Telescope Making – Page 4

Cassini’s Optics

I have always enjoyed learning about the history of astronomy, it is a science whose roots can be traced continuously back to the dawn of human history.

One of my Facebook friends is a bit of an old telescope nut, even more so than myself, regularly posting photos of historic observatories and in particular old refactors. I too have a soft spot for these historic instruments, going out of may way to visit Greenwich Observatory in London, to drive up Mt. Hamilton to see the beautiful old refractor at Lick Observatory, or flying across the country to see one of William Herchel’s telescopes on display at the Smithsonian.

Ovidiu Cotcas recently posted a link to a fun research paper analyzing the telescope optics of Cassinni’s telescopes. These instruments were state of the art in the mid-1600’s, a period when the first telescopes were being used to provide the first good look at astronomical objects, revolutionizing our understanding of the universe. Only five decades after Galileo astronomers across Europe were attempting to build ever better instruments to provide views of the planets that had only recently been nothing but moving lights in the heavens. These early telescopes showed that planets were worlds, opening a whole new realm to observation and study.

Paris Observatory XVIII Century — Paris Observatory in the times of Cassini during the late 1600’s showing the very long focal length refracting telescopes of the day. Credit: Wikimedia Commons

Prior to the invention of the achromatic doublet in 1758 the main limitation of refracting telescopes was chromatic error. A single lens is also a prism, focusing the different colors of light at different focal lengths. The only solution to this was to make objective lenses with very long focal lengths. Today’s telescopes use compound lenses of two or three elements in the objective with different types of glass. This combination of lenses can be cleverly arranged to cancel out chromatic error resulting in an achromatic lens.

The long focal lengths of those first singlet lens telescopes appear absurd by modern standards, huge instruments with long tubes suspended from masts or with the objective lenses mounted upon tall towers while the observer and eyepiece were at ground level. Telescopes were thirty or even a hundred feet long. Unlike today’s convention of referring to a telescope’s aperture, telescopes were referred to by focal length. Cassini’s primary instruments had focal lengths of between 17 and 40 feet, with one having the incredible focal length of 150ft! As familiar as I am with using small telescopes I shudder at the challenges of aligning and aiming such an instrument, much less tracking a target across the sky.

Continue reading “Cassini’s Optics”

SBIG ST-i Camera Mount

The SBIG ST-i is a useful little camera, I have enjoyed using mine. I usually use the camera for autoguiding, but it can also be used for basic astrophotography and even some science. In preparation for using the ST-i with a group of students I had need to make a couple additional mounts. After I go to the effort of designing a simple solution to my problem I may as well share the solution.

SBIG STi Mount — A simple mount for an SBIG STi camera for use in a piggyback setup

The ST-i camera is designed to slip into a standard telescope focuser in place of an eyepiece, as it is the same diameter at 1.25″. The camera can also be used in a “piggyback” style, mounted atop a telescope and fitted with a standard c-mount lens. Using a lens the camera will have a much larger field of view. The front of the camera is threaded for c-type 1″ threads to accomodate this. The SBIG guiding kit provides such a lens and a ring style mount. If you do not have the kit, or already have a suitable c-mount lens you still need a suitable mount.

ST-i camera mount mechanical drawing

I have included the mechanical drawing for the mount in the link above. I machined this from a block of aluminum. There is no reason it could not be made of wood or plastic to allow fabrication with whatever equipment is available. For wood you may need to make the block a little longer and use inserts for the threading. Plastic could be done pretty much as drawn.

The version I made was milled from a solid chunk of aluminum, but a good version could be easily cut from wood and assembled with brass inserts. The design could also be 3D printed without much loss in mechanical robustness.

To mount a c-mount lens you will need the adapter ring sold by SBIG to convert the 1.25″ filter thread found on the camera to the 1″ c-mount thread. Still, at $40 this ring is a lot less than the $350 guiding kit. Good c-mount lenses can be found from many sources for less than $100. You will need a focal length between 75 and 150mm for a nice image scale and as wide an f/ratio as you can find. The kit includes a 100mm f/2.8 lens which I find is quite useful in guiding my Televue 76mm or the AT6RC.

Makaʻiki – A 6-inch Travel Telescope

A six-inch f/5 telescope designed to allow maximum portability. A simple travel telescope that can go anywhere, small enough to fit in an airline carry-on bag while leaving room for clothing. Large enough to provide good views of celestial targets.

