An Indoor Observatory, My 10 Inch Window Scope

After seeing Oscar Knab’s article years ago in Sky & Telescope I always thought it would be neat to have such a scope with an indoor eyepiece location especially in winter when my observing is frankly almost nil. Imagine sitting in a comfortable chair indoors at the eyepiece when the temperature outside is numbing cold! Over the years I never had a house with a southern exposure or the right conditions. Finally however I have a house with a southern exposure but with a lot of trees and light polluted skies. Even so I do have enough of a section of sky that I can do some planetary observing.

Fig.1 Optical Layout

Design

Oscar’s polar scope has some advantages but I couldn’t see carrying a long refractor and flat assembly in and out of the house nor was it something I wanted to leave outside permanently out on my patio (my wife wouldn’t go for that). I eventually settled on the design in fig.1. The flat is mounted on an alt-azimuth mount and feeds light into the primary mirror. The flat is right outside my window and as a result is blocked from seeing anything north of overhead by the house. Even so it isn’t a problem at least for planetary observing. The design is a catadioptric Herschellian. The primary was to be mounted in a box mounted on two posts at the edge of my patio. The distance to my dining room window to this location determined the focal length I needed. At f/17 the tilted primary mirror will have too much astigmatism though and needs correcting elements. I had planned to use a subdiameter Maksutov corrector but later discovered that two tilted lenses worked better especially in the field. These are a weak positive and negative lens that I made. This is a different design than the Maksutov because the primary should actually be parabolic but for me (having already made the primary spherical) at f/17 the difference is quite insignificant. The telescope should give outstanding better than APO performance.

The most difficult part of this project was the flat. You can sometimes find these for sale on Astromart if you are lucky. An ATM can also make a flat and you can read my July 1990 article in Sky on the Raleigh water test which is how I made mine. My flat is 12.5 inches diameter and 2 inches thick Pyrex (actually I’m still using one a friend made but it will be replaced with mine). I measured the distance from the edge of the patio to about 16 inches from my dining room window as 170 inches. I then ground a full thickness 10 inch pyrex mirror to this radius and polished it to a good sphere. I used the Foucault test to obtain a good null making the primary relatively easy to make.

The only other optics in this scope are two correcting lenses. These are Bk7 glass although I could have used many other glass types. I could have tried to use off the shelf lenses since you can find them up to 500 mm focal length and 2 inch diameter. The longer focal length of these lenses the better the performance in the field, however, as long as they are located behind the flat. For this reason I made my own lenses starting with two BK7 optical windows. While protecting the other side with plastic tape I generated the curves on these and then lapped them. I poured a pitch lap and polished them using a small polishing machine in my basement. The concave lens can be tested with the Foucault test and the convex tested by interference against a master. That completed the optics for the scope, I could have AR coated the new curves but since I’ll be primarily observing planets I didn’t think the light loss would matter. I silvered the mirrors by chemical silvering both to try chemical silvering and to do testing and debugging before I finally aluminize the mirrors. The silver coat didn’t look as nice as fresh aluminum but worked quite well anyway.

Mechanics:

Instead of an optical tube assembly there are three components:

1, a permanent mirror box holding the primary

2, a motorized alt-azimuth mount for the flat and shelf with motorized turntable.

3, a window unit and eyepiece holder.

The mirror box is simply a plywood box that I covered with aluminum flashing glued on with contact cement. I also used aluminum tape around some edges that might see moisture. The front panel is hinged and has a weather seal along its back to seal out the weather but can be opened and held in the up position. Inside the box the mirror is supported with a sling and has only three points on the back since the mirror is nearly vertical. The back supports have three adjustment screws with large diameter knobs that can be adjusted from the front side. There are two mirror clips that keep the mirror from falling forward in case the box would accidentally be bumped. Two 4x4 treated posts were sunk in the yard and the mirror box screwed to these posts at roughly 12 degrees from vertical.

