, sliding it, and re-tightening. (When I first built the mechanism, this was impossible, because the nuts were unreachable when the flip-mirror was in place. I changed the pair of nuts to be two threaded holes in a small bar so they were captive.) This is awkward. I should have built a real mechanism to tilt the piano hinge -- perhaps by allowing the ends to be raised and lowered.

    Holding the Flip Mirror in Place

    When the flip mirror is in the "down" position and you are looking through the four-inch objective, it rests against the nylon screw and gravity holds it there.

    Whan the flip mirror is "up", you need something to hold it there. Ideally, you would like a mechanism that holds it but will release with enough force, so that you can move the mirror via its handle without having to unlatch something with your other hand.

    What I use is a small magnet. In the initial photograph, you can see a small piece of black tape on the front of the red six-inch tube. There is a small flat magnet at the bottom-end of the tape, and the tape is attached at the top of the tube. There is a matching magnet on a small standoff on the plate that the flip-mirror is on. When the flip-mirror is "up", the magnets attract and hold it there. I can "flip" the mirror either by using force on the handle to break the magnet grip, or by holding the handle and, with my other hand, just sliding the tape to the side to separate the magnets.

    This is not ideal: Two hands are preferable and with a hard knock the flip-mirror can fall down. I think that a better arrangement would be some sort of slow-the-fall cushion at the bottom (next to the white screw), so you could just release the magnet (or other catch) and let the flip-mirror fall down. It would be like a door-closer that stops a door from slamming. But I haven't tried to build one yet: there isn't a lot of free space inside the six-inch tube, and the mechanism has to allow the flip-mirror to settle on to the white screw to keep it aligned. And note that the down-force will vary depending on the altitude that the telescope is pointed to.

    Remark on Wide-Field Observing

    I use this telescope for wide-field low-pwer observing with nebula filters. It is not suitable for high-power viewing, because the main objectives are simple f/6 doublets.

    In the photo, you can see a device sticking up from the focusser, before the final star-diagonal and eyepice. That is a Lumicon filter slide. I find it valuable to be able to switch between filters without taking my eye away from the field. I can switch between Lumicon UHC, O-III and H-Beta filters (one filter slide) or between a Custom Scientific narrow O-III filter, a Custom Scientific "green continuum" filter, and a H-Beta filter (an alternate filter slide). The latter lets me "blink" nebula between the O-III line or H-Beta line and an adjacent passband of similar width that lacks nebular emission: a great way to see whether you are really observing nebulosity or not.

     

    Contacts

    I would be happy to correspond about this telescope design with interested individuals.

    I can be reached via email as "bob" at this dot-org domain. Or at astroayers@gmail.com