First Light for Makaʻiki — Makaʻiki under the stars for the first time at Hale Pōhaku on the side of Mauna Kea

I call the design Travel6, for obvious reasons. The actual telescope I have dubbed Makaʻiki, or simply “little eye” in Hawaiian. While 6″ may seem big to some, by the standards of amateur telescopes it is quite small. By the standards of the telescopes I work on it is downright miniscule.

Still, a 6″ telescope is quite capable in the right hands, able to give pleasing views of many celestial objects. The design is an RFT, or rich field telescope. A low power, wide angle eyepiece will result in a field rich in stars.

Like all good telescopes the design is based on ideas borrowed from other telescopes I have seen. In this case the basic design is from a very similar telescope by Brett Schaerer he named WikiKea, a telescope I had a chance to examine at Oregon Star Party a couple years ago. He incorporated a clever focus mechanism into that ‘scope that got my attention. This is a design I liked, I would have to build one for myself sometime.

Travel6 Telescope Plans

Of course I had no plans for the ‘scope, just a couple photos. The first task was to draw up a complete set of plans, only then would I be able to understand all of the design issues. I have posted the full mechanical plans for the telescope at the link above. These plans should be enough for anyone with a little workshop savvy to duplicate the telescope.

The design shown could be notably simplified if one chose to do so. I have enough parts left over to build a second scope, and may consider a refined design the second time through based on the lessons of the first pass.

Just a Little Smoke

With aluminum shavings flying about I was enjoying an after work session in the machine shop.

Machining the mirror cell from an aluminum scuba tank on the lathe

Making the heavy cuts to rough out the part I needed to remove a lot of material with a boring bar, I was cutting hard. The tool was also getting bogged down with aluminum sticking to the steel tool, to prevent this I was using a little machine oil while cutting. Predictably things got hot and started smoking, no problem, just stop and let the metal cool as I watched a streamer of smoke waft up from the spinning part.

Then the fire alarm goes off.

A moment of disbelief ensues, this had never happened before, and this was certainly not the first time I had created a little smoke in the machine shop. Nothing to do but call our facilities manager and let him know that I had just set off the fire alarm, not that he could mistake the loud ringing in the background of the phone call.

I also realized that earlier that day the fire alarm company had been about, replacing old sensors on the alarm system. This had to be related. With my phone call complete I joined everyone else outside. Admitting responsibility led to a round of good natured joking at my expense… Yeah, I will hear about this for a while.

Fire alarm reset everyone went back to work. I hope my disruption of the night crew preparing to observe was not significant. It was still quite light out and observations had not yet begun. I returned to finish the part, this time carefully not creating any smoke in the process.

The next day I found out what really happened. It had been decided to replace the heat detectors in the machine shop with smoke detectors. To my mind not a great decision, they will get set off. It is not a question of if, but when and how often. I just find it amusing that this occurred the first day. No worries, I expect it will get fixed. The same person who made that decision gets the phone call when the alarm goes off. Sufficient motivation to deal with the problem I expect.

First Light for Makaʻiki

First light is complete… It works very nicely, the images of rich star fields and sweeping nebulae are worth the hours of work it takes to complete the fabrication of a telescope.

The telescope is Makaʻiki, a new 6″ travel scope. Hours of design, cutting and drilling, sanding and spraying paint. There is nothing particularly hard about building a telescope, but it always takes more work that you remember from the last.

As usual the most difficult part of assembly is the last. Installing and aligning the optics. Adjustment of the secondary mirror for the correct position takes an hour of mucking about with allen keys, a Cheshire eyepiece, and a laser collimator. Get it centered, get the tilt right, correct placement of the secondary is critical for good performance of the telescope.

The final step is first light, the traditional ceremony when a new telescope is aimed at the sky for the first time. My traditional first light target is M42, the Great Orion Nebula. An old friend and telescope maker, Bob Goff, often stated he would like to tour this nebula in spirit after he died. I do not know if he made it, but I remember him each time I commission a new telescope and peer at the beauty of this nebula.

Given the time of year I had to get up before dawn for a view of this nebula. Driving uphill from the village I could see Orion rising above the dark shadow of Mauna Kea. In the predawn darkness I set up near Hale Pohaku for my first light checks. Set up is a bit grandiose for this telescope, there is not much to it. Open the tripod, set the scope on top, take the covers off and aim at the sky.

The first object to focus in the telescope was M42, the nebula appearing quite nice at low power. The 40mm eyepiece gives a 3.3° field at about 19x, wide enough to comfortably fit the entire nebula region in the field, from NGC1980 to NGC1981. From there I wandered about the sky… M1, M78, M79, M41, M35 made for a nice sampling of objects.

Higher powers shows that collimation can be a bit touchy in the ‘scope, not a surprise given the single strut design.

