8-inch scope details

Overview

I built the whole scope except:

My aim was to make this the most convenient telescope possible for my style of observing. As a result, I can carry the whole assembled scope outside in one trip (including the integrated equatorial platform), remove a couple of covers, turn on the cooling fans, collimate whilst looking into the focuser (using the easily accessible collimation rods) and be ready to observe. If I want to use DSCs/tracking, the controls are mounted on my stand within easy reach. If I want to pop in a filter, that just involves a small turn of the filter wheel. If it looks like a dewy night, I plug in one cable which supplies power to all my heaters. It's an extra trip for a heavy-duty battery, but that's only necessary if I'm going to be outside for a long observing session anyway. Everything can by powered by smaller batteries which are usually left hooked up.

Filter Wheel

This is one of my most used accessories. A lot of people use filter slides, but I haven't seen many examples of filter wheels for visual use. My wheel is made of a circle cut from 4mm plywood. It has 6 holes for 1.25" filters. It's actually quite straightforward to tap threads into plywood – I used a cheap moon filter and just screwed it into the slightly undersized holes (done with a 1" spade bit and a file) by force. The centre axle of the wheel is just a bolt which goes through a bracket made from aluminium angle, and it connects to the focuser shelf via a bolt and three set screws. The set screws allow me to adjust the tilt of the wheel (similar to a secondary mirror holder), to ensure the filter surfaces are perpendicular to the focuser draw tube (important since the wavelength of interference depends on the angle of the incident light!). A spring-loaded door catch and notches in the filter wheel provide an index to let me know when each filter is rotated into place.

Currently I have mounted in my wheel: LP-2 (OIII+Hβ) and LP-3 (OIII) filters by Thousand Oaks Optical and an Antares variable polarising filter.

Dew heaters and electrical

I made heaters for my optics using nichrome wire or resistors covered with heat shrink tubing. These connect via speaker cable to RCS jacks in the same way as typical commercial systems. I followed Mark Kaye's method to build the heaters. The eyepiece, quikfinder and optical finder heaters all connect to the simple junction box shown in the photo (left). This is just 4 RCA sockets wired in parallel. Into one of the sockets I plug an RCA cable which carries power from my rocker box. The rocker box contains a PWM circuit which allows me to efficiently control the amount of power going to the heaters. A red LED on the top of the box (with a high-valued resistor to keep the light dim) lets me know that the heaters are on and the brightness indicates the power level.

I also use passive methods to help control dew. The black foam on the Rigel Quikfinder is a hood which can be opened and closed as needed. I have a foam dew cap on my optical finder too, and try to replace the covers when not in use.

The PWM controller, the leads to my fans and my platform controller, etc., are all gathered together in my rocker box and are wired in parallel to a single cigarette lighter jack. This means I can power everything from a single 12V jump starter battery. Alternatively, most of the devices terminate in 9V battery snaps, so PP3 batteries can be used instead if I just want to set up quickly and don't want to carry a 12V battery around. The one exception is the dew guard on my secondary heater (the wires are visible in the filter wheel photo above). The 9V connector from the circuit goes to another 9V battery snap wired up to two electrically-isolated vanes of my wire spider. These connect to a 9V battery and switch on the underside of the upper ring [the power at the upper end of the scope varies depending on the heater settings and the dew guard needs 9-12V].

Fans

To cool the primary mirror, the mirror box has 2 small fans at the front to blow air across both the front and back. There are no exhaust ports since the mirror box is fairly low profile. The bottom of the mirror box is covered with a fine mesh to keep dirt and bugs out. The green wires in the picture to the left run from the mirror box (where they connect to the main power supply) to a 9V battery snap which can be connected to the fans. The fan grills are also visible from the inside of the box in the picture below. Incidentally, the mirror box is covered with 2 layers of birch veneer except around the fans. This side uses 1/64" plywood with one layer of veneer of the top. This allows the face plate to be removed for access to the fans (note the screws at each side).

Collimation rods and mirror cell

This photo, taken during construction, shows the insides of the mirror box. The mirror cell is made from two lengths of aluminium square tubing bolted together in a tee. I started following the design of Mike Lidner's cell. As I went along I came up with a simple method to allow the primary to be collimated whilst looking through the focuser. I thought of extending the collimation bolts (look at the last photo on his page) out through the front of the mirror box. The other difference is how the mirror is held in place. In the past, I've tried both mirror clips in front of the mirror and silicone adhesive on the back of the mirror, but I found this interesting method by Bob Royce. I used three pieces of aluminium angle to make edge clips. I drilled and filed slots in the bottom to allow them to slide back and forth where they attach to the cell by a bolt. On the top surface of the bottom of each L is stuck a piece of teflon on which the mirror sits. On the top of the L, a side clip is made from a large rubber washer, curved to the shape of the mirror and attached to the bracket with silicone adhesive. The mirror is not stuck to the cell, and can be taken out in seconds by loosening the bolt to one clip, sliding the clip outwards and lifting the mirror.

The photo to the right shows the assembled scope (right) next to a Starmaster V8. The collimation rods can be seen. Since this picture was taken, I've split the tubes in half and put an intermediate ring (similar to the ones visible in the V8) in between to add extra stiffness. The V8 is a fantastic scope and I was lucky enough to be able to borrow it whilst building my scope. It gave me lots of ideas, and the versatility of this scope allowed me to swap individual components between this telescope and mine. In the end, it was the view using the fine Zambuto mirror that persuaded me to spend the money on a premium mirror.

Spring counterweights

In order to minimise weight (to meet my goal of the scope being easily carried around in one piece) I use two spring counterweights – one on each side of the rocker box. These comprise bungee cords wrapped around wooden pulleys and connected to kevlar cords which terminate in a keyring at the end. The keyring slips over a bolt on the altitude bearing. These photos illustrate the spring (between the box side braces, made of paler wood) with the scope at three different altitudes. The kevlar cord is yellow and the bungee cord black. I still need to neatly mount the various other wires out of the way. A couple of smaller silver pulleys guide the virtual counterweight.

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