Primero

Every astronomer has a first telescope, mine is a 6″ f/5.1 Newtonian I first built as a teenager. As life progressed I was forced to dismantle the telescope and it dwelt for a time as a pile of parts in a box. Eventually I had an opportunity to rebuild the telescope, but as both my technical capability and my financial means had increased I was able to do a substantially better job.

Primero
Primero setup at TIMPA outside of Tucson
The result is the instrument you see to the right, Primero, or simply ‘first’ in Spanish. The original mirror was used but little else from that first telescope was reused. A completely new mount, a new tube and all new fittings. The only purchased parts are the focuser, the Telrad and the secondary mirror. Several parts of the mount were removed and reworked from a previous mount, this includes both the bearings, shafts, counterweight and saddle. The entire tripod, tube, mirror cell, spider and secondary holder were produced by hand for this scope.

The optical design of the telescope is standard Newtonian with a f/5.1 primary mirror of 6.0 inches giving a focal length of 777mm. The mirror was hand ground when I was a teenager and thanks to expert help during figuring is an excellent mirror.

The RFT design is deliberate and has proven to be a good choice, particularly with modern eyepieces that perform so well in short focal length scopes. With a 35mm Tele-Vue Panoptic eyepiece the scope provides a 22x image with just over a three degree field.

The only finder provided is a Telrad, more than sufficient for an RFT that merely needs to be aimed in the general direction to find the target.

Front End
Nose section of the tube showing the telrad mount as well as the JMI crayford focuser

A JMI crayford focuser gives low profile smooth focusing and properly holds large and heavy eyepieces. The curve on the base of this focuser is for a larger tube, but a piece of high density neoprene foam neatly closes the gap and allows for squaring the focuser to the optical path.

The tube itself is eight inch heavy wall (0.083″) irrigation tube from an irrigation supplier. This aluminum is a good alloy, strong light and durable. A smooth welded seam is visible but not obtrusive. I spent a couple hours wet sanding the tube with a fine grit emery cloth to provide a even matte finish. A coat of automotive wax protects this finish. This gives an attractive tube that also slides easily in the saddle to allow the tube to rotate.

Spider and Baffles
Down the tube showing baffles and hand made spider

A look down the tube shows the baffles added inside the tube to deal with low angle reflection down the optical path. The tube was painted inside with Krylon Ultra Flat Black, a good flat black, but as the photo to the right shows it will reflect some light from a camera flash or a bright object in the field. Two aluminum baffles were cut and bolted inside the tube, one near the front and one just down from the focuser. This appears to work as a bright object like Jupiter gives good contrast and does not show much glare across the field if moved just out of field.

Mirror Cell
Looking in the rear of the tube at the mirror cell

The mirror cell was machined from two plates of aluminum. After they were made to fit the mirror I went on to punch them full of holes to remove as much mass as seemed prudent to permit rapid cooling of the cell and mirror.

The arrangement floats on three small blocks (visible in the photo to the right) that permit adjustment of the cell position in the tube before everything is locked down by means of oversize bolt holes. This arrangement was necessary as the irrigation tubing is not perfectly round and some adjustment was going to be required.

Axis
Detail of the tripod head, latitude adjustment and the RA and declination axis

The mating point between the tripod head and the RA axis is setup to allow latitude adjustment. One of the screws that locks this adjustment down has a couple different holes it can thread into, one hole is set for a latitude of 45° while the other is set for 32° (Tucson), there is room in the metal to drill and tap another hole or two for different latitudes.

To ensure smooth motion of the axis it proved necessary to put a very high tech material between the load bearing collars on the axis. You can see this in the photo as the thin white washers present in the axis assembly. The material, 0.020″ polypropylene sheet, cut from the top of a sour cream tub, it works very well.

The first mount for the OTA worked well mechanically, but the tripod was entirely unsatisfactory. It wobbled and bounced every time the scope was moved and with every breath of wind. The largest problem was the legs. Thus I rebuilt everything from the central hub down, keeping the bearings and shafts, just stripping the paint so they could be anodized to match.

I tried and succeeded in making a tripod that was absolutely steady. Each leg is two lengths of 1″x2″ oak bolted side by side in such a way that the legs can not flex outward. he head is machined from a solid billet of aluminum, the largest and most complex piece of machine work I had ever attempted. It is hard to over design a telescope mount and despite the solid components this mount is still light enough to be easily portable

Spreader
The spreader with an accessory plate and spare eyepiece positions

To keep the mount steady I used a heavy spreader. The center plate of aluminum is bolted to struts that mate with disks between the pieces of the leg. These disks can rotate to meet the correct angle of the spreader and hold a knob with a 1/4-20 threaded shaft that screws into the end of the strut. When in place the entire tripod may as well be a solid piece as there is no substantial flexure anywhere.

The mount is a traditional German equatorial by deliberate decision. I considered a dobsonian mount as it is definitely a simpler mount to construct. I have nothing against the dob design (I have constructed four of them) but opted for the GEM mount with which I could track an object by moving in one direction only. The scope also moves north-south, east-west making star hopping easier as the movement corresponds to chart directions. An additional benefit is the height, as I am 6’2″ this scope is tall enough to match.

Adjustable Leg
Detail of the adjustable foot arrangement

For leveling each leg is provided with a jack. Simply a 1/2″ steel rod attached to each foot that can be extended and locked with a simple clamp knob arrangement.

To preserve the aluminum I had everything anodized at a local coating shop. The bright purple color has become something of a trademark. All telescope parts I have machined since sport this color and bits and pieces of telescopes from Arizona to Washington that I have contributed parts to can be quickly recognized.

Set Up
Primero setup at Jarnac Observatory among a friendly crowd of other telescopes

The result is a classic looking design that would have been right at home in a star party in the mid 20th century, before the dobsonian revolution.

The scope has seen good use and is now the veteran of four messier marathons, scoring 106, 108 and a perfect 110, as well as a tough, cloud challenged, 104. The wide field of view makes finding objects very easy, if the object is in the telrad rings, it is in the field of view.

Whenever I just want to take a fun scope capable of beautiful wide field scanning, I leave the 18″ at home and load up Primero.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai’i.

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