There were also three bright planets in the dawn. The brilliant Venus is shining near its maximum brightness right now, showing a nice crescent in the telescope. Mars is still too far away to show much of anything and Jupiter was still quite low in the brightening dawn, the moons barely visible.

First Light complete I knew that the effort to create a new telescope was worth it. Makaʻiki performs as designed. With a sense of satisfaction I headed uphill for breakfast and to join the crew for a day on the summit.

A Secondary Mount

The secondary mount is often one of the most complex pieces of a telescope build.

The mount must be small, not blocking any more light than the secondary mirror. It must be adjustable to allow tip, tilt, and centering of the secondary mirror. it must be rigid, able to hold the adjustment precisely once the telescope optical alignments are made.

Thus I have put together a new design for myself, to be used in the 6″ travel ‘scope under construction. The design is simple and robust, a pretty solid little device using the lessons I have learned across the years. It draws upon ideas from many other secondary mounts I have seen.

The body is square as this is easier to clamp during each of the manufacturing operations. The square is as large as possible to provide a stable mount, with the corners beveled just enough to stay behind the secondary.

The mount shown here is designed for a 31mm minor axis diagonal mirror. Thus the body of the mount is one inch square with 0.1″ beveled off each edge. This could be scaled up or down for another size diagonal mirror. The overall length of the entire mount is as short as possible. A bar of aluminum was cut to size and beveled first. A four inch bar yielded two complete mounts. I left the machined surfaces a bit rough to break up the smooth surfaces, better to hold paint and to avoid reflections.

There are four adjustment screws here. This allows for a simpler adjustment scheme than a three screw arrangement. With three screws adjustments are often made using all three screws at once. A four screw scheme allows the screws to be adjusted in pairs to effect moves in one axis at a time.

Secondary Mount Drawing — Mechanical drawing for the Travel6 secondary mount

I have made mounts with spring loaded screws in the past. They are easier to adjust, no need to loosen and tighten opposing screws. But they are not as rigid, the careful adjustment more likely to drift. This mount uses pusher screws that must be loosened and tightened in pairs.

Shallow pockets are drilled into the rear of the wedge for the tip of each adjustment bolt. this prevents any rotation of the wedge as long as the adjustment bolts are tight.

Vertical adjustments will be made by adjusting the height of the standoffs used to mount the secondary, or perhaps by shimming with a washer or two. The two mounting screws will sit in slotted holes allowing the secondary to be aligned with the focuser.

The center screw sits in a counterbored hole, only a small section of the shaft near the head is constrained. This allows it to tip slightly and allow the adjustments. A split lockwasher keeps some back tension during adjustments.

The secondary will be attached with RTV adhesive. The corners of the mount line up with the edges of the secondary making alignment quick and simple. All that is needed is a coat of flat black paint on the sides to prevent odd reflections in the optical path.

Building a New Telescope

No, I am not talking about TMT. I have something a bit smaller in process. For the first time in many years I am building a telescope. Just a little one.

The design for a 6″ f/5 travel telescope Makaʻiki

It has been a while, my cred as an ATM is in jeopardy of being revoked.

When I say a little one I am being serious, a 6″ f/5 is the design specification. This is the same optical specification as Primero, my first telescope. The difference here is in building a travel telescope, something that will fit in carry-on baggage with room for clothing.

I am modelling the telescope off a small telescope built by Brett Schaerer he named WikiKea, a telescope I had a chance to examine at Oregon Star Party a couple years ago. He incorporated a clever focus mechanism into that ‘scope that got my attention.

Brett built his telescope for a trip to Hawaiʻi, thus the Hawaiian influence in the name. I will follow suit and have decided that I am building Makaʻiki, which translates as little eye.

I have generated a full set of plans based on a couple photographs of the original telescope. That exercise itself was fun, I love designing things like this. When these plans are complete, a few details are to be settled during building, I will publish them here on DV.

I have manufactured many of the mechanical parts over the last week. A couple more days will be required to make the remainder. I have sourced many of the materials from my stash in the garage, including the plywood, scraps left over from building other telescopes including Deep Violet. I have found that the selection of stainless steel hardware at the Waimea ACE truly sucks, they are out of stock on everything.

Other parts are coming in from eBay merchants, like knobs and HDPE tape. The optics will likewise be from the eBay, most likely GSO. Later today there will be a stop at Home Depot, I am out of wood glue, the last bit thrown out as it had hardened in the bottle. And unlike the local ACE, their hardware selection is pretty good.

Update! The new ‘scope is complete, you can read more about it here.