The mount for the flat sits on a wooden shelf that is attached to the side of the house when needed with masonry anchors and wing nuts. An 11 inch acrylic worm wheel rests on three ball bearing races and rotates about a 1/4 inch steel dowel pin. The mount for the flat rests on this acrylic wheel and pin. The worm wheel is driven by a ¼-20 stainless steel threaded rod and a geared stepper motor. The flat is housed in a wooden box which rotates in altitude on two ½ inch stainless shafts. Another ¼-20 stainless threaded rod is bent into a curved bolt and attached to this box (this forms a curved bolt drive). A nut soldered to gear is threaded on the curve bolt and is captive. Another stepper motor with spur gear drives it which moves the flat in altitude. The flat is located just below the return axis of the primary mirror and was positioned after the window panel and eyepiece focuser was made.

Different views of the shelf and mount for flat

The window panel is just an oak frame made to fit tightly in the window opening. ¼ inch oak plywood was used to keep the panel light. A small box was made with a 15 degree angle to hold the refractor focuser co-axial with the primary. A hole was cut in the panel to hold 3.125 aluminum tubing which will hold the lens holder. The lens holder is made from 3/8 inch maple plywood (below) and has plastic tipped set screws for centering the convex lens in X and the concave lens in Y. The concave lens holder is hinged so that the angle between the lenses can be adjusted. After completing the window panel I found that the focuser and heavy eyepiece was stable enough and the image shook when my wife walked about the house. I decided to redesign the eyepiece holder to hold a lighter focuser and use a piece of aluminum tubing to hold both the focuser and correction lens holder. Also the assembly will rest on the marble sill for extra stability. Show below is a long focal length eyepiece made from lenses I had laying around approx 4 inch focal length. It is an achromatized Huygens and does a pretty nice job with a little astigmatism at the edge. The f/17 focal ratio is easy on eyepieces. Made from the 3.125 inch tubing it covers a full one degree field of view.

Window panel and stepper control box

4 inch FL eyepiece 22mm Nagler 16 mm Brandon

Electronics:

I built an electronic control box to manually adjust the stepper speed and direction to keep an object centered in the eyepiece. I suppose that others would make a more sophisticated drive system or even a GOTO system but I know my limitations when it comes to electronics and so I decided to keep it simple. I bought two MD-1 stepper driver cards from the internet making some small modifications. I bought four boards in all, I ruined two boards in the process but these boards only cost $10 each. Pin 11 controlling motor direction was wired to ground on the board. I had to take an Exacto knife and remove the copper from this pin and pin 10. The pin for half stepping is wired underneath the 5604 chip and can’t be disconnected as easily on this board. Then I wired pin 11 to a slide switch to switch between ground and 5 volts which reverses the motor. The pot R1 on the board changes the motor speed. I cut the copper on the board to this pot and wired a 10k pot on the box cover for each board. Capacitor C1 on the board also controls the motor speed and I wired another slide switch to switch between a 5 mf or 20 mf capacitor for an additional speed range on each motor. The 25 pin output cable plugs into the window panel connector which is just an extension. Outside the cable plugs into one on the shelf and a 9 pin connector connects the altitude motor. I have two slide switches to change the direction of each motor, two slide switches to switch from fast or slow range for each motor and a pot on each to vary the speed in that range.

Stepper control box and MD-1 card

Finder:

How to find anything in such a scope had me stumped for a while. I first thought of an auxiliary scope with a mirror that I would flip in and out or possibly a video camera. Finally a friend suggested a green laser. I tried one and liked this solution. Since there is no obstruction I can just put it in the eyepiece focuser. Outside I have a remote switch wired through the control box to turn on the laser outside. I just rotate the flat manually, disconnect the curve bolt gear clamp and disengage the gear and spin it to a new location (takes just seconds). This requires going outside for a minute to find a new object but it isn’t a problem as long as I don’t accidentally hit an airplane with the laser.

Performance:

I’m really pleased with the way the telescope performed. It is very refractor like in performance but with reflector like color correction and is diffraction limited over a flat half degree field. With a comfortable indoor eyepiece position I find that I really can observe for extended periods without fatigue. I even observed a satellite transit the moon. As a 10 inch schiefspiegler the moon has become a fascinating object with never ending detail. You might chuckle at my lazy armchair astronomy but I never imagined it could open up a whole new observing experience.

Imagine seeing this from your dining room window!