TeleVue-76 to STi Autoguider Mount

A rigid mount to adapt the TeleVue-76 to the SBIG STi autoguider? I need such a solution, I have both of these bits of kit that need to be wed together for the minimal astrophoto setup. As I am unlikely to find such a part commercially, I would have to make it myself.

Another couple hours in the machine shop were in order, another small pile of aluminum chips. This actually went pretty quickly, these are easy cuts to make. No tapping is required, the four holes are simply drilled through. The two hours included design and cleanup for a quick project.

STi to TV-76 Mount — The SBIG STi to TelevVue TV-76 mount

TeleVue Autoguider Mount — Mechanical drawing for a mount to attach an SBIG STi autoguider to a TeleVue refractor

I came up with the design on the fly. A piece of aluminum from the scrap bin and a couple quick measurements of the ‘scope and guider. I simply cut aluminum until it seemed about right. It was only after the fact that I produced a drawing of the finished item.

The TV-76 has a rather non-standard mounting point on top for accessories like this. A pair of #10-32 threaded holes, 0.75″ apart and located in a slot 0.625″ wide milled into the mounting ring. This seems simple enough. An adapter made for the TV-76 should work with any of the TeleVue refractors that use this mounting. Another concern is that the solution must also be very rigid, any flexure between the autoguider and the telescope will result in smeared stars.

The design assumes that you have the additional guiding kit sold by SBIG for the STi autoguider. This provides the mounting rings that clamp the camera body.

The hardware required will be two ¼-20 x 1½” socket head cap screws, and two #10-32 x 1″ socket head cap screws. A pair of plastic press on caps convert the #10 screws to knobs. All parts you can find in a neighborhood hardware store.

The mount is 1.5″ high to set the autoguider away from the main OTA, as to not encounter any vignetting. This also allows space to get fingers onto the mounting knobs. The dimensions are chosen so that standard screws protrude by just the right amount. I messed this up on the one I made, machining the center to far. As a result a few washers are needed for the screws that mate with the refractor. This is fixed in the mechanical drawing.

To co-boresight the TV-76 with the autoguider it was necessary to slip a 1/4″ washer between the mount and the rear ring as a shim. With that in place the same object is centered in both fields of view. The STi has a 2.7° x 2.2° field when used with the 100mm lens supplied in the SBIG accessory kit. With this wide field of view it also serves as a finder to locate and frame the photographic target.

It assembles nicely, a good start. The true test will be the quality of the images produced by the rig.

Rewiring a Celestron NexStar Telescope

The original NexStar telescopes are great instruments. Ours has seen many uses, from dark skies to not so dark skies as it has been set up in the Arizona Desert, the summit of Mauna Kea, or various school yards and resorts for public viewing. It has been used as a visual instrument, a photographic ‘scope, even done a little real work.

For the most part the scope has worked well, and has been well maintained, even updated with the latest hand pad controller. But on occasion there is a problem… connection issues would crop up. The dreaded “No Response 16” or “No Response 17” errors would appear, indicating that the motor control board is not talking. This would result in having to power cycle and realign the telescope.

Lately the errors have become more problematic. The last straw was a public event I recently used the telescope for, setting up the telescope for Halloween. Continual errors plagued the evening, a constant struggle. While the scope usually tracked, I could not use GOTO as each alignment was quickly off by just enough to be useless.

Under the Hood — Under the hood of the NexStar showing the new cables

I have had the telescope apart too often in attempts to fix this, inspecting and re-seating the cables. This usually works, the problem will go away, for a while.

In general I like what I see inside the telescope. A well designed piece of kit with good components. Decades of taking gear apart have provided me so many examples of poor or good design. Inside the NexStar I just like what I see. The telescope is easy to get apart, just a few screws to remove each cover, exposing everything you might need to work on. The designers of this telescope obviously took pride in their work, it shows.

The exception to this is the wiring. There are a number of issues that can create trouble. Or rather there were a few issues, I just took care of that…

The Purple Refractor

Apparently the purple refractor was quite the hit on yesterday’s webcast. Not only did it provide beautiful imagery of the Transit of Venus for everyone to enjoy, but caught the attention of many viewers. There we many comments… “Where can I get one?”, “How much?”, and my favorite… “…paint Keck purple!”

Violet Haze really is a great telescope, an entirely hand-made instrument, with the exception of the focuser. The lens set is a full apochromatic triplet by Roger Ceragioli, a true expert in refractor design and manufacture. The purple optical tube is my own machine work. The result is a truly unique telescope that is a joy to use.

Imaging Venus in the Daytime — Imaging Venus in the daytime a mere 12°44' from the Sun.